* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download chapter- ii literature review
Ising model wikipedia , lookup
Edward Sabine wikipedia , lookup
Lorentz force wikipedia , lookup
Friction-plate electromagnetic couplings wikipedia , lookup
Superconducting magnet wikipedia , lookup
Electromagnetic field wikipedia , lookup
Electromagnetism wikipedia , lookup
Magnetic stripe card wikipedia , lookup
Magnetometer wikipedia , lookup
Earth's magnetic field wikipedia , lookup
Electron paramagnetic resonance wikipedia , lookup
Magnetic monopole wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Magnetic nanoparticles wikipedia , lookup
Magnetotellurics wikipedia , lookup
Electromagnet wikipedia , lookup
Magnetotactic bacteria wikipedia , lookup
Force between magnets wikipedia , lookup
Magnetoreception wikipedia , lookup
Magnetohydrodynamics wikipedia , lookup
History of geomagnetism wikipedia , lookup
Giant magnetoresistance wikipedia , lookup
Multiferroics wikipedia , lookup
Chapter II Literature Review CHAPTER- II LITERATURE REVIEW 2.1 Magnetism of materials The origin of magnetism of materials comes from the orbital and spin motions of electrons, in which the electrons interact with one another. The best way to understand different types of magnetism is to describe how materials respond to magnetic fields. The main different is that the some materials there is no collective interaction of atomic magnetic moments, while in other materials there is a very strong interaction between atomic moments [7]. Magnetism arises due to the moving charges, such as an electric current in a coil of wire. Even when there is no current present in a material, still there are magnetic interactions. The processes which create magnetic field in an atom are: i) Nuclear spin, some nuclei such as in hydrogen atom, have a net spin which creates a magnetic field. ii) Electron spin, an electron has two types of spin states, which are called spin up and spin down. iii) Electron orbital motion. There is a magnetic field due to the orbital motion of the electron. Each of these magnetic fields interacts with one another and with external magnetic fields. Some of these interactions are strong; others are negligible weak. In most case, magnetic interaction with nuclear spin is a very minor effect. But interactions between the intrinsic spin of one electron and the intrinsic spin of another electron are strongest for very heavy elements such as the actinides. This is called spin-spin coupling. For these elements this coupling can shift the electron orbital energy levels. The interaction between on electron’s intrinsic spin and its orbital motion is called spin-orbit coupling has a significant effect on the energy levels of the orbital in many inorganic compounds. 2.2 Magnetic Hysteresis The lag or delay of a magnetic material known commonly as Magnetic Hysteresis, relates to the magnetisation properties of a material by which it firstly becomes magnetised and then de-magnetised. We know that the magnetic flux generated by an electromagnetic coil is the amount of magnetic field or lines of force produced within a given area and that it is more commonly called "Flux Density” given the symbol 7 Chapter II Literature Review B with the unit of flux density being the Tesla, T. It is known that the magnetic strength of an electromagnet depends upon the number of turns of the coil, the current flowing through the coil or the type of core material being used and if we increase either the current or the number of turns we can increase the magnetic field strength, symbol H. The relative permeability, symbol μr is the product of the absolute permeability μ and the permeability of free space μo (a vacuum) and this was given as a constant. However, the relationship between the flux density, B and magnetic field strength, H can be defined by the fact that the relative permeability, μr is not a constant but a function of the magnetic field intensity thereby giving magnetic flux density as: B = μ H. Then the magnetic flux density in the material will be increased by a larger factor as a result of its relative permeability for the material compared to the magnetic flux density in vacuum, μoH and for an air-cored coil this relationship is given as: B A and B 0 H For ferromagnetic materials the ratio of flux density to field strength (B/H) is not constant but varies with flux density. However, for air cored coils or any non-magnetic medium core such as woods or plastics, this ratio can be considered as a constant and this constant is known as μo, the permeability of free space, (μo = 4.π.10-7 H/m). By plotting values of flux density, (B) against the field strength, (H) we can produce a set of curves called Magnetisation Curves, Magnetic Hysteresis Curves or more commonly B-H Curves for each type of core material used as shown below. 8 Chapter II Literature Review 2.2.1 Magnetic Hysteresis Loop Figure 2.1 Magnetic Hysteresis of a ferromagnetic core. The Magnetic Hysteresis loop above (Fig 2.1) shows the behavior of a ferromagnetic core graphically as the relationship between B and H is non-linear. Starting with an unmagnetised core both B and H will be at zero, point 0 on the magnetisation curve. If the magnetisation current, i is increased in a positive direction to some value the magnetic field strength H increases linearly with i and the flux density B will also increase as shown by the curve from point 0 to point a as it heads towards saturation. Now if the magnetising current in the coil is reduced to zero the magnetic field around the core reduces to zero but the magnetic flux does not reach zero due to the residual magnetism present within the core and this is shown on the curve from point a to point b. To reduce the flux density at point b to zero we need to reverse the current flowing through the coil. The magnetising force which must be applied to null the residual flux 9 Chapter II Literature Review density is called a Coercive Force. This coercive force reverses the magnetic field rearranging the molecular magnets until the core becomes unmagnetised at point c. An increase in the reverse current causes the core to be magnetised in the opposite direction and increasing this magnetisation current will cause the core to reach saturation but in the opposite direction, point d on the cure which is symmetrical to point b. If the magnetising current is reduced again to zero the residual magnetism present in the core will be equal to the previous value but in reverse at point e. Again reversing the magnetising current flowing through the coil this time into a positive direction will cause the magnetic flux to reach zero, point f on the curve and as before increasing the magnetisation current further in a positive direction will cause the core to reach saturation at point a. Then the B-H curve follows the path of a-b-c-d- e-f-a as the magnetising current flowing through the coil alternates between a positive and negative value such as the cycle of an AC voltage. This path is called a Magnetic Hysteresis Loop. The effect of magnetic hysteresis shows that the magnetisation process of a ferromagnetic core and therefore the flux density depends on which part of the curve the ferromagnetic core is magnetised on as this depends upon the circuits past history giving the core a form of "memory". Then ferromagnetic materials have memory because they remain magnetised after the external magnetic field has been removed. However, soft ferromagnetic materials such as iron or silicon steel have very narrow magnetic hysteresis loops resulting in very small amounts of residual magnetism making them ideal for use in relays, solenoids and transformers as they can be easily magnetised and demagnetised. Since a coercive force must be applied to overcome this residual magnetism, work must be done in closing the hysteresis loop with the energy being used, being dissipated as heat in the magnetic material. This heat is known as hysteresis loss, the amount of loss depends on the material's value of coercive force. By adding additive's to the iron metal such as silicon, materials with a very small coercive force can be made that have a very 10 Chapter II Literature Review narrow hysteresis loop. Materials with narrow hysteresis loops are easily magnetised and demagnetised and known as soft magnetic materials. 