Anisotropic magnetic response in the superconducting mixed state of UPt₃

Equilibrium magnetization M_eq of the superconducting mixed state of UPt₃ has been measured as a function of field. At low temperatures, the discontinuity of dM_eq/dH at the upper critical field H_c2 exhibits marked anisotropy between the two principal directions of the hexagonal crystal, being indiscernibly small for H ⊥ c, where the normal state paramagnetic susceptibility is largest. The results are not simply explained by the effective mass anisotropy nor by the ordinary paramagnetic effect of a spin-singlet pairing; they are rather in favor of an odd-parity pairing with an appreciable anisotropy in the pair-spin correlation.