arXiv subjects

Gøran J. Nilsen

Publications and source records attributed to Gøran J. Nilsen.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic properties and excitation spectrum of the S = 1/2 three-legged spin ladder, LiMoOP2O7

We report a comprehensive experimental study including density functional theory calculations detailing the magnetic ground state of the $S = 1/2$ Mo$^{5+}$ pyrophosphate, LiMoOP2O7, and its magnetic excitations. Neutron powder diffraction measurements confirm the $P2_1/n$ structural model which is comprised of a three-legged ladder of distorted Mo$^{5+}$ octahedra. Magnetometry and specific heat reveal a long-range magnetic ordering transition at $T_\textrm{N} = 3.5$~K and a field-induced transition at $μ_{0}$H$_\textrm{c}$ = 1 T. This zero-field long-range magnetic order was determined to be a commensurate $κ= 0$ order with an ordered moment that is reduced from the full $S = 1/2$ magnetic moment by 30%. Through inelastic neutron scattering measurements and density functional theory calculations, we define the minimal microscopic model and resolve the spin Hamiltonian up to the seventh leading exchange parameter, resulting in a model with dominant antiferromagnetic leg interactions, weaker ferromagnetic rung interactions, and a significant interlayer ferromagnetic coupling.

cond-mat.str-el↗

arXiv2026

Observation of Switchable Chiral Magnons in an Altermagnet

Chiral magnons, the quanta of handed spin waves, transport spin angular momentum without energy loss due to Joule heating. The recently discovered altermagnets were proposed to host chiral magnons arising from a non-relativistic exchange mechanism, similar to that in ferromagnets but without net magnetization, offering a stray-field-free platform for efficient magnon spin-current manipulation. In this work, we directly observed chiral magnons in the altermagnetic prototype MnTe using polarized inelastic neutron scattering. Furthermore, the magnon chirality was found to be reversibly switched by magnetic-field control, establishing a robust foundation for functional altermagnetic magnonics.

cond-mat.str-el↗

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↗

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↗

arXiv2023

Dynamics of the antiferromagnetic spin ice phase in pyrochlore spinels

Motivated by the classical spin-nematic state observed in the breathing pyrochlore spinel LiGa$_{0.95}$In$_{0.05}$Cr$_4$O$_8$, we theoretically discuss spin dynamics in models of spin-lattice coupling in these materials. Semiclassical dynamical simulations successfully recover the key features of inelastic neutron-scattering experiments on LiGa$_{0.95}$In$_{0.05}$Cr$_4$O$_8$: a broad finite-energy peak alongside a continuum of scattering near the (200) wave vector that extends from the elastic line to high energies. To interpret this result, we develop a small-fluctuation theory for the spin-ice-like nematic ground states, analogous to linear spin-wave theory for conventionally ordered magnets, which reproduces the numerical simulation results quantitatively. In particular, we find that the inelastic peak is well explained by collective modes confined to ferromagnetic loops of the underlying nematic order. In addition, we find a sharp, linearly dispersing mode in the dynamical structure factor, which originates in long-wavelength fluctuations of the nematic director: We believe identifying this mode will be an interesting target for future experiments on these materials. We also outline potential future applications of our methods to both pyrochlore spinels and other spin-nematic systems.

cond-mat.str-el↗

arXiv2023

One-dimensional magnetism in synthetic Pauflerite, $β$-VOSO$_4$

We have synthesized single-crystal samples of $β$-VOSO$_4$ and fully characterized their magnetic properties. Our magnetic susceptibility, high field magnetization and powder inelastic neutron scattering results are in excellent agreement with theoretical expressions for a one-dimensional spin-1/2 Heisenberg chain with an exchange parameter of $3.83(2)$ meV. Ab initio calculations identify the superexchange pathway, revealing that the spin-chain does not run along the expected crystallographic chain $a$ direction but instead between V$^{4+}$O$_{6}$ octahedra that are linked via SO$_{4}$ tetrahedra along the $b$ axis. We do not detect any phase transition to a long-range magnetic order within our experimental conditions, indicating $β$-VOSO$_4$ is very close to an ideal one-dimensional magnetic system.