2.2.2 Magnetic Hysteresis Loops for Soft and Hard Materials Figure 2.2 Magnetic Hysteresis Loops for soft and Hard Materials In Fig 2.2, Magnetic Hysteresis results in the dissipation of wasted energy in the form of heat with the energy wasted being in proportion to the area of the magnetic hysteresis loop. Hysteresis losses will always be a problem in AC transformers where the current is constantly changing direction and thus the magnetic poles in the core will cause losses because they constantly reverse direction. Rotating coils in DC machines will also incur hysteresis losses as they are alternately passing north the south magnetic poles. As said previously, the shape of the hysteresis loop depends upon the nature of the iron or steel used and in the case of iron which is subjected to massive reversals of magnetism, for example transformer cores, it is important that the B-H hysteresis loop is as small as possible. 11 Chapter II Literature Review 2.3 The origin of Atomic moments, spin and orbital states of Electrons The Schrödinger equation has led to information on the energy levels that can be occupied by the electrons [8]. The states that electrons can have are characterized by four quantum numbers. i. The principal quantum number (n) with values 1, 2, 3,…. determines the size of the orbits and describes its energy. Electrons in orbits with n=1, 2, 3,… are referred to as occupying the K, L, M,…. shells respectively. ii. The orbital angular momentum quantum number ( l ) describes the angular momentum of the orbital motion. The number ‘ l ’ can take one of the integral values 0, 1, 2,….,n-1 depending on the shape of orbit. The electros with a certain quantum number ‘ l ’ are referred as s, p, d… Electrons. iii. The magnetic quantum number ml describes the component of the orbital angular momentum l along a particular direction. In most cases, this direction is chosen along the applied magnetic field. For a given l , ml l , l 1,...,0,..., l 1, l. iv. The spin quantum number ms describes the electron’s spin s along a particular direction, usually along he applied magnetic field. The allowed values of ms are + ½ and – ½ According to Pauli’s principle, it is not possible for two electrons to occupy the same state, that is the states of two electrons are characterized by different sets of the quantum numbers n, l , ms and ml . The maximum number of electrons occupying a given shell is given by, n 1 2 (2l 1) 2n2 (2.6) l 0 Basically the moving electron can be considered as a current flowing in a wire that coincides with the electron orbit. The corresponding magnetic effects can then be derived by considering the equivalent magnetic shell. An electron with an orbital angular momentum l has an associated magnetic movement l e 2m l B l , Where B Bohr magnetron. 12 (2.7) Chapter II Literature Review The absolute value of the magnetic moment is given by l B l (l 1), (2.8) and its projection along the direction of the applied field is lz ml B, (2.9) The situation is different for the spin angular moment is s ge e 2m s g e B s, (2.10) Where ge (=2.002290) is the spectroscopic splitting factor or g-factor for the free electron. The component in the field direction is sz ge ms B, (2.11) The energy of the magnetic moment in a magnetic field H is given by the Hamiltonian H o .H .B (2.12) Where B is the flux density or magnetic induction. o 4 107 TMA1 is the vacuum permeability. The lowest energy Eo , the ground –state energy, is reached for and H parallel using Eq. (2.11) and ms 1 , one finds for one single electron 2 1 Eo o sz H ge ms o B H ge o B H 2 (2.13) 1 For an electron with spin quantum number, ms 1 , the energy equals ge o B H . 2 2 This corresponds to an antiparallel alignment of the magnetic spin moment with respect to the field. In the absence of a magnetic field, the two states characterized by ms 1 2 are degenerate, that is, they have the same energy. However, under an applied field this degeneracy is destroyed and the splitting in energy levels can be observed. This is showed in Fig 2.3 13 Chapter II Literature Review Figure 2.3 Effect of a magnetic field on the energy levels of the two electron states with ms 1 and ms 1 . 2 2 However, when describing the atomic origin of magnetism, one has to consider orbital and spin motion of the electros and the interaction between them. The total orbital (L) angular momentum of a given atom is defined as L li (2.14) i where the summation symbol includes all electrons. The same argument applies to the total spin angular momentum, defined as S si (2.15) i Here, one has to bear in mind that the summation over a complete shell is zero, the only contribution coming from incomplete shells. The resultants S and L thus formed are loosely coupled through the spin-orbit interaction to the resultant total angular momentum J : J LS 14 (2.16) Chapter II Literature Review Figure 2.4 Resultant total angular momentum J This type of coupling is called as Russell-Saunders coupling. This coupling is applicable to most magnetic atoms. Most of the magnetic properties of different types of materials depend on the applied magnetic field and temperature [8]. At zero temperature, the situation is simple because for any of the number of atoms (N) only the lowest level will be occupied. In this case, one obtains for the magnetization of the system M Ng J mB Ng J J B (2.17) However, at finite temperatures, higher lying levels will become occupied. The extent to which this happens depends on the temperature but also on the energy separation between the ground-state level and the excited levels, that is, on the field strength. The magnetization M of the system can then be found from the statistical average z of the magnetic moment z g J mB . This statistical average is obtained by weighing the magnetic moment µz of each state by the probability that this state is occupied and summing over all states: M N z 15 (2.18) Chapter II By substituting x Literature Review g J B o H kT into Eq. (2.18) and using the relation d ln x x 1dx and demx memx dx, after carrying out the differentiation, one finds M Ng B JBJ ( y) (2.19) Where BJ ( y) is the so-called Brillouin function, given by BJ ( y) 2J 1 (2 J 1) y 1 y coth coth , 2J 2J 2J 2J y With gJ B o H kT (2.20) (2.21) Keep in mind that in this expression ‘H’ is the field responsible for splitting the energy levels [8]. 2.4 Classification of magnetic materials In many atoms the electrons are completely paired, that is, for each electron spinning in one direction, there is an electron spinning in the opposite direction. The same situation exists in regard to the orbital motion of the electrons. Thus, the net current circulating about any given axis is zero and there are no Amperian current in such substances, consequently the resultant magnetic movement of the atom is also zero. Such substances show very “weak magnetic effects” that they are called “non-magnetic”. However, vacuum is the only true non-magnetic medium [9]. All materials show some magnetic effects with the exception of ferromagnetic group these effects are weak. It has been established both theoretically and experimentally that all mater may be classified into three groups, according to its fundamentally different behavior under the action of a magnetic field. These are classified as i) diamagnetic ii) paramagnetic and iii) ferromagnetic. The diamagnets and paramagnets consist of atoms with completely occupied electrons shells. In the absence of an external magnetic field, they do not possess a finite magnetization. The ferromagnets have spontaneous magnetic order. These are usually transition metal (3d) atoms with partially filled d and f electron shells. 16 Chapter II Literature Review Figure 2.5 Types of magnetism: (A) paramagnetism (B) ferromagnetism (C) antiferromagnetism (D) ferrimagnetisms (E) enforced ferromagnetism As mentioned already, the magnetic moment of a free atom has three principal sources: i) the electron spin ii) the electron orbital angular movement and iii) the change in the orbital momentum induced by an applied magnetic field. The first two effects give rise to paramagnetic contributions to the magnetization, while the third gives a contribution to the diamagnetism. The magnetization M is defined as the magnetic dipole moment per unit volume. The magnetic susceptibility per unit volume is defined as M B Where M – Magnetization B-Magnetic field intensity 17 (2.22) Chapter II Literature Review It is a dimensionless quantity. Substance with a positive susceptibility is called paramagnets, while materials with negatives susceptibility are diamagnets. The ferromagnetic materials have a spontaneous magnetic moment, that is, a magnetic moment exists even when the applied magnetic field is zero. In ferromagnetic crystals, the spontaneous moments suggests that electron spins and magnetic moments are arranged in a regular manner. From the thermodynamic point of view, the entropy of any magnetic materials increases with temperature and decrease with applied magnetic field. Therefore the spontaneous magnetization must decrease with temperature. There is a transition temperature Tc above which the spontaneous magnetization vanishes. This particular transition temperature is called the curieTemperature. Above the Curie temperature, the ferromagnetic materials become paramagnetic. The paramagnetic susceptibility above Tc is given as [10] C T TC (2.23) Where C is curie constant. The above Eq. (2.23) is called curie-Weiss law. In antiferromagnetic material spins are ordered in an anti-parallel configuration with zero net moment at temperatures below the temperature TN, which is called the Neel temperature. 