Hard-Axis Magnetization in Iron Borate Single Crystal

Iron Borate (FeBO₃) is a transparent green ferromagnet with a rhombohedral calcite-type structure. It holds great potential for use in magneto-optical devices due to its rare combination of room temperature ferromagnetism and optical transparency. Neutron diffraction studies have shown FeBO₃ is an antiferromagnet below T_N = 348 K and exhibits weak ferromagnetism due to Dzyaloshinskii-Moriya interaction, all in the crystallographic a-b plane. However, recent work has shown evidence of c-axis magnetism in FeBO₃ at low temperatures. In this study, we present structural (bulk and micro), heat capacity, and magnetic measurements of single crystal FeBO₃ that confirm c-axis magnetization with novel behavior including nontraditional paramagnetism above T_N, magnetic hysteresis, and rich magnetic entropy change. A qualitative explanation of these behaviors is proposed, stemming from magnetic frustration due to the interplay of long-range surface anisotropy and magnetic inhomogeneity with strong short-range local antiferromagnetism.