Intrinsic ferromagnetism and valley polarization in hydrogenated group V transition-metal dinitride (MN2H2, M = V/Nb/Ta) nanosheets: insights from first-principles

Abstract

Due to the extraordinary electronic and magnetic properties, transition-metal dinitrides (TMDNs) and their derivatives are gaining importance in low-dimensional layered materials. In this work, through first-principles calculations, we have comprehensively investigated the structural and electronic properties of hydrogenated group V TMDN nanosheets. We find that surface hydrogenation can well stabilize the H-, T- and M-phase structures of group V TMDNs, for which the formed MN₂H₂ nanosheets have robust energetic, dynamical and thermal stabilities. Different from pristine MN₂ systems, the H-phase system has become the most favorable structure of MN₂H₂ nanosheets. Intrinsic ferromagnetism is present in these H-MN₂H₂ nanosheets, which even exhibit bipolar magnetic semiconducting behaviors. More interestingly, large spontaneous valley polarization occurs in the H-MN₂H₂ nanosheets, and is attributed to the coexistence of remarkable spin–orbit coupling and magnetic exchange interactions according to the k·p model analysis. Among them, the H-NbN₂H₂ nanosheets are found to be a promising ferrovalley material, whose valley polarization value reaches as large as 0.11 eV and the Curie temperature is up to 225 K. Besides that, versatile electronic properties are obtained in the T- and M-phase structures of the MN₂H₂ nanosheets, which will be magnetic/nonmagnetic metals/semiconductors depending on the metal species and phase structures. Our study demonstrates that the hydrogenation can bring robust structural stabilities and unconventional electronic properties into the group V TMDN nanosheets, which enable many potential applications in spintronics and valleytronics.