Design, calibration, and performance of a uniaxial pressure cell for neutron scattering studies of quantum magnetism.

We provide an overview of a pressure cell designed to apply uniaxial pressure to single crystals for the study, by neutron scattering techniques, of strongly correlated magnetic systems and, in particular, quantum magnets. A detailed overview of the pressure cell components, their requirements, and links to the scientific and technical specifications are presented. The pressure cell is able to accommodate a 200 mm3 single crystal that can be pressurized up to 2 GPa at cryogenic temperatures. The pressure cell is consistent with the requirements of inelastic neutron scattering and, importantly, neutron polarization analysis. A particular strength of the uniaxial pressure cell is the highly uniform and low background for a wide scattering angle of 360° horizontally and ±20° vertically. We show the performance of the uniaxial pressure cell using a relevant neutron scattering instrument, the polarized diffuse scattering instrument, D7. The experiments confirm that the cell complies with the scientific and technical requirements. This uniaxial pressure cell will provide a useful additional tool in the sample environment suite available for the study of quantum magnetism.

Experimental Evidence for the Spiral Spin Liquid in LiYbO_{2}.

Spiral spin liquids are an exotic class of correlated paramagnets with an enigmatic magnetic ground state composed of a degenerate manifold of fluctuating spin spirals. Experimental realizations of the spiral spin liquid are scarce, mainly due to the prominence of structural distortions in candidate materials that can trigger order-by-disorder transitions to more conventionally ordered magnetic ground states. Expanding the pool of candidate materials that may host a spiral spin liquid is therefore crucial to realizing this novel magnetic ground state and understanding its robustness against perturbations that arise in real materials. Here, we show that the material LiYbO_{2} is the first experimental realization of a spiral spin liquid predicted to emerge from the J_{1}-J_{2} Heisenberg model on an elongated diamond lattice. Through a complementary combination of high-resolution and diffuse neutron magnetic scattering studies on a polycrystalline sample, we demonstrate that LiYbO_{2} fulfills the requirements for the experimental realization of the spiral spin liquid and reconstruct single-crystal diffuse neutron magnetic scattering maps that reveal continuous spiral spin contours-a characteristic experimental hallmark of this exotic magnetic phase.

Small-angle neutron scattering study of mesoscale magnetic disordering and skyrmion phase suppression in the frustrated chiral magnet Co6.75Zn6.75Mn6.5.

Co-Zn-Mn chiral cubic magnets display versatile magnetic skyrmion phases, including equilibrium phases stable far above and far below room temperature, and the facile creation of robust far-from-equilibrium skyrmion states. In this system, compositional disorder and magnetic frustration are key ingredients that have profound effects on the chiral magnetism. Reported here are studies of the magnetism in Co6.75Zn6.75Mn6.5 by magnetometry, small-angle neutron scattering (SANS), magnetic diffuse neutron scattering and Lorentz transmission electron microscopy (LTEM). While features in magnetometry and LTEM often give standard indications for skyrmion formation, they are not readily observed from the measurements on this system. Instead, skyrmion lattice correlations are only revealed by SANS, and they are found to form an orientationally disordered structure in a minority fraction of the sample. The majority fraction of the sample always displays orientationally disordered helical spin correlations, which undergo further disordering along the radial direction on cooling below the critical temperature (T c ≃ 102 K). The near-complete suppression of the skyrmion phase, and the process of disordering on cooling, are attributed to competing magnetic interactions that dominate over the ferromagnetic interaction expected to favour chiral magnetism in this system. These competing interactions start to develop above T c and become further enhanced towards low temperatures. The present observations of co-existing and disordered magnetic correlations over multiple length scales are not unique to Co6.75Zn6.75Mn6.5 but are seemingly common to the family of Co-Zn-Mn compounds with finite Mn, and their accurate description presents a challenge for theoretical modelling. In addition, this study highlights a need for neutron instrumentation capable of the comprehensive measurement of magnetic correlations over expanded ranges of momentum transfer in such multiple-length-scale magnets.

