Magnetic and superconducting properties of single-crystal ${\mathrm{TmNi}}_2$${\mathrm{B}}_2$C

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⊥c) leading to a smaller induced magnetization than the magnetization for the magnetic field applied parallel to the c axis (H∥c). This anisotropy is attributed to crystalline electric field (CEF) splitting of the J=6 manifold of the ${\mathrm{Tm}}^{+3\mathrm{}}$ ion. From the inverse susceptibility [1/χ(T)] for H∥c and H⊥c, the CEF parameter, ${\mathit{B}}_2^0$, is found to be (-1.15±0.02) K. The superconducting state magnetization for H≊${\mathit{H}}_{\mathit{c}2}$(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${\mathrm{\Vert }}_{\mathit{T}\mathit{c}}$ values. The superconducting properties in this temperature region are similar to those of the nonmagnetic superconductor ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C, which has been shown to be an isotropic conventional type-II superconductor. For T≤6 K, ${\mathit{H}}_{\mathit{c}2}$(T) shows highly anisotropic behavior: ${\mathit{H}}_{\mathit{c}2}^{\mathrm{\perp }\mathit{c}}$≊2${\mathit{H}}_{\mathit{c}2}^{\mathrm{\parallel }\mathit{c}}$. For both H∥c and H⊥c, ${\mathit{H}}_{\mathit{c}2}$(T) reaches a broad maximum near 4 K and decreases as T approaches ${\mathit{T}}_{\mathit{N}}$=(1.52±0.05) K, indicating the interplay between superconductivity and magnetism. The broad maximum in ${\mathit{H}}_{\mathit{c}2}$(T) of ${\mathrm{TmNi}}_2$${\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.