We investigate the magnetic response of thin NiI2 flakes for temperatures above 80~K. Since no magnetic ordering is expected for bulk NiI2, we observe clear paramagnetic response for massive NiI2 single crystals. In contrast, thin NiI2 flakes show well-defined ferromagnetic hysteresis loop within ±2~kOe field range. The value of the response does not scale with the sample mass, ferromagnetic hysteresis can be seen for any flake orientation in the external field, so it originates from the sample surface, possibly, due to the anisotropic exchange (Kitaev interaction). The observed ferromagnetism is weakly sensitive to temperature up to 300~K. If a flake is multiply exposed to air, ferromagnetic hysteresis is accompanied by the periodic modulation of the magnetization curves, which is usually a fingerprint of the multiferroic state. While NiI2 flakes can not be considered as multiferroics above 80~K, surface degradation due to the crystallohydrate formation decreases the symmetry of NiI2 surface, which produces the surface ferroelectric polarization in addition to the described above ferromagnetic one.
@article{arxiv.2309.08392,
title = {Ferromagnetic response of thin NiI$_2$ flakes up to room temperatures},
author = {N. N. Orlova and A. A. Avakyants and A. V. Timonina and N. N. Kolesnikov and E. V. Deviatov},
journal= {arXiv preprint arXiv:2309.08392},
year = {2024}
}