Epitaxial strain effect on transport properties in Ca_2-xSr _xRuO₄ thin films

We have grown Ca_2-xSr_xRuO₄ (x = 0, 0.1, 0.5, and 2) epitaxial thin films using a pulsed laser deposition method and characterized their structures and magnetotransport properties. We find that the x = 0, 0.1, and 0.5 films grown on LaAlO₃ substrates exhibit coherent strain with tetragonal structure. The nature of strain is dependent on Sr content: the Ca₂RuO₄ (x = 0) film features biaxial compressive strain, while the x = 0.5 film shows biaxial tensile strain. The strain in the x = 0.1 film is relatively weak and strongly anisotropic, with compressive strain along the a axis and tensile strain along the b axis. In contrast, the Sr₂RuO₄ films show strain relaxation. The epitaxial strain effect leads the properties of the x=0, 0.1, and 0.5 films to be distinct from those of bulk materials. The bulk material shows antiferromagnetic Mott-insulating properties for x < 0.2 and a nearly ferromagnetic state for x ∼ 0.5, whereas the film displays itinerant ferromagnetism for x = 0 and 0.1 and paramagnetic metal for x = 0.5. Furthermore, in the x = 0 and 0.1 films, we observed distinct fourfold ferromagnetic anisotropy, with the minimum magnetoresistivity along the diagonal directions for x = 0 and a and b directions for x = 0.1. Such evolution of magnetic anisotropy may be associated with the tuning of the spin-orbit coupling by the epitaxial strain.