Term
| What conditions are necessary for superexchange |
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Definition
| this tends to occur in insulators where there are no free electrons available |
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Term
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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. |
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Term
| What is susceptibility and how is it related to M, B, and H |
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Definition
the materials response to an applied magnetic field
μ=1+Χ, Χ=M/H |
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Term
| what determines the degree of magnetization |
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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 |
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Term
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Definition
the potential energy of a magnetized body
Based on E=∫τdΦ, τ=μmxH, Ms=mN/V |
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Term
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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 |
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Term
| how is magnetization effected by the density of states |
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Definition
| higher DoS, greater magnetization. therefore most rare earths/d-band metals are magnets |
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Term
| What is the significance of stoner criterion |
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Definition
| it uses the number of spin up and spin down electrons amongst otehr parameters to determine under what conditions magnetism will occur |
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Term
| How does magnetic anisotropy arise and what is its significance |
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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 |
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Term
| Why does hysteresis take place |
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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 |
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Term
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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 |
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Term
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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 |
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Term
| features on hysteresis loops |
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Definition
saturation
remnance
coercivity |
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Term
| what are eddy currents and how they can they be reduced |
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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 |
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Term
| Name a typical hard magnet used in practice |
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Definition
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Term
| conditions for magnetic data storage |
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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 |
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Term
| What is super paramagnetism and how can it be reduced |
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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 |
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Term
| what are the competiting factors in magnetisation? |
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Definition
| magnetism is always a competition between thermal excitation and magnetocrystallinity |
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Term
| How is magnetic data read |
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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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