Abstract
Extensive work on single molecule magnets has identified a fundamental mode of relaxation arising from the nuclear-spin assisted quantum tunnelling of nearly independent and quasi-classical magnetic dipoles. Here we show that nuclear-spin assisted quantum tunnelling can also control the dynamics of purely emergent excitations: magnetic monopoles in spin ice. Our low temperature experiments were conducted on canonical spin ice materials with a broad range of nuclear spin values. By measuring the magnetic relaxation, or monopole current, we demonstrate strong evidence that dynamical coupling with the hyperfine fields bring the electronic spins associated with magnetic monopoles to resonance, allowing the monopoles to hop and transport magnetic charge. Our result shows how the coupling of electronic spins with nuclear spins may be used to control the monopole current. It broadens the relevance of the assisted quantum tunnelling mechanism from single molecular spins to emergent excitations in a strongly correlated system.
Highlights
Extensive work on single molecule magnets has identified a fundamental mode of relaxation arising from the nuclear-spin assisted quantum tunnelling of nearly independent and quasiclassical magnetic dipoles
To investigate the effect of nuclear spins on the magnetic relaxation in spin ice, we studied four spin ice samples: Ho2Ti2O7, with I = 7/2 and three Dy2Ti2O7 samples spanning a range of nuclear spin composition from I = 0 to I = 5/2
It is more problematic to cool samples containing Ho, due to the large Ho nuclear spin which results in a Schottky heat capacity of 7 J mol−1 K−1 at 300 mK
Summary
Extensive work on single molecule magnets has identified a fundamental mode of relaxation arising from the nuclear-spin assisted quantum tunnelling of nearly independent and quasiclassical magnetic dipoles. We show that nuclear-spin assisted quantum tunnelling can control the dynamics of purely emergent excitations: magnetic monopoles in spin ice. Our low temperature experiments were conducted on canonical spin ice materials with a broad range of nuclear spin values. Our result shows how the coupling of electronic spins with nuclear spins may be used to control the monopole current It broadens the relevance of the assisted quantum tunnelling mechanism from single molecular spins to emergent excitations in a strongly correlated system. In the effective ground state, the spins describe a flux with closed-loop topology and critical correlations, that may be described by a local gauge symmetry rather than by a traditional broken symmetry[6] This strongly correlated spin ice state is stabilised by a remarkable self-screening of the dipole interaction[7,8]. The dynamic properties can be described by assuming an effective monopole mobility[11,12,13], but there have been few studies of the microscopic origin of the monopole motion[14]
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