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MAGNETIC METALSSTRUCTURE –PROPERTIES
APPLICATIONS
Howard H. Liebermann, Ph.D.
Fundamentals
Structure of Metals On atomic level, regular arrangement of atoms immersed
in “sea” of “free electrons”. Results of this:
Metallic bond Electrical, thermal conductivity Ductility
Typical arrangements of atoms: BCC, FCC, HCP
Atypical arrangement of atoms: Amorphous
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Cubic Crystal Systems
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FCCBCCSimple Cube
Basic Magnetics
Electron has negative charge Orbiting of electrons about atom induces magnetic
moment (vector) These magnet moments can interact
with one another with an external applied magnetic field
Extent of interaction determines what kind of magnetism (exchange vs. anisotropy)
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Magnetic Axes in BCC (Fe)
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Magnetic Axes in FCC (Ni)
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Kinds of Magnetism
Ferromagnetism: magnetic spin interaction is large – applied external magnetic field doesn’t affect this
Paramagnetism: magnetic spins tend to align in the direction of applied field
Diamagnetism: magnetic spins tend to align in the direction away from applied field
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Some Magnetic Characteristics
Exchange - strong interaction between magnetization vectors
Anisotropy – preferential direction for magnetization vector in a material
Magnetostriction – interaction between stress (applied, residual, etc.) and magnetization vector direction
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Curie Temperature
Temperature above which sample magnetization ceases.
True for ferromagnetic, paramagnetic, etc. Potential in sensor applications.
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Exchange Interaction
Quantum mechanical effect:
Tendency for adjacent magnetic vectors to align directionally.
Affected by thermal energy.
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Magnetic Anisotropy
Origin: Tropy – direction Iso – constant An – not
Conclusion – not constant with direction in an alloy.
Magnetic anisotropy result of: Crystal structure of alloy. Shape of sample being tested. Magnetic field induced.
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Magnetostriction
Link between change in magnetic sample dimensions (stress) and applied magnetic field.
Reciprocity abounds.
Stress can result from numerous causes: Forces applied to magnetic sample. Residual forces resulting from cooling on heat treating. Forces arising during use of a device.
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Schematic Example
Iron
Cobalt
Iron + Cobalt
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Magnetic Domain Wall Width
w ~
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εK
Magnetization Loop
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Magnetic Domain Wall Motion
No external field (applied, residual, etc.) and magnetization vector direction
Low external field
High external field
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Other Domain Wall Mechanisms
Rotational
Reverse domain nucleation
Eddy current generation Magnetic losses Electrical losses Heat losses
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Hard vs. Soft Magnets
Hard Soft
• robust magnetic field • easily demagnetized
• largely impervious to external fields
• high permeability
• can be costly • switching applications
• provide strong field
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Applications of Magnetic Mat’ls
Hard Soft• stators/rotors • low loss transformers• motors/generators • inductors (various)
• EAS (bias alloy) • Invar (Fe-Ni) alloys• refridge door gaskets, etc. • nanocrystalline• toys • Maglev train
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Applications of Hard Magnets
Field of Application
Products Requirements Materials
• Permanent Magnets
• Loudspeakers• Small
generators/motors• Sensors
• Large H C and MR • Fe-based• Fe+ ~(0.7–5)% Si• Fe +~(35-50)%Co
• Analog Data Storage
• Video tape• Audio tape
• Medium H C and MR (hysteresis loop square)
• Fe/Co/Ni/Al/Cu = 50/24/14/9/3
• SmCo5
• Sm2Co17
• Nd2Fe14B
• Digital Data Storage
• Hard, floppy disc• Bubble memory
• Special magnetic domain structure
• NiCo, NiCoFe• CrO2
• Fe2O3
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Field of Application Products Requirements Materials
• Analog Data Storage
• Video tape• Audio tape
• Medium H C and MR (square hysteresis loop)
• Fe/Co/Ni/Al/Cu = 50/24/14/9/3
• SmCo5
• Sm2Co17
• Nd2Fe14B
• Digital Data Storage
• Hard, floppy disc
• Bubble memory
• Special magnetic domain structure
• Ni/Co, Ni/Co/Fe• CrO2
• Fe2O3
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Applications of Hard Magnets
Field of Application Products Requirements Materials
• Power Conversion• Motors• Generators• Electromagnets
• Large MR
• Small Hc
• Losses low
• Fe-based• Fe+ ~(0.7–5)% Si• Fe +~(35-50)%Co
• Power Adaptation • Power Transformers
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Applications of Soft Magnets
Field of Application Products Requirements Materials
• Signal Transfer
• Other Transformers • Linear M-H
curve
• LF (<100kHz) • Low conductivity
• Fe+36%Fe/Ni/Co = 20/40/40
• HF (>100kHz) • Very low conductivity
• Ni–Zn ferrites
• Magnetic Shielding/EAS
• Permalloy• Mu metal
• Large dM/dH @ H=0
• Ni/Fe/Cu/Cr ~77/16/5/2
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Applications of Soft Magnets
Summary
Wide variety of materials/applications. Elementary concepts of materials science as they
apply to magnetic materials. Aspects of alloy design (chemistry) and resulting
effects on magnetic properties.
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