Sudden field rotations in spin-ice

FERREYRA, MARÍA VICTORIA; BORZI, RODOLFO; GRIGERA, SANTIAGO ANDRÉS

Berlín

Workshop; Third annual workshop of the Helmholtz Virtual Institute ?New states of matter and their excitations?; 2015

Helmholtz Virtual Institute ?New states of matter and their excitations?

Spin ice systems have a pyrochlore structure, where the magnetically active sites form a lattice of corner-sharing tetrahedra. These moments behave Ising-like at low temperatures, pointing in or out of the elementary tetrahedra. The combination of exchange and dipolar interactions leads an effective ferromagnetic interaction between nearest neighbours and gives origin to ice rules, analog to Bernal-Fowler?s: the two-in two-out configuration is the lowest energy state. Excitations - defects, i.e., tetrahedra that violate the ice rules - can be considered as magnetic monopoles which can move through the system by flipping magnetic moments. The dynamics of these excitations have already been studied by Castelnovo et al [1,2], by monitoring the monopole annihilation process in thermal and field quenches. In both quenches, they found a dynamical arrest that slows the annihilation process and leads to an interesting time-dependence of the monopole density. We are interested in these dynamics and their experimental measurement. The suggested experiments for detecting monopoles present difficulties due to the short times involved in the density decays compared with the times of thermal and field quenches. We propose a different point of view: a field quench through a sudden rotation of magnetic field. If a spin ice system is placed in a strong [111] magnetic field, the lowest energy state is one in which every tetraedron has a 3in-1out or 3out-1in configuration. In the saturated regime, the system is a perfect ionic crystal of magnetic monopoles. On the other hand, a [100] magnetic field benefits the magnetic monopole annihilation. In the present work, we studied by Monte Carlo simulations the dynamic behaviour of spin ice under the action of a magnetic field applied in the [100] direction, starting from a magnetic monopole crystal ([111] magnetic field) and turning the field in the [100] direction. [1] Castelnovo C., Moessner R. and Sondhi S. L. (2010) Thermal quenches in spin ice. Phys. Rev. Lett. [2] Mostame, S., Castelnovo C., Moessner R. and Sondhi S. L. (2014) Tunable non-equilibrium dynamics: field quenches in spin ice. Proc. Natl. Acad. Sci USA