Magnetic and superconducting properties of single-crystal TmNi2B2C.

Abstract

The temperature (T) and applied magnetic field (H) dependent magnetization has been measured for a single crystal of ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C in order to study the interplay of superconductivity and the magnetism of the Tm sublattice. The normal-state magnetization of ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C is anisotropic from 2 to 300 K with the magnetic field applied normal to the c axis (H\ensuremath{\perp}c) leading to a smaller induced magnetization than the magnetization for the magnetic field applied parallel to the c axis (H\ensuremath{\parallel}c). This anisotropy is attributed to crystalline electric field (CEF) splitting of the J=6 manifold of the ${\mathrm{Tm}}^{+3}$ ion. From the inverse susceptibility [1/\ensuremath{\chi}(T)] for H\ensuremath{\parallel}c and H\ensuremath{\perp}c, the CEF parameter, ${\mathit{B}}_{2}^{0}$, is found to be (-1.15\ifmmode\pm\else\textpm\fi{}0.02) K. The superconducting state magnetization for H\ensuremath{\approxeq}${\mathit{H}}_{\mathit{c}2}$(T) obeys the Ginzburg-Landau theory which is used to evaluate the upper critical magnetic field ${\mathit{H}}_{\mathit{c}2}$(T) and ${\mathit{dH}}_{\mathit{c}2}$/dT${\mathrm{\ensuremath{\Vert}}}_{\mathit{T}\mathit{c}}$ values. The superconducting properties in this temperature region are similar to those of the nonmagnetic superconductor ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C, which has been shown to be an isotropic conventional type-II superconductor. For T\ensuremath{\le}6 K, ${\mathit{H}}_{\mathit{c}2}$(T) shows highly anisotropic behavior: ${\mathit{H}}_{\mathit{c}2}^{\mathrm{\ensuremath{\perp}}\mathit{c}}$\ensuremath{\approxeq}2${\mathit{H}}_{\mathit{c}2}^{\mathrm{\ensuremath{\parallel}}\mathit{c}}$. For both H\ensuremath{\parallel}c and H\ensuremath{\perp}c, ${\mathit{H}}_{\mathit{c}2}$(T) reaches a broad maximum near 4 K and decreases as T approaches ${\mathit{T}}_{\mathit{N}}$=(1.52\ifmmode\pm\else\textpm\fi{}0.05) K, indicating the interplay between superconductivity and magnetism. The broad maximum in ${\mathit{H}}_{\mathit{c}2}$(T) of ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C is likely a result of the increasing Tm sublattice magnetization at ${\mathit{H}}_{\mathit{c}2}$(T) with decreasing temperature, rather than of antiferromagnetic fluctuations.