arXiv subjects

T. Lancaster

Publications and source records attributed to T. Lancaster.

At least 19 recordsLinked to original sources

arXiv2026

Local magnetic properties of the rare-earth intermetallics $R$Mn$_2$Ge$_2$ ($R$=Ce, Pr, Nd)

We present an investigation of the rare-earth intermetallic materials, $R$Mn$_2$Ge$_2$ ($R$=Ce, Pr, Nd), reported to host a lattice of skyrmionic bubbles at room temperature. The magnetism of all three materials is characterised by the onset of local fluctuations as the magnetic state changes from a conical magnetic structure to a collinear antiferromagnetic one as temperature $T$ is increased. In this $T$ regime, where skyrmion-bubble textures have been reported, we see dynamics similar to those observed in other skyrmion-hosting materials. The level of disorder above this transition increases across the series from $R$=Ce to Nd. At low temperatures the magnetism is affected by the ordering of the rare-earth ions, resulting in distinct behaviour for the different members of the series.

cond-mat.str-el↗

arXiv2025

Heterometallic spin-1/2 quantum magnet under hydrostatic pressure

We investigate the properties of CuVOF$_4$(H$_2$O)$_6$$\cdot$H$_2$O, in which two different spin species, Cu(II) and V(IV), form antiferromagnetic spin-1/2 dimers with weak interdimer coupling provided via hydrogen bonding. Using radio-frequency susceptometry and electron-spin resonance (ESR), we show how the temperature-magnetic field spin-dimer phase diagram evolves as a function of applied hydrostatic pressure and correlate this with pressure-induced changes to the crystal structure. These results, coupled with pressure-tuned DFT calculations, confirm the prior prediction that the primary exchange interaction is mediated via an unusual mechanism in which the V(IV) ions provide considerable spin density to the oxygen that joins the two spins in each dimer and which lies along the Jahn-Teller axis of the Cu(II) ion. In addition, the dissimilarity in the spins that make up each dimer unit leads to a non-linear field dependence of the electronic energy levels as detected in the ESR measurements.

cond-mat.str-el↗

arXiv2025

Single-ion anisotropy driven chiral magnetic order in a spin-1 antiferromagnetic chain

Chirality in magnetic systems gives rise to a wide range of exotic phenomena, yet its influence in $S=1$ chains remains largely unexplored. Here, we present a comprehensive experimental study of a chiral antiferromagnetic (AFM) $S=1$ chain, [Ni(pym)(H$_{2}$O)$_{4}$]SO$_{4} \cdot$ H$_{2}$O (pym = pyrimidine), where the Ni(II) octahedral orientation exhibits a four-fold chiral periodicity. Muon spin rotation measurements indicate the onset of long-range magnetic order below $T_{\rm N} = 1.82(2)\,\mathrm{K}$. Neutron diffraction measurements reveal a chiral AFM order driven by a chiral modulation of the easy-axis anisotropy direction, rather than the typical scenario of Dzyaloshinskii-Moriya interactions, geometrical frustration or higher-order interactions. Inelastic neutron scattering (INS) measurements reveal dispersive spin-wave excitations well described by linear spin-wave theory, with Hamiltonian parameters $J_{0} = 6.81(1)\,\mathrm{K}$ (intrachain exchange), $J'_{1\rm a} = -0.091(1)\,\mathrm{K}$ (interchain exchange), and $D = -3.02(1)\,\mathrm{K}$ (easy-axis single-ion anisotropy). These parameters are further validated by Monte Carlo simulations of the magnetisation. Additionally, the INS data reveal multiple dispersionless bands, suggesting the presence of further excitations beyond the scope of our linear spin-wave theory.

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arXiv2025

Magnetism in $M_{1/3}$NbS$_2$ ($M$ = Fe, V, Mn): insight into intercalated transition-metal dichalcogenides using $\mu$SR

We present the results of muon-spin relaxation ($\mu$SR) measurements of the static and dynamic magnetism of $M_{1/3}$NbS$_2$ ($M$ = Fe, V, Mn), three intercalated transition-metal dichalcogenides. Transitions to long-range magnetic order are observed in all three materials and local magnetic fields at muon sites are compared to dipole field calculations. Measurements on Fe$_{1/3}$NbS$_2$ capture the evolution of two coexisting magnetic phases. In V$_{1/3}$NbS$_2$ we observe a peak in the dynamic response at $9$ K, coincident with previous reports of a possible low-temperature phase transition. The observation of high-frequency muon precession in Mn$_{1/3}$NbS$_2$ suggests the existence of an additional muon site that implies a difference in electronic energy landscape compared to the other materials in the series. Taken together, this demonstrates that the change in intercalant species drives significant variations in magnetism, highlighting the $M_{1/3}$NbS$_2$ ($M$ = Fe, V, Mn) series as an ideal group of materials for investigating a wide range of magnetic phenomena.

