Itinerant Ferromagnetism in Cr Doped Perovskite Ruthenates*

We report results of structural, magnetic and transport properties of single crystal CaRu$_{1-x}$Cr$_{x}$O$_{3}$ (0$\le $x$\le $0.36) and SrRu$_{1-x}$Cr$_{x}$O$_{3}$ (0$\le $x$\le $0.20). Cr substitution as low as x=0.08 drives CaRu$_{1-x}$Cr$_{x}$O$_{3}$ from paramagnetic state to an itinerant ferromagnetic state with an abrupt jump in isothermal magnetization leading to a sizeable saturation moment ($\sim $0.4$\mu _{B}$/f.u. within the ab plane). Ferromagnetism occurs abruptly and reaches as high as T$_{C}$=123 K for x=0.22. The Cr-driven ferromagnetism is highly anisotropic, suggesting an important role for spin-orbit coupling. Lattice constant and magnetic measurements strongly support the valence of the Cr to be tetravalent (Cr$^{4+}$, 3d$^{2}$ configuration). The Cr substitution for Ru in SrRuO$_{3}$ (T$_{C}$=165 K) enhances the itinerant ferromagnetism, consistent with Cr-induced ferromagnetism in paramagnetic CaRuO$_{3}$, but without significant magnetic anisotropy. Preliminary pressure-dependent magnetization of CaRu$_{0.85}$Cr$_{0.15}$O$_{3}$ shows strong enhancement of the saturation magnetization (25{\%} for P$\sim $0.7 GPa). All results indicate a coupling of Ru 4d and Cr 3d electrons that is unexpectedly favorable for itinerant ferromagnetism which often exists delicately in the ruthenates.