Electronic and magnetic properties of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ epitaxial thin films*

Strongly correlated Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (CSRO) has attracted much attention for its rich physical properties such as Mott metal-insulator (MI) transition, antiferromagnetism (AFM), and spin-triplet superconductivity. We have grown epitaxial CSRO thin films on LaAlO$_{3}$ (001) substrates using a pulsed laser deposition method and investigated their electronic and magnetic properties. Ca$_{2}$RuO$_{4}$ thin films show strong compressive strain leading to an itinerant ferromagnetic (FM) phase coexisting with insulating AFM phase in the ground state and a suppressed broad and gradual MI transition. This is in sharp contrast to bulk Ca$_{2}$RuO$_{4}$, which exhibits an AFM Mott-insulating ground state and sharp MI transition. While the $x$=0.1 and 0.5 CSRO films also exhibit coherent strain, the MI transition and itinerant ferromagnetism are partially suppressed in the $x$=0.1 film and fully suppressed in the $x$=0.5 film. In contrast, Sr$_{2}$RuO$_{4}$ thin films are not susceptible to strain on any perovskite substrates including LaAlO$_{3}$; superconductivity in these films is suppressed due to disorders resulting from strain relaxation.