arXiv subjects

Gavin B. G. Stenning

Publications and source records attributed to Gavin B. G. Stenning.

12 recordsLinked to original sources

arXiv2025

Polarized Neutrons at ISIS: Recent Developments And Highlights

We present two recent projects which aim to improve the performance of polarized neutron scattering experiments using hyperpolarized $^{3}He$ spin filters at ISIS. The first is the optimization of a new compact magnetostatic cavity ("Magic Box") to house the $^{3}He$ spin filters based on an existing design. With a length of only 380 mm, it provides a field gradient relaxation time for the $^{3}He$ cell of 421 h in ambient conditions. It also contains a radiofrequency coil for adiabatic fast passage flipping. The second project is dedicated to the improvement of the $^{3}He$ relaxation time inside the spin filter cell. We have developed a chamber which allows for the deposition of alkali metal coatings on the surface of substrates. This emulates the spin filter cell walls, as well as subsequent heat treatment, thus mimicking the preparation of a new spin filter cell. The chamber is air-tight and has transparent windows, so that the structure resulting from the deposition of alkali metal on the surface of the wafer can be studied by X-ray or neutron reflectometry. We plan to continue this work by performing a systematic study at various conditions, which should help to shed light on the long-standing mystery of how alkali metal coatings help to improve relaxation time of $^{3}He$ cells. The first results are discussed in the text.

physics.ins-det↗

arXiv2025

The Role of 11B4C Interlayers in Enhancing Fe/Si Multilayer Performance for Polarized Neutron Mirrors

This study investigates the effects of incorporating 11B4C interlayers into Fe/Si multilayers, with a focus on interface quality, reflectivity, polarization, and magnetic properties for polarized neutron optics. It is found that the introduction of 1 Å and 2 Å 11B4C interlayers significantly improves the interface sharpness, reducing interface width and preventing excessive Si diffusion into the Fe layers. X-ray reflectivity and polarized neutron reflectivity measurements show enhanced reflectivity and polarization, with a notable increase in polarization for 30 Å period multilayers. The inclusion of interlayers also helps prevent the formation of iron-silicides, improving both the magnetic properties and neutron optical performance. However, the impact of interlayers is less pronounced in thicker-period multilayers (100 Å), primarily due to the ratio between layer and interface widths. These results suggest that 11B4C interlayers offer a promising route for optimizing Fe/Si multilayer performance in polarized neutron mirrors.

cond-mat.mtrl-sci↗

arXiv2024

Kitaev Interactions Through an Extended Superexchange Pathway in the jeff = 1/2 Ru3+ Honeycomb Magnet, RuP3SiO11

Magnetic materials are composed of the simple building blocks of magnetic moments on a crystal lattice that interact via magnetic exchange. Yet from this simplicity emerges a remarkable diversity of magnetic states. Some reveal the deep quantum mechanical origins of magnetism, for example, quantum spin liquid (QSL) states in which magnetic moments remain disordered at low temperatures despite being strongly correlated through quantum entanglement. A promising theoretical model of a QSL is the Kitaev model, composed of unusual bond-dependent exchange interactions, but experimentally, this model is challenging to realise. Here we show that the material requirements for the Kitaev QSL survive an extended pseudo-edge-sharing superexchange pathway of Ru3+ octahedra within the honeycomb layers of the inorganic framework solid, RuP3SiO11. We confirm the requisite jeff = 1/2 state of Ru3+ in RuP3SiO11 and resolve the hierarchy of exchange interactions that provide experimental access to an unexplored region of the Kitaev model.

cond-mat.str-el↗

arXiv2022

Anomalous evolution of the magnetocaloric effect in dilute triangular Ising antiferromagnets $Tb_{1-x}Y_{x}(HCO_{2})_{3}$

We investigate the effects of diamagnetic doping in the solid-solution series $Tb_{1-x}Y_{x}(HCO_{2})_{3}$, in which the parent $Tb(HCO_{2})_{3}$ phase has previously been shown to host a combination of frustrated and quasi-1D physics, giving rise to a triangular Ising antiferromagnetic ground state that lacks long range 3D order. Heat capacity measurements show three key features: (i) a low temperature Schottky anomaly is observed, which is constant as a function of x; (ii) the transition temperature and associated entropy change are both surprisingly robust to diamagnetic doping; and (iii) an additional contribution at T < 0.4 K appears with increasing x. The origin of this unusual behaviour is rationalised in terms of the fragmentation of quasi-1D spin chains by the diamagnetic $Y^{3+}$ dopant. Magnetocaloric measurements show a nonlinear dependence on x. The mass-weighted magnetocaloric entropy decreases across the series from the promising values in $Tb(HCO_{2})_{3}$; however, the magnetocaloric entropy per magnetic $Tb^{3+}$ ion first decreases then increases with increasing x. Our results establish $Tb_{1-x}Y_{x}(HCO_{2})_{3}$ as a model system in which to explore the functional ramifications of dilution in a low-dimensional magnet.

cond-mat.str-el↗

arXiv2022

Neutron diffraction evidence of the 3-dimensional structure of Ba2MnTeO6 and misidentification of the triangular layers within the face-centred cubic lattice

