8/18/2019 CBE455 Lecture14 Magnetism2 Handout

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The characteristic time, !, for a magnetized domain to flip is given by:

( where K u is the magnetocrystalline anisotropy,

V is the volume of the nanocrystal (4 nm in “diameter”),

T is the temperature,

k is Boltzmann’s constant (8.61×10-5 eV/K) ,

 f 0 is a frequency factor (use 10-9 sec-1) )

what is the minimum magnetocrystalline anisotropy ( K u) that is needed

for a memory storage application at room temperature (kT  = 26 meV)?

Assume that the flip time should be longer than 100 sec.

1

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=   f 0 exp  #K uV 

kT 

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We want to have a small size magnetic bit. (small V)

Magnetic energy scales with V.

To keep the magnet reliably magnetic, we need to either lower T or increase KV.