Supercooled Spin Liquid States in the Pyrochlore Magnets Dy2Ti2O7 and Ho2Ti2O7

The ground states and low temperature characteristics of frustrated magnetic systems offer a variety of novel magnetic states. Among these are the pyrochlore magnetic insulators Dy2Ti2O7 and Ho2Ti2O7, for which despite a well-ordered crystal structure and strong magnetic interactions between the Dy or Ho ions, no long-range magnetic order has been detected [1]. The low-temperature magnetic state in these materials is governed by spin-ice rules, in analogy to water ice. These constrain the Ising-like spins in the materials, yet does not result in a global broken symmetry state.

To explore the actual magnetic phases in these spin-ice materials, we performed magnetic susceptibility measurements, employing boundary-free geometries. We demonstrate a distinctive behavior of the magnetic susceptibility of both compounds, that is indistinguishable in form from the permittivity of supercooled dipolar liquids. Moreover, we show that the microscopic magnetic relaxation times for both materials increase along a super-Arrhenius trajectory also characteristic of supercooled glass-forming liquids. Both materials therefore exhibit characteristics of a supercooled spin liquid.

Supercooled liquids develop when a solid does not crystallize upon cooling below its ordering temperature and, instead, the microscopic relaxation times diverge so rapidly that equilibration eventually becomes impossible. Since supercooled liquids are glass forming liquids, this finding indicates the existence of a novel magnetic glass state in a translationally-invariant nominally disorder-free frustrated spin system for the measured pyrochlores. Differences in the spin dynamics between the two materials investigated, specifically in their measured time scales, are consistent with a strongly-correlated dynamics of magnetic monopole excitations. A possible connection to many-body localization will also be discussed.

[1] J. S. Gardner, et al., Rev. Mod. Phys., 82, 53 (2010).