2.5 Semiconductor materials Now a day’s semiconductor materials have been playing an important role in the electronic industry. Because of their application in devices such computers and mobile phones, the search for new semi conductor materials and the improvement of existing materials has been continuous. The key property of a semi conductor material is that it can be doped with impurities that alter its electronic property in a controllable way. The semi conductor material most commonly used is crystalline inorganic solids. These materials are classified according to the periodic table groups from which their constituent atoms come. Table.1 shows a list of some most common semiconductors. In this table it is important to note that oxide semi conductors, such as SnO2, ZnO or TiO2 are being widely used as DMS material. 18 Chapter II Literature Review Table.1 Some common semi conducting materials Semi conductor Group Silicon (Si) IV Germanium (Ge) IV Gallium arsenide (GaAs) III-V Indium nitride (InN) III-V Cadmium telluride (CdTe) II-VI Zinc Selenide (ZnSe) II-VI SnO2 Oxide semiconductor TiO2 Oxide semiconductor ZnO Oxide semiconductor 2.5.1 Transport phenomena in semi conductors In the second half of the 1940s, Bardeen, Brattain and Shockley discover the transistor effect. After the discovery of the transistor effect, a break in the behavior of doped Germanium conductivity at low temperature was observed. In 1956, Conwell [2] and Mott [3] suggested a model for a “new” process of conduction in which charge carriers conducts the electric current by thermally activated tunneling from an occupied site to an empty site. This process has been known as “phonon assisted hopping” and was the starting point of a number of transport theories, such as the model of miller and Abrahams [4]. This model became the most widely accepted theory of conduction between localized states and is the source of the variable range hopping (VRH) theory of Mott[5]. According to this model, electrons are “intelligent” since in the conduction process they hop from an initial state “i” to another “j” with energy as low as possible. For such an energy the site “j” is statistically located for from “i”, involving a distance rij=R, which generally is much larger than the decay length of the wave function. According to Mott, the conductivity ‘σ’ depends on two factors (i) Boltzmann W factor exp and (ii) overlap of the wave functions exp 2 R , where k K - Boltzmann constant 19 Chapter II Literature Review T - Temperature W - Hopping Energy 1 - Wave function decay length R - Distance separating Sites “i” and “j” W Mott consider that the conductivity “ ” behaves as the factor exp 2 R . kT So it may be written as o exp 2 R W kT (2.1) Where σo is a factor that is weakly temperature dependent. The idea of Mott was to maximize the hopping probability. So the express for the hopping energy W may be written in the form W 3 4 R N ( EF ) (2.2) 3 This gives the minimum value of “R” as R3 D 3 1 2 8 N ( EF )k 4 T 1 1 (2.3) 4 where N (EF) is the density of states at the Fermi level. It then appears that the distance of a hop increases as T decrease. Substituting Eq. (2.2) and Eq.(1.3) into Eq. l (2.1) we get 1 the T 4 Mott conductivity for non-crystalline semiconductor as 3D o 3D T 3D 14 exp o T (2.4) This mechanism is a powerful tool to explain and characterize electrical transport in semiconductors. Unfortunately not all semiconductors can be fitted under this model. The nearest neighbor hopping or Granular metal mechanism are just a few examples of various mechanism that have been introduced as an alternative to variable range hopping theory. In many cases the transport properties measured show a composite of different 20 Chapter II Literature Review transport mechanism. Thus different models can be applied at different temperatures, which prove that the transport in a semiconductor is a very complicated matter. 2.6 Dilute magnetic semi conductors Room temperature ferromagnetism in semiconductor materials has been studied for few years and has received renewed attention, to some extent because of possible uses in spintronic application [11]. The research in this field is primarily focused on transition metal doped II-VI, IV-VI, II-V and III-V compound semiconductors. Strong ferromagnetic interactions between localized spin have been observed in Mn-doped II-VI compounds. However, recently the group of materials researched has been extended to various oxide semiconductors as well. Among these oxides the strong favorite is SnO2. Recently room temperature ferromagnetism has been reported for SnO2 doped with transition metals, showing the potential for achieving room temperature spintronic technologies [12]. 2.6.1 Ferromagnetism in oxide dilute magnetic semiconductors In dilute solid solutions of a magnetic ion in a non magnetic metal crystal (such as Mn in Sn) the exchange coupling between the ion and the conduction electron has important consequences. The conduction electron gas is magnetized in the vicinity of the magnetic ion, with the spatial dependence. This magnetization causes an indirect exchange interaction between two magnetic ions. The interaction is known as the Fried or Ruder man, Kittle, Kasuya and Yosida (RKKY) interaction. A consequence of the magnetic ion-conduction electron interaction is the Kondo effect. The occurrence of a resistance minimum is connected with existence of localized magnetic moments on the impurity atoms. Where a resistance minimum is found, there is a inevitably a local moment. The central result is that the spin-dependent contribution to the resistivity is Spin C M 1 3zJ ln T C o C 1 ln T EF where J - exchange energy 21 (2.23) Chapter II Literature Review Z -number of nearest neighbors C -Concentration M -Measure of the strength of the exchange scattering We see that the spin resistivity increases toward low temperature if J is negative. If the phonon contribution to the electrical resistivity goes as T5 in the region of interest and if the resistivities are additive, then the total resistivity has the form aT 5 C o C 1 ln T (2.24) d C 5aT 4 1 0 T dT (2.25) C 1 5a (2.26) With a minimum at Hence 15 Tmin The temperature at which the resistivity is a minimum varies as the one-fifth power of the concentration of the magnetic impurity, in agreement with experiment at least for Fe in Cu. Recently S.J. Liu et al [13] have discussed a situation where ferromagnetism in magnetic semiconductors originates from Itinerant-Electron magnetism. This model is suitable for semiconductors that are rich in charge carriers. The localized moments arises when the magnetic atoms form part of an intermetallic compound. In these cases, the unpaired electrons responsible for the magnetic moment are no longer localized and accommodated in energy levels belonging exclusively to a given magnetic atom. Instead, the unpaired electrons are delocalized; the original atomic energy levels having broadened into narrow energy bands. Extend of this broadening depends on the interatomic separation between the atoms. According to a calculation made by Heine (1967), the following relation applies between the width of the energy bands W and the interatomic separations (r). W α r-5 (2.27) The most prominent examples of itinerant-electron system are metallic system based on 3d transition elements, with the 3d electrons responsible for the magnetic properties. For a discussion of the magnetism of the 3d electron bands, we will make the simplifying 22 Chapter II Literature Review assumption that these 3d bands are rectangular. This means that the density of electron states N(E) remains constant over the whole energy range spanned by the band width W. A maximum of ten 3d electrons per atom, that is, five electrons of either spin direction, can be accommodated in the 3d band. In the case of cu metal, because each cu atom provides ten 3d electrons, the 3d band will be completely