Spin Transfer Switching

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Spin Transfer Switching is a new technique that is currently being researched as a method of writing in MRAMs. By using this technique it is believed that the current needed to switch an MRAM cell will be greatly reduced. This in turn will lead to higher density MRAMs

[edit] Description of the Process

A stream of conducting electrons moving through a magnetic material of fixed orientation are spin polarized (i.e. most of the electrons' spins become aligned to that of the magnetic material). When these spin-polarized electrons pass through another nanomagnetic layer (of free magnetic orientation) they get repolarized. In repolarizing, the nanomagnet experiences a torque associated with the change in angular momentum resulting from the rotation of the spins. This torque pumps enough energy to the nanomagnet for its magnetic moment to precess (move at microwave frequencies around the symmetry axis with ever increasing amplitude till it reverses its orientation).

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The damping force keeps the nanomagnet in static equilibrium. The current density determines amount of spin torque. Based on the current density (and hence the spin torque), the precession can lead to three scenarios

  • If the spin torque is small,damping overcomes precession and the magnet settles to static equilibrium.
  • If the spin torque equals the damping force persistent precession takes place.
  • If the spin torque is much greater than the damping force, precession takes place with increasing amplitude and magnetization reverses direction.
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