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Register a new userExtremely slow non-equilibrium monopole dynamics in classical spin ice
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We report on the non-equilibrium monopole dynamics in the classical spin ice Dy$_2$Ti$_2$O$_7$ detected by means of high-resolution magnetostriction measurements. Significant lattice changes occur at the transition from the kagome-ice to the saturated-ice phase, visible in the longitudinal and transverse magnetostriction. A hysteresis opening at temperatures below 0.6 K suggests a first-order transition between the kagome and saturated state. Extremely slow lattice relaxations, triggered by changes of the magnetic field, were observed. These lattice-relaxation effects result from non-equilibrium monopole formation or annihilation processes. The relaxation times extracted from our experiment are in good agreement with theoretical predictions with decay constants of the order of $10{^4}$ s at 0.3 K.
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One of the most remarkable examples of emergent quasi-particles, is that of the fractionalization of magnetic dipoles in the low energy configurations of materials known as spin ice, into free and unconfined magnetic monopoles interacting via Coulombs 1/r law [Castelnovo et. al., Nature, 451, 42-45 (2008)]. Recent experiments have shown that a Coulomb gas of magnetic charges really does exist at low temperature in these materials and this discovery provides a new perspective on otherwise largely inaccessible phenomenology. In this paper, after a review of the different spin ice models, we present detailed results describing the diffusive dynamics of monopole particles starting both from the dipolar spin ice model and directly from a Coulomb gas within the grand canonical ensemble. The diffusive quasi-particle dynamics of real spin ice materials within quantum tunneling regime is modeled with Metropolis dynamics, with the particles constrained to move along an underlying network of oriented paths, which are classical analogues of the Dirac strings connecting pairs of Dirac monopoles.
Frustration in magnetic systems is fertile ground for complex behaviour, including unconventional ground states with emergent symmetries, topological properties, and exotic excitations. A canonical example is the emergence of magnetic-charge-carrying quasiparticles in spin-ice compounds. Despite extensive work, a reliable experimental indicator of the density of these magnetic monopoles in spin-ice systems is yet to be found. Here, using measurements on single crystals of Ho$_{2}$Ir$_{2}$O$_{7}$ in combination with dipolar Monte Carlo simulations, we show that the magnetoresistance is highly sensitive to the density of monopoles. Moreover, we find that for the orientations of magnetic field in which the monopole density is enhanced, a strong coupling emerges between the magnetic charges on the holmium sublattice and the antiferromagnetically ordered iridium ions, leading to an ability to manipulate the antiferromagnetic domains via a uniform external field. Our results pave the way to a quantitative experimental measure of monopole density and provide a powerful illustration of the interplay between the various magnetic and electronic degrees of freedom in the frustrated pyrochlore iridates. This interdependence holds promise for potential functional properties arising from the link between magnetic and electric charges, as well as for the control of antiferromagnetic domain walls, a key goal in the design of next-generation spintronic devices.
We report a positive muon spin relaxation and rotation (muSR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature T_c at which the specific heat displays an intense peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above T_c up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the muSR spectra recorded above T_c. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c. Considering previously reported ac susceptibility, neutron scattering and muSR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.
Cyclotron spin-flip excitation in a nu=2 quantum Hall system, being separated from the ground state by a slightly smaller gap than the cyclotron energy and from upper magnetoplasma excitation by the Coulomb gap [S. Dickmann and I.V. Kukushkin, Phys. Rev. B 71, 241310(R) (2005) ; L.V. Kulik, I.V. Kukushkin, S. Dickmann, V.E. Kirpichev, A.B. Vankov, A.L. Parakhonsky, J.H. Smet, K. von Klitzing, and W. Wegscheider, Phys. Rev. B 72, 073304 (2005)] cannot relax in a purely electronic way except only with the emission of a shortwave acoustic phonon (k~3*10^7/cm). As a result, relaxation in a modern wide-thickness quantum well occurs very slowly. We calculate the characteristic relaxation time to be ~1s. Extremely slow relaxation should allow the production of a considerable density of zero-momenta cyclotron spin-flip excitations in a very small phase volume, thus forming a highly coherent ensemble - the Bose-Einstein condensate. The condensate state can be controlled by short optical pulses (<1 mcs), switching it on and off.
Magnetic monopoles have eluded experimental detection since their prediction nearly a century ago by Dirac. Recently it has been shown that classical analogues of these enigmatic particles occur as excitations out of the topological ground state of a model magnetic system, dipolar spin ice. These quasi-particle excitations do not require a modification of Maxwells equations, but they do interact via Coulombs law and are of magnetic origin. In this paper we present an experimentally measurable signature of monopole dynamics and show that magnetic relaxation measurements in the spin ice material $Dy_{2}Ti_{2}O_{7}$ can be interpreted entirely in terms of the diffusive motion of monopoles in the grand canonical ensemble, constrained by a network of Dirac strings filling the quasi-particle vacuum. In a magnetic field the topology of the network prevents charge flow in the steady state, but there is a monopole density gradient near the surface of an open system.
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