arXiv subjects

D. D. Khalyavin

Publications and source records attributed to D. D. Khalyavin.

At least 37 records · Page 2Linked to original sources

arXiv2023

Magnetic inhomogeneities in the quadruple perovskite manganite [Y$_{2-x}$Mn$_x$]MnMnMn$_4$O$_{12}$

A combination of competing exchange interactions and substitutional disorder gives rise to magnetic inhomogeneities in the [Y$_{2-x}$Mn$_x$]MnMnMn$_4$O$_{12}$ $x = 0.23$ and $x = 0.16$ quadruple perovskite manganites. Our neutron powder scattering measurements show that both the $x = 0.23$ and $x = 0.16$ samples separate into two distinct magnetic phases; below T$_{1}$ = 120 $\pm$ 10 K the system undergoes a transition from a paramagnetic phase to a phase characterised by short range antiferromagnetic clusters contained in a paramagnetic matrix, and below T$_{2}$ $\sim$ 65 K, the system is composed of well correlated long range collinear ferrimagnetic order, punctuated by short range antiferromagnetic clusters. A sharp increase in the antiferromagnetic phase fraction is observed below $\sim$ 33 K, concomitant with a decrease in the ferrimagnetic phase fraction. Our results demonstrate that the theoretically proposed AFM phase is stabilised in the [Y$_{2-x}$Mn$_x$]MnMnMn$_4$O$_{12}$ manganites in the presence of dominant B-B exchange interactions, as predicted.

cond-mat.str-el↗

arXiv2022

Hidden magnetic order on a kagome lattice for KV3Sb5

KV3Sb5 has recently attracted a considerable attention, due to its low temperature superconducting properties, which are heralded by a charge density wave. The apparent presence of a very weak magnetism does not result in long range ordering. We propose a model compatible with a detectable internal magnetic field with no evidence of magnetic long-range order. It invokes higher order terms in the vanadium magnetization density compatible with the presence of a "hidden order" of Dirac (polar) multipoles. The Dirac dipole, known as an anapole or toroidal dipole, is one of a family of electronic multipoles visible in x-ray and magnetic neutron diffraction while undetectable with standard laboratory-based techniques. Two plausible hidden orders are studied with a view to testing in future x-ray and neutron Bragg diffraction experiments whether they are trustworthy. One candidate is magneto-electric and restricted to the linear type, while the other candidate cannot show a magneto-electric effect of any type. The latter hosts a vanadium entity that is both a true scalar and magnetic, and its presence epitomizes an absence of loop currents. Diffraction patterns for the two proposed hidden magnetic orders are distinctly different, fortunately. Corresponding scattering amplitudes for resonant x-ray and neutron Bragg diffraction are calculated with standard chemical and magnetic symmetry tools, and atomic physics.

cond-mat.str-el↗

arXiv2021

Magnetic properties of ruthenium dioxide (RuO2) and charge-magnetic interference in Bragg diffraction of circularly polarized x-rays

Rutile-type RuO2 likely supports a simple antiferromagnetic structure which can be verified by x-ray Bragg diffraction. Three magnetic motifs that do not break translation symmetry are explored in calculations of amplitudes suitable for diffraction enhanced by tuning the primary x-ray energy to a ruthenium atomic resonance. Coupling to x-ray helicity through a charge-magnetic interference is common to all motifs, together with magnetic and charge intensities in quadrature in the rotated channel of polarization. Necessary conditions for these diffraction phenomena are a centrosymmetric crystal structure, null magnetic propagation vector, and absence of a linear magnetoelectric effect. Published x-ray diffraction data for RuO2 was analysed by the authors against a magnetic motif that does not satisfy the conditions. A polarized neutron study of antiferromagnetic domains can be achieved with a sample that meets the stated crystal and magnetic symmetries.

cond-mat.str-el↗

arXiv2021

Collinear antiferromagnetic order in URu$_2$Si$_{2-x}$P$_x$ revealed by neutron diffraction

