Full Electric Control of Exchange Bias

We report the creation of a multiferroic field effect device with a ${\mathrm{BiFeO}}_3$ (BFO) (antiferromagnetic-ferroelectric) gate dielectric and a ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (LSMO) (ferromagnetic) conducting channel that exhibits direct, bipolar electrical control of exchange bias. We show that exchange bias is reversibly switched between two stable states with opposite exchange bias polarities upon ferroelectric poling of the BFO. No field cooling, temperature cycling, or additional applied magnetic or electric field beyond the initial BFO polarization is needed for this bipolar modulation effect. Based on these results and the current understanding of exchange bias, we propose a model to explain the control of exchange bias. In this model the coupled antiferromagnetic-ferroelectric order in BFO along with the modulation of interfacial exchange interactions due to ionic displacement of ${\mathrm{Fe}}^{3+}$ in BFO relative to ${\mathrm{Mn}}^{3+/4+}$ in LSMO cause bipolar modulation.

Figure 1

(a), (b) Two devices with a gated hall bar geometry. In (a) current goes in the [100] direction, while in (b) the current goes along [110]. Voltage pulses ($V_G$) are applied to the Au top gate electrode to ferroelectrically polarize BFO and the four point magnetoresistivity of LSMO is measured under applied fields in both the $x$ and $y$ directions. (c) An optical photograph of the device.

Figure 2

(a) Gate pulse sequence applied before carrying out magnetoresistance measurements (green, left arrow) and the corresponding sheet resistance of the LSMO (purple, right arrow). (b), (c) Measurements of exchange bias and coercivity taken at 5.5 K after gate pulse shown in (a). Panels (b) and (c) represent the data when voltage pulses are applied in positive and negative remanent magnetization, respectively. Current was applied along the [110] direction with applied magnetic field in the $B_x$ [100] direction.

Figure 3

Exchange bias as a function of temperature for both remanent magnetization states of LSMO for a device oriented in the [110] direction with magnetic field applied in the $B_x$ [100] direction.

Figure 4

(a) Magnetic hysteresis curve of LSMO before and after BFO ferroelectric polarization reversal. Numbers and arrows represent the progression of time as magnetic field is swept. (b) Depiction of interfacial spins for each number in (a). AFM anisotropy is reduced at the interface between a FM (orange, top) and AFM (blue, bottom) creating an interface layer (red, center) with rotatable AFM spins (dashed red arrows) that rotate with the FM when magnetic field is applied and pinned AFM spins (bold red arrows) that are not affected by applied magnetic field.