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Magnetism in Semiconducting Molybdenum Dichalcogenides

Transition metal dichalcogenides (TMDs) are interesting for understanding the fundamental physics of two-dimensional (2D) materials as well as for applications to many emerging technologies, including spin electronics. Here, we report the discovery of long-range magnetic order below TM = 40 and 100 K in bulk semiconducting TMDs 2H-MoTe2 and 2H-MoSe2, respectively, by means of muon spin rotation (μSR), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. 

The μSR measurements show the presence of large and homogeneous internal magnetic fields at low temperatures in both compounds indicative of long-range magnetic order. DFT calculations show that this magnetism is promoted by the presence of defects in the crystal. The STM measurements show that the vast majority of defects in these materials are metal vacancies and chalcogen-metal antisites, which are randomly distributed in the lattice at the subpercent level. 

DFT indicates that the antisite defects are magnetic with a magnetic moment in the range of 0.9 to 2.8 μB. Further, we find that the magnetic order stabilized in 2H-MoTe2 and 2H-MoSe2 is highly sensitive to hydrostatic pressure. These observations establish 2H-MoTe2 and 2H-MoSe2 as a new class of magnetic semiconductors and open a path to studying the interplay of 2D physics and magnetism in these interesting semiconductors.

Transition metal dichalcogenides (TMDs), a family of two-dimensional (2D) layered materials like graphene, have been a subject of tremendous amounts of experimental and theoretical studies due to their exciting electronic and optoelectronic properties. TMDs share the same formula, MX2, where M is a transition metal and X is a chalcogen. They have a layered structure and crystallize in several polytypes, including 2H-, 1T-, 1T′-, and Td-type lattices. Much interest has focused on the cases of M = Mo or W, because the 2H forms of these compounds are semiconducting and can be mechanically exfoliated to a monolayer. 

In bulk form, 2H-MoTe2 is a semiconductor with an indirect bandgap of 0.88 eV. The unique properties of TMDs especially in the monolayer form have shown great promise in device applications such as magnetoresistance and spintronics, high on/off ratio transistors, optoelectronics, valley optoelectronics, superconductors, and hydrogen storage. Many of these interesting properties arise on account of the strong spin-orbit interaction present in these materials due to the heavy metal ion. 

While there are many studies focused on the spin-orbit coupling and the interesting consequences for electrical and optical properties in these systems, there are very limited, and mostly theoretical, studies on intrinsic magnetism in these materials. Theoretical and experimental work shows that in the absence of crystalline imperfections, the Mo-based TMDs are nonmagnetic. The ability to add magnetism into the properties of these materials can open up a host of new opportunities as tunable magnetic semiconductors.

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