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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