Order by Disorder in an antiferromagnetic Ising pyrochlore

FERREYRA, MARÍA VICTORIA; GURUCIAGA, PAMELA CAROLINA; BORZI, RODOLFO; GRIGERA, SANTIAGO ANDRÉS

Trieste

Otro; School in Computational Condensed Matter Physics: From Atomistic Simulations to Universal Model Hamiltonians; 2015

The Abdus Salam International Centre for Theoretical Physics (ICTP)

We present a study of an antiferromagnetic Ising pyrochlore by means of computational simulations. The Ising-like spins live on the vertices of a corner-sharing tetrahedra lattice (pyrochlore lattice), pointing in or out  of the elementary tetrahedra along the local directions <111>.  The spin configuration can be mapped into a simpler system by replacing the magnetic dipoles of the pyrochlore sites (spins) by dumbbells consisting of equal and opposite magnetic charges situated at the vertices of a diamond lattice . In this way, depending on the spin configuration, the diamond lattice can host four types of charge (magnetic monopoles): positive or negative, single or double. In our system, the ground state is a crystal of alternating double monopoles ( all-in/all-out configuration). If the system is placed in a [110] magnetic field, the lowest energy state has every tetrahedron in a 3in-1out or 3out-1in configuration, i.e., the ground state consists of single monopoles. The field along the [110] direction couples only two spins per tetrahedron, since the other spins are orthogonal to that direction. The double monopoles are now the lowest excitations. At T=0 the ground state is disordered, but at finite (low) temperature a charge crystalline order  appears. This effect is known as order by disorder.  We study this phenomena by Monte Carlo simulations, using the Wang-Landau algorithm to approximate the density of state of the system, and the Metropolis algorithm to complete the characterization.