Resonant X-ray Scattering Study of Epitaxial Ca₂RuO₄ Thin Films

We report a comprehensive study of epitaxial Ca₂RuO₄ thin films using resonant X-ray scattering at the Ru-L₂ and L₃ absorption edges, which directly probe the Ru 4d valence states. The strong modification of the electronic properties by biaxial strain is readily seen in transport measurements, which reveal a tuning from an insulator (on NdCaAlO₄(110) substrates) to a metal (on LaSrAlO₄(001)). Using magnetometry and resonant X-ray scattering, we can identify ferromagnetic phases in films on LaSrAlO₄(001), LaSrAlO₄(110) and LaAlO₃(100) substrates and antiferromagnetic phases in films on NdCaAlO₄(110), LaSrAlO₄(110) and LaAlO₃(100). The antiferromagnetic phases are most compatible with the B-Type magnetic structure found in the pressurized bulk system exhibiting a T_N≈150 K. While the magnetic moment direction is typically along (010) in bulk Ca₂RuO₄, a polarization analysis of magnetic reflections of the film on NCAO(110) shows an unusual magnetic moment direction towards (-102).