Nanopillars consisting of two magnetic layers separated by an insulating or non-magnetic metal can exhibit oscillations: a constant current can make the magnetization in one layer precess with frequencies in the gigahertz range. The physics behind this is the so-called spin torque effect, through which conduction electrons transfer angular momentum to the magnetization in a layer. If the current is adjusted such that the rate of transfer of angular momentum from the conduction electrons to the magnetization in the second magnetic layer is precisely balanced by the loss of angular momentum due to dissipation in the magnetic layer, the oscillations become very precise and long-lived. Such devices are called spin torque oscillators, and are very interesting from the point of view of various applications, since they the convert a dc current to gigahertz oscillations and can be readily integrated with other electronics. A fundamental question, which is also important for applications, is how long time the oscillations can be maintained without becoming imprecise. This time is called the coherence time. Researchers at Argonne national Laboratory and the University of Gothenburg, Sweden, and the Royal Institute of Technology, Sweden have shown1 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 oscillator 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 by carefully tuning the current. A theoretical analysis shows that the mechanism governing the mode-hopping is very similar to the mechanism responsible for mode-hopping in semiconductor lasers. The mode-hopping arises from fundamental and intrinsic interactions in the nanomagnets, and the understanding and analogy with semiconductor ring lasers is an important step towards applications of spin torque oscillators.
||Figure 1: The left panels show the frequency of a spin torque oscillations as a function of time for near anti-parallel alignment of the magnetization (top) panel, as depicted in the top right cartoon, and at approximately 45° away from anti-parallel alignment, as shown in the bottom right cartoon|
P. K. Muduli, O. Heinonen, and J. Åkerman, Decoherence and Mode Hopping in a Magnetic Tunnel Junction Based Spin Torque Oscillator, URL: http://link.aps.org/doi/10.1103/PhysRevLett.108.207203
POC: Olle Heinonen, Group Leader, Interfacial Materials Group (