Torque magnetometry of an amorphous-alumina/strontium-titanate interface

We report torque magnetometry measurements of an oxide heterostructure consisting of an amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ thin film grown on a crystalline ${\mathrm{SrTiO}}_3$ substrate ($a$-AO/STO) by atomic layer deposition. We find a torque response that resembles previous studies of crystalline ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (LAO/STO) heterointerfaces, consistent with strongly anisotropic magnetic ordering in the plane of the interface. Unlike crystalline LAO, amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ is nonpolar, indicating that planar magnetism at an oxide interface is possible without the strong internal electric fields generated within the polarization catastrophe model. We discuss our results in the context of current theoretical efforts to explain magnetism in crystalline LAO/STO.

Figure 1

Torque signal for different angles. (a) Small $\varphi$. The torque traces increase in amplitude monotonically with increasing angle $\left|\varphi \right|$. The torque traces invert about $\varphi =0^{\circ }$. Inset: Schematic depicting the interface and applied field. (b) Large $\varphi$. For $\varphi >{27}^{\circ }$, the torque signal monotonically decreases with increasing $\varphi$. The torque traces are proportional to $H_{\perp }=Hcos\varphi$. As $\varphi$ approaches ${90}^{\circ }$, the torque amplitude vanishes. Inset: Picture of a brass cantilever with the sample. The dark ring around the sample is a calibration loop.

Figure 2

Magnetic moment calculation. The magnetic moment $m\left(H\right)$ was calculated from the torque traces by dividing by the perpendicular applied field: $m\left(H\right)=\tau \left(H\right)/{\mu }_{0}Hcos\theta$. The trace corresponding to $\varphi ={90}^{\circ }$ was excluded due to the loss of sensitivity around $\varphi \sim {90}^{\circ }$. Data below 50 mT have been suppressed due to loss of torque sensitivity at low field. The net magnetic moment has been expressed in units of Bohr magnetons ${\mu }_{\mathrm{B}}$ per 2D STO unit cell (u.c.).

Figure 3

Temperature dependence of torque. Torque traces at constant $\varphi =-{11}^{\circ }$ from 400 mK to 30 K. There is no quantitative change in torque signal below 0.5 T throughout the accessible temperature range. The curvature at the higher field is due to an overall diamagnetic background.