Materials Science C

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Materials Science C

• Magnetic properties

Based on B.S. Mitchell, An introduction to
materials engineering and science for chemical
and materials engineers pp600-625
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Magnetic Materials
Examples compass, ferrite, tapes
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Magnetic Properties of Metals and Alloys
Molecular origin of magnetism
Magnetic dipole moment
Pole strength
Separation distance
In an external magnetic field

• Intrinsic magnetic dipole moment
• Orbital motion of electrons around nucleus
• Net spin of unpaired electrons

Bohr magneton ?B 9.27 ? 10-24 A?m2
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Molecular origin of magnetism
Vacuum
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Different magnetic materials
? lt -10-5
Diamagnetic (independent of T and H)
Paramagnetic (electron spins)
? 10-6 10-2
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Paramagnetic moments in metals

• Only electrons with energy near the Fermi level
  are responsible for spin paramagnetism

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Types of magnetism

• Very large positive susceptibility below Curie
  temperature, ?c

Ferromagnetism

• Small susceptibility in general, due to equal
  antiparallel spin magnetic moments
• Maximum susceptibility at Néel temperature, ?N

Antiferromagnetism

• Parallel and antiparallel spin magnetic moments
  dont cancel each other net spin moment exists
• Very large susceptibility below Néel temperature,
  ?N

Ferrimagnetism
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Types of magnetism
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Types of magnetism
Ferromagnetism
Antiferromagnetism
Ferrimagnetism
T gt ?C paramagnetism
T gt ?N paramagnetism
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Types of magnetism
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Magnetic Domains

• Typical domain wall 100 nm thick
• Net magnetization zero, in the absence of
  external magnetic field

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Magnetic Hysteresis
Ms Saturation magnetization, at Bs Br
Remnant induction Hc Coercivity
Area or hysteresis loop Energy loss per unit
volume of material per cycle
Eddy current applied magnetic field varies
cyclically
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Soft and Hard Magnets

• Soft magnets
• High magnetic permeability
• Low coercivity
• Can be easily magnetized and demagnetized

• Hard magnets
• High remnant induction
• High coercivity
• Large hysteresis loss
• Related to microstructure

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Soft Magnets
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Magnetism in Alloys
Outer-shell energy bands in Ni
Absolute zero
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Magnetism in Alloys
Saturation magnetization Cu-Ni alloy
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Magnetism in Alloys
Saturation magnetization of Ni alloys
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Hard Magnets
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Magnetic Properties of Ceramics and Glasses
Diamagnetic most ceramic/glasses, due to
closed electronic shells Paramagnetic from
unpaired electrons due to transition, rare-earth,
actinide elements (of little importance) Ferrim
agnetic typical example Ferrite (Fe3O4 ?
FeO?Fe2O3 / MIIO?Fe2O3) Antiferromagnetic
exchange between unpaired electrons causing
antiparallel spin alignments MnO, FeO, NiO,
CaO (NaCl structure)
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Antiferromagnetic Spin Alignments (FeO)
(111) plane
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Antiferromagnetic ceramics
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Ferrites
General formula MOFe2O3 M divalent
element, Fe, Cu, Mn,

• Typical ferrites
• Familiar magnetite FeOFe2O3
• Three crystal structures inverse spinel, garnet,
  hexagonal
• Soft, hard, semihard magnets
• Superparamagnetism

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Most important types of ferrites
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Ferrites
Spinel structure
Inverse spinel unit cell
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Magnetite (Fe3O4) electron configuration
Fe(III)
No preference
Favors octahedral
Fe(II)
Fe3O4 is inverse spinel Fe3Fe2,Fe3O4
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Ferrites electron configurations
Magnetic moments of ions in tetrahedral sites are
oriented opposite to those of ions in octahedral
sites
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Summary

• Molecular origin of magnetism
• Diamagnetic/paramagnetic materials
• Magnetic susceptibility
• Three types of magnetism ferromagnetism,
  antiferromagnetism, and ferrimagnetism
• Magnetic hysteresis
• Ferrites