arXiv subjects

S W Lovesey

Publications and source records attributed to S W Lovesey.

12 recordsLinked to original sources

arXiv2025

Magnetic structure of the kagome metal YbFe6Ge6 in view of Bragg diffraction

A material in possession of localized 4f-electron magnetism and delocalized 3d-electron or band magnetism can often present A material in possession of localized 4f-electron magnetism and delocalized 3d-electron or band magnetism can often present enigmatic physical phenomena, and there has been a longstanding interest in the kagome metal YbFe6Ge6. More recently, because of an investigation of a so-called anomalous Hall effect, or topological Hall effect, and magnetic neutron Bragg diffraction [W. Yao et al., Phys. Rev. Lett. 134, 186501 (2025)]. Iron moments in the two-dimensional layers of a hexagonal nuclear structure undergo collinear antiferromagnetic order below a temperature 500 K. The moments depart from the c axis in a spontaneous transition at 63 K to an orthorhombic structure. The magnetism of Yb ions appears to behave independently, which can be confirmed using resonant x-ray diffraction enhanced by a Fe atomic resonance. The inferred magnetic space group is a P(parity)T(time)-symmetric (anti-inversion collinear antiferromagnet. A linear magnetoelectric effect is allowed, as in historically important chromium sesquioxide, and Kerr rotation and the piezomagnetic effect are forbidden. Symmetry informed Bragg diffraction patterns for future x-ray and neutron experiments are shown to be rich in Fe magnetic properties of orthorhombic YbFe6Ge6, including space-spin correlations, anapoles and Dirac quadrupoles familiar in high-Tc ceramic superconductors.

cond-mat.str-el↗

arXiv2025

Double perovskite Ba2NaOsO6: Magnetic structure, Kramers doublet and x-ray magnetic dichroic signals

Of all the oxides of osmium, Ba2NaOsO6 is the only one with a substantial ferromagnetic moment. A fresh theoretical study of bulk magnetic properties of the double perovskite is accomplished with an appropriate space group, and an atomic model of a Kramers doublet formed by the single osmium d-electron. It possesses symmetry demanded by the Wyckoff position assigned to Os in a canted ferromagnet. Whereupon, magnetic moments in the paramagnetic and magnetically ordered states of Ba2NaOsO6 are unified. Analytic x-ray magnetic dichroic signals (XMCD) at osmium L2 and L3 absorption edges are presented, and shown to agree with hitherto unexplained recent measurements.

cond-mat.str-el↗

arXiv2022

Polar magnetism and chemical bond in alpha-RuCl3

Ruthenium trichloride is the subject of many recent experimental and theoretical studies. Johnson et al. infer a candidate for the magnetic structure (Cc2/m) from results gathered in an extensive set of experiments on an untwined sample of alpha-RuCl3 [R. D. Johnson et al., Phys. Rev. B 92, 235119 (2015)]. The proposed zigzag antiferromagnetic ground state of Ru ions does not respond to bulk magnetic probes. Properties of the candidate magnetic structure not previously explored include polar magnetism that supports Ru Dirac multipoles, e.g., a ruthenium anapole. In a general case, Dirac dipoles are capable of generating interactions between magnetic ions, as in an electrical Dzyaloshinskii-Moryia interaction [T. A. Kaplan and S. D. Mahanti, Phys. Rev. B 83, 174432 (2011) and H. J. Zhao et al., Nat. Mat. 20, 341 (2021)]. Dirac multipoles contribute to the diffraction of both x-rays and neutrons, and a stringent test of the magnetic structure Cc2/m awaits future experiments. From symmetry-informed calculations we show that, the magnetic candidate permits Bragg spots that arise solely from Dirac multipoles. Stringent tests of Cc2/m can also be accomplished by performing resonant x-ray diffraction with signal enhancement from the chlorine K-edge. X-ray absorption spectra published for alpha-RuCl3 possess a significant low-energy feature [K. W. Plumb et al., Phys. Rev. B 90, 041112(R) (2014)]. Many experimental studies of other Cl-metal compounds concluded that identical features hallmark the chemical bond. Using a monoclinic Cc2/m structure, we predict the contribution to Bragg diffraction at the Cl K-edge absorption. Specifically, the variation of intensity of Bragg spots with rotation of the sample about the reflection vector. The two principal topics of our studies, polar magnetism and the chemical bond in alpha-RuCl3, are brought together in a minimal model of magnetic Ru ions in Cc2/m.

