Spin-torque oscillators (STOs) are nanoscale devices that use variations in magnetic field direction to produce microwaves. However, the microwaves generated by any individual device are extremely weak for practical applications.
Physicists have tried—and have constantly failed so far—to create reliable microwave fields by coupling large ensembles. Michael Zaks from Humboldt University of Berlin and Arkady Pikovsky from the University of Potsdam in Germany have now demonstrated why cascading these devices cannot be possible and, at the same time, proposed other ways to investigate. Their study was recently reported in EPJ B.
Spin-torque oscillations have the same underlying physics as that of the hard disk drive of a computer. It is a quantum mechanical effect called “giant magnetoresistance,” where varying the external magnetic field around a stack of layers of alternating ferromagnetic and non-magnetic metals leads to considerable variations in electrical resistance.
When the electric force generated is adequately strong and the magnetic layers are free to rotate, magnetic oscillation takes place and microwaves are produced; this is referred to as the STO effect. However, only synchronized oscillations from large ensembles of oscillators can generate microwaves that are strong enough to be useful. Zaks and Pikovsky’s work demonstrates why it has known to be very difficult to synchronize them.
To perform this, the physicists reproduced the motion of an ensemble of consecutively connected STOs using the equations of non-linear dynamics. Their analysis showed that the ensembles were always too unstable for the oscillations to remain coherent.
Particularly, they discovered that the random variations of electric current that influence all oscillators simultaneously—so-called “common noise”—do not alleviate the oscillations, as some had predicted. Instead, in certain instances, adequately powerful fluctuations were able to subdue the oscillations altogether.
Zaks and Pikovsky have named this newly identified phenomenon “noise-induced oscillation death.” Fortified with new theoretical know-how on this system, they are now exploring other techniques for coupling these nanoscale machines to generate robust microwaves on the large scale.