Andrew S. Wills

Toward Perfection: Kapellasite, Cu3Zn(OH)6Cl2, a New Model S = 1/2 Kagome Antiferromagnet

The crystal structure and preliminary magnetic properties of the new S = 1/2 kagome antiferromagnet Kapellasite, Cu3Zn(OH)6Cl2, are presented. Susceptibility data reveal a moment collapse below T ≈ 25 K that is compatible with the spins condensing into a nonmagnetic single state.

Magnetic ordering in the XY pyrochlore antiferromagnet Er2Ti2O7: a spherical neutron polarimetry study

Er2Ti2O7 has been proposed as a realization of the pyrochlore antiferromagnet with dipolar interactions, where the spins of Er3+ lie perpendicular to the local axes. Below a N?el temperature of TN = 1.173?K magnetic order with the propagation vector occurs. Previous powder neutron diffraction studies were not able to determine details of the magnetic ordering beyond its symmetry due to powder averaging. In an attempt to resolve the questions as regards the ordering in this model magnet we performed a spherical neutron polarimetry experiment using CRYOPAD. The analysis of these data and a proposed magnetic order are presented.

Magnetic ordering in diluted kagome antiferromagnets

Potassium iron jarosite, KFe3(SO4)2(OH)6, provides a model kagome Heisenberg antiferromagnet with which to study the behaviour of this highly frustrated system. Magnetic susceptibility, powder neutron diffraction and muon spin relaxation measurements have been performed on samples of protonated and deuterated potassium iron jarosite in which some of the iron is replaced by aluminium ions to study the effect of diamagnetic dilution. The two successive magnetic ordering transitions of the pure material drop in temperature on doping. In all cases the low temperature phase appears to have long-range ordering of all moments in an array which is coplanar or nearly coplanar with the crystallographic ab axes, while in the intermediate phase there is a component of moment along the c-axis and some degree of dynamic character.

Ground state and intrinsic susceptibility of the kagome antiferromagnet vesignieite as seen by V-51 NMR

The intrinsic magnetic susceptibility and local magnetization of the near-kagome quantum magnet vesignieite, Cu3BaV2O8(OH)(2), are presented as measured using V-51 NMR. The NMR line shift gives an accurate measurement of the intrinsic susceptibility of the kagome sites which closely resembles that of the quantum spin liquid herbertsmithite [A. Olariu et al., Phys. Rev. Lett. 100, 087202 (2008)]. It is therefore surprising that, at T-C similar or equal to 9K, a transition to a heterogeneous ground state is observed. A gradual wipeout of half the NMR intensity indicates a slowly fluctuating spin liquid component, and a detailed analysis of the linewidth reveals the onset of static magnetism at the remaining half of the sites. It is proposed that this transition and unusual ground state originate from a nearby quantum critical point induced by the Dzyaloshinskii-Moriya interaction.

Magnetic ordering in the spin-ice candidate Ho2Ru2O7

Neutron scattering measurements on the spin-ice candidate material Ho2Ru2O7 have revealed two magnetic transitions at T approximately 95 and approximately 1.4 K to long-range ordered states involving the Ru and Ho sublattices, respectively. Between these transitions, the Ho3+ moments form short-ranged ordered spin clusters. The internal field provided by the ordered S=1 Ru4+ moments disrupts the fragile spin-ice state and drives the Ho3+ moments to order. We have directly measured a slight shift in the Ho3+ crystal field levels at 95 K from the Ru ordering.

Magnetic ordering in Gd2Sn2O7 the archetypal Heisenberg pyrochlore antiferromagnet

Low-temperature powder neutron diffraction measurements are performed in the ordered magnetic state of the pyrochlore antiferromagnet Gd2Sn2O7. Symmetry analysis of the diffraction data indicates that this compound has the ground state predicted theoretically for a Heisenberg pyrochlore antiferromagnet with dipolar interactions. The difference in the magnetic structure of Gd2Sn2O7 andof nominally analogous Gd2Ti2O7 is found to be determined by a specific type of third-neighbour superexchange interaction on the pyrochlore lattice between spins across empty hexagons.

Spin dynamics in the S=1/2 quantum kagome compound vesignieite, Cu3Ba(VO5H)(2)

We report the study of high quality samples of the frustrated S = 1/2 kagome antiferromagnet vesignieite, Cu3Ba(VO5H)2. Neutron powder diffraction measurements evidence the high quality of the kagome lattice and show no sign of a transition to long-range order. A kink in the susceptibility below T = 9 K is matched to a reduction in paramagnetic-like correlations in the diffraction data and a slowing of the spin dynamics observed by MuSR. Our results point to an exotic quantum state below 9 K with coexistance of both dynamical and small frozen moments \sim 0.1 \muB. We propose that Dzyaloshinsky-Moriya interaction is large enough in this system to stabilize this novel quantum ground state.

Dzyaloshinsky-Moriya interaction in vesignieite: A route to freezing in a quantum kagome antiferromagnet

We report an electron spin resonance investigation of the geometrically frustrated spin-1/2 kagome antiferromagnet vesignieite, BaCu

Spin-liquid ground state in the frustrated kagome antiferromagnet MgCu3(OH)6Cl2

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Anisotropy-driven spin glass transition in the kagome antiferromagnet hydronium jarosite, (H3O)Fe3(SO4)2(OH)6

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