filled. However, in the case of other 3d metals, less 3d electrons are available per atom so that the 3d band will be partially empty. Such a situation is shown in Fig 2.6.a. In Fig.2.6.a, we have indicated that there is no discrimination between electrons of spinup and spin-down direction with respect to band filling. Both types of electrons will therefore be present in equal amount, meaning that there is no magnetic moment associated with the 3d band in this case. However, this situation is not always a stable one, as will be discussed below. It is possible to define effective exchange energy U eff per pair of 3d electrons. This can be regarded as the energy gained when switching from antiparallel to parallel spin. In order to realize such gain in energy, electrons have to be transferred, say, from the spin-down sub band into the spin-up sub band. This can be seen in Fig.2.6.b, this implies an increase in Kinetic energy, which counteracts this electron transfer. However, it will be shown below that such transfer is likely to occur if U eff is large and density of states at the Fermi level EF is high. After the transfer, there will be more spin-up electrons than spin-down electrons, and the magnetic moment, which has arisen, will be equal to (n1 n2 )B . First we will derive a simple band model, which accounts for the existence of ferromagnetism. The interaction Hamiltonian, can be written as H U eff n1n2 (2.28) Where n1 and n2 represents the number of electrons per atom for each spin state, and where the total number of 3d electrons per atom equals n n1 n2 . Because Ueff is a positive quantity, Eq. (2.28) will lead to the lowest energy if the product n1n2 is as small as possible. For equally populated sub bands, this product has its maximum value and hence the highest energy, consequently, electron transfer is always favorable for the lowering of the 23 Chapter II Literature Review exchange energy and this electron transfer will come to an end only if one of the two spin sub bands is empty or has become completely filled up. It is defined that N(E) as the density of states per spin sub band, and P as the fraction of electrons that has moved from the spin-down band to the spin-up band. This means 2 pn n1 n2 (2.29) Let us assume that the interaction Hamiltonian (Eq.2.28) leads to an increase in the number of spin-up electrons at the cost of the number of spin -down electrons. The corresponding gain in magnetic energy is then [8]. EM U eff n1n2 U eff 1 2 n , 4 EM U eff n2 p 2 (2.30) This energy gain is accompanied by an energy loss in the form of the amount of energy EC needed to fill the states of higher kinetic energy in the band. For a small displacement E EF E2 E1 EF (see Fig. 2.6.b) this kinetic energy loss can be written 1 as EC E (n1 n2 ) Enp . 2 The total energy variation is 1 then EC EM U eff n2 p 2 Enp . Since, N ( EF ) E (n1 n2 ), one may write, 2 E EC EM n2 p 2 1 U eff N ( EF ) N ( EF ) (2.31) If 1 U eff N (EF ) 0 , the state of lowest energy corresponds to p=0 and the system is non-magnetic. However, if 1 U eff N ( EF ) 0 , the 3d band is exchange split (p>0), which corresponds to ferromagnetism, this is the stoner criterion for ferromagnetism, which is frequently stated in the more familiar form (stoner, 1946) If U eff N (EF ) 1 , by means of this model, it can be understood that 3d magnetism leads to non-integral moment values if expressed in Bohr magnetrons per 3d atoms, 2 pnB . The conditions favoring 3d moments in metallic system are obviously; a large value for U eff , but also a large value for N (EF). The density of states of the S- and P-electron bands is considerably smaller than that of the d band, which explains why 24 Chapter II Literature Review band magnetism is restricted to elements that have a partially empty d band. However, not all of the d-transition elements give rise to d-band moments. For example, in the 4d metal Pd, the stoner criterion is not met, although it comes very close to it. 1→E1, 2→EF , 3→E2 Figure 2.6 Schematic representation of a partially depleted 3d band: (a) Paramagnetism, (b) Weak ferromagnetism, (c) Strong ferromagnetism with n 5 , (d) Strong ferromagnetism with n 5 . 2.6.2 3d elements doped Oxide semiconductor: SnO2 Recently SnO2 has been most widely researched oxide semiconductor for DMS materials. SnO2 is a direct wide band-gap semiconductor with applications in photovoltaic devices, gas detectors and transparent electronic devices such as transparent widow heaters. The band gap of SnO2 is ~3.7eV at room temperature. Another advantage of SnO2 is that this band gap can be easily controlled by introducing interstitial ions, such as CO-, Fe-, Ni-, V- and W- by varying Sn composition, the direct energy band gap can be adjusted from 3.7 to 4.15 eV [13]. A controllable band gap is very important for spin life time and spin transport in semiconductor based spintronic devices [14]. SnO2 has a Rutile (tetragonal), tp6 type crystal structure (2, 4) with lattice parameter a= 4.737 Ao and C= 3.186 Ao. The mineral form of SnO2 is called cassiterite and this is the main ore of tin. This colourless, diamagnetic Solid is amphoteric SnO2 is usually 25 Chapter II Literature Review regarded as an oxygen-deficient n-type semiconductor. “Stannic acid” refers to hydrated Tin dioxide, SnO2, which is also “stannic hydroxide”. Figure 2.7 crystal structure of SnO2 For spintronic, theoretical prediction [15] suggests that room temperature ferromagnetism is possible in Mn doped SnO2. Experimental results on 3d metal doped SnO2 are very controversial. In fact, Mn-doped SnO2 films showed room temperature ferromagnetism in some studies [17], while other observed Para magnetism [16]. Mn is just one of many transition metal ions that have been used in the synthesis of magnetic SnO2. The samples were synthesized in the research for the best candidate in the family of materials. So far, Ni-doped [18, 23, 29, 31], V-doped [20, 21, 22], Codoped [27, 25], W-doped [35] SnO2 have been reported. They were reported to be both ferromagnetic and non-ferromagnetic even for the same sample or compositions. Soft chemical technique or sol-gel is the most effective method that can help to avoid the formation of unneeded impurities [36]. Such synthesis can provide better control over the composition of materials than is obtained with high temperature vacuum deposition or solid state synthesis technique. On the other hand, it is a simple, lower-cost method, and easier. The solution made by this chemical technique could provide a homogeneous distribution of dopant ions throughout the sample. Furthermore, the whole synthesis process was performed in the absence of vacuum, which could prevent the formation of metal nanoparticles. In the present research work, the 3d-metal doped and codoped SnO2 nanocrystalline DMS were synthesized by soft chemical techniques or solgel method. 26 Chapter II Literature Review The recent research progress on oxide dilute magnetic semiconductors has been summarized in this section. It is evident that, many research groups have reported observation of room temperature ferromagnetism in oxide semiconductors. However, reports on the absence of ferromagnetism or impurity mediated ferromagnetism in such materials are also numerous. It is of very great importance to synthesis and analyzes materials that would be pure intrinsic magnetic semiconductors. From all of the oxide semiconductors reported, SnO2 is probably the best candidates for spintronic applications. 