Collective Magnetic Phases with Pentavalent Iridates

Abstract

Magnetism and magnetism-driven phenomena have always remained a central theme of modern condensed matter physics. Conventional magnetic transition, such as para magnetic to ferromagnetic phase, involves symmetry breaking and is described within the framework of Landau’s theory of phase transition. In recent years, one of the major themes of magnetism is study of quantum spin liquid (QSL). This is an exotic phase of magnetic materials where the spins continue to fluctuate without any symmetry breaking down to zero temperature in spite of having sufficient magnetic interactions among themselves. The spins are entangled over a long range in QSL phase. These systems can host fractional excitations, which are a potential platform for topological quantum computation. However, materials realization, experimental identification, and theoretical description of such highly entangled QSL phases are extremely challenging. This thesis focuses on the experimental realization and manipulation of such QSL phases. The popular approach to realize QSL phase is stabilizing S = 1/2 ions on a two dimensional geometrically frustrated lattice such as triangular, Kagome, etc. Over the past 15 years, several compounds have been reported as QSL candidates following this design principle. Among the handful of reports on QSL with spin S = 1, examples with magnetic ions on a three-dimensional (3D) magnetic lattice are extremely rare since both the larger spin and higher dimension tend to suppress quantum fluctuations. In this thesis, we explore a completely new strategy to achieve 3D QSL with a high spin by utilizing two types of transition metal ions; both are magnetically active but located at crystallographically inequivalent positions. For this, we focus on hexagonal oxides with general formulae Ba3MM’2O9, where M and M’ are transition metal ions. In the first part, we investigated Ba3NiIr2O9 consisting of interconnected corner-shared NiO6 octahedra and face-shared Ir2O9 dimer, both having triangular arrangements in the ab-plane. Ni, in its +2-oxidation state, has a spin of 1 (S = 1). While a nonmagnetic J = 0 state is expected due to strong spin-orbit coupling effect for Ir5+ ions, they exhibit finite moments in reality due to presence of a non-cubic crystal field. Detailed electrical, magnetic, and thermodynamic measurements confirm the presence of significant exchange interactions in three dimensions without any magnetic ordering down to 100 mK. Our findings of linear dependence of magnetic specific heat with temperature in this insulating compound implies presence of gapless spinon excitations with a Fermi surface. Muon spin rotation (µSR) measurements further demonstrated the presence of dynamically fluctuating magnetic moments down to 100 mK. All these experimental observations are in-line with the expected behaviors of a QSL. Next, we investigate the effect of disorder on the QSL state. Generally, it is believed that the QSL phase is very susceptible to disorder as disorder and randomness in a lattice break frustration, leading to frozen or glassy dynamics. As we have two magnetic ions in Ba3NiIr2O9, we replaced them individually and checked their effect on the ground state. First, we have doped nonmagnetic Zn in place of Ni as nearly similar ionic radius of Zn & Ni allows us to perform such doping uniformly without modifying the crystal structure. The decreasing Ni concentration reduces the dimensionality of the exchange. Our thermodynamic measurements and µSR experiments down to 100 mK confirm the absence of any magnetic ordering. Finally, the disorder effects at the Ir-site have been investigated. For this, we have replaced Ir with Ru partially, as Ru can be stabilized in face-shared dimer units with a +5 oxidation state. This allows to maintain +5 oxidation state of Ir. Moreover, the SOC strength of Ru is much smaller compared to Ir due to its lower atomic number. Interestingly, we have found that the effective SOC strength of Ir5+ also reduces with Ru doping, which is likely to be connected to a change in structural parameters and electronic hopping strength. We have observed that the magnetic Ru (Ru5+: S=3/2) substitution drastically changes the ground state of Ba3NiIr2O9 and results in long-range magnetic order.