Field-tuning order by disorder in the antiferromagnetic Ising pyrochlore

PAMELA C. GURUCIAGA; M. VICTORIA FERREYRA; SANTIAGO A. GRIGERA; RODOLFO A. BORZI

Lyon

Seminario; Theoretical Physics Seminar; 2015

Laboratoire de Physique, École Normale Supérieure de Lyon

In the pyrochlore lattice, Ising-like spins occupy the vertices of corner-sharing tetrahedra, pointing along the local directions. Via the dumbbell model, this spin configuration can be mapped into a system of non-conserved magnetic charges ("magnetic monopoles"), with four types of charge: positive or negative, single or double. In this way, the rotation of a magnetic moment is equivalent to the creation, annihilation or translation of a monopole in a discrete lattice. This mapping is commonly used in the context of the frustrated magnetic materials known as spin ices; in this case, we will address their antiferromagnetic counterpart and consider only nearest-neighbour interactions.Due to the system's geometry, a magnetic field applied along the [110] direction of the pyrochlore lattice couples to two spins per tetrahedron, but does not affect the other two which are perfectly orthogonal. In this situation, there is a range of field intensity in which the ground state consists of single monopoles, with double monopoles playing the role of the lowest excitations. Although the system is charge-disordered at T=0, a single monopole crystal is found at finite (low) temperature --a phenomenon known as order by disorder. We use Wang-Landau algorithm to find the density of states of the system and show that it is lead to order by the excitations. Also, we perform Monte Carlo simulations with Metropolis algorithm to characterise other transitions present.