Unusual magnetic response in superconducting mixed state of Sr2RuO4

We report on detailed measurements of the magnetization of Sr 2 RuO 4 in the superconducting mixed state, performed on a high-quality single crystal in a magnetic field parallel to the [100] axis. A small but distinct kink structure is found at around 9 kOe in the magnetization curve, indicating the presence of an additional superconducting phase transition that has been theoretically predicted to be evidence of the superconductivity with a two-component order parameter. An anomalous increase of magnetization is also observed just below the upper critical field. These anomalies disappear when the field is tilted from the [100] axis by only 3–5°. Possible origins of such magnetization behavior are discussed from the viewpoint of a spin-triplet pairing scenario with a two-component order parameter.