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arXiv2019

Anapole Correlations in Sr2IrO4 Defy the jeff = 1/2 Model

Zel'dovich (spin) anapole correlations in Sr2IrO4 unveiled by magnetic neutron diffraction contravene the spin-orbit coupled ground state used by the jeff = 1/2 (pseudo-spin) model. Specifically, spin and space know inextricable knots which bind each to the other in the iridate. The diffraction property studied in the Letter is enforced by strict requirements from quantum mechanics and magnetic symmetry. It has not been exploited in the past, whereas neutron diffraction by anapole moments is established. Entanglement of the electronic degrees of freedom is captured by binary correlations of the anapole and position operators, and hallmarked in the diffraction amplitude by axial atomic multipoles with an even rank.

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arXiv2019

Magnetic Multipoles in a Ruthenate Ca3Ru2O7

Compulsory Dirac multipoles in the bilayer perovskite Ca3Ru2O7 are absent in published analyses of experimental data. In a first step at correcting knowledge of the magnetic structure, we have analysed existing Bragg diffraction patterns gathered on samples held well below the Néel temperature at which A-type antiferromagnetic order of axial dipoles spontaneously develops. Patterns were gathered with neutrons, and linearly polarized x-rays tuned in energy to a ruthenium atomic resonance. Neutron diffraction data contains solid evidence of Dirac dipoles (anapoles or toroidal moments). No such conclusion is reached with existing x-ray diffraction data, which instead is ambiguous on the question. To address this shortcoming by future experiments, we calculated additional diffraction patterns. Chiral order of Dirac multipoles is allowed by magnetic space-group PCna21, and it can be exposed in Bragg diffraction using circularly polarized x-rays. Likewise, a similar experiment can expose a chiral order of axial dipoles. A magnetic field applied parallel to the b-axis creates a ferrimagnetic structure in which bulk magnetization arises from field-induced nonequivalent Ru sites (magnetic space-group Pm'c'21).

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arXiv2017

Magnetoelectric properties of the layered room-temperature antiferromagnets BaMn2P2 and BaMn2As2

Properties of two ThCr2Si2-type materials are discussed within the context of their established structural and magnetic symmetries. Both materials develop collinear, G-type antiferromagnetic order above room temperature, and magnetic ions occupy acentric sites in centrosymmetric structures. We refute a previous conjecture that BaMn2As2 is an example of a magnetoelectric material with hexadecapole order by exposing flaws in supporting arguments, principally, an omission of discrete symmetries enforced by the symmetry of sites used by Mn ions and, also, improper classifications of the primary and secondary order-parameters. Implications for future experiments designed to improve our understanding of BaMn2P2 and BaMn2As2 magnetoelectric properties, using neutron and x-ray diffraction, are examined. Patterns of Bragg spots caused by conventional magnetic dipoles and magnetoelectric (Dirac) multipoles are predicted to be distinct, which raises the intriguing possibility of a unique and comprehensive examination of the magnetoelectric state by diffraction. A roto-inversion operation in Mn site symmetry is ultimately responsible for the distinguishing features.

cond-mat.str-el↗

arXiv2017

Electronic and magnetic properties of multiferroic ScFeO3

Experimental techniques using Bragg diffraction may elucidate electronic and magnetic properties of ferric ions in multiferroic ScFeO3 that are puzzling. A collinear magnetic motif of dipole moments is not expected for s-state ions with an acentric parent (paramagnetic) structure, because single-site anisotropy is negligible and antisymmetric exchange interactions promote an inhomogeneous magnetic motif. However, current indications are that ScFeO3 supports G-type antiferromagnetism, with an unknown moment direction, which implies that ferric ions are not in the high-spin 6S state. It is argued that significant insight on these open questions can be achieved by confronting experimental data with calculated structure factors for resonant x-ray and magnetic neutron Bragg spots, and we report results for G-type antiferromagnetism allowed by the corundum-like parent R3c structure. Calculated structure factors include Dirac multipoles that are time-odd and parity-odd, e.g., dipoles that are often called anapoles or toroidal moments.

