Exotic surface magnetotransport phenomena in the antiferromagnetic Mott insulator Ni S2

Sami El-Khatib, Faisal Mustafa, Mehmet Egilmez, Bhaskar Das, Yu Tao, Moumita Maiti, Yeon Lee, Chris Leighton

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

The pyrite-structure transition-metal disulfide NiS2 is in principle a model cubic antiferromagnetic Mott insulator that can be doped through insulator-metal transitions with both electrons and holes (in Ni1-xCuxS2 and Ni1-xCoxS2), eventually inducing superconductivity and ferromagnetism, respectively. Magnetism and transport have proven challenging to understand in NiS2, however. The antiferromagnetic spin structure below ∼39K is complex due to frustration, while unexplained weak ferromagnetism emerges below ∼30K. Surface conduction is also now understood to dominate in NiS2 at low temperatures, raising questions about the interpretation of decades of prior data. Here, we present a complete study of the surface magnetotransport phenomena that emerge at low temperatures in high-quality single-crystal NiS2, which turn out to be strikingly rich. On cooling, isotropic magnetoresistance due to a field-induced shift of the first-order weak ferromagnetic ordering transition is first uncovered, i.e., metamagnetic magnetoresistance. At lower temperatures, larger, anisotropic magnetoresistance effects arise due to distinct switching events associated with the weak ferromagnetism. Strong evidence is presented that this is due to a field-driven in-plane to out-of-plane reorientation of surface spins, likely correlated with surface steps and terraces. In-plane exchange bias accompanies these effects, further supporting this interpretation. At the lowest temperatures, the spin reorientation field eventually exceeds the 9-T measurement window, generating strongly field-asymmetric magnetoresistance. Some of these unusual phenomena also manifest in the Hall channel, culminating in a sizable anomalous Hall effect at low temperatures. These results significantly demystify recent magnetoresistance and magnetic microscopy observations in NiS2 crystals and nanoflakes, and constitute an important step in elucidating the complex electronic and magnetic properties of this pivotal antiferromagnetic Mott insulator.

Original languageEnglish (US)
Article number104401
JournalPhysical Review Materials
Volume7
Issue number10
DOIs
StatePublished - Oct 2023

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© 2023 American Physical Society.

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