Evidence of impurity and boundary effects on magnetic monopole dynamics in spin ice

Revell, Hannah; Yaraskavitch, L. R.; Mason, J.D.; Ross, K. A.; Noad, Hilary; Dąbkowska, H. A.; Gaulin, B. D.; Henelius, Patrik; Kycia, J. B.

doi:10.1038/nphys2466

Cited by 86 publications

(123 citation statements)

References 25 publications

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“…Although extrinsic defects can influence the dynamics of spin ice at low temperature 23 , it is clear that they do not cause the behaviour we observe (see Supplementary Section 1.3). The irrelevance of extrinsic defects to our experiments are consistent with creation of monopole-rich states, in which the monopole density far exceeds that of extrinsic defects.…”

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confidence: 84%

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Far-from-equilibrium monopole dynamics in spin ice

et al. 2014

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Condensed matter in the low-temperature limit reveals exotic physics associated with unusual orders and excitations, with examples ranging from helium superfluidity 1 to magnetic monopoles in spin ice 2,3 . The far-from-equilibrium physics of such low-temperature states may be even more exotic, yet to access it in the laboratory remains a challenge. Here we demonstrate a simple and robust techniquethe 'magnetothermal avalanche quench'-and its use in the controlled creation of non-equilibrium populations of magnetic monopoles in spin ice at millikelvin temperatures. These populations are found to exhibit spontaneous dynamical effects that typify far-from-equilibrium systems and yet are captured by simple models. Our method thus opens new directions in the study of far-from-equilibrium states in spin ice and other exotic magnets.The normal way of controlling the temperature of a system is to connect it thermally to a second body with a much larger thermal mass, which then acts as a thermal reservoir. If it is desired to force thermal excitations out of equilibrium by a rapid temperature quench, then the simplest strategy would be to heat the sample, and then abruptly remove the heating so that the sample is cooled rapidly by the reservoir. However, direct heating of the sample will also tend to heat the reservoir, and this becomes a particular problem in low-temperature devices: for example, in a 3 He-4 He dilution refrigerator it may entail heating of the mixing chamber, and ultimately limit the speed of any thermal quench that can be practically achieved. Our technique gets round this problem by using the natural tendency of magnets to undergo magnetothermal 'avalanches' at low temperature [4][5][6][7] . It is illustrated in Fig. 1 and discussed further in Supplementary Section 1.4. The essential principle is that magnetic work done on the sample is abruptly converted into internal heat, which causes a sudden increase in temperature inside the sample (T int ). The sample then finds itself at a relatively high temperature but connected to a cold thermal bath. The ensuing quench is as efficient and rapid as possible as it involves minimal heating of the sample environment, which remains at the reservoir temperature, T .Magnetothermal avalanches typically occur at low temperature (T < 1 K), a regime also notable for the occurrence of exotic magnetic states based on long-range interactions, quantum effects and magnetic frustration [8][9][10][11][12][13][14] . Hence, the avalanche quench technique could be generally used to drive such systems out of equilibrium. We focus on the case of spin ice, a nearly ideal realization of a magnetic ice-type or vertex model 8 . The far-from-equilibrium physics of vertex models is a subject of great intrinsic interest 15 In spin-ice materials such as Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 , the frustrated pyrochlore lattice geometry and local crystal field combines with a self-screening dipole-dipole interaction to give a local 'ice rule' that controls low-energy spin configurations 8,16...

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confidence: 84%

“…This has been discussed in terms of intrinsic kinetic effects 22 , slowing down of the monopole hop rate 18,23,24 , and the trapping of monopoles on extrinsic defects 23 , and it is likely that all such factors play a role. However, the degree of non-equilibration has not hitherto been brought under experimental control.…”

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confidence: 99%

Far-from-equilibrium monopole dynamics in spin ice

et al. 2014

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“…58 Definite progress in understanding dipolar Ising spin glasses has recently been made. 38,41,43 Other problems of disorder are rel-evant for spin ices, for example, evidence 37,59 that, in image furnace grown single crystals, there is a small level (O(1%)) of substitution of the Ti 4+ transition metal ions by trivalent rare-earth ions − a phenomenon referred to as "stuffing". 60 Other examples of disorder include the mixing of different types of ions on the rare-earth site, 61 different non-magnetic ions at the B site 62,63 and oxygen vacancies.…”

Section: Discussionmentioning

confidence: 99%

“…[29][30][31][32] Yet, the same dipolar interactions should give rise to long-range order at a critical temperature T c ≪ T peak if true thermal equilibrium can be maintained, 30,31 as observed in Monte Carlo simulations of a simple dipolar spin ice model (DSIM) 29 that employ non-local loop moves. 33,34 To this date, however, no experiment has found a transition to long-range order in spin ice materials, 26,35 presumably because of a dynamical arrest in spin flips 36 and the associated relaxation times growing exponentially fast below a temperature of about 1 K. 26,37 Hence, it is perhaps reasonable to imagine that a slight dilution of the magnetic Dy 3+ and Ho 3+ ions could lower the kinematic barriers for spin flips, thus accelerating the spin dynamics, and help promote a transition to long-range order without significantly affecting the broken discrete symmetry long-range ordered ground state of dipolar spin ice. 33,34 Luckily, magnetic site-dilution in spin ices can be realized rather straightforwardly in the Dy 2−x Y x Ti 2 O 7 and Ho 2−x Y x Ti 2 O 7 compounds.…”

Section: Introductionmentioning

confidence: 99%

Nonmonotonic residual entropy in diluted spin ice: A comparison between Monte Carlo simulations of diluted dipolar spin ice models and experimental results

