Magnetism and superconductivity in single-crystal ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C

The resistivity ${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$(T) and anisotropic magnetization M(T) of ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C crystals were measured as a function of applied magnetic field (H) from 2 to 300 K to study the magnetic and superconducting properties with H parallel and perpendicular to the c axis (H∥c and H⊥c). Low-temperature specific-heat measurements for H=0 show a lambda-shaped anomaly associated with antiferromagnetic (AF) ordering at ${\mathit{T}}_{\mathit{N}}$=(5.85±0.15) K. The ${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$(T) and M(T) data show a superconducting transition at ${\mathit{T}}_{\mathit{c}}$=10.5 K, long-range AF ordering at ${\mathit{T}}_{\mathit{N}}$=6.0 K, and coexistence of superconductivity and antiferromagnetism below ${\mathit{T}}_{\mathit{N}}$. From the ${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$(T) and M(T) data, the ${\mathit{T}}_{\mathit{N}}$ was found to be independent of H up to H=20 kG for H∥c, whereas for H⊥c ${\mathrm{T}}_{\mathit{N}}$ decreased as ∼${\mathit{H}}^2$ from 6.0 K at H=0 to ${\mathit{T}}_{\mathit{N}}$=3.2 K at H=18 kG. The M(T) data show a change in the easy axis direction from H∥c above 150 to H⊥c below 150 K. This change in anisotropy is associated with the anomalously small magnitude of the crystalline electric field ${\mathit{B}}_2^0$ term.

The superconducting upper critical magnetic fields ${\mathit{H}}_{\mathit{c}2}$(T) for H∥c and H⊥c, determined from M(T) and ${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$(H,T) data, show anomalies for both field orientations near ${\mathit{T}}_{\mathit{N}}$. However, the local minimum of ${\mathit{H}}_{\mathit{c}2}$(T) near ${\mathit{T}}_{\mathit{N}}$, seen previously for both H⊥c and H∥c in a ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C crystal, was found only for H∥c in ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C. This anisotropy in ${\mathit{H}}_{\mathit{c}2}$(T) is likely a result of the anisotropy of the Er sublattice magnetization, specifically the anisotropy of ${\mathit{T}}_{\mathit{N}}$(H). The depth of the local minimum in ${\mathit{H}}_{\mathit{c}2}$ for H∥c for ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C near ${\mathit{T}}_{\mathit{N}}$ is comparable to the one for ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C at 5 K.