[en] The alternating susceptibility of Ho₂Ti₂O₇ and Ho_1.9La_0.1Ti₂O₇ and specific heat of Dy₂Ti₂O₇ and Dy_1.9Y_0.1Ti₂O₇ are reported. Temperature dependence of the relaxation time obtained from susceptibility data of both Ho₂Ti₂O₇ and Ho_1.9La_0.1Ti₂O₇ can be described assuming thermally activated relaxation. No indication of λ-like anomaly is observed in specific heat of Dy_1.9Y_0.1Ti₂O₇. The obtained results suggest that although 5% magnetic dilution reduces the relaxation time it is not sufficient to induce long-range ordering at least down to 100 mK

[en] The spinel series Cu_1-xZn_xCr₂Se₄ has been investigated by AC susceptibility measurements under applied pressures of up to 6 kbar. Applied pressure is found to cause a decrease in the antiferromagnetic ordering temperature T_N (∼10% at 6 kbar). We argue that this is evidence for T_N being determined by the near-neighbour coupling which in these materials is ferromagnetic, despite the overall antiferromagnetic order

[en] We have studied the spin relaxation in diluted spin ice Ho_2-x Y_x Ti₂O₇ by means of neutron spin echo spectroscopy. Remarkably, the geometrical frustration is not relieved by doping with non-magnetic Y, and the dynamics of the freezing is unaltered in the spin echo time window up to x ≅ 1.6. At higher doping with non-magnetic Y (x (ge) 1.6) a new relaxation process at relatively high temperature (up to at least T ≅ 55 K) appears which is more than 10 times faster than the thermally activated main relaxation process. We find evidence that over the whole range of composition all Ho spins participate in the dynamics. These results are compared to a.c. susceptibility measurements of the diluted Ho and Dy spin ice systems. X-ray absorption fine structure (EXAFS) spectra and x-ray diffraction show that the samples are structurally well ordered

[en] M. W. Long and collaborators ( Phys. Rev. B 83 054422 (2011)] recently proposed magnetic structures for gadolinium titanate that differ from those previously reported by us ( J. R. Stewart, G. Ehlers, A. S. Wills, S. T. Bramwell and J. S. Gardner J. Phys.: Condens. Matter 16 L321 (2004)]. In this Comment, we show that the calculated structure factors, S(Q), of the newly proposed models are inconsistent with our neutron powder diffraction data. Long and colleagues were led to reconsider the magnetic structure of gadolinium titanate on the basis of a number of theoretical and experimental assumptions. We argue that these assumptions have no basis in fact and conclude that they provide no reason to doubt our published magnetic structures.

[en] In R₂Ti₂O₇ compounds, rare earth R ions with well localized 4f moments occupy a pyrochlore lattice with corner sharing tetrahedra. This lattice is highly frustrated for antiferromagnetic interactions, and for ferromagnetic interactions between ions with a strong local Ising anisotropy. The high frustration leads to exotic magnetic states, such as spin liquid or spin ice states, whose stability is governed by a delicate balance between exchange, anisotropy and dipolar interactions. At ambient pressure, Ho₂Ti₂O₇ behaves as a 'dipolar spin ice', with a magnetic state mapping that of the ice lattice. In contrast, Tb₂Ti₂O₇ behaves as a 'spin liquid', retaining short range liquid-like spin correlations down to at least 70 mK, despite a Curie Weiss temperature of ∼19 K. The energy balance is modified by pressure, since magnetic interactions depend on interatomic distances in different ways. We have studied the influence of an applied pressure for the first time, up to 9 GPa, in the temperature range 1.4 K < T < 300 K, by high pressure neutron diffraction. In Ho₂Ti₂O₇, the spin ice state remains stable up to at least 5 GPa. In Tb₂Ti₂O₇, the spin liquid state also remains stable up to 5 GPa, but under higher pressures novel features appear in the neutron spectra, pointing out drastic changes in the microscopic spin arrangement. From our first results, high pressure shows as a new and essential tool to investigate the magnetic interactions responsible for the novel magnetic states observed in highly frustrated magnets. This study was performed using a technique recently developed in Laboratoire Leon Brillouin, which allows one to determine magnetic order up to much higher pressures than in conventional neutron measurements. (author)

[en] The precession of muons implanted in a non-magnetic sample at a distance from a magnetized body has been investigated experimentally. Specifically, the muons were implanted into a silver sheet several millimeters from a magnetized nickel sheet. An exact theoretical expression for the field from a uniformly magnetized body was found to provide an excellent description of the average muon precession as a function of distance between the two samples. This experiment illustrates the possibility of using an easy modification to the standard μSR experiment to provide a bulk probe of the sample magnetization with the same time dependence as the usual local probe. This may help in the interpretation of certain μSR experiments as well as controlling for sample perturbation by the muons.

[en] Neutron scattering has been used to investigate the magnetic correlations in the spin-ice material dysprosium titanate, Dy₂Ti₂O₇. An isotopically enriched sample was used to minimise neutron absorption. In zero field no magnetic order was observed down to 50 mK but the magnetic diffuse scattering was in qualitative agreement with that expected for the disordered low-temperature state of dipolar spin ice. Application of a field of ∼0.8 T in the [100] direction led to long-range order. With the field applied in the [1 anti 10] direction a coexistence of long-range ferromagnetic and short-range antiferromagnetic order was observed. This is attributed to the pinning of only half the spins by the field. The hysteresis loops in both field orientations displayed unusual steps and plateaus. (orig.)

[en] We report a detailed study of the pyrochlore Ho₂Ti₂O₇, in which the magnetic ions (Ho³⁺) are ferromagnetically coupled with J∼1 K . We show that the presence of local Ising anisotropy leads to a geometrically frustrated ground state, preventing long-range magnetic order down to at least 0.05K. However, unlike in the case of a frustrated antiferromagnet, this disorder is principally static. In a magnetic field, the ground-state degeneracy is broken and ordered magnetic phases are formed which display an unusual history dependence due to the slow dynamics of the system. These results represent the first experimental evidence for geometrical frustration in a ferromagnetic system. copyright 1997 The American Physical Society

[en] Er₂Ti₂O₇ has been suggested to be a realization of the frustrated <111> XY pyrochlore lattice antiferromagnet, for which theory predicts fluctuation-induced symmetry breaking in a highly degenerate ground state manifold. We present a theoretical analysis of the classical model compared to neutron scattering experiments on the real material, both below and above T_N=1.173(2) K. The model correctly predicts the ordered magnetic structure, suggesting that the real system has order stabilized by zero-point quantum fluctuations that can be modeled by classical spin wave theory. However, the model fails to describe the excitations of the system, which show unusual features

[en] The influence of the sample quality on the magnetic properties of the heavy-fermion superconductor URu₂Si₂ has been studied by neutron scattering, specific heat, electrical resistivity, and magnetic susceptibility measurements. Two single crystals prepared under identical conditions received different heat treatments. One crystal was annealed, the other was used as-grown. While the macroscopic properties and the magnetic excitations are essentially the same for the two crystals, the magnetic Bragg peak intensity have completely different temperature dependences. Despite this, the low-temperature magnetic moment is identical for the two samples, showing that the small moment of 0.023(3) μ_B is intrinsic. The finite correlation length (∝500 A) appears to be related to defects. We discuss the relevance of itinerant versus localized behavior of the 5f electrons responsible for the magnetism, and the possibility of two successive phase transitions. (orig.)