cond-mat.str-el↗

arXiv2014

Anapoles and other magneto-electric multipoles in BaFe2Se3

We submit that the magnetic space-group Cac (#9.41) is consistent with the established magnetic structure of BaFe2Se3, with magnetic dipole moments in a motif that uses two ladders [Caron J M et al 2011 Phys. Rev. B 84 180409(R)]. The corresponding crystal class m1' allows axial and polar dipoles and forbids bulk ferromagnetism. The compound supports magneto-electric multipoles, including a magnetic charge (monopole) and an anapole (magnetic toroidal dipole) visible in the Bragg diffraction of x-rays and neutrons. Our comprehensive simulation of neutron Bragg diffraction by BaFe2Se3 exploits expressions of a general nature that can be of use with other magnetic materials. Electric toroidal moments are also allowed in BaFe2Se3. A discussion of our findings for resonant x-ray Bragg diffraction illustrates the benefit of a common platform for neutron and x-ray diffraction.

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arXiv2014

Strange magnetic multipoles and neutron diffraction by an iridate perovskite (Sr2IrO4)

A theoretical investigation of a plausible construct for electronic structure in iridate perovskites demonstrates the existence of magnetic multipoles hitherto not identified. The strange multipoles, which are parity-even, time-odd and even rank tensors, are absent from the so-called jeff = 1/2 model. We prove that the strange multipoles contribute to magnetic neutron diffraction, and we estimate their contribution to intensities of Bragg spots for Sr2IrO4. The construct encompasses the jeff = 1/2 model, and it is consistent with the known magnetic structure, ordered magnetic moment, and published resonant x-ray Bragg diffraction data. Over and above time-odd quadrupoles and hexadecapoles, whose contribution changes neutron Bragg intensities by an order of magnitude, according to our estimates, are relatively small triakontadipoles recently proposed as the primary magnetic order-parameter of Sr2IrO4.

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arXiv2014

Magneto-electric operators in neutron scattering

We succeed in deriving an exact expression for the magnetic interaction of neutrons and electrons including magneto-electric operators, allowed in the absence of a centre of inversion symmetry. Central characters are a spin anapole and an orbital (toroidal) analogue, in addition to familiar parity-even operators. A simulation of neutron diffraction by antiferromagnetic copper oxide makes full use of information inferred from a thorough investigation with resonant x-ray Bragg diffraction.

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arXiv2013

X-ray diffraction by magnetic charges (monopoles)

Magnetic charges, or magnetic monopoles, may form in the electronic structure of magnetic materials where ions are deprived of symmetry with respect to spatial inversion. Predicted in 2009, the strange magnetic, pseudoscalars have recently been found different from zero in simulations of electronic structures of some magnetically ordered, orthorhombic, lithium orthophosphates (LiMPO4). We prove that magnetic charges in lithium orthophosphates diffract x-rays tuned in energy to an atomic resonance, and to guide future experiments we calculate appropriate unit-cell structure factors for monoclinic LiCoPO4 and orthorhombic LiNiPO4.

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arXiv2013

Melting of chiral order in terbium manganate (TbMnO3) observed with resonant x-ray Bragg diffraction

Resonant Bragg diffraction of soft, circularly polarized x-rays has been used to observe directly the temperature dependence of chiral-order melting in a motif of Mn ions in terbium manganate. The underlying mechanism uses the b-axis component of a cycloid, which vanishes outside the polar phase. Melting is witnessed by the first and second harmonics of a cycloid, and we explain why the observed temperature dependence is different in the two harmonics. Our direct observation of melting is supported by a solid foundation of evidence, derived from extensive studies of the azimuthal-angle dependence of intensities with both linear and circular polarization.

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