Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires

Matteo Franchin, Andreas Knittel, Maximilian Albert, Dmitri Chernyshenko, Thomas Fischbacher, Anil Prabhakar, and Hans Fangohr, Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires, Physical Review B 84 094409 (9 pages) (2011)

Online: http://link.aps.org/doi/10.1103/PhysRevB.84.094409

Open access preprints available: pdf, also at http://arxiv.org/abs/1104.3010

Motivation

• It is important to study transverse domain walls in cylindrical nanowires, as recent theoretical studies have been reporting desirable properties for such systems, such as zero depinning current and no walker breakdown,

• Moreover the cylindrical geometry is likely to play an important role in the coming years. Indeed, a convenient way of densely filling a given volume with nanowires is by engineering two dimensional arrays of nanowires (basically drilling cylindrical holes in an insulating film and filling it with ferromagnetic conductive material).

• It is important to understand how domain walls interact with potential barriers (geometric barriers or other kinds of pinning centers), as these are necessary in order to control the domain wall motion and make it more reliable

Summary

• We study the field-driven and current-driven motion of transverse domain walls through potential barriers in cylindrical nanowires

• We study how the critical field or critical current density required in order to push the domain wall through the barrier depends on the direction of magnetization pinning in the barrier

• We show that the critical current density is lowered drastically for barriers which allow free rotation of the domain wall around its axis. In particular, the critical current density required to traverse a barrier with a given height in energy, can change by a factor ~130 depending on whether or not the domain wall is free to rotate during its motion through the barrier.

• We show that the efficiency of the energy pumping is related to the spin torque being always directed opposite to the damping term, leading to an accumulation of the effect. The result thus suggests that it is important to engineer systems where the magnetization trajectory allows coherent accumulation of the spin transfer torque effect.

• Results (click images to enlarge):

  

  Figures (2 and 10) from the publication showing (left) the equilibrium position of the domain wall in potential barriers with different pinning direction and (right) the current density that one needs to apply in order to push the domain wall through a potential barrier with a given height in energy.

• An animation showing the system (avi), and the dynamics after application of a spin-polarised current

Related presentations

• Talk (pdf) presented at MMM 2010 in Atlanta.

  

  Provides

  • Study of field-driven and current-driven motion of domain wall through potential barriers

  • Study of importance of pinning effect of barrier on magnetisation

  • Analytical model predicting current required to overcome given energy barrier

• Talk (pdf) presented at CMMP 2009 in Warwick (UK).

  

  Provides

  • Study of rotationally free domain walls

  • Show how rotationally free domain walls can do things that conventional domain walls cannot

  • Propose a prototype memory element based on this