cond-mat.str-el↗

arXiv2023

Inelastic Neutron Scattering Study of the Spin Dynamics in the Breathing Pyrochlore System LiGa0.95In0.05Cr4O8

The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present here an inelastic neutron scattering study of the spin dynamics in this composition. Above Tf , the dynamical scattering function S(Q,E) is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below Tf , the spectral weight splits between a broad inelastic feature at 5.8 meV and toward the elastic line. The former feature can be ascribed to spin precessions within antiferromagnetic loops, lifted to finite energy by the effective biquadratic spin-lattice term in the spin Hamiltonian.

cond-mat.str-el↗

arXiv2022

One-Dimensional Quantum Magnetism in the S = 1/2 Mo(V) system, KMoOP2O7

We present a comprehensive experimental and ab-initio study of the $S=1/2$ Mo$^{5+}$ system, KMoOP$_2$O$_7$, and show that it realizes the $S = 1/2$ Heisenberg chain antiferromagnet model. Powder neutron diffraction reveals that KMoOP$_2$O$_7$ forms a magnetic network comprised of pairs of Mo$^{5+}$ chains within its monoclinic $P2_1/n$ structure. Antiferromagnetic interactions within the Mo$^{5+}$ chains are identified through magnetometry measurements and confirmed by analysis of the magnetic specific heat. The latter reveals a broad feature centred on $T_\textrm{N} = 0.54$ K, which we ascribe to the onset of long-range antiferromagnetic order. No magnetic Bragg scattering is observed in powder neutron diffraction data collected at 0.05 K, however, which is consistent with a strongly suppressed ordered moment with an upper limit $μ_\textrm{ord} < 0.15 μ_\textrm{B}$. The one-dimensional character of the magnetic correlations in KMoOP$_2$O$_7$ is verified through analysis of inelastic neutron scattering data, resulting in a model with $J_\textrm{1} \approx 34$ K and $J_\textrm{2} \approx -2$ K for the intrachain and interchain exchange interactions, respectively. The origin of these experimental findings are addressed through density-functional theory calculations.

cond-mat.str-el↗

arXiv2022

Magnetic, electronic, and structural investigation of the strongly correlated Y$_{1-x}$Sm$_x$Co$_5$ system

SmCo$_5$ and YCo$_5$ are isostructural compounds both showing large magnetocrystalline anisotropy, where the former originates mainly from the crystal-electric field and magnetic interactions. We investigate the contribution of both interactions by partially substituting Y by Sm in as-cast polycrystalline Y$_{1-x}$Sm$_{x}$Co$_5$ (with $x$=0, 0.1, 0.2, 0.3, and 0.4) and measuring their structural, magnetic, and electrical properties through X-ray diffraction, magnetization, and electrical transport measurements. Our results suggest an interplay between microstructure strain in as-cast samples and the electronic and magnetic interactions.

cond-mat.str-el↗

arXiv2021

Magnetic frustration in a van der Waals metal CeSiI

The realization of magnetic frustration in a metallic van der Waals (vdW) coupled material has been sought as a promising platform to explore novel phenomena both in bulk matter and in exfoliated devices. However, a suitable material platform has been lacking so far. Here, we demonstrate that CeSiI hosts itinerant electrons coexisting with exotic magnetism. In CeSiI, the magnetic cerium atoms form a triangular bilayer structure sandwiched by van der Waals stacked iodine layers. From resistivity and magnetometry measurements, we confirm the coexistence of itinerant electrons with magnetism with dominant antiferromagnetic exchange between the strongly Ising-like Ce moments below 7 K. Neutron diffraction directly confirms magnetic order with an incommensurate propagation vector k ~ (0.28, 0, 0.19) at 1.6 K, which points to the importance of further neighbor magnetic interactions in this system. The presence of a two-step magnetic-field-induced phase transition along c axis further suggests magnetic frustration in the ground state. Our findings provide a novel material platform hosting a coexistence of itinerant electron and frustrated magnetism in a vdW system, where exotic phenomena arising from rich interplay between spin, charge and lattice in low dimension can be explored.

