Nonvolatile Electric Field Control of Thermal Magnons in the Absence of an Applied Magnetic Field

Spin transport through magnetic insulators has been demonstrated in a variety of materials and is an emerging pathway for next-generation spin-based computing. To modulate spin transport in these systems, one typically applies a sufficiently strong magnetic field to allow for deterministic control of magnetic order. Here, we make use of the well-known multiferroic magnetoelectric, ${\mathrm{BiFeO}}_3$, to demonstrate nonvolatile, hysteretic, electric-field control of thermally excited magnon current in the absence of an applied magnetic field. These findings are an important step toward magnon-based devices, where electric-field-only control is highly desirable.

Figure 1

Experimental setup. (a) Optical and PFM images of nonlocal device structure. Out-of-plane and in-plane PFM images reveal a well-ordered 109° domain structure. Arrows in inset (OOP) show IP projection of spontaneous polarization, $\mathbf{P}$. (b) Measurement configuration. (c) Pulsing configuration.

Figure 2

Bistable states of thermal magnon current. (a) Cross sectional device schematic. As shown by the interfacial spin (black), the detected voltage along the detector (left) wire is dependent on the magnon spin polarization component orthogonal to the length of the Pt wires. (b) Experimental protocol and results of “half” hysteresis loops confirming the stability of the ferroelectric state after electric field poling. (c) Measured lock-in second harmonic voltage, $V_{nl}(2\omega )$, measuring magnon current, as a function of time. 100 sec of data are collected after each electric-field pulse. Data reflect relative changes upon poling, i.e., a small ($\sim 10\mathrm{s}$ of nV) dc offset is subtracted from both positively and negatively poled signals. Histogram combining results from 10 trials confirms two distinct states of magnon current. Fits are to normal distributions. An $800\mu \mathrm{A}$ charge current was used to generate the thermal gradient for SSE.

Figure 3

Switching mechanism. (a) Schematic of twin 109° domains showing ferroelectric polarization vector, $\mathbf{P}$ (red), Néel vector $\mathbf{L}$ (blue), and canted magnetization vector $\mathbf{M}$ (green). (b) In-plane phase PFM images after $+350$ and $-350\mathrm{kV}/\mathrm{cm}$ applied across the channel. The change in contrast indicates reversal of the net in-plane polarization (and therefore canted magnetization). A (green) circle marks an external reference domain pattern for comparison. (c) Schematic of 71° IP switch showing reversal of the both the net ferroelectric polarization, $\mathbf{P}$, and net canted moment, $\mathbf{M}$.

Figure 4

Hysteretic response. (a) Hysteretic magnon current measurement protocol. (b) Observation of hysteresis in nonlocal second harmonic signal in BFO showing excellent agreement with the associated ferroelectric hysteresis loop. Identical measurements on PSTO and YIG are also shown. (c) Magnitude of differential nonlocal voltage as a function of injector (heater) current, as measured through several different means.