Reversal of nanomagnets by propagating magnons in ferrimagnetic yttrium iron garnet enabling nonvolatile magnon memory

Abstract

Despite the unprecedented downscaling of CMOS integrated circuits, memory-intensive machine learning and artificial intelligence applications are limited by data conversion between memory and processor. There is a challenging quest for novel approaches to overcome this so-called von Neumann bottleneck. Magnons are the quanta of spin waves. Their angular momentum enables power-efficient computation without charge flow. The conversion problem would be solved if spin wave amplitudes could be stored directly in a magnetic memory. Here, we report the reversal of ferromagnetic nanostripes by spin waves which propagate in an underlying spin-wave bus. Thereby, the charge-free angular momentum flow is stored after transmission over a macroscopic distance. We show that the spin waves can reverse large arrays of ferromagnetic stripes at a strikingly small power level. Combined with the already existing wave logic, our discovery is path-breaking for the new era of magnonics-based in-memory computation and beyond von Neumann computer architectures.

Fig. 1. Magnonic memory effect–Reading and writing of magnetic bits by spin waves.

a Two lattices of Py nanostripes (bistable magnetic bits) underneath coplanar waveguides (CPWs) on an insulating YIG film. b Depending on the spin-wave (SW) amplitude bit writing (I and II), reading (III) and data replication (IV) are achieved without charge flow. c–e Transmission signals Mag(S21) are taken at three different power levels P_irr by applying electromagnetic (em) waves via a vector network analyzer (VNA). Analyzing signal strengths of mode branches (blue and orange dashed lines), fields H_C1 and H_C2 are extracted reflecting bit reversal. The red dashed lines indicate + 14mT. Yield of bit writing (dark) underneath f CPW1 and g CPW2 by propagating SWs at μ₀H_B = + 14 mT. The 50% transition power levels P_C1 and P_C2, respectively, are marked as black dots. The error bars indicate the 70% and 30% transitions (Methods). h Mag(S11) at P_sens and i Mag(S21) taken at +14 mT. In i, bits at CPW1 and CPW2 were magnetized to state 1.

Fig. 2. Nonvolatile storage of spin-wave signals over macroscopic distance.

MFM measurements taken after the sample with erased bits (state 0, shown as white end) was irradiated at μ₀H_B = + 14 mT with different powers a P_irr = − 25 dBm, b − 15 dBm, and c − 5 dBm in the frequency range 0.1 GHz to 12.5 GHz. In this range, spin waves are excited in YIG. Magnetic bits (highlighted by broken lines) which are reversed appear with a black end. In c, beyond x = 37 μm (marked by a yellow arrow) stripes remained magnetized in state 0 (white end), consistent with the decay of the SW amplitude when propagating away from the emitter CPW1.

Fig. 3. Efficiency analysis of reversal field reduction by linear and nonlinear spin waves in YIG.

a Dependence of critical fields H_C1 (solid blue line) and H_C2 (solid orange line) extracted from VNA spectra on P_irr applied in a broad frequency range from 0.1 GHz to 12.5 GHz. The power region attributed to nonlinear effects in the SW modes is shaded in light gray. b H_C1 and H_C2 as a function of the evaluated in-plane dynamic field amplitude μ₀h_rf,x (rms-value) in the linear SW regime. The error bars represent the 30% and 70% switching field values. The straight lines are guides to the eye.