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Spin ices are frustrated magnets, a class of magnetic material well known for supporting exotic excitations, both in experiment (9,10) and in theory (11-13). The spin ice family (5-8) is [...], The transport of electrically charged quasiparticles (based on electrons or ions) plays a pivotal role in modern technology as well as in determining the essential functions of biological organisms. In contrast, the transport of magnetic charges has barely been explored experimentally, mainly because magnetic charges, in contrast to electric ones, are generally considered at best to be convenient macroscopic parameters (1,2), rather than well-defined quasiparticles. However, it was recently proposed that magnetic charges can exist in certain materials in the form of emergent excitations that manifest like point charges, or magnetic monopoles (3). Here we address the question of whether such magnetic charges and their associated currents--'magnetricity'--can be measured directly in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of electrolytes (4), we show that this is indeed possible, and devise an appropriate method for the measurement of magnetic charges and their dynamics. Using muon spin rotation as a suitable local probe, we apply the method to a real material, the 'spin ice' [Dy.sub.2][Ti.sub.2][O.sub.7] (refs 5-8). Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb potential, and have measurable currents. We further characterize deviations from Ohm's law, and determine the elementary unit of magnetic charge to be 5 [µ.sub.B] [Å.sup.-1], which is equal to that recently predicted using the microscopic theory of spin ice (3). Our measurement of magnetic charge and magnetic current establishes an instance of a perfect symmetry between electricity and magnetism.

Spin-ice materials are magnetic substances in which the spin directions map onto hydrogen positions in water ice. Their low-temperature magnetic state has been predicted to be a phase that obeys a Gauss' law and supports magnetic monopole excitations: in short, a Coulomb phase. We used polarized neutron scattering to show that the spin-ice material [Ho.sub.2][Ti.sub.2][O.sub.7] exhibits an almost perfect Coulomb phase. Our result proves the existence of such phases in magnetic materials and strongly supports the magnetic monopole theory of spin ice. 10.1126/science.1177582

Theorists are continually challenged by the statistical treatment of treatment of three-dimensional turbulent flow. An analogy with equilibrium phase transitions has been suggested. The idea is approached experimentally, providing a strong analogy between statistical behaviour and that of a model of the critical behaviour of a ferromagnet. The large fluctuations experienced by both systems were limited only by system size.

The probability distribution function for the magnetic order parameter M within the low temperature stage of the two-dimensional XY model, is examined. The thermal value of the M parameter has abnormally large finite size corrections, and an exact result within a spin wave calculation gives (M)=(1/2N)T/8piJ. Asymmetry is shown to come from the three-spin and higher correlations, and the distribution is Gaussian if only a two-spin correlation is considered.

The triangular lattice antiferromagnet has many unusual properties, such as large quantum fluctuation effects and novel critical exponents. Materials having layered crystal structures related to that of the mineral Yavapaiite KFe(So(sub4))(sub2) are realizations of a model quasi-two-dimensional triangular lattice antiferromagnet. Low temperature magnetic structures adopted by tile compounds have been determined and compared with the theoretical expectations for row and triangular models.

A method where the rounded universal jump demonstrated universal properties in two-dimensional XY magnets was described. The universal jump, which was associated with spin-wave stiffness, was a primary feature of the Kosterlitz-Thouless-Berezinskii phase transition. Phase transition properties depended on system size.

Anisotropic planar Heisenberg pyrochlore lattice antiferromagnet spin model indicates the discontinuous phase transition-induced order by thermal disorder of the lattice. The zero temperature degenerates macroscopically and the finite temperature of the system is analyzed using Monte Carlo computations.

Spontaneous order in two-dimensional magnetic systems was investigated. The results showed that two-dimensional magnetic behavior in thin films and layered magnets can be grouped into two universality classes: the two-dimensional Ising model and the finite size XY model. It was shown that correlations below the Kousterlitz-Thouless transition temperature decay at a slow rate such that magnetization occurs in any given reliable system.

