Spins vs. magnetic charges in spin ice: to what extent can the monopole picture describe these systems?

PAMELA C. GURUCIAGA; RODOLFO A. BORZI

Villa Carloz Paz, Córdoba

Congreso; XIII Latin American Workshop on Nonlinear Phenomena (LAWNP); 2013

FaMAF, UNC

Particles analogous to magnetic monopoles have been recently found - not in vacuum, but as collective excitations of a kind of frustrated magnetic materials known as spin ices. The ground states of these systems present a degeneracy that grows exponentially with their size, giving rise to a finite entropy even at zero degrees kelvin. A direct mapping between the spin degrees of freedom in spin ice and proton positions in conventional water ice makes their residual entropies identical (motivating the name of these magnetic materials). Excitations of the multiple ground states are local defects in the magnetic structure, which interact approximately following a Coulomb law. They resemble non conserved magnetic charges, and hence their association with magnetic monopoles with four different types of charge: positive or negative, single or double. Therefore, the rotation of magnetic moments is equivalent to the creation, annihilation or translation of magnetic charge in a discrete lattice. How far can we extend the analogy between these monopoles and conventional electric charges? May magnetic crystals (similar to ionic crystals) exist? To answer these and other questions we study spin ice systems, performing Monte Carlo simulations over two models: one with nearest-neighbour interactions between spins, and another taking into account the full long-range dipolar interactions. The non-trivial phase diagrams we find can be understood in terms of the usual phases found in ionic systems. However, we will see that the mechanism leading to charge correlations is more subtle than a simple interaction between charges.