Understanding 2D van der Waals Antiferrmagnets: MnPS3 and CrSBr

Abstract

Low-dimensional material shows unique properties as compared to their 3D counterpart, making them extremely fascinating to study. Since 2004 when graphene was first exfoliated by Andre Geim and Konstantin Novoselov, the field of 2d van der Waals material bloomed. 2d van der Waals material contains a wide range of materials such as metals, insulators, semiconductors, superconductors, and topological materials. After 2016, researchers’ interest in 2D magnets increased as they can used as multifunctional material with a wide range of phenomena and promising candidates for next-generation information technology. The recent discovery of 2D magnetic ordering in CrI3, Cr2Ge2Te6, VSe2, and Fe3GeTe2 has stimulated to expansion of the scope of 2D magnets. In this thesis, we will discuss two such materials which are antiferromagnetic in nature. MnPS3 is an antiferromagnetic semiconductor with a wide band gap of 2.64 eV and a Neel temperature of 78 K. CrSBr is an antiferromagnetic semiconductor with a band gap of 1.5 eV and Neel temperature of 132 K. In the first part of the thesis, both MnPS3 and CrSBr were characterized using an Optical microscope, atomic force microscopy, and Raman Spectroscopy. Raman mode found for MnPS3: 275 cm-1 and 383 cm-1 and CrSBr: 114 cm-1, 244 cm 1, 344 cm-1showing comparing vibrational modes in two different samples. HR-TEM and elemental mapping of CrSBr show Cr and S have a lower atomic concentration than that of bromine. In the second part, Low-temperature STM measurements were performed on a CrSBr single crystal at 77K in the antiferromagnetic phase to understand its electronic correlation. Then, an investigation of intrinsic defects was performed and tried to understand the role played in electron doping. A general trend of valence band upward shift was observed in those defect regions giving a clear indication of an increase in charge carrier density. MnPS3 thin flakes color on SiO2/Si was quantified showing quasi-periodic oscillatory behavior. The optical absorbance spectra were taken at room temperature for thin flakes to understand the light-matter interaction. An optical band gap of 2.7eV was observed in all the flakes.