2.6.3 Mn-doped SnO2 K. Gopinadhan et al. (2009) have investigated that it is possible to induce room temperature ferromagnetism, in tin oxide thin films by introducing manganese in a SnO2 lattice; observed temperature dependence of the magnetization predicts a curie temperature exceeding 550 k. The generation of additional free electrons by F doping in Mn-doped SnO2 does not cause any increase in the magnetic moment per Mn ion, suggesting no significant role of electron in bringing about the magnetic ordering [15]. Davinder Kaur et al. (2009) synthesized Mn doped SnO2 thin films. Magnetic studies reveal RTFM, a systematic change in magnetic behavior from ferromagnetic to paramagnetic was observed with increase in substrate temperature from 500 to 700o C for Mn doped SnO2 film. The presence of room-temperature ferromagnetism could be obtained by controlling the substrate temperature and Mn doping concentration [16]. Susmita kunda et al. [32] has reported that room temperature ferromagnetism in highly transparent Mn (II) - doped ITO films deposited on soda lime silica glass and pure silica glass. The Mn-O bond length and the Mn-Mn pair exchange interaction is found to be depend on the substrate smaller bond length and stronger exchange interaction is found in the case of films deposited on soda lime glass substrate and larger bong length and weaker exchange interaction in the case of films deposited on pure silica glass substrate. They achieved both magnetic ions doping in nanocrystals of In2O3 as well as strong room temperature ferromagnetism without any degradation of optical transparency which could be useful for many opto-spintronics based devices [32]. Z.M.Tian et al. (2008) prepared Mn doped SnO2 nanoparticles by chemical co-precipitation method. They systematically investigated the microstructure and magnetic properties of the prepared samples. The X- 27 Chapter II Literature Review ray diffraction reveals that all samples are pure rutile type tetragonal phase. They found that the cell parameters a and c decrease monotonously with the increase of Mn content which indicated that Mn ions substitute into the lattice of SnO2. The XPS studies indicate the mixture state of Mn3+ and Mn4+ in the samples. The magnetic investigations demonstrate that the magnetic properties strongly depend on both the sintering temperature and doping content. They found that no ferromagnetism was observed for samples sintered at high temperature irrespective of the doping content. Also the samples sintered at low temperature posses’ room temperature ferromagnetism when the doping content is below 5% [34]. Yanxue chen et al. (2009) has investigated to the influence of the growth temperature on the electrical conductance, the crystal structure, the surface morphology and the optical property of Mn doped SnO2. X-ray diffraction pattern show that the thin films grown at lower substrate temperature were amorphous, while those grown at higher temperatures had only a single orientation. Atomic force microscopy results indicate that the films grown at low temperature were extremely flat and they became rougher as substrate temperature increased. Optical transmission reveals that the absorption edge shifts towards the blue side and become steeper at increasing substrate temperature. High- resolution transmission electron microscopy analysis clearly indicates that the samples have a well-arranged tetragonal structure without any detectable secondary phase. Inhomogeneous incorporation of Mn atoms into the SnO2 lattice was observed [37]. B. Sathyaseelan et al. (2010) synthesized pure and transition metal ion doped SnO2 nanoparticles using a simple chemical precipitation method. (Mn and Co) of 1, 3, and 5 mol% were doped in order to study the influence of structural and optical properties. The structural, chemical and optical properties of the samples were analyzed using x-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, and UV-Vis spectroscopy techniques. The SnO2 crystallites were found to exhibit tetragonal rutile structure with lattice parameters, revealing that the metal ions get substituted in the SnO2 lattice. It is observed that the peak position (110) get shifted to lower angles by increasing the dopant concentration. Hence the lattice parameters and the cell volume get decreased. The SEM photograph shows that the grain size of pure SnO2 is larger that of the metal-doped SnO2 indicating grain grown upon doping. TEM photograph revels that by increasing the content of metal ions, average grain size 28 Chapter II Literature Review decreased. A significant red shift in the UV absorbing band edge was observed with the increase in the amount of Mn and Co contents [38]. H. Kimura et al. (2001) fabricate Mn doped SnO2 Epitaxial films by pulsed laser deposition method. Thin films with (101) and (100) orientations were grown on r-(102) and c (001) sapphire substrates, respectively. They found that Mn ions are soluble into SnO2 films up to 30 mol%. Transmission spectra show d-d transition absorption in mid –gap region due to presence of the Mn ion. Additional doping of Sb induces an n-type conduction with a carrier concentration as high as 6.9x1019cm-3 at 300k. The resistivity rapidly increases with decreasing the temperature below 50k, where considerable increase of the resistivity is observed in a magnetic field below 20k [39]. Feng Gu et al. (2003) studies the room temperature photo luminescent properties of Mn- doped tin oxide nanoparticles. The samples are crystalline with a tetragonal rutile structure of tin oxide. The FTIR and the UV-Vis absorption spectra of the samples have also been investigated. The photo luminescence spectra are measured at room temperatures as a function of different sintering temperatures and the Mn2+ concentration, respectively. The luminescence processes are associated with oxygen vacancies in the host and related with the recombination of electrons in single occupied oxygen vacancies with photo excited holes in the valence band [40]. A.J Nadolny et al. (2002) fabricated the thin layers of semi magnetic Sn1-xMnxTe semiconductor with Mn content x≤0.04 and layer thickness 0.22µm were grown by molecular beam epitaxy(MBE) on BaF2(111) substrates with SnTe and Mn fluxes, they employed an additional Te flux to provide the efficient way of controlling the deviation from stoichiometry in the layers. By manipulating the MBE conditions they obtained SnMnTe crystalline layer with the conducting hole concentration in the range from P=5x1019 cm-3 to 2x1021 cm-3. In the layers grown under extra Te flux condition they observe ferromagnetic transition with the Curie temperature Tc ≤ 6k. Layers with the same Mn content but grown with no extra Te flux are paramagnetic. This dramatic change of magnetic properties, referred to as a carrier concentration induced ferromagnetic transition, can be understood in the frame work of a model attributing the ferromagnetic interactions in SnMnTe to the mechanism of indirect exchange via conducting holes [41]. Y.W.Heo et al. (2004) studies the effects of high dose implantation of Ni, Fe, Co and Mn ions into SnO2. The x-ray diffraction showed no 29 Chapter II Literature Review evidence of secondary phase formation in the SnO2. The Mn –implanted SnO2 remained paramagnetic, as also reported for samples doped during thin film growth, but the Fe, Co, and Ni–implanted SnO2 showed evidence of hysteresis with approximate curie temperature of 300k. The carrier density in the implanted region appears to be too low to support carrier-mediated origin of the ferromagnetism and formation of bound magnetic polarons may be one explanation for the observed magnetic properties [42]. K.Gopinadhan et al. (2007) induce room temperature ferromagnetism, exhibiting high transition temperature dependence of the magnetization in a SnO2 lattice. The temperature dependence of the magnetization study predicts a curie temperature exceeding 550 k in these films. X-ray and electron diffraction investigations, in combination with magnetization studies, reveal that the RTFM is not due to any ferromagnetic impurities. X-ray structure factor calculations suggest that Mn has been incorporated in the SnO2 matrix. Also, the optical absorption study predicts a random alloy formation for Sn1-xMnxO2-δ systems in conformity with the predictions of the other observed properties. Different levels of F doping in Sn0.09Mn0.10O2-δ have been carried out, and the results indicate that the RTFM is not carrier mediated [50]. 2.6.4 Ni-doped SnO2 Nguyen Hoa hong et al. (2005) had developed Ni-doped SnO2 thin films on different substrates. All the sample shows room-temperature ferromagnetism and films grown on LAO (LaAlO3) substrates have a large magnetic moment, about one order larger than that of films grown under the same conditions on STO (SrTiO3) and Al2O3 substrates [17]. Junying zhang et al. (2010) reported that magnetic moment per Ni ion decreases with the increase of Ni doping because antiferromagnetic Super– exchange interaction takes places in the nearest neighbour Ni2+ ions for the samples with high Ni content and also the annealing at oxidizing and reducing atmosphere shows oxygen vacancies play a crucial role in producing ferromagnetism [18]. C.M.Liu et al.(2007) had investigated the influence of Nickel dopant on the microstructure and optical properties of SnO2 nanopowders. XRD measurements indicate that the grain size becomes larger and the structure of lattice grows better with low nickel content and the obtained samples are polycrystalline with rutile structure of pure SnO2, no impurity phase cannot present. 