Extrinsic room-temperature ferromagnetism in MoS₂

We report stable room-temperature ferromagnetism in commercially available MoS₂ powder with a nominal purity greater than 98%. In order to assess the origin of the unexpected ferromagnetic signal, we carried out thorough characterization of the samples, by a combination of X-ray diffraction, Raman spectroscopy, electron microscopy, X-ray photoelectron spectroscopy and superconducting quantum interference device magnetometry. Using secondary ion mass spectrometry, we infer that up to 1.6% of a pool of different external dopants, including 0.8% of Fe and others, are present in the MoS₂ samples. We find very low value of magnetic moment per unit formula that, together with the small density of magnetic dopants, and the room-temperature magnetic order, leads us to conclude that ferromagnetism is not hosted at the MoS₂ crystal but can be ascribed to secondary phase of transition metal atoms’ clusters that aggregate. Our results stress the need of a careful characterization of transition metal dichalcogenides in the study of magnetism and spintronics involving either nominally pure MoS₂ as a diamagnetic semiconductor substrate or as a host material for diluted magnetic alloying.