Ba2MnTeO6 was first characterised using X-ray diffraction and reported to show a small distortion1 from the idealised cubic perovskite which displays face-centred cubic arrangement of Mn2+. A recent report has asserted that this leads to a layered configuration of Mn2+ that serves as an example of a triangular lattice, i.e. a 2D structure containing discreet layers.2 Here we show how neutron scattering gives great confidence in establishing the crystal structure being an undistorted cubic phase and how this can be mis-assigned as a triangular layered structure. This has profound implications for the understanding of the magnetic properties of the system.

cond-mat.str-el↗

arXiv2021

Site-selective $d^{10}$/$d^0$ substitution in a $S = 1/2$ spin ladder Ba$_2$CuTe$_{1-x}$W$_x$O$_6$ ($0 \leq x \leq 0.3$)

Remarkably, doping isovalent $d^{10}$ and $d^0$ cations onto the $B$'' site in $A_2B$'$B$''O$_6$ double perovskites has the power to direct the magnetic interactions between magnetic $B$' cations. This is due to changes in orbital hybridization, which favors different superexchange pathways, and leads to the formation of alternative magnetic structures depending on whether $B$'' is $d^{10}$ or $d^0$. Furthermore, the competition generated by introducing mixtures of $d^{10}$ and $d^0$ cations can drive the material into the realms of exotic quantum magnetism. Here, a W$^{6+}$ $d^0$ dopant was introduced to a $d^{10}$ hexagonal perovskite Ba$_2$CuTeO$_6$, which possesses a spin ladder geometry of Cu$^{2+}$ cations, creating a Ba$_2$CuTe$_{1-x}$W$_x$O$_6$ solid solution ($x$ = 0 - 0.3). Neutron and synchrotron X-ray diffraction show that W$^{6+}$ is almost exclusively substituted for Te$^{6+}$ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. This means the intra-ladder interactions are selectively tuned by the $d^0$ cations. Bulk magnetic measurements suggest this suppresses magnetic ordering in a similar manner to that observed for the spin-liquid like material Sr$_2$CuTe$_{1-x}$W$_x$O$_6$. This further demonstrates the utility of $d^{10}$ and $d^0$ dopants as a tool for tuning magnetic ground states in a wide range of perovskites and perovskite-derived structures.

cond-mat.str-el↗

arXiv2020

Investigation of the magnetic ground state of the ordered double perovskite Sr2YbRuO6: a tale of two transitions

Comprehensive muon spin rotation/relaxation (muSR) and neutron powder diffraction (NPD) studies supported via bulk measurements have been performed on the ordered double perovskite Sr2YbRuO6 to investigate the nature of the magnetic ground state. Two sharp transitions at TN1 ~ 42 K and TN2 ~ 36 K have been observed in the static and dynamic magnetization measurements, coinciding with the heat capacity data. In order to confirm the origin of the observed phase transitions and the magnetic ground state, microscopic evidences are presented here. An initial indication of long-range magnetic ordering comes from a sharp drop in the muon initial asymmetry and a peak in the relaxation rate near TN1. NPD confirms that the magnetic ground state of Sr2YbRuO6 consists of an antiferromagnetic (AFM) structure with interpenetrating lattices of parallel Yb3+ and Ru5+ moments lying in the ab-plane and adopting a A-type AFM structure. Intriguingly, a small but remarkable change is observed in the long-range ordering parameters at TN2 confirming the presence of a weak spin reorientation (i.e. change in spin configuration) transition of Ru and Yb moments, as well as a change in the magnetic moment evolution of the Yb3+ spins at TN2. The temperature dependent behaviour of the Yb3+ and Ru5+ moments suggests that the 4d-electrons of Ru5+ play a dominating role in stabilizing the long range ordered magnetic ground state in the double perovskite Sr2YbRuO6 whereas only the Yb3+ moments show an arrest at TN2. The observed magnetic structure and the presence of a ferromagnetic interaction between Ru- and Yb- ions are explained with use of the Goodenough-Kanamori-Anderson (GKA) rules. Possible reasons for the presence of the second magnetic phase transition and of a compensation point in the magnetization data are linked to competing mechanisms of magnetic anisotropy.

cond-mat.str-el↗

arXiv2020

Diamagnetic d-orbitals Drive Magnetic Structure Selection in the Double Perovskite Ba$_2$MnTeO$_6$

B-site ordered A$_2$B'B''O$_6$ double perovskites have a variety of applications as magnetic materials. Here we show that diamagnetic $d^{10}$ and $d^0$ B'' cations have a significant effect on the magnetic interactions in these materials. We present a neutron scattering and theoretical study of the Mn$^{2+}$ double perovskite Ba$_2$MnTeO$_6$ with a $4d^{10}$ Te$^{6+}$ cation on the B''-site. It is found to be a Type I antiferromagnet with a dominant nearest-neighbor $J_1$ interaction. In contrast, the $5d^0$ W$^{6+}$ analogue Ba$_2$MnWO$_6$ is a Type II antiferromagnet with a significant next-nearest-neighbor $J_2$ interaction. This is due to a $d^{10}$/$d^0$ effect, where the different orbital hybridization with oxygen 2p results in different superexchange pathways. We show that $d^{10}$ B'' cations promote nearest neighbor and $d^0$ cations promote next-nearest-neighbor interactions. The $d^{10}$/$d^0$ effect could be used to tune magnetic interactions in double perovskites.

