Anisotropic magnetic and superconducting phase diagrams of RNi[]B[]C (R = Y, Lu, Ho)

Abstract

Measurements of the upper critical field, H[]₂(T), are reported for YNi₂B₂C, T[] = 15.6 K, and LuNi₂B₂C, T[] = 16.1 K as well as a metamagnetic phase diagram for HoNi₂B₂C, T[] = 8.6 K. Resistance measurements, R(T, H), were taken on single crystal samples of RNi₂B₂C (R = Y, Lu) with the magnetic field applied along the crystallographic directions of (110) and (100) to determine the upper critical field. The resulting curves show a strong upward curvature near the superconductors transition temperature, T[], and a linear behavior with respect to decreasing temperature. Unlike most upper critical field curves, H[]₂(T), these show little sign of saturation down to temperatures below 2 K. Nonlocal extensions to the Ginzburg-Landau equations and numerical calculations of the coupled Eliashberg equations for an isotropic two band model will be used to interpret the interesting behavior exhibited by these compounds. HoNi₂B₂C has three magnetic transitions below the superconductors transition temperature which appear in zero applied magnetic field at 6.0, 5.5, and 5.2 K. From measurements of magnetic moment, M(T, H), and AC magnetic susceptibility, x(T, H), the magnetic phase boundaries have been determined to temperatures below 2 K and for applied magnetic gelds up to about 1 T. These measurements were taken with the applied magnetic field along the crystallographic directions of (110) and (100) and have been used to construct a magnetic phase diagram as a function of applied magnetic field, temperature, and in-plane field direction. A theoretical model which includes effects of the crystal electric field (CEF) helps explain the orientation of the magnetic moments for the various magnetic phases at low temperature, and the data presented here is supported by neutron scattering experiments, reported separately, which have determined incommensurate modulated phases at higher temperature.