Magnetic Ceramics

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Magnetic Ceramics

• EBB443-Technical Ceramics
• Dr. Sabar D. Hutagalung
• School of Materials Min. Res. Eng.,
• Universiti Sains Malaysia

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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.

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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.

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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.

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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.

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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
  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.

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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 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.

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Magnetic Field

• When magnetic fields inside of materials are
  calculated using Amperes 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

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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

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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.

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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

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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 to
  determine the amount of magnetic energy stored in
  a given geometry.

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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.

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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.

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Hysteresis Loop or BH Loop
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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.

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Soft Hard Magnetic

• Hard magnetic high remanent magnetization (Br),
  high coercivities (Hc), difficult to demagnetize,
  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, (MeOtransition metal
  oxide).
• Garnet 5 Fe2O3 3 Me2O3 (Me2O3rare earth metal
  oxide)
• Magnetoplumbite 6 Fe2O3 1 MeO (MeOdivalent
  metal oxide from group II, BaO, CaO, SrO).

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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.

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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 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.

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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.

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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.

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• 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.

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Intrinsic Magnetoresistance

• SrRuO3
• Tl2Mn2O7
• CrO2
• La0.7(Ca1-ySry)0.3MnO3
• Fe3O4
• CaCu3Mn4O12 (CCMO)