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.