Site and Symmetry Defined Magnetic Ground States of Intercalated Divalent Transition Metal Diselenides

Abstract

Transition metal dichalcogenides (TMDs) are a vast family of compounds with various structural types. In this thesis, we focus on the 2H-type TMD structure. The number “2” describes the number of two-dimensional layers needed to describe the hexagonal unit cell, which is denoted by the letter “H”. We exploit the versatility of the interstitial site between the two TMD layers to introduce magnetic cations – specifically cobalt. While the 2H-phase is the most stable structure for the host TMD, we find that the intercalant can have many site arrangements leading to distinct superlattice arrangements. These superlattices will have severe implications on the symmetry of the crystal, where at low intercalation ratios we find that centrosymmetry of the host is preserved; further increasing the intercalant, this symmetry operation is broken, leading to a non-centrosymmetric structure. Each of these symmetries has direct implications on both observed electronic and magnetic properties. The host lattice made of TMD has partially filled electronic states, leading to various electronic properties and instabilities. Filling these available states through intercalation leads to an intricate interplay between the host electronic properties and the “guest’s” (intercalated cation) magnetism. Here, we explore the conditions under which particular symmetry (centrosymmetric or non-centrosymmetric) crystalize and map the magnetostructural phase diagram producing each magnetic phenomena. These magnetic properties are mainly antiferromagnetic for the present case of intercalating cobalt, and include altermagnetism at low cobalt densities, and spin density waves at higher occupations. We particularly focus on theeffects of intercalating within NbSe2 and TaSe2, as we have found them to be underexplored at the time of this study, while being canonical examples of exhibiting electronic instabilities.