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arXiv2024

Magnetic structure and crystal field states of antiferromagnetic CeNiGe$_3$: Neutron scattering and $\mu$SR investigations

We present the results of microscopic investigations of antiferromagnetic CeNiGe$_3$, using neutron powder diffraction (NPD), inelastic neutron scattering (INS), and muon spin relaxation ($\mu$SR) measurements. CeNiGe$_3$ crystallizes in a centrosymmetric orthorhombic crystal structure (space group: $Cmmm$) and undergoes antiferromagnetic (AFM) ordering. The occurrence of long-range AFM ordering at $T_{\rm N} \approx 5.2$~K is confirmed by magnetic susceptibility, heat capacity, neutron diffraction, and $\mu$SR measurements. The NPD data characterize the AFM state with an incommensurate helical magnetic structure having a propagation vector $k$ = (0, 0.41, 1/2). In addition, INS measurements at 10~K identified two crystal electric field (CEF) excitations at 9.17~meV and 18.42~meV. We analyzed the INS data using a CEF model for an orthorhombic environment of Ce$^{3+}$ ($J=5/2$) and determined the CEF parameters and ground state wavefunctions of CeNiGe$_3$. Moreover, zero-field $\mu$SR data for CeNiGe$_3$ at $T< T_{\rm N}$ show long-range AFM ordering with three distinct oscillation frequencies corresponding to three different internal fields at the muon sites. The internal fields at the muon-stopping sites have been further investigated using density functional theory calculations.

cond-mat.str-el↗

arXiv2024

Magnetic properties of a staggered $S=1$ chain Ni(pym)(H$_{2}$O)$_{2}$(NO$_{3}$)$_{2}$ with an alternating single-ion anisotropy direction

Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states which are sensitive to the single-ion-anisotropy (SIA) energy ($D$), and intrachain ($J_{0}$) and interchain ($J'_{i}$) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of Ni(pyrimidine)(H$_{2}$O)$_{2}$(NO$_{3}$)$_{2}$, a chain compound where the tilting of Ni octahedra leads to a 2-fold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at $T_{\text{N}}=2.3$\,K and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find $J_{0} = 5.107(7)$\,K, $D = 2.79(1)$\,K, $J'_{2}=0.18(3)$\,K and a rhombic anisotropy energy $E=0.19(9)$\,K. Mean-field modelling reveals that the ground state structure hosts spin canting of $\phi\approx6.5^{\circ}$, which is not detectable above the noise floor of the elastic neutron diffraction data. Monte-Carlo simulation of the powder-averaged magnetization, $M(H)$, is then used to confirm these Hamiltonian parameters, while single-crystal $M(H)$ simulations provide insight into features observed in the data.

cond-mat.str-el↗

arXiv2024

Muon spectroscopy investigation of anomalous dynamic magnetism in NiI$_2$

We present the results of muon-spin relaxation ($\mu^{+}$SR) measurements of the van der Waals magnet NiI$_2$, which probe magnetic phase transitions at $T_{\mathrm{N1}}=73$~K and $T_{\mathrm{N2}}=60$~K. Supporting density functional theory (DFT) calculations allow the determination of a single muon stopping site whose magnetic environment is consistent with the proposed ground-state magnetic structure. $\mu^{+}$SR measurements of the dynamics reveal behavior consistent with spin-wave excitations below $T_{\mathrm{N2}}$. In the region $T_{\mathrm{N2}}<T<T_{\mathrm{N1}}$ the character of the dynamics changes qualitatively, resulting in an unusual region of temperature-independent fluctuations.

cond-mat.str-el↗

arXiv2024

Pseudo-easy-axis anisotropy in antiferromagnetic $S=1$ diamond-lattice systems Ni$X_{2}$(pym)$_{2}$

We investigate the magnetic properties of $S=1$ antiferromagnetic diamond lattice, Ni$X_{2}$(pyrimidine)$_{2}$ ($X$ = Cl, Br), hosting a single-ion anisotropy (SIA) orientation which alternates between neighbouring sites. Through neutron diffraction measurements of the $X$ = Cl compound, the ordered state spins are found to align collinearly along a pseudo-easy-axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseudo-easy-axis is also present for $X$ = Br. We estimate the Hamiltonian parameters by combining analytical calculations and Monte-Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseudo-easy-axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseudo-easy-axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields.