Magnetic-field-controlled spin fluctuations and quantum critically in Sr3Ru2O7.

When the transition temperature of a continuous phase transition is tuned to absolute zero, new ordered phases and physical behaviour emerge in the vicinity of the resulting quantum critical point. Sr3Ru2O7 can be tuned through quantum criticality with magnetic field at low temperature. Near its critical field Bc it displays the hallmark T-linear resistivity and a [Formula: see text] electronic heat capacity behaviour of strange metals. However, these behaviours have not been related to any critical fluctuations. Here we use inelastic neutron scattering to reveal the presence of collective spin fluctuations whose relaxation time and strength show a nearly singular variation with magnetic field as Bc is approached. The large increase in the electronic heat capacity and entropy near Bc can be understood quantitatively in terms of the scattering of conduction electrons by these spin-fluctuations. On entering the spin-density-wave ordered phase present near Bc, the fluctuations become stronger suggesting that the order is stabilised through an "order-by-disorder" mechanism.

Spin Dynamics and Unconventional Coulomb Phase in Nd_{2}Zr_{2}O_{7}.

We investigate the temperature dependence of the spin dynamics in the pyrochlore magnet Nd_{2}Zr_{2}O_{7} by neutron scattering experiments. At low temperature, this material undergoes a transition towards an "all-in-all-out" antiferromagnetic phase and the spin dynamics encompass a dispersionless mode, characterized by a dynamical spin ice structure factor. Unexpectedly, this mode is found to survive above T_{N}≈300 mK. Concomitantly, elastic correlations of the spin ice type develop. These are the signatures of a peculiar correlated paramagnetic phase which can be considered as a new example of Coulomb phase. Our observations near T_{N} do not reproduce the signatures expected for a Higgs transition, but show reminiscent features of the "all-in-all-out" order superimposed on a Coulomb phase.

High-resolution x-ray scattering from epitaxial thin films of Y/Nb on Al2O3.

During the 1990s, Roger Cowley had a strong interest in the crystal and magnetic structures of rare-earth superlattices as a means to understand the rich and exotic magnetic properties of the rare-earth metals. High-quality samples can be grown by molecular beam epitaxy on sapphire substrates by first depositing a thin epitaxial layer of niobium, then a layer of yttrium or lutetium as a seed. High-resolution x-ray scattering is an excellent probe to characterise the crystal quality and was used to study the structure of the niobium layer. However, relatively little attention was paid to the seed layer. This article summarises some of the x-ray experiments performed by the Cowley group to study the structure of epitaxial [Formula: see text] niobium on [Formula: see text] sapphire, and extends the work to report some results on the structure of thin [Formula: see text] yttrium seed layers. The structure of the yttrium films is shown to have a strong dependence on the thickness of the niobium buffer, with the buffer needing to be thicker than a critical value of ∼80 [Formula: see text] for the formation of misfit dislocations at the Nb/Al2O3 interface before highly coherent Y films can be grown. Yttrium films grown on Nb buffers thinner than ∼500 [Formula: see text] show a similar two-peak line shape in [Formula: see text] scans through their specular Bragg peaks to that seen in the specular Nb Bragg peaks, with a resolution-limited feature on a broader diffuse peak. The resolution-limited feature depends on the thickness of the yttrium film, becoming weaker and having a stronger decay with increasing [Formula: see text] as the film thickness increases, while the width of the yttrium broad peak evolves as the square root of the width of the niobium Bragg peak. The data are discussed within the context of theories describing the scattering from films with misfit dislocations.

Tuning dimensionality in van-der-Waals antiferromagnetic Mott insulators TMPS3.