cond-mat.str-el↗

arXiv2020

Pairing symmetry of an intermediate valence superconductor CeIr3 investigated using muSR measurements

We have investigated the bulk and microscopic properties of the rhombohedral intermediate valence superconductor CeIr$_3$ by employing magnetization, heat capacity, and muon spin rotation and relaxation ($μ$SR) measurements. The magnetic susceptibility indicates bulk superconductivity below $T_\mathrm{C} = 3.1$~K. Heat capacity data also reveal a bulk superconducting transition at $T_\mathrm{C} = 3.1$~K with a second weak anomaly near 1.6~K. At $T_{\mathrm{C}}$, the jump in heat capacity $ΔC$/$γT_{\mathrm{C}} \sim 1.39(1)$, is slightly less than the BCS weak coupling limit of 1.43. Transverse-field $μ$SR measurements suggest a fully gapped, isotropic, $s$-wave superconductivity with 2$Δ(0)/k_{\mathrm{B}}T_{\mathrm{C}} = 3.76(3)$, very close to 3.56, the BCS gap value for weak-coupling superconductors. From the temperature variation of magnetic penetration depth, we have also determined the London penetration depth $λ_{\mathrm{L}}(0) = 435(2)$~nm, the carriers' effective mass enhancement $m^{*} = 1.69(1)m_{\mathrm{e}}$ and the superconducting carrier density $n_{\mathrm{s}} = 2.5(1)\times 10^{26}$ carriers m$^{-3}$. The fact that LaIr$_3$, with no $4f$-electrons, and CeIr$_3$ with $4f^{n}$ electrons where $n \le 1$-electron (Ce ion in a valence fluctuating state), both exhibit the same $s$-wave gap symmetry indicates that the physics of these two compounds is governed by the Ir-$d$ band near the Fermi-level, which is in agreement with previous band structure calculations.

cond-mat.supr-con↗

arXiv2019

On the Origin of Superconductivity at Nickel-Bismuth Interfaces

Unconventional superconductivity has been suggested to be present at the interface between bismuth and nickel in thin-film bilayers. In this work, we study the structural, magnetic and superconducting properties of sputter deposited Bi/Ni bilayers. As-grown, our films do not display a superconducting transition, however, when stored at room temperature, after about 14 days our bilayers develop a superconducting transition up to 3.8 K. To systematically study the effect of low temperature annealing on our bilayers, we perform structural characterization with X-ray diffraction and polarized neutron reflectometry, along with magnetometry and low temperature electrical transport measurements on samples annealed at $70\,^\circ$C. We show that the onset of superconductivity in our samples is coincident with the formation of ordered NiBi$_3$ intermetallic alloy, a known $s$-wave superconductor. We calculate that the annealing process has an activation energy of $(0.86\pm 0.06)$eV. As a consequence, gentle heating of the bilayers will cause formation of the superconducting NiBi$_3$ at the Ni/Bi interface, which poses a challenge to studying any distinct properties of Bi/Ni bilayers without degrading that interface.

cond-mat.supr-con↗

arXiv2019

Long and short range magnetism in the frustrated double perovskite Ba2MnWO6

The structural and magnetic properties of the face-centered cubic double perovskite Ba2MnWO6 were investigated using neutron powder diffraction, DC-magnetometry, muon spin relaxation and inelastic neutron scattering. Ba2MnWO6 undergoes Type II long-range antiferromagnetic ordering at a Neel temperature of 8(1) K with a frustration index, f = 8. Inelastic neutron scattering was used to identify the magnetic coupling constants J1 and J2, which were found to equal -0.080 meV and -0.076 meV respectively. This indicated that both of the magnetic coupling constants were antiferromagnetic with similar magnitudes, which is in contrast to other known 3d metal double perovskites Ba2MWO6. Above the Neel temperature, muon spin relaxation measurements and inelastic neutron scattering techniques identify a short-range correlated magnetic state that is similar to that observed in the archetypical face-centered cubic lattice antiferromagnet MnO.

cond-mat.mtrl-sci↗

arXiv2019

Investigations of the superconducting ground state of Zr$_{3} $Ir: Introducing a new noncentrosymmetric superconductor

Superconductivity in materials whose crystal structure lacks inversion symmetry is a prime candidate for unconventional superconductivity. A new noncentrosymmetric compound Zr$_{3}$Ir crystallizes in a tetragonal $ α$-V$_{3} $S structure. The magnetization, specific heat and muon spin rotation confirm s-wave superconductivity, having a transition temperature T$_{c}$ = 2.3 K. Muon spin relaxation confirms the preservation of time reversal symmetry in the superconducting ground state.

cond-mat.supr-con↗