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Decoherence, Mode Hopping, and Mode Coupling in Spin Torque Oscillators

Journal article
Authors O. G. Heinonen
Pranaba Muduli Kishor
Ezio Iacocca
Johan Åkerman
Published in Ieee Transactions on Magnetics
Volume 49
Issue 7
Pages 4398-4404
ISSN 0018-9464
Publication year 2013
Published at Department of Physics (GU)
Pages 4398-4404
Language en
Links dx.doi.org/10.1109/tmag.2013.224286...
Keywords Magnetoresistive devices, microwave oscillators, spin polarized transport, spin valves, tunneling, MAGNETIC MULTILAYERS, POLARIZED CURRENT, NANO-OSCILLATORS, MODULATION, DRIVEN, GENERATION, EXCITATION, WAVES
Subject categories Physical Sciences

Abstract

Spin torque oscillators (STOs) often exhibit multiple modes, leading to complex behavior. One example is mode hopping between different eigenmodes of a magnetic tunnel junction (MTJ) STO. This mode hopping is a strong function of current and angle between the magnetization in the free and fixed layers, and away from anti-parallel configuration, mode hopping can be the dominant decoherence process. Another example is the linewidth of a nanocontact STO that can be a complex non-monotonic function of temperature in regions where two or more modes are excited by the oscillators. These phenomena require a generalization of the single-mode nonlinear STO theory to include mode coupling. We derive equations describing the slow time evolution of the coupled system and show they describe a dynamically driven system, similar to other systems that exhibit mode hopping in the presence of thermal fluctuations. In our description, mode coupling also leads to additional coupling between power and phase fluctuations, which can in certain limited cases lead to longer relaxation times for power fluctuations, and consequently to larger linewidths through the nonlinear frequency shift.

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