cond-mat.str-el↗

arXiv2020

Realising square and diamond lattice $S=1/2$ Heisenberg antiferromagnet models in the $α$ and $β$ phases of the coordination framework, KTi(C$_2$O$_4$)$_2\cdot$\textit{x}H$_2$O

We report the crystal structures and magnetic properties of two psuedo-polymorphs of the $S=1/2$ Ti$^{3+}$ coordination framework, KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O. Single-crystal X-ray and powder neutron diffraction measurements on $α$-KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of Ti$^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with $J_1\approx7$ K and $J_2/J_1=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below $1.8$ K, $α$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of Ti$^{3+}$ in $α$ is suppressed from its spin-only value to $0.62(3)~μ_B$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $β$-KTi(C$_2$O$_4$)$_2\cdot$2H$_2$O, on the other hand, realises a three-dimensional diamond-like magnetic network of Ti$^{3+}$ moments within a hexagonal $P6_222$ structure. An antiferromagnetic exchange coupling of $J\approx54$ K -- an order of magnitude larger than in $α$ -- is extracted from magnetometry and specific heat data. $β$ undergoes Néel ordering at $T_N=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79~μ_B$ per Ti$^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $α$ and $β$ psuedo-polymorphs. Given their observed magnetic behaviors, we propose $α$-KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O and $β$-KTi(C$_2$O$_4$)$_2\cdot$2H$_2$O as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.

cond-mat.str-el↗

arXiv2020

Zero-energy excitation in the classical kagome antiferromagnet NaBa$_{2}$Mn$_{3}$F$_{11}$

We performed inelastic neutron scattering measurements on a polycrystalline sample of a classical kagome antiferromagnet NaBa$_{2}$Mn$_{3}$F$_{11}$ to investigate the possibility of a dispersionless zero-energy excitation associated with rotation of spins along the chains. The observed spectra indeed exhibit such an excitation with strong intensity at low energy, as well as dispersive excitations with weak intensity at high energy. Combining the measurements with calculations from linear spin-wave theory reveals that NaBa$_{2}$Mn$_{3}$F$_{11}$ is a good realization of the classical kagome antiferromagnet which exhibits a dispersionless mode lifted by the magnetic dipole-dipole interaction.

cond-mat.str-el↗

arXiv2019

Large Easy Axis Anisotropy in the One-Dimensional Magnet BaMo(PO$_4$)$_2$

We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound BaMo(PO$_4$)$_2$. The magnetic susceptibility reveals strong antiferromagnetic interactions $θ_{CW} = -167$~K and long-range magnetic order at $T_N=22$~K, in agreement with a recent report. Powder neutron diffraction furthermore shows that the order is collinear, with the moments near the $ac$ plane. Neutron spectroscopy reveals a large excitation gap $Δ= 15$~meV in the low-temperature ordered phase, suggesting a much larger easy-axis spin anisotropy than anticipated. However, the large anisotropy justifies the relatively high ordered moment, Néel temperature, and collinear order observed experimentally, and is furthermore reproduced in a first principles calculations using a new computational scheme. We therefore propose BaMo(PO$_4$)$_2$ to host $S=1$ antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.

cond-mat.str-el↗

arXiv2019

A quantum liquid of magnetic octupoles on the pyrochlore lattice

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles$^1$. Theories typically define such states using gauge fields and deconfined quasiparticle excitations that emerge from a simple rule governing the local ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice in rare-earth pyrochlores - a state described by a lattice gauge theory of quantum electrodynamics$^{2-4}$. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders$^{5-6}$. Here we show that the correlated ground state of a Ce$^{3+}$-based pyrochlore, Ce$_2$Sn$_2$O$_7$, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with the formation of a fluid-like state of matter, but the intensity distribution is weighted to larger scattering vectors, which indicates that the correlated degrees of freedom have a more complex magnetization density than that typical of magnetic dipoles in a spin liquid. The temperature evolution of the bulk properties in the correlated regime below 1 Kelvin is well reproduced using a model of dipole-octupole doublets on a pyrochlore lattice$^{7-8}$. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule. Our work identifies Ce$_2$Sn$_2$O$_7$ as a unique example of a material where frustrated multipoles form a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

cond-mat.str-el↗

arXiv2017

Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCo$_2$F$_7$ and NaSrCo$_2$F$_7$ with magnetic pair distribution function analysis

