| Geometric magnetic frustration is a ubiquitous phenomenon within condensed matter physics. The magnetic ground states are highly degenerate, leading to competing low temperature phases and enhanced quantum fluctuations. The nature of the magnetic fluctuations in these systems is not well understood. However, recent theoretical advances have been made in understanding the excitations in so called "spin ice" model magnetic systems. Spin ice systems combine the rich phenomenology associated with geometric magnetic frustration with the greater theoretically tractable nature of Ising models, drawing on a close analogy with the statistical mechanics of proton disorder in water ice. The spin flip excitations have been described classically in terms of pairs of magnetic charges or monopoles, which are free to move apart within the lattice. We have addressed questions arising from the recent work of S. T. Bramwell et al, Nature 461, 956 (2009), which describes the observation of such magnetic monopoles in the compound Dy2Ti2O7, an experimental realisation of a spin ice. Rather than the highly temperature dependent and field-induced nucleation processes of bound and unbound pairs previously reported by Bramwell et al, we report temperature independent fluctuations which survive well into the spin ice state and necessarily imply non-Ising like contributions to the spin Hamiltonian. |
|