arXiv subjects

A. S. Wills

Publications and source records attributed to A. S. Wills.

34 records · Page 2Linked to original sources

arXiv2009

Indexing magnetic structures and crystallographic distortions from powder diffraction: Brillouin zone indexing

A considerable challenge is faced by researchers wishing to identify the propagation vector(s) associated with a magnetic structure or a lattice distortion from powder diffraction data, due to the severe destruction of information by powder averaging and search algorithms based on extracted peak positions. In this article a new method is introduced that is based on the points, lines and planes of the Brillouin zone of the crystal structure before the transition. These correspond to different classifications of the translational symmetry of the resultant order and of the Bloch wave that is used to describe the magnetic structure or phonon mode. An automated search algorithm for the study of magnetic structures is described based on reverse-Monte Carlo refinement of moment orientations for a given k vector that markedly reduces the number of false-positives and allows the straight forward analysis of systems that contain contributions from several unrelated k vectors.

cond-mat.mtrl-sci↗

arXiv2008

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

The search for the resonating valence bond (RVB) state continues to underpin many areas of condensed matter research. The RVB is made from the dimerisation of spins on different sites into fluctuating singlets, and was proposed by Anderson to be the reference state from which the transition to BCS superconductivity occurs. Little is known about the state experimentally, due to the scarcity of model materials. Theoretical work has put forward the S = 1/2 kagome antiferromagnet (KAFM) as a good candidate for the realization of the RVB state. In this paper we introduce a new model system, the S = 1/2 KAFM Kapellasite, Cu3Zn(OH)6Cl2. We show that its crystal structure is a good approximation to a 2-dimensional kagome antiferromagnet and that susceptibility data indicate a collapse of the magnetic moment below T = 25 K that is compatible with the spins condensing into the non-magnetic RVB state.

cond-mat.str-el↗

arXiv2008

Field induced magnetic order in the frustrated magnet Gadolinium Gallium Garnet

Gd3Ga5O12, (GGG), has an extraordinary magnetic phase diagram, where no long range order is found down to 25 mK despite Θ_CW \approx 2 K. However, long range order is induced by an applied field of around 1 T. Motivated by recent theoretical developments and the experimental results for a closely related hyperkagome system, we have performed neutron diffraction measurements on a single crystal sample of GGG in an applied magnetic field. The measurements reveal that the H-T phase diagram of GGG is much more complicated than previously assumed. The application of an external field at low T results in an intensity change for most of the magnetic peaks which can be divided into three distinct sets: ferromagnetic, commensurate antiferromagnetic, and incommensurate antiferromagnetic. The ferromagnetic peaks (e.g. (112), (440) and (220)) have intensities that increase with the field and saturate at high field. The antiferromagnetic reflections have intensities that grow in low fields, reach a maximum at an intermediate field (apart from the (002) peak which shows two local maxima) and then decrease and disappear above 2 T. These AFM peaks appear, disappear and reach maxima in different fields. We conclude that the competition between magnetic interactions and alternative ground states prevents GGG from ordering in zero field. It is, however, on the verge of ordering and an applied magnetic field can be used to crystallise ordered components. The range of ferromagnetic and antiferromagnetic propagation vectors found reflects the complex frustration in GGG.

cond-mat.str-el↗

arXiv2006

Intermediate phase in the oxidative hydrothermal synthesis of potassium jarosite, a model kagome antiferromagnet

The jarosite family of minerals contain antiferromagnetically coupled Fe3+ ions that make up the kagome network. This geometric arrangement of the Fe3+ ions causes magnetic frustration that results in exotic electronic ground states, e.g. spin glasses and spin liquids. Synthesic research into jarosites has focused on producing near perfect stoichiometry to eliminate possible magnetic disorder. An new oxidative synthesis method has been devel-oped for the potassium, sodium, rubidium and ammonium jarosites that leads to high Fe coverage. We show through the identification of a meta-stable intermediate, using powder X-ray diffraction, how near perfect Fe coverage arises using this method. Understanding this new mechanism for jarosite formation suggests that is it possible to synthesis hydronium jarosite, an unconventional spin glass, with a very high Fe coverage.

cond-mat.mtrl-sci↗

arXiv2006

The origin of the spin glass transition in a model geometrically frustrated magnet

