Tuning Magnetic Coupling in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ Thin Films with Epitaxial Strain

We report x-ray resonant magnetic scattering and resonant inelastic x-ray scattering studies of epitaxially strained ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin films. The films were grown on ${\mathrm{SrTiO}}_3$ and $({\mathrm{LaAlO}}_3{)}_{0.3}({\mathrm{Sr}}_2{\mathrm{AlTaO}}_6{)}_{0.7}$ substrates, under slight tensile and compressive strains, respectively. Although the films develop a magnetic structure reminiscent of bulk ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, the magnetic correlations are extremely anisotropic, with in-plane correlation lengths significantly longer than the out-of-plane correlation lengths. In addition, the compressive (tensile) strain serves to suppress (enhance) the magnetic ordering temperature $T_N$, while raising (lowering) the energy of the zone-boundary magnon. Quantum chemical calculations show that the tuning of magnetic energy scales can be understood in terms of strain-induced changes in bond lengths.

Figure 1

$L$-scan profile of magnetic x-ray diffraction along (0, 1, L) for SIO-LSAT (a) and SIO-STO (b). Polarization dependence for SIO-LSAT (c) and SIO-STO (d), in the $\sigma \sigma$ channel (filled circles) and the $\sigma \pi$ channel (empty circles). Incident energy dependence for SIO-LSAT (e) and SIO-STO (f). All data in (c)–(f) were obtained at $T=5\mathrm{K}$ at the (0, 1, 14) reflection.

Figure 2

SIO-LSAT: Scans of the (0, 1, 14) magnetic peak along the $K$ (a) and $L$ directions (b). SIO-STO: Scans of the (0, 1, 14) magnetic peak along the $H$ (c) and $L$ directions (d). The baseline of the profiles in (a)–(d) is shifted. The solid lines through the data points are the results of the fit described in the text. The solid black lines depict the $H$, $K$, and $L$ scans of the (1, 0, 18) magnetic peak for bulk ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ (the peak intensity is rescaled and the $L$ scan is shifted from 18 to 14).

Figure 3

(a) The half-width at half maximum (fitting parameter $\kappa$) is plotted as a function of temperature, along the $H$ and $L$ directions for SIO-STO, and along the $K$ and $L$ directions for SIO-LSAT. (b) The change in integrated intensity with temperature for the (0, 1, 14) reflection, for SIO-LSAT (red triangles) and SIO-STO (blue squares). The black arrow indicates the magnetic transition temperature of SIO bulk, and the red and blue dotted lines are provided as a guide to the eye. The background coloring depicts the structural transitions of the STO substrate. Anomalous temperature dependence below 110 K is due to structural transitions in the STO substrate, and will not be discussed here. The inset shows the magnetic transition temperature as a function of strain.

Figure 4

Epitaxial strain effect on the low-lying magnetic excitations of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. A comparison of Ir $L_3$-edge RIXS spectra collected at room temperature for bulk SIO, SIO-LSAT, and SIO-STO demonstrates that compressive (tensile) strain significantly raises (lowers) the energy of the ($\pi$,0) zone-boundary magnon. The solid lines represent Gaussian fits to the data (described in the Supplemental Material [30]). The dashed vertical lines represent the fitted values for the magnon energies.