Spin torque oscillators

Spin torque oscillators (STOs)

[Research output: IEEE Magnetics Letters, 8, 3509504, (2017); Journal of Magnetism and Magnetic Materials, 414, 227-242, (2016); Physical Review B, 93, 094431 (2016); Nanoscale Research Letters, 9, 597 (2014); IEEE Transactions on Magnetics,50,1400104 (2014); Phys. Rev. B. Rapid Communication, 87, 020409(R) (2013); Phys. Rev. B., 86, 014418 (2012); Physical Review B, 84, 104414 (2011); Physical Review B, 83, 174424 (2011); Integrated Ferroelectrics, 125, 147-154 (2011); Physical Review B (Rapid Communication), 82, 140407(R) (2010); IEEE Trans. Magn., 45, 2773, (2009); Applied Physics Letters, 94, 112503, (2009); Journal of Applied Physics, 105, 07D116, (2009); Applied Physics Letters, 92, 262508, (2008); Applied Physics Letters, 92, 092505, (2008); Journal of Applied Physics, 101, 09A510, (2007); Journal of Applied Physics, 101, 09A510, (2007)]

In 1996, John Slonczewski and Luc Berger theoretically predicted that a current of spin polarized electrons can exert enough torque on a magnetic material to switch its state and also induce magnetic excitations. In 1999, these theoretical predictions were experimentally verified in GMR nanopillars of Co/Cu/Co. The excitations are coherent and at well-defined GHz frequencies and furthermore tunable both via the dc current level or the strength of the applied magnetic field. The concept of a current- and field-controlled Spin Torque Oscillator (STO) was consequently born. The large degree of coherence leads to Q values as high as 18200. While STOs have been demonstrated with a current- and field-tunable range of 5 – 40 GHz, the upper limit is so far only set by measurement limitations and the intrinsic frequency should continue well above 100GHz. This nano-patterned ferromagnetic device, in which high-quality tunable microwave oscillations can be generated by a small DC current, opens new perspectives in microwave engineering design. Much attention has been focused on increasing the ~mV signals generated in GMR-based STOs. In 2004 the same effects were demonstrated in MTJs, which significantly increases the output voltage thanks to the much higher tunneling magnetoresistance (TMR). The signal can also increase through coherent phase-locking of parallel STOs, where the RF power was doubled when two individual STOs phase-locked via magnetostatic interactions in one of the magnetic layers. The signal increase is accompanied by a further increased Q value. During my PHD at KTH and my independent research career at HKU, I have demonstrated that phase-locking also occurs through the RF current itself in serially connected chains of STOs. Besides increasing the RF signal and Q, they conveniently introduce an averaging of the particular frequency, amplitude and tunability property of each individual STO, and hence significantly reduce process variations, improve the manufacturability, and raise the potential for the commercialization of STO-based radio- and microwave components.