Exploring structure-property relationships in layered perovskite iridates

Abstract

Geometrically frustrated perovskites- where the symmetry of the underlying crystal structure inhibits long range magnetic ordering- is the most prominent structural class, where the search for non-trivial spin structures appears to bear fruit. The physics of these systems is especially rich, when they comprise of 4d and 5d transition metals, where the large spin-orbit coupling is known to be responsible for the stabilization of a number of novel magnetic and electronic ground states. During the course of this thesis, we have investigated a number of Iridium based systems crystallizing in the triple perovskite structure of the form Ba3MIr2O9 (M=Co, Ni or Na) with the aim of understanding their structure-property relationships. Our results show that the ground state of these systems are remarkably diverse, with factors like multiple valence states of M and Ir, Jahn-Teller distortion, anti-symmetric Dzyaloshinskii-Moriya interaction, multiple super-exchange pathways, crystal field splitting, spin-orbit coupling, and hybridization being the driving forces. For instance, Ba3CoIr2O9 and Ba3NiIr2O9, both stabilize in a hexagonal symmetry at room temperature with Ir in the +5 oxidation state. Ba3CoIr2O9 exhibits a magneto-structural transition to a monoclinic phase at 107K - the highest known amongst all the triple perovskite iridates - and transforms to a monoclinic phase with even lower symmetry at 70K, with both these phases coexisting down to the lowest measured temperatures. Interestingly, Ba3NiIr2O9 retains the hexagonal symmetry down to the lowest measured temperature and exhibits two magnetic transitions at 106K and 6K that are attributed to antiferromagnetic ordering. Ba3NaIr2O9, with a mixed valent state of Ir+5 and Ir+6, is a geometrically frustrated antiferromagnet exhibiting a co-operative paramagnetism starting at around 200K. On reducing the temperature, one observes a strain induced structural transition to an orthorhombic phase nucleating at 40K. Below 6K, it exhibits glassy dynamics, originating as a consequence of mixed interactions and phase coexistence. The results obtained would add significantly to our current understanding of the structure-property relationships in this class of materials with strong spin-orbit coupling. We have also explored a hitherto unreported family of Sr based triple perovskites where we have successfully found many new potential quantum spin liquid candidates. In addition, we have also investigated the possible existence of a multiglass state- where both the magnetic and polar orders are frozen- in the doped quantum paraelectric SrTiO3 and KTaO3. The feasibility of multiglass state in doped quantum paraelectrics has been under debate for a long time. By doping of magnetic transition metals (Mn, Fe, Co, or Ni), we observe that the doped quantum paraelectric appear to remain intrinsically paramagnetic down to the lowest measured temperatures, settling the conflict and thus providing a concrete proof on the absence of multiglass state in these doped quantum paraelectrics.