Dynamical crossover in  hot  spin ice

Abstract

The magnetic dynamics of the spin ice material Ho2Ti2O7 in its paramagnetic (‘hot’) phase have been investigated by a combination of neutron spin echo and ac-susceptibility techniques. Relaxation at high temperatures (T > 15 K) is proved to occur by a thermally activated single-ion process that is distinct from the process that dominates at lower temperatures (1 K < T < 15 K). It is argued that the low-temperature process must involve quantum mechanical spin tunnelling, as quasi-classical channels of relaxation are exhausted in this temperature range. Our results resolve a mystery in the physics of spin ice: why has a 15 K ac-susceptibility peak been observed in Dy2Ti2O7 but not in Ho2Ti2O7 or Ho2Sn2O7?

Quantum and thermal spin relaxation in the diluted spin iceDy2−xMxTi2O7Snyder1, Ueland2, Mizel3 et al. 2004Phys. Rev. B7311592View full textAdd to dashboardCite

“…Previously published data obtained by our group (for x ഛ 0.4) showed the freezing transition disappearing with dilution and thus suggested that the transition was associated with spin-spin correlations. 5 The present data, showing the re-emergence of the freezing transition with increasing dilution and its presence up to 99% dilution (i.e., x = 1.98), imply instead that the freezing is fundamentally a single-ion phenomenon, 6 akin to a superparamagnetic blocking transition (albeit a phenomenon affected by the spin-spin interactions, as described below).…”

“…1 Of particular recent interest are the rare-earth pyrochlores spin ice materials such as Dy 2 Ti 2 O 7 , Ho 2 Ti 2 O 7 , and Ho 2 Sn 2 O 7 . [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] In these materials the lattice geometry and spin symmetry lead to frustration of ferromagnetic and dipolar interactions [17][18][19][20] and an unusual disordered ground state that has been demonstrated experimentally through neutron scattering studies 3,10,14 and through measurements of the magnetic specific heat 4,12 and the ac magnetic susceptibility. 16 While the spin entropy only freezes out below T ice ϳ 4 K in Dy 2 Ti 2 O 7 , ac magnetic susceptibility studies show a strongly frequency-dependent spin-freezing at T f ϳ 16 K, 5,7 below which the high frequency susceptibility ͑f Ͼ 100 Hz͒ is suppressed.…”

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

“…The analysis of that behavior implies the presence of the relaxation process with the typical time scales satisfying Arrhenius law τ ∼ exp(E a /T ) with activation energies E a ∼ 200 K, which agrees with recent muon spin relaxation studies [173,174], which give the muon relaxation rate of similar form ∝ exp(−E a /T ) with E a ∼ 220 K. It means that such a relaxation involves transitions to the higher-energy single-ion multiplets of rare earth ions. On the other hand, another dynamical processes in spin ices in the temperature range between 5 K and 10 K did not show any significant temperature dependence [173,174,175], which has been interpreted as caused by the quantum tunneling effect between up-and down-directed states of effective Ising spins. Notice that µSR (muon spin relaxation) and ac susceptibility measurements observed relaxation rates different of each other up to three orders of magnitude, perhaps, because of the local character of the µSR probe.…”

Out-of-equilibrium dynamics and extended textures of topological defects in spin ice

“…Among other consequences, the magnetic relaxation time diverges at low temperature in Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 spin-ice materials, as measured by a variety of experimental probes, such as AC-magnetic susceptibility [11][12][13] , thermal transport 14 , neutron spin echo 15 , neu-tron scattering 16 and muon spectroscopy 17 . This spin freezing is due to the rarefaction of defects 18,19 , mediated by impurities and surface effects 20 .…”