Exploring magnetic anisotropy and robustness of the Jeff=1/2 state under substantial orthorhombic distortion in S r2Ir O4 thin films

We present a comprehensive study revealing the intricate interplay of the magnetic anisotropy and orthorhombic distortion in thin films of Sr2IrO4 through a Ca3Ru2O7 substrate. By inducing a pronounced orthorhombic distortion along the direction of oxygen octahedral edges, we effectively modulated the uniaxial magnetic anisotropy in the system. Remarkably divergent responses along the easy and hard magnetic axes were unveiled through x-ray magnetic circular dichroism (XMCD) measurements under magnetic fields. Specifically, the spin flop transition observed when the magnetic field aligns with the hard axis allows us to estimate the magnetic anisotropy energy, which is around 14.2 μeV, close to that estimated from the single magnon peak measured via Raman spectroscopy. The observed anisotropy energy remains notably lower than the linear estimates derived from the strain-anisotropy energy relationship outlined in H.-H. Kim, Nat. Commun. 13, 6674 (2022)10.1038/s41467-022-34375-6. This underscores the enduring preservation of the isotropic character of the Jeff=1/2 states. This is also supported by the negligible XMCD intensity ratio at the L2 edge compared to that of the L3 edge. Furthermore, the branching ratio determined from x-ray absorption spectroscopy shows that the expectation value of the spin-orbit coupling is similar to that of bulk Sr2IrO4 single crystals. Our findings indicate that even under a substantial anisotropic biaxial distortion, Sr2IrO4 remains remarkably proximate to the Jeff=1/2 state. This study not only provides valuable information in understanding the interplay between magnetic anisotropy and strain but also the robustness of the Jeff=1/2 state under octahedral distortion within materials exhibiting emergent quantum phenomena.