Interlayer superexchange in the two-dimensional ferromagnet Rb2CrCl4-xBrx was investigated using quasielastic neutron scattering analysis. The results showed that the material ordered between 52.2 and 57.0 K. Quasielastic diffuse scattering was detected along the (001) direction. The wave vector dependent modulation of the width and the intensity of diffuse scattering was attributed to the effects of weak interplanar exchange coupling.

The series of magnetic rare-earth pyrochlore stannates R[sub 2] Sn[sub 2] O[sub 7] (R = rare earth, except Ce and Pm) have been investigated by powder susceptibility measurements down to T = 1.8 K. The results are compared with results for the analogous titanate series, which are well known frustrated magnets. Unlike the titanates, the whole series can be formed in the cubic pyrochlore structure. Possible experimental advantages of studying the stannates are discussed. PACS Nos.: 75.10Nr, 75.50Ee, 75.50LkNous avons étudié la série de stannates pyrochlores magnétiques de terres rares R[sub 2] Sn[sub 2] O[sub 7] (R = terres rares, sauf Ce et Pm) par des mesures de susceptibilité dans les poudres à des températures descendant jusqu'à T = 1,8 K. Nous comparons ces résultats avec ceux obtenus sur des titanates, qui sont des aimants frustrés bien connus. Contrairement aux titanates, toute la série peut être logée dans la structure cubique pyrochlore. Nous discutons les avantages possibles résultant de l'étude expérimentale des stannates. [Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

We present an experimental investigation of the structural and magnetic properties of Ho[sub 3] SbO[sub 7] and Dy[sub 3] SbO[sub 7] . These compounds adopt the Y[sub 3] TaO[sub 7] structure, space group C222[sub 1] . The magnetic rare-earth ions occupy an intricate lattice related to the pyrochlore lattice that occurs in Ho[sub 2] Ti[sub 2] O[sub 7] and Dy[sub 2] Ti[sub 2] O[sub 7] . The crystal structure of Ho[sub 3] SbO[sub 7] is determined by Rietveld refinement of the powder X-ray diffraction pattern at ambient temperature, and that of the Dy analogue is inferred to be similar. Magnetic susceptibility measurements show that Ho[sub 3] SbO[sub 7] and Dy[sub 3] SbO[sub 7] have negative Curie–Weiss temperatures: –8.4 K (Ho) and –9.2 K (Dy). Magnetic transitions have been detected at 2.0 K (Ho) and 3.0 K (Dy). We discuss the results in terms of the ``dipolar spin ice model'' that has been used to describe Ho[sub 2] Ti[sub 2] O[sub 7] and Dy[sub 2] Ti[sub 2] O[sub 7] . PACS Nos.: 75.25+z, 75.50Ee, 61.10NzNous étudions expérimentalement la structure et les propriétés magnétiques de Ho[sub 3] SbO[sub 7] et Dy[sub 3] SbO[sub 7] . Ces composés adoptent la structure du Y[sub 3] TaO[sub 7] , le groupe spatial C222[sub 1] . Les ions magnétiques des terres rares occupent un réseau complexe relié au réseau pyrochlore que l'on retrouve dans Ho[sub 2] Ti[sub 2] O[sub 7] et Dy[sub 2] Ti[sub 2] O[sub 7] . La structure cristalline de Ho[sub 3] SbO[sub 7] est déterminée par une méthode améliorée de Rietveld de cristallographie à rayons X sur poudre à température ambiante et nous déduisons que celle du Dy est similaire. Les mesures de susceptibilité magnétique montrent que Ho[sub 3] SbO[sub 7] et Dy[sub 3] SbO[sub 7] ont des températures de Curie–Weiss négatives : –8,4 K (Ho) et –9,2 K (Dy). Nous analysons ces résultats dans le cadre du modèle de glace de spin dipolaire utilisé pour décrire Ho[sub 2] Ti[sub 2] O[sub 7] et Dy[sub 2] Ti[sub 2] O[sub 7] . [Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]