Superconductivity and magnetism in the Ho1−xErxRh4B4 alloy system

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Abstract
We have used neutron scattering techniques to examine the magnetic transitions in the Ho1−xErxRh4B4 pseudoternary system. A wide variety of behavior is found, ranging from mean‐field behavior for HoRh4B4 to complicated behavior for ErRh4B4, where superconductivity and long‐range ferromagnetic order coexist between 0.7 and 1.2 K. Long‐range ferromagnetic order is found at the lowest temperatures for all alloy compositions. Alloys with more than about 30% Ho order magnetically along the c axis and superconductivity is destroyed in these alloys in a sharp transition coincident with magnetic ordering. Alloys near the ErRh4B4 composition order in the basal plane but undergo a complex ordering process through a sinusoidally modulated state. Because Ho and Er have competing orthogonal magnetic anisotropies, compounds near the composition Ho0.25Er0.75Rh4B4 are near a multicritical point in the magnetic phase diagram.