Evidence for unconventional superconductivity in UPt₃ from magnetic torque studies

We studied the anisotropic magnetic response of the internal superconducting phases of UPt₃ and its anisotropic magnetic susceptibility with a capacitive torque meter which is very sensitive in high fields. Experiments were performed at temperatures down to 20 mK and at various angles between the c axis (hexagonal structure) and B⃗, ranging from 16° to 82°. The samples were four single crystals grown with different methods and subjected to different annealing procedures. The normal state susceptibility has a maximum around 20 K for B⃗ in the a-b plane which we followed up to 14 T. It may arise from hybridized uranium ion states split by the hexagonal crystal field. The magnetization curves in the superconducting (SC) regime show strong irreversibilities which are highly sample dependent. They are not correlated with the internal SC phase lines but continue up to a line of fields that lies parallel to the B_c2 curve and even follows its kink at the tetracritical point (T^⋆,B^⋆). In the cleanest sample this line is shifted to fields well below the B-C internal phase line which then manifests itself in a pronounced kink of the magnetization curve indicating an enhanced Ginzburg-Landau parameter κ. In another sample the B-C phase line between two of the three internal SC states could be detected even in the hysteretic region. The enhanced Ginzburg-Landau parameter κ means a larger penetration depth and/or a shorter coherence length, clear evidence for the unconventional character of the B-C phase transition. With our cleanest sample we also observe an anomalous peak effect, a region of enhanced flux pinning near B_c2, which is probably related to the Fulde-Ferrell-Larkin-Ovchinnikov state. In yet another sample we find a crossing of the up-down magnetization curves, also near B_c2, but with reversed orientation of the magnetization loops. We interpret this in terms of different flux pinning in the two main crystal directions, possibly in relation to the peak effect which is, however, masked in this sample by strong irreversibilities.