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Magnetic Ceramics
EBB443-Technical Ceramics
Dr. Sabar D. Hutagalung
School of Materials & Min. Res. Eng.,
Universiti Sains Malaysia
Introduction
• Materials may be classified by their response
to externally applied magnetic fields as
–diamagnetic,
–paramagnetic, or
–ferromagnetic.
• These magnetic responses differ greatly in
strength.
Introduction
• Diamagnetism is a property of all materials and
opposes applied magnetic fields, but is very weak.
• Most materials are diamagnetic and have very
small negative susceptibilities (about 10-6).
• Example: Inert gases, hydrogen, many metals (Bi,
Ag, Cu, Pb), most non-metals and many organic
compounds.
• A superconductor will be a perfect diamagnet
since there is no resistance to the forming of the
current loops.
Introduction
• Paramagnetism is stronger than diamagnetism
and produces magnetization in the direction of
the applied field, and proportional to the applied
field.
• Paramagnetics are those materials in which the
atoms have a permanent magnetic moment
arising from spinning and orbiting electrons.
• The susceptibilities are therefore positive but
again small (in range of 10-3 – 10-6).
• The most strongly paramagnetic substances are
compound containing transition metal or rare
earth ions and ferromagnetics above Tc.
Introduction
• Ferromagnetic effects are very large,
producing magnetizations sometimes
orders of magnitude greater than the
applied field and as such are much larger
than either diamagnetic or paramagnetic
effects.
Relative Permeability
• The magnetic constant, m0 = 4p x 10-7 T m/A is
called the permeability of space.
• The permeabilities of most materials are very
close to m0 since most materials will be classified
as either paramagnetic or diamagnetic.
• But in ferromagnetic materials the permeability
may be very large and it is convenient to
characterize the materials by a relative
permeability.
Relative Permeability
Some representative relative permeabilities:
• Magnetic iron: 200
• Nickel: 100
• Permalloy (78.5% Ni, 21.5% Fe): 8,000
• Mumetal (75% Ni, 2% Cr, 5% Cu, 18% Fe):
20,000
Magnetic Field
• The magnetization of a material is expressed
in terms of density of net magnetic dipole
moments, m in the material.
• We define a vector quantity called the
magnetization M by
M = mtotal/V
• Then the total magnetic fields B in the material is
given by
B = B0 + m0M
• where m0 is the magnetic permeability of space and
B0 is the externally applied magnetic field.
Magnetic Field
• When magnetic fields inside of materials are calculated
using Ampere’s law or the Biot-Savart law, then the m0 in
those equations is typically replaced by just m with the
definition
m = Kmm0
where Km is called the relative permeability.
• If the material does not respond to the external
magnetic field, then Km = 1.
• Another commonly used magnetic quantity is the magnetic
susceptibility which specifies how much the relative
permeability differs from one.
• Magnetic susceptibility, cm = Km - 1
Magnetic Field
• For paramagnetic and diamagnetic materials the
relative permeability is very close to 1 and the
magnetic susceptibility very close to zero.
• For ferromagnetic materials, these quantities may
be very large.
• Another way to deal with the magnetic fields which
arise from magnetization of materials is to introduce
a quantity called magnetic field strength, H.
• It can be defined by the relationship
H = B0/m0 = B/m0 - M
Magnetic Field
• The relationship for B above can be written in the
equivalent form
B = m0(H + M)
• H and M will have the same units, amperes/meter.
• Ferromagnetic materials will undergo a small
mechanical change when magnetic fields are applied,
either expanding or contracting slightly.
• This effect is called magnetostriction.
Flux Magnet
• By definition, magnetic energy is the product of the flux
density in the magnetic circuit and the magnetizing force
it took to excite the material to that flux level.
• Energy = B x H
• The unit of energy in the SI system is the Joule, in the
CGS system it is the ERG.
• In permanent magnet design a special energy density, or
energy product, is also used to indicate energy and
storage properties per unit volume.
• The CGS unit of energy product is the Gauss-Oersted,
the SI unit is the Joule Per Meter3.
• 1 joule = 107 ergs
• 1 joule per meter3 = 125.63 gauss-oersted
Flux Magnet
Tesla in SI units:
• 1 Tesla = 10,000 Gauss
• 1 Tesla = 1 Weber/m2
• 1 Gauss = 1 Maxwell/cm2
• Flux density is one of the components used to
determine the amount of magnetic energy
stored in a given geometry.
Ferromagnetism
• Iron, nickel, cobalt and some of the rare earths
(gadolinium, dysprosium) exhibit a unique magnetic
behavior which is called ferromagnetism because
iron (ferric) is the most common and most dramatic
example.
• Ferromagnetic materials exhibit a long-range
ordering phenomenon at the atomic level which
causes the unpaired electron spins to line up
parallel with each other in a region called a domain.
• Ferromagnetism manifests itself in the fact that a
small externally imposed magnetic field can cause
the magnetic domains to line up with each other
and the material is said to be magnetized.
