Discovery of New Low Temperature Phase Transitions in Barium Hexaferrite

Abstract

The study of phase transitions in frustrated magnetic systems has been a frontline area of research in solid-state and material sciences. The most commonly investigated source of frustration is frozen-in substitutional disorder leading to a competition between ferromagnetic (FM) and antiferromagnetic (AFM) interactions. Such a competition and randomness in certain situations can prevent the emergence of long-range ordered (LRO) phases and instead give rise to a unique spin-glass state in which the spins are randomly frozen below a frequency (and#969;) and field (H) dependent peak, usually called as spin-glass freezing temperature (Tf), in the temperature dependence of magnetic susceptibility as a result of ergodic symmetry breaking. Frustration may also arise due to nearest neighbour AFM interactions alone due to the geometry of the lattice for spins arranged on the edge shared triangular (e.g. YbZnGaO4), corner shared triangular or kagome (e.g. (H3O)Fe3(SO4)2(OH)6 and SrGa12-xCrxO19), pyrochlore (e.g. Tb2Mo2O7), and spinels (e.g. MAl2O4 with M = Co, Fe, and Mn) lattices. Such geometrically frustrated systems have evinced enormous attention in recent years as many of them exhibit exotic spin liquid, spin ice and spin-glass transitions even in the absence of any apparent site-disorder. newlineRecent theoretical calculations seem to suggest that in the absence of any site-disorder, and hence randomness, the ground state of the geometrically frustrated systems has macroscopic degeneracy with no phase transition down to the absolute zero temperature. Such systems, however, have the possibility of the degeneracy getting lifted by quantum or thermal fluctuations through an intriguing mechanism known as order by disorder even in the absence of site disorder.