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo$_2$F$_7$ and NaSrCo$_2$F$_7$, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously-proposed model of short-range XY-like correlations with a length scale of 10-15 Å, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing a novel opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.

cond-mat.str-el↗

arXiv2017

Phase diagram of multiferroic KCu$_3$As$_2$O$_7$(OD)$_3$

The layered compound KCu$_3$As$_2$O$_7$(OD)$_3$, comprising distorted kagome planes of $S=1/2$ Cu$^{2+}$ ions, is a recent addition to the family of type-II multiferroics. Previous zero field neutron diffraction work has found two helically ordered regimes in \kns, each showing a distinct coupling between the magnetic and ferroelectric order parameters. Here, we extend this work to magnetic fields up to $20$~T using neutron powder diffraction, capacitance, polarization, and high-field magnetization measurements, hence determining the $H-T$ phase diagram. We find metamagnetic transitions in both low temperatures phases around $μ_0 H_c \sim 3.7$~T, which neutron powder diffraction reveals to correspond to a rotation of the helix plane away from the easy plane, as well as a small change in the propagation vector. Furthermore, we show that the sign of the ferroelectric polarization is reversible in a magnetic field, although no change is observed (or expected on the basis of the magnetic structure) due to the transition at $3.7$~T. We finally justify the temperature dependence of the polarization in both zero-field ordered phases by a symmetry analysis of the free energy expansion.

cond-mat.str-el↗

arXiv2016

From 3D static order to 2D dynamic correlations in the tetragonal double perovskite Sr2CuWO6

\SCWO is a tetragonal ($I4/m$) double perovskite with diamagnetic W$^{6+}$ (5d$^0$) and (Jahn-Teller-active) Cu$^{2+}$ (3$d^9$) in a rocksalt-ordered arrangement on the B sites, modelling an anisotropic fcc-lattice antiferromagnet with $S=1/2$. We have studied the magnetic structure, dynamics and thermodynamic properties of this material above and below the \Neel{}-ordering transition at $T_{\textrm{N}} = 24$~K. Muon spin relaxation spectroscopy, neutron diffraction and neutron spectroscopy experiments show that the transition at $T_{\textrm{N}}$ on heating is from static 3D type-2 antiferromagnetism to 2D dynamic correlations that remain detectable up to 100~K. Above $T_{\textrm{N}}$ the muon relaxation is described by a compressed exponential decay, revealing a strong correlation between the spin fluctuation rate and the 2D correlation length. The low-temperature muon data and the entropy release around $T_{\textrm{N}}$ as obtained from the heat capacity data point to small magnetic domains, probably of the order of 60~Å. We note that the material behaves more like a quasi-2D square lattice antiferromagnet than could be expected considering its structure only. A possible explanation might lie in the unusual strong spin-orbit-coupled magnetism of the W 5$d$ levels, as the dominant exchange interaction is via the Cu-O-W-O-Cu pathway.

cond-mat.str-el↗

arXiv2015

Magnetic Phase Diagram of the Breathing Pyrochlore Antiferromagnet LiGa1-xInxCr4O8

The spinel oxides LiGaCr4O8 and LiInCr4O8 contain size-alternating pyrochlore lattices of spin-3/2 Cr3+ tetrahedra with different magnitudes of alternation. We show here that the solid solutions LiGa1-xInxCr4O8 between these two 'breathing' pyrochlore compounds display (i) rapid suppression of magnetic and structural transitions upon doping the end members, (ii) spin-glass-like freezing above 2 K in the range 0.1 $\lesssim$ x $\lesssim$ 0.6, and (iii) apparent spin-gap behavior for x $\gtrsim$ 0.7. Furthermore, no transitions are observed above 2 K at x ~ 0.9, where magnetic susceptibility remains finite at 2 K and magnetic heat capacity shows a quadratic temperature dependence at 1-5 K. Our work shows that breathing pyrochlore compounds provide a unique opportunity for studying both geometrical frustration and bond alternation.

cond-mat.mtrl-sci↗