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arXiv2024

Field-orientation-dependent magnetic phases in GdRu$_2$Si$_2$ probed with muon-spin spectroscopy

Centrosymmetric GdRu$_2$Si$_2$ exhibits a variety of multi-Q magnetic states as a function of temperature and applied magnetic field, including a square skyrmion-lattice phase. The material's behavior is strongly dependent on the direction of the applied field, with different phase diagrams resulting for fields applied parallel or perpendicular to the crystallographic $c$ axis. Here, we present the results of muon-spin relaxation ($\mu^+$SR) measurements on single crystals of GdRu$_2$Si$_2$. Our analysis is based on the computation of muon stopping sites and consideration of zero-point motion effects, allowing direct comparison with the underlying spin textures in the material. The muon site is confirmed experimentally, using angle-dependent measurements of the muon Knight shift. Using transverse-field $\mu^+$SR with fields applied along either the [001] or [100] crystallographic directions, we distinguish between the magnetic phases in this system via their distinct muon response, providing additional evidence for the skyrmion and meron-lattice phases, while also suggesting the existence of RKKY-driven muon hyperfine coupling. Zero-field $\mu^+$SR provides clear evidence for a transition between two distinct magnetically-ordered phases at 39 K.

cond-mat.str-el↗

arXiv2023

Anisotropic skyrmion and multi-$q$ spin dynamics in centrosymmetric Gd$_2$PdSi$_3$

Skyrmions are particle-like vortices of magnetization with non-trivial topology, which are usually stabilized by Dzyaloshinskii-Moriya interactions (DMI) in noncentrosymmetric bulk materials. Exceptions are centrosymmetric Gd- and Eu-based skyrmion-lattice (SkL) hosts with zero DMI, where both the SkL stabilization mechanisms and magnetic ground states remain controversial. We address these here by investigating both the static and dynamical spin properties of the centrosymmetric SkL host Gd$_2$PdSi$_3$ using muon spectroscopy ($\mu$SR). We find that spin fluctuations in the non-coplanar SkL phase are highly anisotropic, implying that spin anisotropy plays a prominent role in stabilizing this phase. We also observe strongly-anisotropic spin dynamics in the ground-state (IC-1) incommensurate magnetic phase of the material, indicating that it hosts a meron-like multi-$q$ structure. In contrast, the higher-field, coplanar IC-2 phase is found to be single-$q$ with nearly-isotropic spin dynamics.

cond-mat.str-el↗

arXiv2023

Unusual magnetism of the axion-insulator candidate Eu$_5$In$_2$Sb$_6$

Eu$_5$In$_2$Sb$_6$ is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of Eu$_5$In$_2$Sb$_6$ single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic X-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of Eu$_5$In$_2$Sb$_6$ results in coplanar noncollinear magnetic structure with an Ising-like net magnetization along the $a$ axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-$q$ structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.

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arXiv2023

Quantum spin liquids

A glance at recent research on magnetism turns up a curious set of articles discussing, or claiming evidence for, a state of matter called a quantum spin liquid (QSL). These articles are notable in their invocation of exotic notions of topological physics, quantum entanglement, fractional quantum numbers, anyon statistics and gauge field theories. So what is a QSL and why do we need this complicated technical vocabulary to describe it? Our aim in this article is to introduce some of these concepts and provide a discussion of what a QSL is, where it might occur in Nature and why it is of interest. As we'll see, this is a rich subject which is still in development, and unambiguous evidence for the realisation of the QSL state in a magnetic material remains hotly debated. However, the payoff in terms of the special nature of quantum entanglement in the QSL, and its diverse spectrum of unusual excitations and topological status will (at least to some extent) justify the need to engage with some powerful, occasionally abstract, technical material.

cond-mat.str-el↗

arXiv2023

Asymmetric phase diagram and dimensional crossover in a system of spin-1/2 dimers under applied hydrostatic pressure

We present the magnetic and structural properties of [Cu(pyrazine)$_{0.5}$(glycine)]ClO$_4$ under applied pressure. As previously reported, at ambient pressure this material consists of quasi-two-dimensional layers of weakly coupled antiferromagnetic dimers which undergo Bose-Einstein condensation of triplet excitations between two magnetic field-induced quantum critical points (QCPs). The molecular building blocks from which the compound is constructed give rise to exchange strengths that are considerably lower than those found in other $S = 1/2$ dimer materials, which allows us to determine the pressure evolution of the entire field-temperature magnetic phase diagram using radio-frequency magnetometry. We find that a distinct phase emerges above the upper field-induced transition at elevated pressures and also show that an additional QCP is induced at zero-field at a critical pressure of $p_{\rm c} = 15.7(5)$ kbar. Pressure-dependent single-crystal X-ray diffraction and density functional theory calculations indicate that this QCP arises primarily from a dimensional crossover driven by an increase in the interdimer interactions between the planes. While the effect of quantum fluctuations on the lower field-induced transition is enhanced with applied pressure, quantum Monte Carlo calculations suggest that this alone cannot explain an unconventional asymmetry that develops in the phase diagram.

