N°15 - 23 September 2016 - Enhanced modulation rates via field modulation in spin torque nano-oscillators
Spin Transfer Nano-Oscillators (STNOs) are promising candidates for telecommunications applications due to their frequency tuning capabilities via either a dc current or an applied field. This frequency tuning is of interest for Frequency Shift Keying (FSK) concepts to be used in wireless communication schemes or in read head applications. For these technological applications, one important parameter is the characterization of the maximum achievable rate at which an STNO can respond to a modulating signal, such as current or field. Previous studies of in-plane magnetized STNOs on frequency modulation via an rf current revealed that the maximum achievable rate is limited by the amplitude relaxation rate Γp, that gives the time scale over which amplitude fluctuations are damped out. This might be a limitation for applications. Here we demonstrate via numerical simulation that application of an additional rf fieldis an alternative way for modulation of the in-plane magnetized STNO configuration, that has the advantage that frequency modulation is not limited by the amplitude relaxation rate, so that higher modulation rates above GHz are achievable. This occurs when the modulating rf field is oriented along the easy axis (longitudinal rf field). Tilting the direction of the modulating rf field in-plane and perpendicularly with respect to the easy axis (transverse rf field), the modulation is again limited by the amplitude relaxation rate similar to the response observed in current modulation.
Appl. Phys. Lett. 108, 122402 (2016)
FIG. 1: (a) Schematics of the STNO configuration with an in-plane magnetized free layer and an in-plane polarized spin current (Japp>0) stabilizing an in-plane precession trajectory (in red). The projection on the x-y plane is indicated as well as the in-plane angle b. (b) Comparison between the numerical (black line) and analytical (red and green lines) instantaneous frequency fi(t) for longitudinal rf field (Hrfx) modulation at fm=50 MHz. The red line considers only the contribution from amplitude modulation and the green line considers both amplitude and direct frequency modulation.
(c-f) Double logarithmic plots of the amplitude noise PSDδa and frequency noise PSDδf of the modulated signal for (c,d) longitudinal Hrf//x and (e,f) transverse Hrf//y rf fields. The envelope of the background level due to thermal noise is indicated by the red dotted line and the envelope due to the modulation signal by the blue dotted