30 Chapter II Literature Review From the optical studies the band gap diminishes with the increase of nickel content. The nickel doping does not change the PL peak position. The sintering temperature increases the ratio of UV/blue decreases, the band gap increases and finally saturated [29]. C.M. Liu et al. (2009) explored the role played by nickel doping in magnetic property and PL of SnO2 nanopowders. They found that there is no need for transition metal doping to change the defects into magnetic polarization as the undoped SnO2 nanopowders are magnetic and nickel doping disturbs the magnetic interaction of pure SnO2 nanopowders. The PL peaks position changes little with dopant content, UV emission is separated into two luminescence peaks at higher doping [31]. X.Liu et al. (2010) fabricated Ni- doped SnO2 hollow spheres and characterized by x-ray diffraction, x-ray photoelectron spectroscopy, inductively coupled plasma–optical emission spectroscopy and transmission electron microscopy[44]. 2.6.5 Fe-doped SnO2 and Cu-doped SnO2 Aditya Sharma et al. (2011) had developed Fe and Ni doped SnO2 nanoparticles using simple wet chemical method. The XRD pattern indicates that Fe and Ni are incorporated into SnO2 lattice without forming any oxide phases. UV-Visible absorption spectra results indicate the decrease of optical band gap with increasing concentration. From the room temperature magnetization measurements, they observed ferromagnetic hysteresis loops and magnetic moments per Fe\Ni ions were found to decrease with increase of Fe and Ni concentrations. The decrease in the magnetic moment with increase in concentration is consistent with the known super-exchange interaction among the transition metal ions [26]. C. B. Fitzgerald et al. (2004) synthesized SnO2 doped (Mn, Fe, or Co) dilute magnetic semiconductors and they found room temperature ferromagnetism in it. They found the formation of a rutile-structure phase from x-ray diffraction. For (Sn0.95Fe0.05) O2 the saturation magnetization is 0.2 and 1.8 Am2kg-1. The Curie temperature is found to be 340 and 360 k respectively. From the x-ray diffraction pattern, the magnetization cannot be attributed to any impurity phase. The 57 Fe Mossbauer spectra of the Fe-doped SnO2 samples, recorded at room temperature and 16k, show that about 85% of the iron is in a magnetically ordered high spin Fe3+ State, the remainder being paramagnetic [33]. L. M. Fang et al. (2009) prepared Fe3+-doped 31 Chapter II Literature Review SnO2 nanoparticles by sol-gel-hydrothermal route. The SnO2 crystalline with tetragonal rutile structure could form directly during the hydrothermal process without calcination Fe formed stable solid solutions in SnO2 nanoparticles. Compared to sol gel calcination route, sol-gel hydrothermal route led to better disperse spherical Fe3+-doped SnO2 nano particles with narrow size distribution and larger specific surface area. The composite nanoparticles prepared by the sol-gel-calcination route. Large surface areas of Fe3+-doped SnO2 nanoparticles prepared by sol-gel-hydrothermal route make it particularly appealing in applications for gas sensors and optoelectronics devices [36]. C. Van komen et al. (2008) reports the structural and magnetic behavior of transition metal doped SnO2 nanopowders by sol-gel based chemical method. Relatively stronger ferromagnetism was observed in Co, Fe and Cr ions doped samples. In these systems, strong correlations between the changes in the structural and ferromagnetic properties were observed. A limiting dopant concentration XL was observed below which the SnO2 lattice contracts and ferromagnetic magnetization strengthens, both in proportion to x. This is attribution due to substitutional incorporation of the transition metal ions as the dominant process the SnO2 lattice expands and the ferromagnetic behavior is destroyed. They attribute this behavior to additional interstitial incorporation of the dopant ions and depletion of the free carriers [43]. J. M. D. Coe et al. (2003) fabricated thin films by pulsed –laser deposition. The Fe-doped SnO2 thin films are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610k and 2.2 Am2kg-1 respectively. The 57 Fe Mossbauer spectra show the iron is all high-spin Fe3+. But the films are magnetically inhomogeneous on an atomic scale, with only 23% of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8µB, is greater than for any simple iron oxide with super exchange interactions. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy is proposed to explain the high Curie temperature [45]. S. Sambasivam et al. (2011) prepared a series of Fe-doped SnO2 nanoparticles by sol-gel method. Structure of the samples is found to be rutile and the crystalline size is found to decrease with increase of Fe content. The g- values of the ESR spectra reveal that the nature of Fe in the prepared samples is isolated rhombic Fe3+ ions in rutile phase, and the line width of ESR increase with increase in Fe3+-content due to the ion induced disorder effect. The temperature dependent magnetization confirms 32 Chapter II Literature Review that the Fe3+-ions have weak antiferromagnetic interactions. However, for the samples with high Fe content, an extra spin-pumping is observed below 250k. The optical absorption spectra of Fe doped SnO2 nanoparticles showed a blue shift in the band gap compared to the undoped SnO2 samples [47]. Junko sakuma et al. (2007) prepared transparent conducting SnO2 powders by a polymerized complex method. It is confirmed from XRD data that the powder samples prepared from acidic solution have the rutile structure of The 57 SnO2 and that the annealing temperature affects the structural parameters. Fe Mossbauer spectra consist of only doublets, including no magnetic sextets. However, the samples prepared at 500◦c and 600◦c showed ferromagnetism. The magnetization of the sample at 550◦c has larger magnetization than the sample at 600◦c [48]. Khaled melghit et al. (2005) prepared nanosize solid solution of Fe-doped SnO2 at low temperature. The XRD data of the samples shows that the sample is rutile phase without any trace of an extra phase. SEM and TEM reveal a homogeneous composition of the sample. The size was found to increase slightly with temperature that Fe3+ substitute for Sn4+ [49]. 2.6.6 Co-doped SnO2 K. Srinivas et al. (2008) had investigated the structural, optical and magnetic properties of nanocrystalline Co doped SnO2 based diluted magnetic semiconductors. From the XRD pattern of all the samples reveals that all the samples have tetragonal rutile structure. The average crystalline size varies from 15 to 40 nm. From the FTIR spectra it is clear that there are clear changes in the position, size and shape of IR peaks indicating the Co ions incorporate in SnO2 host. From the magnetization studies they conclude that cobalt doping enhanced the specific magnetization value. The Room temperature ferromagnetism have been achieved by modifying oxygen vacancies and electronic structure through stabilization of unstably bonded Co2+ ions by controlling the particle size, shape and surface stoichiometry. In the case of ferromagnetic nanocrystals, the strength of exchange coupling between magnetization of adjacent ferromagnetic grains also depends on the magnetic nature of the matrix [27]. Weibing chen et al. (2011) reported that SnO2 doped with less than 5% Co synthesized by sol-gel-Hydrothermal method exhibits room temperature ferromagnetism. From the XRD pattern any other 33 Chapter II Literature Review secondary phases or clusters of cobalt cannot present and the sample holds a curie temperature as high as 400K and shows a combining character of ferromagnetism and antiferromagnetism [28]. C. B. Fitxgerald et al. (2006) reported that the high temperature ferromagnetism in thin films of transition metal doped SnO2 is an intrinsic effect. The moments at low concentrations approach the spin-only values, and they cannot be explained in terms of secondary impurity phases. Nor can the anisotropy of the magnetization of the films be explained in these terms. Absence of anomalous Hall Effect indicates that there is negligible 3d density of states at EF. Much of the magnetic moment is associated with electronic defects of lattice defects. The dopants somehow serve to activate the defect moment, since the undoped SnO2 is net ferromagnetic. The high Curie temperature observed for low dopant implys that the defect related magnetism is inhomogeneous [51]. X. F. Liu et al. (2009) fabricate Co-doped SnO2 insulating films using magnetron sputtering. X-ray photoelectron spectrum reveal a solid solution of Co dopants in SnO2 lattice, where Co is in 2+ oxidation state and substitutes for Sn4+. The films exhibit clearly room-temperature ferromagnetism and with more structural defects show higher saturated magnetic