Phase Transitions and Finite Temperature Magnetism: Experiment and Analysis

Abstract

Abstract Magnetic excitations, prevailing in different temperature regimes that lie below the magnetic ordering temperature, and static critical phenomena near the thermally‐driven magnetic order–disorder phase transitions in spin systems in which the electrons responsible for magnetism are either localized or itinerant are reviewed from the experimental point of view. Besides bringing out clearly the importance of the energy and time scales governing various physical processes connected with the finite temperature magnetism and phase transitions in such systems, a brief account of the relevant theoretical background and crucial steps in the data analyses is given so as to prepare the ground for a meaningful comparison between theory and experiment. Such a comparison, in turn, is shown to provide improved understanding of the nature and origin of magnetic excitations and critical phenomena in localized‐spin and itinerant‐electron systems. In addition, this review highlights both the power as well as the limitations of the experimental techniques employed in the literature to study magnetic excitations and deals extensively with the magnon‐to‐fracton crossover in systems with localized spins and crossovers between different critical regimes in isotropic spin systems with long‐range interactions or spin systems in which long‐range interactions occur in association with magnetic anisotropy.