Signature of magnetic monopole and Dirac string dynamics in spin ice

Abstract

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 Maxwell's 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

Magnetic correlations in HoxTb2−xTiChang1, Schweika2, Kao3 et al. 2011Phys. Rev. B174140View full textAdd to dashboardCite

“…16,33 Terbium doping, at least lightly, can be considered a defect that facilitates the emergence of quasiparticles known as magnetic monopole. 18,19 These elemental excitations are defects in the two-in, two-out spin configuration on a given tetrahedron. Tb doping would probably relax this constraint, but this idea should be examined further with low-temperature heat-capacity measurements on well-characterized samples.…”

“…A thermally independent process continues to drive the system into the locally ordered state of two in, two out, which is completed by ∼2 K. Below 2 K, the system re-enters a thermally active regime, and this has been associated with the mobility of defects in the ice rules and the dynamics of so-called magnetic monopoles. 18,19 In Tb 2 Ti 2 O 7 , the system remains in a paramagnetic state, albeit sluggish, to temperatures as low as 15 mK, but shortrange (∼5Å) correlations develop below 100 K, which are characteristics associated with a cooperative paramagnetic or spin-liquid state. 5,12,14 The Curie-Weiss law fit through the high-temperature susceptibility of Tb 2 Ti 2 O 7 estimates the Tb 3+ moment of 9.6 μ B , almost the full free moment, and the Curie-Weiss temperature of −19 K. 12 Further studies on the Y diluted Tb 2 Ti 2 O 7 samples successfully estimated the interactions of the crystal field, dipolar, and exchange contributions to the Curie-Weiss temperature as −6, −2, and −11 K, respectively.…”

Crystal-field states of Pr3+in the candidate quantum spin ice Pr2Sn2

“…Spin ice is a very special magnetically frustrated ground state in which the spins at the corners of each tetrahedron on the pyrochlore lattice freeze into a two-in-two-out configuration, analogous to the proton correlations in water ice. Interest in spin ice materials has burgeoned thanks to the prediction and subsequent experimental detection of magnetic monopole-like quasiparticles, which are the fundamental excitations of the spin-ice ground state [3][4][5][6] .…”

Quantum fluctuations in spin-ice-like Pr2Zr2O7

“…In classical spin ice, monopole dynamics is diffusive-only activated thermally or by external magnetic field 5,6,10 as solitons in semiclassical spin chains 11 . The classical nature of Ising spins precludes attainment of thermal equilibrium at temperatures (T) below the effective nearest neighbor energy scale J ff .…”