We present an overview of our recent work in tuning and controlling the structural, magnetic and electronic dimensionality of 2D van-der-Waals antiferromagnetic compounds (Transition-Metal)PS3. Low-dimensional magnetic systems such as these provide rich opportunities for studying new physics and the evolution of established behaviours with changing dimensionality. These materials can be exfoliated to monolayer thickness and easily stacked and combined into functional heterostructures. Alternatively, the application of hydrostatic pressure can be used to controllably close the van-der-Waals interplanar gap and tune the crystal structure and electron exchange paths towards a 3D nature. We collect and discuss trends and contrasts in our data from electrical transport, Raman scattering and synchrotron x-ray measurements, as well as insight from theoretical calculations and other results from the literature. We discuss structural transitions with pressure common to all materials measured, and link these to Mott insulator-transitions in these compounds at high pressures. Key new results include magnetotransport and resistivity data in the high-pressure metallic states, which show potentially interesting qualities for a new direction of future work focussed on low temperature transport and quantum critical physics.

Role of defects in determining the magnetic ground state of ytterbium titanate.

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.

Dipolar Spin Ice States with a Fast Monopole Hopping Rate in CdEr_{2}X_{4} (X=Se, S).

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr_{2}Se_{4} is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three orders of magnitude faster than the canonical spin ice Dy_{2}Ti_{2}O_{7}. In this Letter we use diffuse neutron scattering to show that both CdEr_{2}Se_{4} and CdEr_{2}S_{4} support a dipolar spin ice state-the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to those in Dy_{2}Ti_{2}O_{7}, i.e., dilute and uncorrelated, and so cannot provide three orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er^{3+} ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr_{2}X_{4} (X=Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr_{2}X_{4} offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.

Pressure-Induced Electronic and Structural Phase Evolution in the van der Waals Compound FePS_{3}.

Two-dimensional materials have proven to be a prolific breeding ground of new and unstudied forms of magnetism and unusual metallic states, particularly when tuned between their insulating and metallic phases. Here we present work on a new metal-to-insulator transition system FePS_{3}. This compound is a two-dimensional van der Waals antiferromagnetic Mott insulator. We report the discovery of an insulator-metal transition in FePS_{3}, as evidenced by x-ray diffraction and electrical transport measurements, using high pressure as a tuning parameter. Two structural phase transitions are observed in the x-ray diffraction data as a function of pressure, and resistivity measurements show evidence of the onset of a metallic state at high pressures. We propose models for the two new structures that can successfully explain the x-ray diffraction patterns.

The magnetic properties and structure of the quasi-two-dimensional antiferromagnet CoPS3.

The magnetic properties and magnetic structure are presented for CoPS3, a quasi-two-dimensional antiferromagnet on a honeycomb lattice with a Néel temperature of [Formula: see text] K. The compound is shown to have XY-like anisotropy in its susceptibility, and the anisotropy is analysed to extract crystal field parameters. For temperatures between 2 K and 300 K, no phase transitions were observed in the field-dependent magnetization up to 10 Tesla. Single-crystal neutron diffraction shows that the magnetic propagation vector is k = [Formula: see text] with the moments mostly along the [Formula: see text] axis and with a small component along the [Formula: see text] axis, which largely verifies the previously-published magnetic structure for this compound. The magnetic Bragg peak intensity decreases with increasing temperature as a power law with exponent [Formula: see text] for [Formula: see text].

Magnetic correlations in the magnetocaloric materials Mn3GaC and Mn3GaC0.85N0.15 studied by neutron polarization analysis and neutron depolarization.

Partially substituting carbon by nitrogen in the antiperovskite compound Mn3GaC increases the first order antiferromagnetic/ferromagnetic transition temperature and at the same time causes the high-temperature long-range ferromagnetism to weaken. To show that the weakening is related to the diminishing of ferromagnetic domain formation, we undertake neutron depolarization and neutron polarization analysis experiments on Mn3GaC and Mn3GaC0.85N0.15. Polarization analysis experiments show that strong ferromagnetic correlations are present at high temperatures in the paramagnetic states of both Mn3GaC and Mn3GaC0.85N0.15 and that these correlations vanish in the antiferromagnetic state. Neutron depolarization studies show that above the first order transition temperature, ferromagnetic domain formation is present in Mn3GaC but is absent in Mn3GaC0.85N0.15. The relationship between ferromagnetic domain formation and transitional hysteresis is brought forward for these two important magnetocaloric materials.

