Chapter 6-Magnetic Ceramics

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Magnetic CeramicsEBB443-Technical Ceramics

Dr. Sabar D. HutagalungSchool of Materials & Min. Res. Eng.,

Universiti Sains Malaysia


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Introduction

Materials may be classified by their responseto externally applied magnetic fields as

diamagnetic,paramagnetic, or

ferromagnetic.

These magnetic responses differ greatly instrength.


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Introduction

Diamagnetism is a property of all materials andopposes 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 organiccompounds.

A superconductor will be a perfect diamagnetsince there is no resistance to the forming of thecurrent loops.


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Introduction

Paramagnetism is stronger than diamagnetismand produces magnetization in the direction ofthe applied field, and proportional to the appliedfield.

Paramagnetics are those materials in which theatoms have a permanent magnetic momentarising 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 arecompound containing transition metal or rareearth ions and ferromagnetics above Tc.


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Introduction

Ferromagneticeffects are very large,producing magnetizations sometimes

orders of magnitude greater than the

applied field and as such are much largerthan either diamagnetic or paramagnetic

effects.


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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 classifiedas 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.


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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


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Magnetic Field

The magnetization of a material is expressedin terms of density of net magnetic dipolemoments,min the material.

We define a vector quantity called the

magnetization M byM = mtotal/V

Then the total magnetic fields B in the material isgiven by

B = B0 + m0M where m0 is the magnetic permeability of space and

B0 is the externally applied magnetic field.


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Magnetic Field

When magnetic fields inside of materials are calculatedusingAmperes law or the Biot-Savart law, then the m0 inthose equations is typically replaced by just m with thedefinition

m = Kmm0where Km is called the relative permeability.

If the material does not respond to the externalmagnetic field, then Km = 1.

Another commonly used magnetic quantity is the magneticsusceptibility which specifies how much the relativepermeability differs from one.

Magnetic susceptibility, cm = Km - 1


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Magnetic Field

For paramagnetic and diamagnetic materials therelative permeability is very close to 1 and themagnetic 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 introducea quantity called magnetic field strength, H.

It can be defined by the relationship

H = B0/m0 = B/m0 - M


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Magnetic Field

The relationship for B above can be written in theequivalent form

B = m0(H + M)

H and M will have the same units, amperes/meter.

Ferromagnetic materials will undergo a smallmechanical change when magnetic fields are applied,

either expanding or contracting slightly. This effect is called magnetostriction.


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By definition, magnetic energy is the product of the fluxdensity in the magnetic circuit and the magnetizing forceit 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 theCGS system it is the ERG.

In permanent magnet design a special energy density, orenergy product, is also used to indicate energy andstorage 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


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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 todetermine the amount of magnetic energy storedin a given geometry.


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Ferromagnetism

Iron, nickel, cobalt and some of the rare earths(gadolinium, dysprosium) exhibit a unique magneticbehavior which is called ferromagnetism becauseiron (ferric) is the most common and most dramatic

example. Ferromagnetic materials exhibit a long-range

ordering phenomenon at the atomic level whichcauses the unpaired electron spins to line up

parallel with each other in a region called a domain. Ferromagnetism manifests itself in the fact that asmall externally imposed magnetic field can causethe magnetic domains to line up with each otherand the material is said to be magnetized.


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Ferromagnetism

Ferromagnets will tend to stay magnetized to some extentafter being subjected to an external magnetic field.

This tendency to "remember their magnetic history" is calledhysteresis.

The fraction of the saturation magnetization which is retained

when the driving field is removed is called the remanence ofthe material, and is an important factor in permanentmagnets.

All ferromagnets have a maximum temperature where theferromagnetic property disappears as a result of thermal

agitation. This temperature is called the Curie temperature (Tc).

Ferromagnetic materials are spontaneously magnetizedbelow a temperature term the Curie temperature.


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Hysteresis Loop or BH Loop


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Soft magnetic, or core products, do have theability to store magnetic energy that has beenconverted from electrical energy; but it isnormally short-term in nature because of theease to demagnetize.

This is desirable in electronic and electricalcircuits where cores are normally used becauseit allows magnetic energy to be converted easilyback into electrical energy and reintroduced to

the electrical circuit. Hard magnetic materials (PMs) arecomparatively difficult to demagnetize, so theenergy storage time frame should be quite long.

Soft & Hard Magnetic


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Soft & Hard Magnetic

Hard magnetic: highremanent

magnetization (Br),high coercivities (Hc),difficult todemagnetize, broad

B-H hysterisis loop.


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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.


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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.


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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 metaloxide)

Magnetoplumbite: 6 Fe2O3 : 1 MeO (MeO=divalentmetal oxide from group II, BaO, CaO, SrO).


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Ferrites

The spinel ferrite are isostructural with the naturallyoccuring spinel MgAl2O4 and conform to generalformula AB2O4.

The realatively large oxygen anions are arranged incubic close packing, with octahedral and tetrahedralinterstitial site occupied by transistion metal cations.

The rare earthyittrium iron garnet, Y3Fe5O12 (YIG) isprototypical of the rare earth ferromagnetic insulators.


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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 angleqH.

The deflection of the current paths leads to an increase

in the resistance of the semiconductor.

For small angles ofqHthe resistance Ris:

R R0(1 + tan qH2 ) The applicationsare in magnetic sensors.


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Giant Magnetoresistance (GMR)

The giant magnetoresistance (GMR) is the change in electricalresistance of some materials in response to an appliedmagnetic field.

GMR effect was discovered in 1988 by two European

scientists working independently: Peter Gruenbergof the KFAresearch institute in Julich, Germany, andAlbert Fertof theUniversity of Paris-Sud .

They saw very large resistance changes - 6 percent and 50percent, respectively - in materials comprised of alternating

very thin layers of various metallic elements. These experiments were performed at low temperatures and

in the presence ofvery high magnetic fields.


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Giant Magnetoresistance (GMR)

It was discovered that the application of amagnetic field to magnetic metallic multilayerssuch as Fe/Cr and Co/Cu, in which

ferromagnetic layers are separated bynonmagnetic spacer layers of a few nm thick,results in a significant reduction of the electrical

resistance of the multilayer.


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In the absence of themagnetic field themagnetizations of the

ferromagnetic layersare antiparallel.

Applying themagnetic field, whichaligns the magneticmoments andsaturates themagnetization of themultilayer, leads to adrop in the electricalresistance of themultilayer.


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Intrinsic Magnetoresistance

SrRuO3

Tl2Mn2O7

CrO2 La0.7(Ca1-ySry)0.3MnO3

Fe3O4

CaCu3Mn4O12 (CCMO)

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Chapter 6-Magnetic Ceramics