Anisotropic Curie temperature materials

TL;DR

This paper experimentally validates the long-standing prediction of anisotropic Curie temperatures, demonstrating that certain crystal directions in Fe7S8 remain magnetic at much higher temperatures than others, enabling novel device applications.

Contribution

It provides experimental evidence for anisotropic Curie temperatures in Fe7S8, confirming Callen's theory and revealing large directional dependence in magnetic phase transitions.

Findings

Basal plane directions remain ordered up to 603 K

C-axis becomes paramagnetic at 225 K

Large anisotropy enables new magnetic devices

Abstract

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.