Magnetic and superconducting properties of holmium-rich (Er1−xHox)Rh4B4

Neutron scattering, susceptibility, and resistivity measurements have been carried out on (${\mathrm{Er}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ho}}_{\mathrm{x}}$)${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$ for concentrations x=1.0, 0.89, 0.84, and 0.75, which span the ‘‘multicritical’’ point ($x_c$≊0.9) where the magnetic and superconducting phase boundaries meet. For all concentrations studied a transition to long-range ferromagnetic order is observed, with the Ho moments ordering along the tetragonal axis. The spins are in fact locked along the c axis by strong crystal-field anisotropies, yielding Ising-like magnetic behavior in this concentration region. The temperature dependence of the order parameter is mean-field-like, suggesting that the range of the magnetic interactions is large, and we attribute this behavior to the importance of dipolar interactions. In the concentration range x$x_c$ where superconductivity is observed, the magnetic transition appears to be continuous