Intra-unit-cell magnetic correlations near optimal doping in YBa2Cu3O6.85.

The pseudo-gap phenomenon in copper oxide superconductors is central to any description of these materials as it prefigures the superconducting state itself. A magnetic intra-unit-cell order was found to occur just at the pseudo-gap temperature in four cuprate high-Tc superconducting families. Here we present polarized neutron-scattering measurements of nearly optimally doped YBa2Cu3O6.85, carried out on two different spectrometers, that reveal several features. The intra-unit-cell order consists of finite-sized planar domains that are very weakly correlated along the c axis. At high temperature, only the out-of-plane magnetic components correlate, indicating a strong Ising anisotropy. An aditional in-plane response develops at low temperature, giving rise to an apparent tilt of the magnetic moment. The discovery of these two regimes puts stringent constraints, which are tightly bound to the pseudo-gap physics, on the intrinsic nature of intra-unit-cell order.

Generalization of the classical xyz-polarization analysis technique to out-of-plane and inelastic scattering.

The technique of longitudinal ("xyz") polarization analysis has been used successfully for many years to study disordered magnetic materials in thermal and cold neutron diffraction experiments. The technique allows the simultaneous and unambiguous separation of the nuclear, magnetic, and nuclear spin-incoherent contributions to the scattering. The technical advances seen in recent years, such as the availability of polarized (3)He analyzer cells to cover a large detector solid angle, the ability to detect out-of-plane scattering in a multi-detector, and a significant increase of the usable beam divergence, call for a generalization of the method. A general treatment of the formalism for carrying out neutron polarization analysis will be given in this paper, which describes a possible method of usage at a future, modern diffractometer or inelastic spectrometer with large area multi-detector coverage.

Symmetric and asymmetric instability of buried polymer interfaces.

We demonstrate using neutron reflectometry that the internal interfaces in a trilayer system of two identical thick polystyrene layers sandwiching a much thinner (deuterated) poly(methyl methacrylate) layer 15 nm thick (viscosity matched with the polystyrene layers) increase in roughness at the same rate. When the lower polystyrene layer is replaced with a layer of the same polymer of much greater molecular mass, two different growths of the interfaces are observed. From the growth of the interface for this asymmetric case in the solid regime using the theoretical prediction of the spinodal instability including slippage at the interface, a value of the Hamaker constant of the system has been extracted in agreement with the calculated value. For the symmetric case the rise time of the instability is much faster.

The magnon dynamics and spin exchange parameters of FePS3.

The spin waves in a powdered sample of a quasi-two-dimensional antiferromagnet, FePS(3), have been measured using neutron inelastic scattering. The data could be modelled and the exchange interactions determined using a two-dimensional Heisenberg Hamiltonian with single ion anisotropy. A suitable fit to the data could only be achieved by including magnetic interactions up to the third nearest neighbour, which is consistent with the findings for other members of the MPS(3) family (M=transition metal). The best fit parameters at 6 K were J(1) = 1.49 meV, J(2) = 0.04 meV, J(3) =- 0.6 meV, with an anisotropy of Δ = 3.7 meV. Measurements as a function of temperature give a coarse measure of the behaviour of the anisotropy and the nature of the phase transition.

Surfactant induced symmetric and thermally stable interfaces in Cu/Co multilayers.

In this work we studied Ag surfactant induced growth of Cu/Co multilayers. The Cu/Co multilayers were deposited using Ag surfactant by the ion beam sputtering technique. It was found that Ag surfactant balances the asymmetry between the surface free energies of Cu and Co. As a result, the Co-on-Cu and Cu-on-Co interfaces become sharp and symmetric and thereby improve the thermal stability of the multilayer. On the basis of obtained results, a mechanism leading to symmetric and stable interfaces in Cu/Co multilayers is discussed.

Sperimagnetism in Fe78Er5B17 and Fe64Er19B17 metallic glasses: I. Moment values and non-collinear components.

