Strain-tunable extraordinary magnetocrystalline anisotropy in ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ epitaxial films

We report the discovery of extraordinarily large anisotropy fields and strain-tunable magnetocrystalline anisotropy in ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ epitaxial films. We determine the strain-induced tetragonal distortions and octahedral rotations in ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ epitaxial films grown on (${\mathrm{LaAlO}}_3$)${}_{0.3}$(${\mathrm{Sr}}_2{\mathrm{AlTaO}}_6$)${}_{0.7}$ (LSAT), ${\mathrm{SrTiO}}_3$ (STO), and ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_3$/LSAT substrates using x-ray diffraction and density functional theory. The structural distortions drive dramatic changes in magnetocrystalline anisotropy. We use magnetometry measurements and first principles calculations to determine the atomic origins of the large anisotropy observed. These techniques elucidate the interplay between structural deformations and magnetic behavior and lay the groundwork for the study of other strongly correlated systems in this class of ferromagnetic oxides.

Figure 1

XRD scans around the ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ (004) (left panels) and ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ (022) (right panels) peaks for 90 nm ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ films grown on (a) LSAT, (b) ${\mathrm{SrTiO}}_3$, and (c) a ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_3$ buffer layer on LSAT. The off-normal ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ (022) peaks are measured at a tilt angle $\Psi ={45}^{\circ }$ for in-plane characterization.

Figure 2

Schematics of (a) Cr-O (counterclockwise) and Re-O (clockwise) octahedral rotations in the ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ lattice and (b) an oxygen (red) octahedron surrounding Cr or Re with two different bond lengths, $d^{\text{eq}}$ and $d^{\text{ax}}$. (c) Octahedral rotation angle $\theta$ and (d) bond lengths of Cr-O and Re-O octahedra as functions of the tetragonal distortion $c/a$ of the ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ lattice.

Figure 3

In-plane and out-of-plane magnetic hysteresis loops for the ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ films grown on (a) LSAT, (b) ${\mathrm{SrTiO}}_3$, and (c) ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_3$/LSAT in magnetic fields up to 7 T at $T=20$, 100, and 300 K. Magnetic anisotropy fields can be obtained from the intercept of linear extrapolation of the hard-axis hysteresis loops and the saturation magnetization as shown in the left panels.

Figure 4

Differences ($\Delta m$) between out-of-plane ([001] direction) and in-plane (average of [100] and [010] directions) (a) total magnetic moments, (b) total orbital and spin magnetic moments, and (c) spin and (d) orbital magnetic moments for Re (squares), Cr (diamonds), and O (circles). The lines are guides for the eye.