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. the electrons become magnetized in the same direction as the magnet). This is slightly analogous to what a polarized lens does to light. 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 until it reverses its orientation).

The current density, better known as amps, determines the amount of spin torque supplied by the electrons after passing through the first magnet. For simplicity from a system-wide standpoint, we shall refer to this as current density.

The nanomagnet of free orientation has some interesting properties. It prefers to stay in "static equilibrium". We shall call this desire the "damping force". The "damping force" can be overcome by the current density.

Based on the current density, the precession can lead to three scenarios

  • If the current density is small, the nanomagnet settles to static equilibrium. This is analogous to a spinning top situated, for the most part, upright.
  • If the current density equals the damping force persistent precession takes place. This is analogous to a top that has begun to lose its balance but manages to stay spinning.
  • If the current density is much greater than the damping force, precession takes place with increasing amplitude and magnetization reverses direction. This is analogous to a spinning top that has lost its balance and then regained it upside down.

This image shows the first case, second case and third case respectively.