Highly frustrated systems have macroscopically degenerate ground states that lead to novel properties. In magnetism its consequences underpin exotic and technologically important effects, such as, high temperature superconductivity, colossal magnetoresistence, and the anomalous Hall effect. One of the enduring mysteries of frustrated magnetism is why certain experimental systems have a spin glass transition and its exact nature, given that it is not determined by the strength of the dominant magnetic interactions. There have been some suggestions that real systems possess disorder of the magnetic sites or bonds that are responsible. We show that the spin glass transition in the model kagome antiferromagnet hydronium jarosite arises from a spin anisotropy. This weaker energy scale is much smaller than that of the magnetic exchange, yet it is responsible for the energy barriers that are necessary to stabilise a glassy magnetic phase at finite temperature. The resultant glassy phase is quite unlike those found in conventional disordered spin glasses as it is based on complex collective rearrangements of spins called "spin folds". This simplifies hugely theoretical treatment of both the complex dynamics characteristic of a spin glass and the microscopic nature of the spin glass transition itself.

cond-mat.str-el↗

arXiv2006

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 magnetic structures of Gd2Sn2O7 and of nominally analogous Gd2Ti2O7 is found to be determined by a specific type of third-neighbor superexchange interaction on the pyrochlore lattice between spins across empty hexagons.

cond-mat.mtrl-sci↗

arXiv2004

Magnetic Ordering in the Spin-Ice Candidate Ho$_2$Ru$_2$O$_7$

Neutron scattering measurements on the spin-ice candidate material Ho$_2$Ru$_2$O$_7$ have revealed two magnetic transitions at T $\sim$ 95 K and T $\sim$ 1.4 K to long-range ordered states involving the Ru and Ho sublattices, respectively. Between these transitions, the Ho$^{3+}$ moments form short-ranged ordered spin clusters. The internal field provided by the ordered S=1 Ru$^{4+}$ moments disrupts the fragile spin-ice state and drives the Ho$^{3+}$ moments to order. We have directly measured a slight shift in the Ho$^{3+}$ crystal field levels at 95 K from the Ru ordering.

cond-mat.stat-mech↗

arXiv2003

Er2Ti2O7: Evidence of Order by Disorder in a Frustrated Antiferromagnet

Er_2Ti_2O_7 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 stabilised by zero-point quantum fluctuations that can be modelled by classical spin wave theory. However, the model fails to describe the excitations of the system, which show unusual features.

cond-mat↗

arXiv2002

Kagome spin ice

A new model of localized highly frustrated ferromagnetism is presented: kagome spin ice. By use of analytical and Monte Carlo calculations its massive groundstate entropy is evaluated. Monte Carlo calculations are also used to explore the phases in the presence of further-neighbor interactions and applied fields. The importance of thermal spin fluctuations and their ability to stabilize a variety of magnetic structures is clearly manifested. Remarkably, some of these phases are only partially ordered and present both disordered and ordered sublattices.

cond-mat.mtrl-sci↗

arXiv2002

Comment on "Magnetic field effects on neutron diffraction in the antiferromagnetic phase of UPt3"

Moreno and Sauls [Phys. Rev. B 63, 024419 (2000)] have recently tried to reanalyze earlier neutron scattering studies of the antiferromagnetic order in UPt3 with a magnetic field applied in the basal plane. In their calculation of the magnetic Bragg peak intensities, they perform an average over different magnetic structures belonging to distinct symmetry representations. This is incorrect. In addition, they have mistaken the magnetic field direction in one of the experiments, hence invalidating their conclusions concerning the experimental results.

cond-mat.str-el↗

arXiv2001

Symmetry and magnetic structure determination: Developments in refinement techniques and examples

Group Theory techniques can aid greatly the determination of magnetic structures. The integration of their calculations into new and existing refinement programs is an ongoing development that will simplify and make more rigorous the analysis of experimental data. This paper presents an overview of the practical application of symmetry analysis to the determination of magnetic structures. Details are given of the different programs that perform these calculations and how refinements can be carried out using their results. Examples are presented that show how such analysis can be important in the interpretation of magnetic diffraction data, and to our reasoning of the causes for the observed ordering.