moment, while increase of annealing time, crystallinity of the films is improved and the defects decrease, leading to the decrease of ferromagnetism. This shows that the ferromagnetism in Co-doped SnO2 is intrinsic and can be considerably influenced by the concentration of structural defects [52]. Hongxia Wang et al (2009) reported the effects of Co-dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO2 are studied by the first principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximation. The Co atoms favorably substitute on neighboring sites of the metal sub lattice. For Co-doped SnO2 without oxygen vacancies, spin polarization occurs mainly at Co site and Co is in its low spin state, which is independent of the concentration and distribution of Co atoms. Furthermore only nearest-neighbor Co atoms are ferromagnetically coupled through direct exchange and super exchange interaction. Oxygen vacancy prefers to locate near the Co atom and strongly increases the local magnetic moment of Co atom and strongly increases the local magnetic moment of Co atoms. The presence of the vacancy cause calculated magnetic moment to depend sensitively on the concentration and distribution of Co atoms and induce the long-range 34 Chapter II Literature Review ferromagnetic couplings between Co atoms with large distance through the spin-split impurity band exchange model which contribute to the room temperature ferromagnetism observed experimentally in the Co-doped SnO2 [53]. L. Yan et al. (2009) has reported the Co-doped SnO2 thin films deposited on Si substrates by pulsed- laser deposition. The XRD results showed that the 5at% Co-doped SnO2 with rutile- structure could be well grown on Si substrate. The Co-doped SnO2 thin film has good room temperature magnetic property with the saturated magnetic moment of 1.3µB/Co at 293 k. The x-ray photoelectron spectroscopy measurements for the Co-doped SnO2 thin film on Si predicted that Co has oxidation states of +3. The good magnetic property of the Codoped SnO2 may be deduced the exchange between Co3+ and Co3+ through the oxygen vacancy [54]. Hongxia Wang et al. (2009) studies the electronic structure and ferromagnetic stability of Co-doped SnO2 using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective Uco transforms the ground state of Co-doped SnO2 to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+Uco calculations show that the pure substitutional Co defects in SnO2 cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+Uco show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and large spin split of Co-3d and impurity states induced by the addition of Uco enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a coulomb U0 on Oxygen 2s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+Uco calculation are not changed by the correction of band gap [55]. Yongbin xu et al. (2009) prepared Co-doped SnO2 diluted magnetic semiconductor nanocrystals successfully and obtained roomtemperature ferromagnetism. The magnetic field-assisted approach has been used in the synthesis of Co-doped SnO2 diluted magnetic semiconductor nanocrystals. They found that the external high magnetic field can influence the growth behavior of Co2+ doped 35 Chapter II Literature Review SnO2 nanocrystals, and ferromagnetic properties would improve obviously various techniques such as x-ray diffraction, transmission electron microscope, UV-Visible spectrometry, Raman spectrometry and Vibrating sample magnetometer have been use to characterize the obtained products [56]. N. Brihi et al. (2007) have used the coprecipitation technique to synthesize polycrystalline Co-doped tin oxide diluted magnetic semiconductors of different concentrations. The X-ray diffraction analysis confirmed the rutile structure of SnO2. Transmission electron microscopy confirms the uniform distribution of Co inside the samples. Optical absorption measurements showed an energy band gap which decreases when increasing the concentration of Co. Raman spectroscopy indicates the significant structural modifications and disorder of the SnO2 lattice. Magnetic measurements revealed a mixture of para and antiferro behavior of Codoped SnO2 [60]. 2.6.7 Vanadium (V) -doped SnO2 and Tungsten (W)-doped SnO2 Sang-Do Han et al. (2000) prepared V-doped SnO2 nano crystalline powder by co-precipitation method. The average crystal size decreases with increasing amount of doped Vanadium. The morphology of nanocrystalline vanadium-doped SnO2 particle is spherical and the distribution of crystal size is relatively uniform [20]. Li Zhang et al. (2005) reported that the V-doped SnO2 films on Si (III) shows RTFM with no trace of any impurity phases. The magnetic moment per V drops rapidly with the increase in V content. The annealing study of the films in rich-oxygen and poor-oxygen atmospheres reveals that the rich-oxygen annealing may introduce more oxygen ions, hence reduce oxygen vacancies, which are responsible for the decrease in Ms, while the vacuum annealing may remove oxygen vacancies, and so favor for room- temperature ferromagnetism. Therefore, these results that the oxygen vacancy plays a crucial role in inducing the ferromagnetism in V-doped SnO2 and the origin of ferromagnetism can be explained by the BMP Model [21]. 36 Chapter II Literature Review Figure 2.8 Vanadium atom (green) in the SnO2 rutile structure. Nguyen Hoa Hong at al. (2005) developed V-doped SnO2 thin films were grown on LAO (LaAlO3) , STO (SrTiO3) and R-Cut Al2O3 substrates. The V-doped SnO2 Films grown on Al2O3 do not show ferromagnetism in the whole range of temperature, while the films grown on both LAO and STO show room temperature ferromagnetism. It is noted that the V-doped SnO2 Films grown on LAO substrate is larger than that of the V-doped SnO2 films grown on STO substrate. From the above results the substrate is one of the factors for inducing RTFM [22]. N. Tahir et al. (2009) synthesized V-doped ZnO nanoparticles by sol-gel method. XRD patterns of samples show that all samples exhibit wurtzite structure with no secondary phases. The presence of V-doping was confirmed by the EDX analysis. SEM images reveal that particles are almost of spherical in nature. The TPR analysis indicates the electronically modified material with V doping. The air annealed sample show no ferromagnetism, while samples annealed under reducing atmosphere show clearly room temperature ferromagnetism and there is an increase in saturation magnetization with increase in vanadium concentration. They believe that RTFM is due to the defects or oxygen vacancies. The blue shift in band gap energy has been observed. This might be due to the defects such as oxygen vacancies leading to the Burstein Moss shift [24]. X. L. Wang et al. (2008) had investigated the ferromagnetism in SnO2 doped with transition metals. Their calculation show that ferromagnetism is 37 Chapter II Literature Review obtained by doping SnO2 with Fe and Co ions and paramagnetism is ground state in Vand Mn- doped SnO2. They found that oxygen vacancy has a strong influence on the magnetic properties of the transition metal (TM) ion doped compounds, TM-oxygen-TM groups will be common in Fe- and Co- doped SnO2, but are not formed in V- and Mn doped SnO2. Additional n-type carriers are required to stabilize the ferromagnetic state and the Curie temperature increases with increasing densities of n-type carriers in Vdoped SnO2. They also propose that raising the density of n-type carriers is a possible practical way to obtain high Curie temperature [30]. Yanwei Huang et al. (2009) had investigated on structural, electrical and optical properties of tungsten-doped tin oxide thin films by pulsed plasma deposition method with post annealing. They reported that the electrical conductivity is enhanced greatly, while maintaining high optical transmittance in the range of 400 nm-2500 nm. The polycrystalline films posses’ tetragonal rutile structure of SnO2 and higher-valence element tungsten doping can efficiently improve the conductivity of the thin film. The lowest resistivity of 6.67x104 ohm cm was reproducibly obtained and the average optical transmittance is 86% in the visible region and about 85% in the near-Infra red region [35]. 