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Giant thermoelectric power of ferromagnetic nanocomposites
СoX (Al2O3)1-X in magnetic field
G.V. Lashkarev1, M.V. Radchenko1, M.E. Bugaiova1, A.Ye. Baibara1, W. Knoff2,
T. Story2, L.A. Krushynskaya3, Y.A. Stelmakh3
1I.M.
Frantsevych Institute for Problems of Material Science, National Academy of Sciences of Ukraine, 3 Krzhizhanovskogo
str., Kyiv, Ukraine. [email protected]
2Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
3E.O. Paton Electric Welding Institute, National Academy of Sciences of Ukraine, 68 Antonovich str., Kyiv, Ukraine
Ferromagnetic СoX (Al2O3)1-X nanocomposites (FMNC) represent two phase material in which Co
nanoparticles (NP) are distributed in dielectric matrix Al2O3. The scientific and application importance
of such materials is due to their spin dependent properties. FMNCs attract attention as artificial materials with predicted magnetic structure. Throughout technological control of size, shape, and concentration of single-domain ferromagnetic nanoparticles (NPs) one can impart to FMNC the required
magnetic and electrical properties. Layes of composites were deposited by two the help of two crucible evaporation by electron beams.
Investigation of thermoelectric power (TEP) were carried out in the temperature region of 90300K in magnetic fields up to 5 kOe.
At Co content below percolation threshold (PT) (34 at%) we observed the phenomenon of strong
TEP enlargement (5 times and more) at temperature region (90-300K) at switching on the magnetic
field of 5 kOe.
We suppose that the giant TP in magnetic field is due to a jumping mechanism of the electron
transport via magnetic centres in the conditions of a temperature gradient. Co NPS play role in electron transport below PT.
For dielectrics the main mechanism for electron transport is the hoping electron between the centers of localization [1]. At hopping type conduction with variable range of jumps electron transport in
temperature gradient is spin-dependent, if centers of electron localization are mag0netic ones i.e. contain cobalt. Magnetic moments of such magnetic centres (MC) can be oriented by an external magnetic
field. The jump conduction mechanism realizes via centers of electron localization located near the
Fermi level EF. Energy position of MC without electron is higher than EF, while the centers occupied
by electrons are settled at energies below EF. Their magnetic moments are oriented randomly at H=0.
When an external magnetic field is switched on, magnetic moments of empty magnetic centers as well
as spins of electrons (s) hopping to them, become oriented parallel to one another by the field. In the
last case the potential barrier for electron jumps and a scattering by spin disorder become lower.
The energy of the center, which was left by an electron, increases and its energetic position becomes higher than EF. At the same time MC, which is occupied by an electron lowers its energy to a
position below EF. When a temperature gradient is applied to FMNC electron movement becomes
directional and at switching on a magnetic field the drift velocity increases because of the parallel
directions for electron spins and magnetic moments of centers containing Co, between which these
jumps occur. Such spin-dependent movement of electrons in temperature gradient at the presence of
magnetic field leads to the giant magnetic TEP also because the scattering of electrons becomes less
intensive because of magnetic disorder decrease. Besides that the activation energy εp of jumps in the
case of s↑↑M becomes smaller than in the case of their random orientation εr (εp ‹ εr). The TP magnitude in magnetic field increases with a decrease of a temperature due to the thermal disorientation
weakening of magnetic moments for MCs and electrons.
Discovered unusual phenomenon can be a basis for a development of high magnetosensitive detectors for different applications.
1. N.F. Моtt, E.A. Davis, Electronic processes in non crystalline materials (M., The world: 1974).