Easy-plane ferromagnetism in single-crystal ErB${}_2$ at low temperatures

Abstract

We report a study of single crystals of the hexagonal rare-earth diboride ErB${}_2$ prepared by means of the self-adjusted flux travelling-solvent floating-zone technique. Measurements of the magnetization, ac susceptibility, specific heat, and electrical resistivity consistently establish ferromagnetic order of the Er${}^{3+}$ moments below a second-order phase transition at $T_c=14$~K and a very strong easy-plane anisotropy. Curie--Weiss fits of the ac susceptibility are characteristic of ferromagnetic coupling within the easy hexagonal basal plane, and antiferromagnetic coupling along $⟨001⟩$. Under magnetic field within the basal plane the magnetization is reminiscent of a soft ferromagnet that is polarized in fields above a few tenth of a Tesla, vanishing hysteresis and negligible in-plane anisotropy. Under field along $⟨001⟩$, typical hard-axis behavior is observed with the magnetization increasing only weakly up to a spin-flip transition at $\hat{I}{¼}_0{H}_c=12$~T. The easy-plane anisotropy emerges below a crossover temperature $T_x\approx 50$~K , i.e. a broad paramagnetic temperature range above $T_c$ is governed by strongly anisotropic magnetic fluctuations.