High pressure route to generate magnetic monopole dimers in spin ice

Abstract

The gas of magnetic monopoles in spin ice is governed by one key parameter: the monopole chemical potential. A significant variation of this parameter could access hitherto undiscovered magnetic phenomena arising from monopole correlations, as observed in the analogous electrical Coulomb gas, like monopole dimerization, critical phase separation, or charge ordering. However, all known spin ices have values of chemical potential imposed by their structure and chemistry that place them deeply within the weakly correlated regime, where none of these interesting phenomena occur. Here we use high-pressure synthesis to create a new monopole host, Dy2Ge2O7, with a radically altered chemical potential that stabilizes a large fraction of monopole dimers. The system is found to be ideally described by the classic Debye–Huckel–Bjerrum theory of charge correlations. We thus show how to tune the monopole chemical potential in spin ice and how to access the diverse collective properties of magnetic monopoles.

New physics in frustrated magnets: Spin ices, monopoles, etc. (Review Article)

“…Some low-temperature properties of spin ices were successfully described by magnetic monopoles, e.g., in neutron scattering [211,212], in the behavior of magnetic susceptibility [213] (the latter can be well described by the DebyeHückel theory [214,215,216]), see also [158,217,218,219], in NMR (nuclear magnetic resonance) [220], and in the thermal conductivity [221].…”

Monopole ordered phases in dipolar and nearest-neighbors Ising pyrochlore: From spin ice to the all-in–all-out antiferromagnet

“…At zero magnetic field the density of monopoles is regulated by the sign and magnitude of J eff /T which, in the currently known materials, leads at best to moderately correlated monopole fluids [20,21]. In order to explicitly show the effect of these correlations at low temperatures while stressing the role of charge degrees of freedom, some of us recently reported the results of simulations on a dipolar spin ice model where a new ingredient was introduced: While keeping the dipolar Hamiltonian (1), our approach in the conserved monopole dipolar spin ice model (CDSIM) was to use the density of conserved single monopoles as the main control parameter [22].…”

Frustration under pressure: Exotic magnetism in new pyrochlore oxides

“…15 The germanate series A 2 Ge 2 O 7 and the plumbate series A 2 Pb 2 O 7 are two examples of recent pyrochlores which have been prepared to test the application of chemical pressure to the canonical titanate pyrochlores. [21][22][23][24][25][26] It is these series that will be the focus of the remainder of this review. Throughout this review, the germanates and plumbates are referenced with respect to the pyrochlore structure.…”

“…Although the monopole density can be quite low at low temperatures (on the order of parts per thousand to percent spin defects), one attractive route towards increasing the density is by shrinking the lattice (effectively increasing the exchange and therefore lowering the chemical potential for monopole formation). 21 This was one of the leading arguments for investigating the germanate analogues of the spin ices, where monopole concentrations should increase by an order of magnitude at any given temperatures. 21 Ho 2 Ge 2 O 7 and Dy 2 Ge 2 O 7 were reported as stable, well-ordered pyrochlore structures prepared under high pressure (8 GPa) and at temperatures of 1000…”