cond-mat.mtrl-sci↗

arXiv2001

Conventional and unconventional orderings in the jarosites

The jarosites make up the most studied family of {\it kagomé} antiferromagnets. The flexibility of the structure to substitution of the A and B ions allows a wide range of compositions to be synthesised with the general formula AB3(SO4)2(OH)6 (A = Na$^+$, K$^+$, Ag$^+$, Rb$^+$, H3O$^+$, NH4$^+$, 1/2Ba$^{2+}$, and 1/2Pb$^{2+}$; B = Fe$^{3+}$, Cr$^{3+}$, and V$^{3+}$). Additional chemical tuning of the exchange between layers is also possible by substitution of the (SO4)$^{2-}$ groups by (SeO4)$^{2-}$ or (CrO4)$^{2-}$. Thus, a variety of S = 5/2, 3/2, and 1 systems can be engineered to allow study of the effects of frustration in both the classical and more quantum limits. Within this family both conventional long-ranged magnetic order and more exotic unconventional orderings have been found. This article reviews the different types of magnetic orderings that occur and examines some of the parameters that are their cause.

cond-mat.mtrl-sci↗

arXiv2001

Aging and memory properties of topologically frustrated magnets

The model 2d kagome system (H3O)Fe3(SO4)2(OH)6 and the 3d pyrochlore Y2Mo2O7 are two well characterized examples of low-disordered frustrated antiferromagnets which rather then condensing into spin liquid have been found to undergo a freezing transition with spin glass-like properties. We explore more deeply the comparison of their properties with those of spin glasses, by the study of characteristic rejuvenation and memory effects in the non-stationary susceptibility. While the pyrochlore shows clear evidence for these non-trivial effects, implying temperature selective aging, that is characteristic of a wide hierarchical distribution of equilibration processes, the kagome system does n not show clearly these effects. Rather, it seems to evolve towards the same final state independently of temperature.

cond-mat.dis-nn↗

arXiv2001

Order in the Heisenberg Pyrochlore: The Magnetic Structure of Gd2Ti2O7

The rare earth pyrochlore material Gd2Ti2O7 is considered to be an ideal model frustrated Heisenberg antiferromagnet with additional dipolar interactions. For this system there are several untested theoretical predictions of the ground state ordering pattern. Here we establish the magnetic structure of isotopically enriched (160)Gd2Ti2O7, using powder neutron diffraction at a temperature of 50 mK. The magnetic structure at this temperature is a partially ordered, non-collinear antiferromagnetic structure, with propagation vector k= 1/2 1/2 1/2. It can be described as a set of "q=0" ordered kagome planes separated by zero interstitial moments. This magnetic structure agrees with theory only in part, leaving an interesting problem for future research.

cond-mat↗

arXiv2000

Aging in a topological spin glass

We have examined the nonconventional spin glass phase of the 2-dimensional kagome antiferromagnet (H_3 O) Fe_3 (SO_4)_2 (OH)_6 by means of ac and dc magnetic measurements. The frequency dependence of the ac susceptibility peak is characteristic of a critical slowing down at Tg ~ 18K. At fixed temperature below Tg, aging effects are found which obey the same scaling law as in spin glasses or polymers. However, in clear contrast with conventional spin glasses, aging is remarkably insensitive to temperature changes. This particular type of dynamics is discussed in relation with theoretical predictions for highly frustrated non-disordered systems.

cond-mat.dis-nn↗

arXiv1996

Magnetic correlations in deuteronium jarosite, a model S=5/2 Kagome antiferromagnet

Deuteronium jarosite, (D_3O)Fe_3(SO_4)_2(OD)_6, contains a kagome lattice of Heisenberg spins, S=5/2, with a coverage of 97+-1%. DC and AC susceptibility measurements show strong in-plane antiferromagnetic exchange (θ_CW=-1500+-300K) and a spin-glass transition at T_f=13.8 K, while the magnetic contribution to the specific heat below T_f rises with T as T^2, characteristic of two-dimensional propagating modes. Powder neutron diffraction reveals short-range magnetic correlations (\xsi=19+-2 Å) with a wavevector corresponding to the \sqrt{3}x\sqrt{3} spin structure at 1.9 K.

cond-mat↗