2.6.8 Transition metal codoped SnO2 S. J. Liu et al. reported that the Zn and Mn codoped SnO2 films shows room – temperature ferromagnetism. They deduced that the valence of Mn ions in the (Mn, Zn) codoped SnO2 films is 2+ where as Mn doped SnO2, the valence of Mn ions is 3+. The difficult in the valence state due to the codoping of Zn. The Mn doped SnO2 films exhibit diamagnetic and paramagnetic respectively. By introducing a few percent of Zn (5at %) the codoped films exhibit stable and hysteretic ferromagnetism at room temperature. It is believed that the magnetic moments in the codoped SnO2 films come from the Mn2+ions whose numbers of unpaired 3d electrons (3d5) and ion radius (80pm) are both larger than those of Mn3+ ions (3d4 and 58pm).Thus more unpaired electron and larger ion radius would increase the probability of ferromagnetic coupling between magnetic Mn2+ ions. On the other hand, defects resulted from Zn doping and delocalized carriers originated from oxygen vacancies also enhance the ferromagnetic interaction. It can be seen that the saturated magnetization decrease with decreasing the carrier 38 Chapter II Literature Review concentration. This result reveals that the itinerant carrier plays a dominant role in the ferromagnetic interaction between the magnetic Mn2+ ions. The dependence of the saturated magnetization on the carrier concentration is consistent with the carrier – mediated model [13]. Xing Li et al. (2007) observed room-temperature ferromagnetism in Fe and Ni-codoped In2O3 samples. (InO.9 Fe o.1-x Ni x)2 O3, O< x < 0.1 samples were prepared by citric acid sol-gel method. All of the samples with intermediate x values are ferromagnetic at room- temperature. The Fe-doped sample is found to be weakly ferromagnetic, while the Ni-doped sample is paramagnetic, but the co-doped sample has enhanced room-temperature ferromagnetism. The magnetic and magneto-transport including Hall effects studies on the samples indicate the observed ferromagnetism is intrinsic rather than from the secondary impurity phase [23]. Rezq naji Aljawfi et al. (2011) reported the properties of Co/Ni codoped ZnO based nanocrystalline DMS. There is no change of lattice parameters indicates that the ionic radius of Co2+,Ni2+ and Zn2+ are more or less same and the XRD, XPS spectra shows the incorporation of the Co2+ and Ni2+ ions into the ZnO lattice and replace Zn2+ sites without changing the wurtzite structure. From the UV spectra the absorption edge of sample shifts towards lower energy. This is mainly due to the Co2+ ions got incorporated into the lattice structure of ZnO. From the FTIR spectra, the gradual shift in the absorption frequency with different Co/Ni doping is caused by the difference in the bond lengths that occurs when Co2+ and Ni2+ ions replace Zn2+ ions. And confirm the incorporation of Co and Ni into ZnO lattice structure. The RTFM has been presented in terms of vacancy in the frame of BPM model, where the Co ions at surface of the nanoparticles can be ferromagnetically coupled and mediated by oxygen vacancies. Because of low carrier concentration in the system, they cannot use the RKKY theory to explain the RTFM [25]. 39 Chapter II Literature Review Reference [1]. Jacob Millman, “Integrated electronics,” Tata McGraw-Hill edition, New Delhi (1999). [2]. E.M. Conwell, Phys. Rev, 103, 51 (1956). [3]. N.F. Mott, J.Phys. 34, 1356 (1956). [4]. A.Miller and E.Abrahams, Phys.Rev. 120, 745 (1960). [5]. N.F.Mott, Phil.Mag. 19, 835 (1969). [6]. C.L.Chien and C.R.Westgate, “The Hall effects and its application,” New York: Plenum (1980). [7]. Saxena , Gupta, Saxena, “Fundamentals of solid (2004). [8]. K.H.J.Buschow and F.R.De Boer, “Physics of magnetism and magnetic materials,” Kluwer Acadecmic/Plenum publishers, New York (2003). [9]. K.K.Tewari, “Electricity and magnetism,” S.Chand& Company Ltd, New Delhi (1998). [10]. Charles Kittel, “Introduction to solid state physics,” John wiley &sons, New York (2000). [11]. Nan zheng, “Introduction to dilute magnetic semiconductors,” University of Tennessee Knoxville (2008). [12]. S.A.Wolf, J.Supercond.13, 195 (2000). [13]. S.J.Liu, C.Y.Liu, J.Y.Juang and H.W.Fang, J.of .App.Phy. 105,013928 (2009). [14]. R.Mattan, J.M.George, H.Jaffres, F.Nguyen Van Dua, A.Fert, B.Lepine , A.Guivarc’h, and G.Jezequel, Phys,Rev, Lett,90,166601 (2003). [15]. Subhasb C.Kashyap, K.Gopinadhan, D.K.Pandya, Sujeet chaudhary, JMMM. 321,957-962 (2009). 40 Chapter II Literature Review [16]. Ajay kaushal, Prerna Bansal, Ritu vishnoi, Nitin chaudhary, Davinder Kaur, Physica B.404, 3732-3738 (2009). [17]. Nguyen Hoa Hong, Antoine Ruyter, W.Prellier, Joe sakai and Ngo Thu Huong, J.Phys.: condens, Matter.17 ,5633-6538 (2005). [18]. Junying Zhang, Qu Yun, Qianghong Wang, Mod.Appl.Sci.4,11 (2010). [19]. J.Dubowik, I.Goscianska, Y.V.Kudryavtsev, A.Szlaferek,JMMM. 310, 2773-2775 (2007). [20]. Sang –Do Han, Hua Yang , Li Wang , Jong- won Kim, Sensors and Actuatirs B. 66,112-115 (2000). [21]. Li Zhang, Shihui Ge, Yalu zuo ,Xueyun Zhou, Yuhua Xiao, Shiming Yan, Xiufeng Han, Zhenchao wen, J.Appl.Phys. 104, 123909 (2008). [22]. Nguyen Hoa Hong, Joe Sakai, Physica B. 358,265-268 (2005). [23]. Xing Li, Changtai Xia, Guangqing Pei, Xiaoli He, JPCS. 68,1836-1840 (2007). [24]. N.Tahir, S.T.Hussain, M.Usman, S.K.Hasanain ,A.Mumtaz, Appl.Sur. Sci. 255, 8506-8510 (2009). [25]. Rezq Naji Aljawfi, S.Mollah, JMMM. 323,3126-1332 (2011). [26]. Aditya Sharma , Mayora Varshney, Shalendra kumar, K.D.Verma and Ravi Kumar, Nanometer.nanotechnol. 1(1),29-33 (2011). [27]. K.Srinivas, M.Vithal, B.Sreedhar, M.Manivel Raja and P.Venugopal Reddy, P.PHYS. Chem.C. 113.3543-3552 (2009). [28]. Weibing chen and Jingbo Li, J. Appl. Phys. 109,083930 (2011). [29]. Liu Chun –Ming ,Fang Li- Mei, Zu Xiao-Tao and Zhou Wei-Lie, Chinese Physics. 16,1,95-99 (2007). 41 Chapter II Literature Review [30]. X.L. Wang , Z.X.Dai and Z.Zeng, J.Phys.: Condens. Matter. 20, 045214 (2008). [31]. C.M.Liu, L.M. Fang, X.T.Zu and W.L.Zhou, Phys.Scr. 80, 065703 (2009). [32]. Susmita Kundu, Dipten Bhattacharya, Jitten Ghosh, Pintu Das, Prasanta K.Biswas, Chemical .Phys.Lett. 469, 313-317 (2009). [33]. C.B.Fitzgerald, M.Venkaesan, A.P.douvalis, S.Huber and J.M.D.Coey, J.Appl.Phys. 95 (2) (2004). [34]. Z.M.Tian, S.L.Yuan, J.H.He, P.Li, S.Q.Zhang , C.H.Wang, Y.Q.Wang, S.Y.Yin, L.Liu, J.All.Com. 466, 26-30 (2008). [35]. YanWei Huang, Guifeng Li,Jiahan Feng, Qun Zhang, Thin solid films. 26409 (2009). [36]. L.M.Fang, X.T.Zu, Z.J.Li, S.Zhu, C.M.Liu, W.L.Zhou. L.M.Wang, J.All.Com. 261267 (2008). [37].Yanxue Chen, Jun Jiao, IJMPB. 23,6&7,1904-1909 (2009). [38].B.Sathyaseelan, K.Senthilnathan, T.Alagesan,R.Jayavel, K.Sivakumar, J.Mat.Che.Phys. 124,1046-1050 (2010). [39].H.Kimura T.Fukumara, H.Koinuma, M.Kawasaki, Physica E. 10,265-267 (2001). [40]. Feng Gu, Shu Fen Wang, Meng Kai Kii, Yong Xin Qi, Guang Jun Zhou, Ding Xu, Duo Rong Yuan , Inorg.Che. Comm. 6, 882-885 (2003). [41]. A.J.Nadolny, J.Sadowski, B.Taliashvili, M.Arciszewska, W.Dobrowoslski, V.Domukhovski, E.Lusakowska, A.Mycielski, V.Osinniy, T.Story, K.Swiatek, R.R Glazka, R.Diduszko, JMMM. 248,134-141 (2002). [42]. Y.W.Heo, J.Kelly, D.P.Norton, A.F.Hebard, S.J.Pearton, J.M.Zavada, L.A.Boatner, Ele.Chem and Solid- State Lett. 7,12,309-312 (2004). 42 Chapter II Literature Review [43]. C.Van Komen , A.Thurbe, K.M .Reddy, J.Hays, A.Punnoose, J.Appl.Phys. 103,07D141 (2008). [44]. Xiaghong Liu, Jun Zhang, Xianzhi Guo, Shihua Wu, Shurong Wang, Sensors and Actuators B.(2011). [45]. J.M.D.Coey, A.P.Douvalis, C.B.Fitzgerald, M.Venkatesan, Appl.Phys.Lett. 84,8,1332-1334 (2003). [46]. Pradip KR Kalita, B.K.Sarma, H.L.Das, Bull.Mater.Sci. 23,313-317 (2000). [47]. S.Sambasivam, Byung Chun choi, J.G.Lin, JSSC. 184,199-203 (2011). [48]. Junko Sakumar, Kiyoshi Nomura , Cesar Barrero, Masuo Takeda, Thin solid films. 515,8653-8655 (2007). [49]. Khaled Melghit, Khalid Bouziance, Mat.Sci and Engi B. 128, 58-62 (2006). [50]. K.Gopinadhan, Subhash C .Kashyap, Dinesh K.Pandya and Sujeet Chaudhary, J.Appl.Phys. 102,113513 (2007). [51]. C.B.Fitzgerald, M.Venkatesan, L.S. Dorneless, R.Gunnung, P.Stamenov, and J.M.D.Coey, Phys.Rev. B. 74,115307 (2006). [52]. X.F.Liu ,W.M.Gong, Javed lqbal ,B.He, R.H.Yu, Thin solid films. 517, 6091-6095 (2009). [53]. Hong Xia Wang, Yu Yan , Y.S.H.Mohammed ,Xiaobo Du, Kai Li, Hanmin Jin, JMMM. 321,337-342 (2009). [54].L.Yan,J.S.Pan ,C.K.Ong,MSEB.128,34-36 (2006). [55]. Hong Xia Wang, Yu Yan ,Y.Sh. Mahammmed, Xiaobo Du, Kai Li, Hanmin Jin, JMMS. 321,3114-3119 (2009). [56]. Yongbin Xu, Yongjun Tang, Chuanjun Li, Guanghui Cao, Welin Ren, Hui Xu, Zhongming Ren, J.All. Com. 481,837-840 (2009). 43 Chapter II Literature Review [57]. T.Krishnakumar, R.Jayaprakash, M.Parthibavarman, A.R.Phani, V.N.Singh, B.R.mehta, Mat.Lett. 63,896-898 (2009). [58]. P.S.Patil, S.B.Sadale, S.H.Mujawar, P.S.Shinde, P.S.Cjigare, App.Su.Sci, 253,685085567 (2007). [59]. M.Parthibavarman, V.HariHaran, C.Sekar, V.N.Singh, J.Opto.Ele. &. Adv.Mat. 12, 9, 1894-1898 (2010). [60]. A.Bouaine and N.Brihi, G.Schmerber, C.Ulhaq-Bouillet, S.Colis, A.Dinia, J.Phys.Chem. 111, 2924-2928 (2007). 44