cond-mat.str-el↗

arXiv2023

DFT+{\mu}: Density Functional Theory for Muon Site Determination

The technique of muon spin rotation ({\mu}SR) has emerged in the last few decades as one of the most powerful methods of obtaining local magnetic information. To make the technique fully quantitative, it is necessary to have an accurate estimate of where inside the crystal structure the muon implants. This can be provided by density functional theory calculations using an approach that is termed DFT+{\mu}, density functional theory with the implanted muon included. This article reviews this approach, describes some recent successes in particular {\mu}SR experiments, and suggests some avenues for future exploration.

cond-mat.mtrl-sci↗

arXiv2023

Band-filling-controlled magnetism from transition metal intercalation in $N_{1/3}$NbS$_2$ revealed with first-principles calculations

We present a first-principles study of the effect of 3$d$ transition metal intercalation on the magnetic properties of the 2H-NbS$_2$ system, using spin-resolved density functional theory calculations to investigate the electronic structure of $N_{1/3}$NbS$_2$ ($N$ = Ti, V, Cr, Mn, Fe, Co, Ni). We are able to accurately determine the magnetic moments and crystal field splitting, and find that the magnetic properties of the materials are determined by a mechanism based on filling rigid bands with electrons from the intercalant. We predict the dominant magnetic interaction of these materials by considering Fermi surface nesting, finding agreement with experiment where data are available.

cond-mat.mtrl-sci↗

arXiv2023

First-principles calculations of magnetic states in pyrochlores using a source-corrected exchange and correlation functional

We present a first-principles investigation of the spin-ice state in Dy$_2$Ti$_2$O$_7$ using a magnetic source-free exchange and correlation functional, implemented in the Castep electronic-structure code. By comparing results from the conventional local spin-density approximation, we show that a spin-ice state in Dy$_2$Ti$_2$O$_7$ can be reliably obtained by removing the magnetic sources from the exchange and correlation contributions to the potential, and we contrast this against the computed ground states of other frustrated pyrochlore magnets.

cond-mat.mtrl-sci↗

arXiv2022

Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting multilayer

Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known about the depth dependence of the magnetism. We address this by reporting the results of circular dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin rotation (LE-$\mu^+$SR) measurements on a stack comprising [Ta/CoFeB/MgO]$_{16}$/Ta on a Si substrate. Energy-dependent CD-REXS shows a continuous, monotonic evolution of the domain-wall helicity angle with incident energy, consistent with a three-dimensional hybrid domain-wall-like structure that changes from N\'eel-like near the surface to Bloch-like deeper within the sample. LE-$\mu^+$SR reveals that the magnetic field distribution in the trilayers near the surface of the stack is distinct from that in trilayers deeper within the sample. Our micromagnetic simulations support a quantitative analysis of the $\mu^+$SR results. By increasing the applied magnetic field, we find a reduction in the volume occupied by domain walls at all depths, consistent with a crossover into a region dominated by skyrmions above approximately 180 mT.

cond-mat.str-el↗

arXiv2022

Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet

We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the molecular-based bulk material [Cu(pz)$_2$(2-HOpy)$_2$](PF$_6$)$_2$. Due to the moderate intralayer exchange coupling of $J/k_\mathrm{B} = 6.8$ K, the application of laboratory magnetic fields induces a substantial $XY$ anisotropy of the spin correlations. Crucially, this provides a significant BKT regime, as the tiny interlayer exchange $J^\prime / k_\mathrm{B} \approx 1$ mK only induces 3D correlations upon close approach to the BKT transition with its exponential growth in the spin-correlation length. We employ nuclear magnetic resonance and $\mu^{+}$SR measurements to probe the spin correlations that determine the critical temperatures of the BKT transition as well as that of the onset of long-range order. Further, we perform stochastic series expansion quantum Monte Carlo simulations based on the experimentally determined model parameters. Finite-size scaling of the in-plane spin stiffness yields excellent agreement of critical temperatures between theory and experiment, providing clear evidence that the nonmonotonic magnetic phase diagram of [Cu(pz)$_2$(2-HOpy)$_2$](PF$_6$)$_2$ is determined by the field-tuned $XY$ anisotropy and the concomitant BKT physics.

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