Digital and analogue modulation and demodulation scheme using vortex-based spin torque nano-oscillators

Abstract

In conventional communications systems, information is transmitted by modulating the frequency, amplitude or phase of the carrier signal, which often occurs in a binary fashion over a very narrow bandwidth. Recently, ultra-wideband signal transmission has gained interest for local communications in technologies such as autonomous local sensor networks and on-chip communications, which presents a challenge for conventional electronics. Spin-torque nano-oscillators (STNOs) have been studied as a potentially low power highly tunable frequency source, and in this report we expand on this to show how a specific dynamic phase present in vortex-based STNOs makes them also well suited as Wideband Analogue Dynamic Sensors (WADS). This multi-functionality of the STNOs is the basis of a new modulation and demodulation scheme, where nominally identical devices can be used to transmit information in both a digital or analogue manner, with the potential to allow the highly efficient transmittance of data.

Figure 1

(a) Schematic representation of the modulation and demodulation scheme and (b) single device schematic and (c) transfer curve showing possible magnetisation states of the free layer, (insets are micromagnetic simulations).

Figure 2

Voltage measured experimentally (a,c,e,g) and the magnetisation component collinear to the reference layer (m_y) from micromagnetic simulations (b,d,f,h) as a function of time for different excitation strengths. Typical trajectories of the vortex core calculated from the micromagnetic simulations are presented, where A and B are the point of core expulsion and renucleation, respectively. (i) Experimental and (j) micromagnetic dynamical state diagram where (i) the resistance and (j) the average magnetisation < m_y > are plotted as a function of the excitation strength ($I_{\mathit{ant}}^{\mathit{rf}}$/$H_y^{\mathit{rf}}$) and excitation frequency.

Figure 3

(a) schematic circuit showing how the signal created by MTJ_source is fed into the antenna of MTJ_detector. The (b) spectral properties of MTJ_source alongside the c) resultant resistance response of MTJ_detector as a function of the current applied to the MTJ_source (I_MTJS). The insets in (c) show example time traces, where the free layer is transitioning between the vortex (green) and QUP (blue) states.

Figure 4

(a) STNO based modulation and demodulation scheme with (b) six discrete dynamic states and (c) analogue variation of frequency and power and the resultant resistance measured across the MTJ_detector as a function of time.