Shared Flashcard Set

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C5
Magnetic Materials
19
Science
Undergraduate 3
06/01/2011

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Cards

Term
What conditions are necessary for superexchange
Definition
this tends to occur in insulators where there are no free electrons available
Term
hysteresis losses
Definition

the integral of the closed loop between H v. M curve

it results in heating and is a problem in transducers, electric motors, etc.

Term
What is susceptibility and how is it related to M, B, and H
Definition

the materials response to an applied magnetic field

μ=1+Χ,  Χ=M/H

Term
what determines the degree of magnetization
Definition

electrons available in the system

if the number of spin up and spin down are the same there is no magnetization and if all bands are filled there is no magnetic moment

Term
Zeeman energy
Definition

the potential energy of a magnetized body

Based on E=∫τdΦ, τ=μmxH, Ms=mN/V

Term
Pauli Paramagnetism
Definition
magnetization in metals comes from band structure.  Pauli says the chemical potential of spin up/down electrons changes with H and when they are unequal - leads to magnetization
Term
how is magnetization effected by the density of states
Definition
higher DoS, greater magnetization.  therefore most rare earths/d-band metals are magnets
Term
What is the significance of stoner criterion
Definition
it uses the number of spin up and spin down electrons amongst otehr parameters to determine under what conditions magnetism will occur
Term
How does magnetic anisotropy arise and what is its significance
Definition

this is the energy required to deflect a moment from the easy to hard axis

-magnetocrystallinity (due to band structure)

-macroscopic shape (there are preferred directions)

with no anisotropy, permanent magnets would not exist and no magnetic data sotrage possible

Ea=K1sin2γ+K2sin4γ+K3sin6γ...

total energy=Ezeeman+K1sin2γ

Term
Why does hysteresis take place
Definition

you must apply an extra field to get M to switch directions as it is trapped in minima (like activation E)

 

much energy is required to encourage grainboundaries to move, they finally do at a steady state then saturate as all the moments are essentially aligned

Term
What is a domain wall
Definition

a region of inhomogeneous magnetization

 

E total=anisotropy energy + exchange energy

at Emin, d=Sqrt[2Js2pi/K1a]

 

if this d is plugged back into anisotropy energy, the energy of the wall is known

Term
domain wall forces
Definition

initially there is reversible wall motion

next there is growth of domains and irreversible wall motion

finally there is reversible dipole rotation

 

domain wall motion gives rise to hysteresis

Term
features on hysteresis loops
Definition

saturation

remnance

coercivity

Term
what are eddy currents and how they can they be reduced
Definition

eddy currents are induced currents that result from the application of magnetic fields

laminates with insulators between layers tend to reduce eddy current losses

Term
Name a typical hard magnet used in practice
Definition
AlNiCo
Term
conditions for magnetic data storage
Definition

requires a stable hysteresis loop with fairly rectangular shape

coercivity is critical for correct operations

higher density requires smaller data bits

Must consider superparamagnetic effect

Term
What is super paramagnetism and how can it be reduced
Definition

says if Ea<kBT, there is no longer permanent storage

to avoid it, one could increase volume or K1 (anisotropy)

Often these changes are increasing coercivity, however, therefore requiring higher energy

Term
what are the competiting factors in magnetisation?
Definition
magnetism is always a competition between thermal excitation and magnetocrystallinity
Term
How is magnetic data read
Definition

spin may be up or down and electrons flow indepeendently

a spin valve is used to sense field direction

if adjacent spins are in the same direction, low R

if in opposite directions, large R

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