HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

β-Co(OH)2 intrinsically is an antiferromagnetic semiconductor with Néel temperature (TN) of ~ 11 K. On the other hand, a semiconducting ferromagnet is indeed a very important component and used as ferromagnetic contacts in spin transistors. Therefore, realization of a 2D ferromagnetic semiconductor with higher Curie temperature and coercivity is a genuine challenge. In earlier work, we have used interface interaction to achieve a transition from antiferromagnetic to ferromagnetic ordering when the antiferromagnetic Ni(OH)2, Co(OH)2 are grown on the MoS2 surface acting as a 2D template. Thus interface interaction largely depends on the thickness of both the magnetic material as well as the template material. The major limitation of the earlier work is that we could not reduce the thickness of the template (MoS2), because of the limitation in the synthesis condition. Therefore, in the present work, we have used WS2 as a 2D template, the thickness of which is reduced to about 3–4 layers. As a consequence, a large enhancement in Curie temperature (131 K), coercivity (1285 Oe) and magnetoresistance (46 %) are observed in the present sample of the ultrathin β-Co(OH)2 phase grown on a thin-layered WS2 sheet.

No abstract

To study the 2D materials, an interface interaction is very important to tune the physical properties because of the large specific surface area. When the antiferromagnetic α-Fe2O3 is grown in a single-crystalline form on a diamagnetic MoS2, enormous changes in magnetic properties are observed in the 2D composite system. Strong ferromagnetism with a giant coercivity of 1.8 T is observed in this single-crystalline α-Fe2O3/MoS2 heterostructure. It is noted that thinner sheets show better coercivity than the thicker sheets, and the coercivity decreases with the increase in temperature. This huge coercivity in larger and thinner single-crystalline α-Fe2O3 sheets grown on a MoS2 arises due to charge transfer from “S” to “Fe” and the surface pinning effect at the interface. A large negative magnetoresistance with a maximum value of 15% is achieved due to the spin-spin interaction, and a positive magnetoresistance is also observed at low field and high temperature as a result of the spin splitting effect.

Because of the important role of two-dimensional (2D) magnetic semiconductors in low-dimensional spintronic devices, the generation of ferromagnetism within an ultrathin semiconducting sheet of a transition metal dichalcogenide is highly desirable. A pristine WS2 sheet is a diamagnetic semiconducting transition metal dichalcogenide with superior electronic properties. In this study, we synthesised a free-standing WS2 sheet by a chemical route followed by electrochemical exfoliation by a giant molecule. During exfoliation of the WS2 crystal, atomic vacancies were created in the sheet with a lower number of layers. To understand the mechanism of exfoliation, we carried out x-ray diffraction, transmission electron microscopy, atomic force microscopy and Raman measurements. The types of atomic vacancies were realised by energy-dispersive x-ray spectroscopy, high-resolution transmission electron microscopy (fast Fourier transform), and x-ray photoelectron spectroscopy studies. We also observed a ferromagnetic ordering within the exfoliated WS2 sheet, which is explained on the basis of the generation of an atomic vacancy induced spin-moment. The transport study of the exfoliated WS2 sheet suggests that the electro-transport behaviour still remains as a semiconductor even after exfoliation. This ferromagnetic semiconducting system will be applicable in spintronic devices and this technique will enrich the literature, particularly for the preparation of a 2D semiconducting ferromagnet in a facile fashion.