Lin

Thesberg

et al. 2014

Phys. Rev. B

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Spin ice materials, such as Dy2Ti2O7 and Ho2Ti2O7, are highly frustrated magnetic systems. Their low temperature strongly correlated state can be mapped onto the proton disordered state of common water ice. As a result, spin ices display the same low temperature residual Pauling entropy as water ice, at least in calorimetric experiments that are equilibrated over moderately long time scales. It was found in a previous study [X. Ke et. al. Phys. Rev. Lett. 99, 137203 (2007)] that, upon dilution of the magnetic rare-earth ions (Dy 3+ and Ho 3+ ) by non-magnetic Yttrium (Y 3+ ) ions, the residual entropy depends non-monotonically on the concentration of Y 3+ ions. A quantitative description of the magnetic specific heat of site-diluted spin ice materials can be viewed as a further test aimed at validating the microscopic Hamiltonian description of these systems. In the present work, we report results from Monte Carlo simulations of site-diluted microscopic dipolar spin ice models (DSIM) that account quantitatively for the experimental specific heat measurements, and thus also for the residual entropy, as a function of dilution, for both Dy2−xYxTi2O7 and Ho2−xYxTi2O7. The main features of the dilution physics displayed by the magnetic specific heat data are quantitatively captured by the diluted DSIM up to 85% of the magnetic ions diluted (x = 1.7). The previously reported departures in the residual entropy between Dy2−xYxTi2O7 versus Ho2−xYxTi2O7, as well as with a site-dilution variant of Pauling's approximation, are thus rationalized through the site-diluted DSIM. We find for 90% (x = 1.8) and 95% (x = 1.9) of the magnetic ions diluted in Dy2−xYxTi2O7 a significant discrepancy between the experimental and Monte Carlo specific heat results. We discuss possible reasons for this disagreement.

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“…Oxygen vacancies introduce Ti 3+ defects instead of Ti 4+ ions, modifying the R 3+ crystal field and inducing local distortions, with various consequences on the low temperature magnetic properties and spin ice physics. In classical spin ice Dy 2 Ti 2 O 7 , oxygen depletion or a low level Dy stuffing in the Ti sites strongly slows down the monopole dynamics at very low temperature 7,8 and affects the nonequilibrium macroscopic properties 9,10 . In the quantum spin ice Yb 2 Ti 2 O 7 , small off-stoechiometry clearly affects the magnetic ground state 11 .…”

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Mesoscopic correlations inTb2Ti2O7spin liquid

et al. 2015

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We have studied the spin correlations with k= ( 1 2 , 1 2 , 1 2 ) propagation vector which appear below 0.4 K in Tb2Ti2O7 spin liquid by combining powder neutron diffraction and specific heat on Tb2+xTi2−xO7+y samples with x=0, 0.01, -0.01. The k= ( 1 2 , 1 2 , 1 2 ) order clearly appears on all neutron patterns by subtracting a pattern at 1.2(1) K. Refining the subtracted patterns at 0.07 K yields two possible spin structures, with spin-ice-like and monopole-like correlations respectively. Mesoscopic correlations involve Tb moments of 1 to 2 µB ordered on a length scale of about 20Å. In addition, long range order involving a small spin component of 0.1 to 0.2 µB is detected for the x= 0 and 0.01 samples showing a peak in the specific heat. Comparison with previous single crystals data suggests that the ( 1 2 , 1 2 , 1 2 ) order settles in through nanometric spin textures with dominant spin ice character and correlated orientations, analogous to nanomagnetic twins.Rare earth pyrochlores with geometrical magnetic frustration are model compounds to investigate classical spin ice as well as quantum spin ice physics. The short range ordered ground states with large degeneracy give rise to a large variety of interesting phenomena such as magnetic monopoles 1,2 , first order transition 3 , or unconventional magnetoelastic excitations 4,5 . These frustrated ground states are highly sensitive to small perturbations and to the presence of defects. Already in the 80s, Villain 6 predicted that for an arbitrary small concentration of defects the spinel lattice with pyrochlore structure could switch its ground state from spin liquid to spin glass. More recently many studies focus on the role of a minute amount of defects in pyrochlores. The crystal structure of R 2 Ti 2 O 7 pyrochlores (where R 3+ is a rare earth ion, Ti 4+ is non magnetic) accommodates oxygen vacancies, small off-stoechiometries in the R 3+ or Ti 4+ lattice, "stuffing" or site inversion. Oxygen vacancies introduce Ti 3+ defects instead of Ti 4+ ions, modifying the R 3+ crystal field and inducing local distortions, with various consequences on the low temperature magnetic properties and spin ice physics. In classical spin ice Dy 2 Ti 2 O 7 , oxygen depletion or a low level Dy stuffing in the Ti sites strongly slows down the monopole dynamics at very low temperature 7,8 and affects the nonequilibrium macroscopic properties 9,10 . In the quantum spin ice Yb 2 Ti 2 O 7 , small off-stoechiometry clearly affects the magnetic ground state 11 . It appears that precise studies of the magnetic defects are needed to resolve controversies concerning these unconventional ground states.Among the pyrochlores, Tb 2 Ti 2 O 7 is one of the most complex, the subject of numerous investigations since the discovery of its spin liquid character in 1999. The Tb 3+ short range correlated moments fluctuate 12 down to 20 mK at the time scale of the muon probe (10 −6 s). The origin of these fluctuations has been highly debated [13][14][15][16][17][18] . Several mechanisms have been pr...

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Evidence of impurity and boundary effects on magnetic monopole dynamics in spin ice

Cited by 86 publications

References 25 publications

Far-from-equilibrium monopole dynamics in spin ice

Far-from-equilibrium monopole dynamics in spin ice

Nonmonotonic residual entropy in diluted spin ice: A comparison between Monte Carlo simulations of diluted dipolar spin ice models and experimental results

Mesoscopic correlations inTb2Ti2O7spin liquid

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