Anisotropic Magnetic Response in the Superconducting Mixed State of UPt3

Equilibrium magnetization $M_{\mathrm{eq}}$ of the superconducting mixed state of UP${\mathrm{t}}_3$ has been measured as a function of field. At low temperatures, the discontinuity of ${\mathrm{dM}}_{\mathrm{eq}}/dH$ at the upper critical field $H_{c2\mathrm{}}$ exhibits marked anisotropy between the two principal directions of the hexagonal crystal, being indiscernibly small for $H\perp c$, 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.