Magnetic phase diagrams of the pyrochlore-based magnets: Landau theory

Abstract

The family of crystals with a pyrochlore lattice, including pyrochlores, spinels, and Laves phases (C15) is a fertile playground for the discovery and scientific study of exotic magnetic properties as well as search for new technological applications. In this work a possible magnetically long-range ordered phases derived from the archetypical pyrochlore lattice were studied within the Landau phase transition theory. A group-theoretical analysis reveals the existence of 20 low symmetrical phases, generated by the magnetic order parameters associated with the center of the Brillouin zone. For each of these phases, the symmetry-related magnetic and structural parameters were determined and discussed in detail. We extend this analysis by means of the thermodynamic theory: on the ground of the sixth-degree thermodynamic potential phase diagrams of the spin-ordered pyrochlore-based magnets are constructed in the exchange approximation. It is shown that an essential feature of all studied phase diagrams is the existence of singular elements: multiphase points, multicritical points of different types, lines of phase transitions of both the first and second order.