Superconducting properties of the 3-K phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T-c, of the impurity-free, intrinsic Sr2RUO4 is as high as 1.50 K. However, we recently showed that T-c is remarkably increased up to 3 K in the Sr2RuO4-Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr2RuO4. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H-c2 for the field parallel to the RuO2 plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr2RuO4-Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, rho(c), at low temperatures and near H-c2, only when the field was applied parallel to the RuO2 plane.