Magnetic transitions and nearly reentrant superconducting properties ofHoNi2B2C

A low-temperature phase diagram H(T) of the 7.8-K superconductor ${\mathrm{HoNi}}_2$ ${\mathrm{B}}_2$C (with an onset of 8.3 K) is generated through characterization of well-prepared samples by various experimental techniques including ac magnetic susceptibility, superconducting quantum interference device dc magnetic susceptibility, magnetic hysteresis, specific heat, and electrical resistivity measurements. The results yield a superconducting upper critical field ${\mathit{H}}_{\mathit{c}2}$(0) of 3.5 kG, a lower critical field ${\mathit{H}}_{\mathit{c}1}$(0) of 250 G, and a Ginzburg-Landau parameter κ of 3.5. A nearly reentrant deep minimum at 5.2 K with very small ${\mathit{H}}_{\mathit{c}2}$ of 400 G and ${\mathit{H}}_{\mathit{c}1}$ of 5 G are observed. Two distinct magnetic transitions are observed with an incommensurate magnetic ordering temperature ${\mathit{T}}_{\mathit{m}}$ of 5.7–6 K and an antiferromagnetic Néel temperature ${\mathit{T}}_{\mathit{N}}$ of 5.2 K. The magnetic entropy Δ(${\mathit{S}}_{\mathit{m}}$+${\mathit{S}}_{\mathit{N}}$) estimated between 2 and 10 K is 10.4 J/mol K. The effective internal field which causes the nearly reentrant behavior is 2 kG at 5.2 K.