Electrical Control of Structural and Physical Properties via Strong Spin-Orbit Interactions in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$

Electrical control of structural and physical properties is a long-sought, but elusive goal of contemporary science and technology. We demonstrate that a combination of strong spin-orbit interactions (SOI) and a canted antiferromagnetic Mott state is sufficient to attain that goal. The antiferromagnetic insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ provides a model system in which strong SOI lock canted Ir magnetic moments to ${\mathrm{IrO}}_6$ octahedra, causing them to rigidly rotate together. A novel coupling between an applied electrical current and the canting angle reduces the Néel temperature and drives a large, nonlinear lattice expansion that closely tracks the magnetization, increases the electron mobility, and precipitates a unique resistive switching effect. Our observations open new avenues for understanding fundamental physics driven by strong SOI in condensed matter, and provide a new paradigm for functional materials and devices.

Figure 1

Single-crystal x-ray diffraction of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ with current $I$ applied within the basal plane. (a) Representative x-ray diffraction pattern of a single crystal. The circled Bragg peak is (0,0,16). Inset: Sample mounting showing electrical leads and cryogenic gas feed [11]. Current-controlled changes in (b) the location and (c) the intensity (counts) of the (0016) peak, 3260 for $I=0$ and 999 for $I=105\mathrm{mA}$.

Figure 2

(a) Current-controlled shifts $\mathrm{\Delta }a/a$ and $\mathrm{\Delta }c/c$ for ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Note that $\mathrm{\Delta }a/a$ closely tracks $M_a$ (right-hand scale). (b) For comparison to (a), $\mathrm{\Delta }a/a$ and $\mathrm{\Delta }c/c$ for ${\mathrm{Sr}}_2{\mathrm{Ir}}_{0.97}{\mathrm{Tb}}_{0.03}{\mathrm{O}}_4$. Note that the scales for $\mathrm{\Delta }a/a$, $\mathrm{\Delta }c/c$, and $M_a$ are the same as those in (a) to facilitate comparisons. (c) Temperature dependence of $\mathit{a}$- and $\mathit{c}$-axis thermal expansion of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ for $I=0$. (d) Temperature-induced shifts $\delta a/a$ and $\delta c/c$ corresponding to (c).

Figure 3

Temperature dependence of the (a) $\mathit{a}$-axis magnetic susceptibility ${\chi }_a(T)$ at a few representative currents and (b) $d{\chi }_a(T)/dT$, which clarifies the decrease in $T_N$ with $I$. (c) $M_a(H)$ at 100 K for a few representative currents. (d) Current dependence of $T_N$ and $M_a$. Diagrams illustrate the current-controlled lattice expansion, angle $\theta$ (red), and Ir moments (black arrows) with increasing $I$.

Figure 4

$I\text{−}V$ curves for ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ for (a) $I$ applied along the $\mathit{a}$ axis, (b) along the $\mathit{c}$ axis, (c) both the $\mathit{a}$ and $\mathit{c}$ axis at $T=100\mathrm{K}$. Arrows show the evolution of the current sweeps in (a)–(c). Temperature dependence of (d) the threshold voltage $V_{\mathrm{th}}$ and $M_a(T)$ (right-hand scale), and (e) the $\mathit{a}$-axis resistivity ${\rho }_a$. Inset: Expanded ${\rho }_a$ for $I=20\mathrm{mA}$. (f) Representative data for $dV/dI$ as a function of dc current at $T=100\mathrm{K}$. Note two slope changes marked at $I=I_{C1}$ and $I_{C2}$. Arrows show the sequence of applied $I$, and the dashed line is a guide to the eye.

Figure 5

(a) $I\text{−}V$ curves (red, blue, green) for the $\mathit{a}$- and $\mathit{c}$-axis lattice parameters at $T=100\mathrm{K}$ and ${\mu }_{o}H=0$ and 5 T along the $\mathit{c}$ axis. Light blue data (upper horizontal axis) show $\mathrm{\Delta }a/a$ at $T=100\mathrm{K}$. Dashed lines are guides to the eye. Note slope changes of $\mathrm{\Delta }a/a$ occur at the two turning points at $I_{C1}$ and $I_{C2}$, respectively. (b) Diagrams (not to scale) illustrate the increasing lattice expansion, decreasing $\theta$ (red), and Ir moment canting (black arrows) with increasing $I$. Schematic increase of electron mobility due to decreased gapping of the electronic structure with $I$.