Existence of anisotropic Curie temperature materials [E. R. Callen, Phys. Rev. 124, 1373 (1961); J. Appl. Phys. 32, S221 (1961)] is a longstanding prediction – materials that become paramagnetic at a lower temperature along certain crystal directions while remaining magnetically ordered in other directions up to a higher temperature. Validating Callen's theory, we show that all directions within the basal plane of monoclinic Fe7S8 (pyrrhotite) single crystal remain ordered up to 603 K while the c-axis becomes paramagnetic at 225 K. Results prompt a re-evaluation of existing magnetic materials with a focus on magnetic characteristics along different crystal orientations above instead of below the ordering temperatures. Theoretical guidelines for identifying new materials with large anisotropy of Curie temperature are also given, and analysis protocol to characterize them in a self-consistent manner is discussed. Materials with such a large directional dependence of Curie temperature open the possibility for uniquely new devices and phenomena, including (spin) transport.
Anisotropic Curie temperature in pyrrhotite (Fe7S8). Notice the large difference in ordering temperature along basal plane (main graph) relative to the c-axis (inset).