Investigation of strongly correlated paramagnetic state at sub-Kelvin regime for S ≥ 1/2 systems: Role of disorder and dimensionality

Abstract

A magnetic system usually orders ferro- or anti-ferromagnetically at temperatures comparable to interaction strength between the spins. Moreover, an interacting spin system tends to order with the increase of dimensionality of the magnetic lattice, as determined by the spin-spin correlation along various directions. However, there are certain lattice types where such orderings are strongly suppressed. A prototypical example is the Ising spin-1/2 on a triangular lattice with a nearest neighbour antiferromagnetic interaction where the triangular arrangement results in competing interactions leading to a large number of distinct states with the same ground state energy and therefore magnetic frustration. When frustration extends over a long range, it can lead to the formation of highly degenerate ground states with spin fluctuations at absolute zero temperature, leading to exotic magnetic ground states such as Quantum Spin Liquid (QSL). In recent times, there is an increasing interest in such magnetically frustrated systems, in search of QSLs which relieve the frustration by entangling the spins instead of ordering. Another approach to achieve a ground state without ordering is to introduce sufficient magnetic disorder in a lattice. Such systems may get stuck in a differently disordered “glassy” state. In this thesis work, we explore what happens when extensive disorder is in the frustrated triangular lattice such that conditions that promote spin liquid coexist with those aiding glassiness. The thesis will present in detail such dynamic correlated paramagnetic states in a few selected materials. These systems differ, not only in their chemical compositions and the nature of the magnetic ions, but also in terms of the dimensionality of the magnetic interactions and the extent of disorder. Using combined experimental and theoretical approaches, detailed investigations have been carried out on a series of magnetic materials, varying the extent of disorder, dimensionality of magnetic interactions, and the magnetic spin moments, at the sub-Kelvin temperature regime, revealing several unexpected behaviours.