Anisotropic Curie temperature materials

Existence of anisotropic Curie temperature materials [E. R. Callen, Phys. Rev. 124, 1373 (1961)] is a longstanding prediction - materials that become paramagnetic along certain crystal directions at a lower temperature 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 single crystals remain ordered up to 603 K while the hard c-axis becomes paramagnetic at 225 K. Materials with such a large directional dependence of Curie temperature opens the possibility of uniquely new devices and phenomena.