Superconductivity and magnetism of a body-centered tetragonal ErRh4B4 single crystal. II. Large anisotropy of H/sub c/2

Upper critical field H/sub c/2 and magnetization are measured on a single crystal of body-centered tetragonal (bct) ErRh4B4 which is an antiferromagnetic superconductor and behaves metamagnetically in the c plane under a magnetic field below T/sub N/. The temperature dependences of H/sub c/2 along the [001] and [100] axes are roughly similar to each other in that both H/sub c/2(T) take maxima with decreasing temperature but the magnitude of H/sub c/2 along the [001] axis is quite large compared to that along the [100] axis. Furthermore, the angular dependence of H/sub c/2 in the c plane shows the fourfold anisotropy above T/sub N/ and the complex twofold anisotropy below T/sub N/. The anisotropy of H/sub c/2 is closely related with that of the magnetization. From the analysis in which the spin polarization due to the s-f exchange interaction and the electromagnetic interaction are taken into account it is made clear that the spin polarization plays a very important role in determining H/sub c/2. The s-f exchange constant and the spin-orbit scattering parameter are also determined. It is indicated that the s-f exchange interaction is ferromagnetic and the spin-orbit scattering is very strong in the bct ErRh4B4 crystal. Finally temperature dependence of H/sub c/2 in the c plane below T/sub N/ is qualitatively discussed, being connected with metamagnetism