Ferromagnetism
• Ferromagnets will tend to stay magnetized to some extent
after being subjected to an external magnetic field.
• This tendency to "remember their magnetic history" is called
hysteresis.
• The fraction of the saturation magnetization which is retained
when the driving field is removed is called the remanence of
the material, and is an important factor in permanent
magnets.
• All ferromagnets have a maximum temperature where the
ferromagnetic property disappears as a result of thermal
agitation.
• This temperature is called the Curie temperature (Tc).
• Ferromagnetic materials are spontaneously magnetized
below a temperature term the Curie temperature.
Hysteresis Loop or BH Loop
Soft & Hard Magnetic
• Soft magnetic, or core products, do have the
ability to store magnetic energy that has been
converted from electrical energy; but it is
normally short-term in nature because of the
ease to demagnetize.
• This is desirable in electronic and electrical
circuits where cores are normally used because
it allows magnetic energy to be converted easily
back into electrical energy and reintroduced to
the electrical circuit.
• Hard magnetic materials (PMs) are
comparatively difficult to demagnetize, so the
energy storage time frame should be quite long.
Soft & Hard Magnetic
• Hard magnetic: high
remanent
magnetization (Br),
high coercivities (Hc),
difficult to
demagnetize, broad
B-H hysterisis loop.
Magnetic Domains
• The microscopic ordering of electron spins characteristic of
ferromagnetic materials leads to the formation of regions of
magnetic alignment called domains.
• The main implication of the domains is that there is already a
high degree of magnetization in ferromagnetic materials
within individual domains, but that in the absence of external
magnetic fields those domains are randomly oriented.
• A modest applied magnetic field can cause a larger degree
of alignment of the magnetic moments with the external field,
giving a large multiplication of the applied field.
Magnetic Ceramics
• All ferro- and ferrimagnetic materials exhibit
the “hysteresis effect” (a nonlinear
realtionship between applied magnetic field,
H and magnetic induction, B).
• Many materials have important magnetic
properties, including elemental metals,
transition metal alloys, rare earth alloys and
ceramics.
• Among the magnetic ceramics, ferrites are
the prodominant class.
Ferrites
• Ferrites using Fe2O3 as the major raw material.
• Ferrites crystallize in a large variety of structures:
– Spinel,
– Garnet, and
– Magnetoplumbite.
• Spinel: 1 Fe2O3 : 1 MeO, (MeO=transition metal oxide).
• Garnet: 5 Fe2O3 : 3 Me2O3 (Me2O3=rare earth metal
oxide)
• Magnetoplumbite: 6 Fe2O3 : 1 MeO (MeO=divalent
metal oxide from group II, BaO, CaO, SrO).
Ferrites
• The spinel ferrite are isostructural with the naturally
occuring spinel MgAl2O4 and conform to general
formula AB2O4.
• The realatively large oxygen anions are arranged in
cubic close packing, with octahedral and tetrahedral
interstitial site occupied by transistion metal cations.
• The rare earth yittrium iron garnet, Y3Fe5O12 (YIG)
is prototypical of the rare earth ferromagnetic
insulators.
MAGNETORESISTIVE EFFECT
• In magnetoresistive effect, the resistance of a material
changes in the presence of magnetic field.
• Similarly as the Hall effect, the magnetoresistive effect is
caused by the Lorentz force which rotates the current
lines by an angle qH.
• The deflection of the current paths leads to an increase
in the resistance of the semiconductor.
• For small angles of qH the resistance R is:
R  R0(1 + tan qH2 )
• The applicationsare in magnetic sensors.
Giant Magnetoresistance (GMR)
• The giant magnetoresistance (GMR) is the change in electrical
resistance of some materials in response to an applied
magnetic field.
• GMR effect was discovered in 1988 by two European
scientists working independently: Peter Gruenberg of the KFA
research institute in Julich, Germany, and Albert Fert of the
University of Paris-Sud .
• They saw very large resistance changes - 6 percent and 50
percent, respectively - in materials comprised of alternating
very thin layers of various metallic elements.
• These experiments were performed at low temperatures and
in the presence of very high magnetic fields.
Giant Magnetoresistance (GMR)
• It was discovered that the application of a
magnetic field to magnetic metallic multilayers
such as Fe/Cr and Co/Cu, in which
ferromagnetic layers are separated by
nonmagnetic spacer layers of a few nm thick,
results in a significant reduction of the electrical
resistance of the multilayer.
• In the absence of the
magnetic field the
magnetizations of the
ferromagnetic layers
are antiparallel.
• Applying the
magnetic field, which
aligns the magnetic
moments and
saturates the
magnetization of the
multilayer, leads to a
drop in the electrical
resistance of the
multilayer.
Intrinsic Magnetoresistance
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SrRuO3
Tl2Mn2O7
CrO2
La0.7(Ca1-ySr y)0.3MnO3
Fe3O4
CaCu3Mn4O12 (CCMO)