Nanopillars consisting of two magnetic layers separated by an insulating or non-magnetic metal can exhibit oscillations: a dc current can make the magnetization in one layer precess with frequencies in the gigahertz range. This is so called spin torque oscillator (STO) is potentially useful for RF and microwave devices e.g., mobile phones due its high frequency tunability, high modulation rate, and nano-size.

In the work published in Physical Review Letters, Researchers from GU, KTH and their collaborator Olle G. Heinonen from Argonne National Laboratory, USA discover that the coherence time depends on the relative orientation of the magnetization in the two magnetic layers, and is longest when the magnetizations in the two layers are arranged to be antiparallel.

Measurements using a very fast oscilloscope show that the STO switches frequency as a function of time, which is called mode hopping. The mode hopping is an important process that limits the coherence away from antiparallel orientation. Near antiparallel orientation, the mode hopping can be reduced to be unimportant.

Theoretical analysis shows that the mechanism governing the mode hopping is similar to mode hopping in semiconductor lasers and arises from intrinsic interactions in the nano-magnets. The understanding and analogy with semiconductor ring lasers is an important step towards applications of STOs.

Article: "Decoherence and Mode Hopping in a Magnetic Tunnel Junction Based Spin Torque Oscillator", P. K. Muduli, O. G. Heinonen, and Johan Åkerman, Phys. Rev. Lett. 108, 207203 (2012).
URL: http://prl.aps.org/abstract/PRL/v108/i20/e207203

Contact:
Pranaba Kishor Muduli, Researcher at Department of Physics, University of Gothenburg
Phone: 031- 786 91 46 or 0700- 4436 25
pranaba dot muduli at physics dot gu dot se