Magnetization measurements have been made on a Fe(64)Er(19)B(17) glass, which exhibits ferrimagnetic compensation at T(comp) = 112 K, and polarized beam neutron scattering measurements have been made on Fe(78)Er(5)B(17) and Fe(64)Er(19)B(17) glasses to supplement the measurements made earlier on Fe(64)Er(19)B(17). The magnetization data were analysed with a phenomenological model, to find the magnetic moments and their components needed to interpret the neutron data. Four spin-dependent scattering cross-sections were obtained in absolute units from each neutron experiment, to determine the atomic-scale magnetic structures of the two glasses. The finite spin-flip cross-sections confirmed that these (Fe,Er)(83)B(17) glasses are non-collinear ferrimagnets. The cross-sections were calculated using a model based on random cone arrangements of the magnetic moments. The moment values and the random cone angles were refined in the calculations, which produced good agreement between the calculated curves and the experimental data. The forward limit of the spin-flip cross-sections |∂σ(±∓)/∂Ω|(Q=0) of the Fe(64)Er(19)B(17) glass which peaked at T(comp) and the temperature variation of the total scattering amplitudes (b(∓)p(∥)(Q)) suggested that the random cone angles open fully so that the collinear components p(∥)(Q) tend to zero at T(comp). The ferrimagnetic compensation is therefore characterized by an equality of the magnetic sublattices; the reversal of the magnetic structure and a compensated sperimagnetic phase which appears at T(comp).

Sperimagnetism in Fe78Er5B17 and Fe64Er19B17 metallic glasses: II. Collinear components and ferrimagnetic compensation.

Magnetization measurements on an Fe(64)Er(19)B(17) glass and polarized-beam neutron scattering measurements on Fe(78)Er(5)B(17) and Fe(64)Er(19)B(17) were described in part I. The finite spin-flip neutron scattering cross sections were calculated using a sperimagnetic structure based on random cone arrangements of the magnetic moments. The temperature variation of the cross sections of Fe(64)Er(19)B(17) suggested that a compensated sperimagnetic phase existed at T(comp).The analysis of the non-spin-flip neutron scattering cross sections is described here in part II. Two spin-dependent total structure factors S(±±)(Q) were defined from these cross sections and, despite the limited range of the data 0.5 Å(-1) < Q < 6.5 Å(-1), their Fourier transform gave reliable spin-dependent radial distribution functions RDF(±±)(r). These were interpreted in terms of the atomic pair correlation functions ρ(±±)(AB)(r) and their weighting factors ω(±±)(AB). The data on Fe(64)Er(19)B(17) at 1.5 K showed, for example, how the directions of the magnetic sublattices can be defined uniquely. The analysis of the RDF(±±)(r) for Fe(64)Er(19)B(17) at 112 K confirmed that the mean collinear components of the magnetic moments , are zero on both sublattices in the compensated sperimagnetic structure at T(comp). The pre-peak in the spin-dependent total structure factors at 112 K showed that it originated in the atomic structure and it may involve Fe-Er-Fe 'collineations' at a radial distance of ≈6.0 Å. Finally, the RDF(±±)(r) of Fe(64)Er(19)B(17) at 180 K and of Fe(78)Er(5)B(17) at 2 K show that both glasses have the (µ(Fe) UP:µ(Er) DOWN) structure like the (Fe,Tb)(83)B(17) collinear ferrimagnets.

The influence of interfacial roughness on the coherence of structure and magnetic coupling across barriers in Fe/MgO multilayers.

Single-crystal Fe/MgO multilayers are model systems in which to study magnetic tunnel junctions. We find that the interfacial roughness leads to the loss of coherence of the crystal structure. For thick MgO layers ferromagnetic (FM) ordering is found using polarized neutron reflectivity (PNR). For thin MgO layers magnetization measurements reveal the presence of antiferromagnetic (AF) interactions, but no long-range AF order is found using PNR. After cycling in a hysteresis loop, FM correlations are found at the coercive point, and this will limit the maximum tunnelling magnetoresistance.