Abstract
We study the dipolar spin-ice model at fixed density of single excitations, ρ, using a Monte Carlo algorithm where processes of creation and annihilation of such excitations are banned. In the limit of ρ going to zero, this model coincides with the usual dipolar spin-ice model at low temperatures, with the additional advantage that a negligible number of monopoles allows for equilibration even at the lowest temperatures. Thus, the transition to the ordered fundamental state found by Melko, den Hertog, and Gingras in 2001 is reached using simple local spin flip dynamics. As the density is increased, the monopolar nature of the excitations becomes apparent: the system shows a rich ρ vs T phase diagram with "charge" ordering transitions analogous to that observed for Coulomb charges in lattices. A further layer of complexity is revealed by the existence of order both within the charges and their associated vacuum, which can only be described in terms of spins--the true microscopic degrees of freedom of the system.
Highlights
We study the dipolar spin-ice model at fixed density of single excitations, using a Monte Carlo algorithm where processes of creation and annihilation of such excitations are banned
Controlling the density of topological defects is a clean way of highlighting their essential role in determining some ordered phases
The strength and beauty of the monopolar picture of this magnetic system appears reinforced by our finding of two phases, which can be understood in terms of the different types of ordering of the attracting monopoles. Adding to this remarkable result, our perspective shows in a unified view the presence of more subtle forms of order: they are related to the many different ways in which both the monopole-free system and a perfect crystal of single monopoles can be assembled in terms of their constituent spins
Summary
We study the dipolar spin-ice model at fixed density of single excitations, , using a Monte Carlo algorithm where processes of creation and annihilation of such excitations are banned. The magnetic properties of spin-ice (SI) materials at low temperatures are well described by the dipolar model Lower inset: Temperature dependence of the order parameter for the spin-ice ground state [17].
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