Read Back Signals in Magnetic Recording - Research Group Fidler

7 OutlookEquation
Section (Next)
Outlook
This work was a first step to take arbitrary current distributions into account in micromagnetic
simulations. The program solves the LLG equation and simultaneously calculates the current
distribution, which can be influenced by the magnetization. So it can be used for read back
signal calculations. Of course it is also possible to calculate the read back signal of
perpendicular recording layers. Here the additional SUL has to be taken into account. Spin
valves with synthetic antiferromagnet can also be calculated, if the interlayer exchange
coupling is introduced.
Possible enhancements of this program are the consideration of eddy currents [41] [42] or
finite temperatures. To take fluctuations at finite temperatures into account, a stochastic,
thermal field H th is added to the effective field in the LLG equation [38]
∂J α ∂J
=−γ J× ( Heff + Hth) + J × . (7.1)
∂t J ∂t
s
Future hard disks will make use of spin valves operating in CPP mode. Therefore simulations
of CPP spin valves become more and more interesting. A current perpendicular to the layer
structure leads to spin transfer, which has to be taken into account by an additional term,
called spin torque term [39]
∂J α ∂J
Cj
=−γ J× H ′
eff + J× + J× J × J. (7.2)
∂t J ∂t
J J′
2
s s s
Here J ′ is the magnetization of a reference layer, which polarizes the electrons. The polarized
current transfers the spin to the actual magnetic polarization J and causes an additional
motion of J. j is the local current density. For CPP spin valves the magnetization is transferred
from the pinned layer to the free layer or vice versa.
The application of this program is not restricted to read head simulations. It can also be
applied, for example, on magnetic tunnel junctions (MTJs), which are used in MRAMs
(Magnetic Random Access Memory).
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