The hidden order phase in URu$_2$Si$_2$ is highly sensitive to electronic doping. A special interest in silicon-to-phosphorus substitution is due to the fact that it may allow one, in part, to isolate the effects of tuning the chemical potential from the complexity of the correlated $f$ and $d$ electronic states. We investigate the new antiferromagnetic phase that is induced in URu$_2$Si$_{2-x}$P$_x$ at $x\gtrsim0.27$. Time-of-flight neutron diffraction of a single crystal ($x=0.28$) reveals $c$-axis collinear $\mathbf{q}_\mathrm{m}=(\frac12,\frac12,\frac12)$ magnetic structure with localized magnetic moments ($\approx2.1\,μ_\mathrm{B}$). This points to an unexpected analogy between the (Si,P) and (Ru,Rh) substitution series. Through further comparisons with other tuning studies of URu$_2$Si$_2$, we are able to delineate the mechanisms by which silicon-to-phosphorus substitution affects the system. In particular, both the localization of itinerant 5$f$ electrons as well as the choice of $\mathbf{q}_m$ appears to be consequences of the increase in chemical potential. Further, enhanced exchange interactions are induced by chemical pressure and lead to magnetic order, in which an increase in inter-layer spacing may play a special role.

cond-mat.str-el↗

arXiv2021

Magnetic order and 5d1 multipoles in a rhenate double perovskite Ba2MgReO6

Structural and magnetic transitions in a double perovskite hosting 5d1 Re ions are discussed on the basis of recently published high-resolution x-ray diffraction patterns [D. Hirai, et al., Phys. Rev. Res. 2, 022063(R) (2020)]. A reported structural transition below room temperature, from cubic to tetragonal symmetry, appears not to be driven by T2g-type quadrupoles, as suggested. A magnetic motif at lower temperature is shown to be composed of two order parameters, associated with propagation vectors k = (0, 0, 1) and k = (0, 0, 0). Findings from our studies, for structural and magnetic properties of Ba2MgReO6, surface in predicted amplitudes for x-ray diffraction at rhenium L2 and L3 absorption edges, and magnetic neutron Bragg diffraction. Specifically, entanglement of anapole and spatial degrees of freedom creates a quadrupole in the neutron scattering amplitude. It would be excluded in an unexpected scenario whereby the rhenium atomic state is a manifold. Also, a chiral signature visible in resonant x-ray diffraction will be one consequence of predicted electronic quadrupole and magnetic dipole orders. A model Re wave function consistent with all current knowledge is a guide to electronic and magnetic multipoles engaged in x-ray and neutron diffraction investigations.

cond-mat.str-el↗

arXiv2021

Dynamic Spin Fluctuations in the Frustrated Spin Chain Compound Li$_3$Cu$_2$SbO$_6$

We report the signatures of dynamic spin fluctuations in the layered honeycomb Li$_3$Cu$_2$SbO$_6$ compound, with a 3$d$ S = 1/2 $d^9$ Cu$^{2+}$ configuration, through muon spin rotation and relaxation ($μ$SR) and neutron scattering studies. Our zero-field (ZF) and longitudinal-field (LF)-$μ$SR results demonstrate the slowing down of the Cu$^{2+}$ spin fluctuations below 4.0 K. The saturation of the ZF relaxation rate at low temperature, together with its weak dependence on the longitudinal field between 0 and 3.2 kG, indicates the presence of dynamic spin fluctuations persisting even at 80 mK without static order. Neutron scattering study reveals the gaped magnetic excitations with three modes at 7.7, 13.5 and 33 meV. Our DFT calculations reveal that the next nearest neighbors (NNN) AFM exchange ($J_{AFM}$ = 31 meV) is stronger than the NN FM exchange ($J_{FM}$ = -21 meV) indicating the importance of the orbital degrees of freedom. Our results suggest that the physics of Li$_3$Cu$_2$SbO$_6$ can be explained by an alternating AFM chain rather than the honeycomb lattice.

cond-mat.str-el↗

arXiv2021

Competing electronic instabilities in the quadruple perovskite manganite PbMn$_{7}$O$_{12}$

Structural behaviour of PbMn$_{7}$O$_{12}$ has been studied by high resolution synchrotron X-ray powder diffraction. This material belongs to a family of quadruple perovskite manganites that exhibit an incommensurate structural modulation associated with an orbital density wave. It has been found that the structural modulation in PbMn$_{7}$O$_{12}$ onsets at 294 K with the incommensurate propagation vector $\mathbf{k}_s=(0,0,\sim2.08)$. At 110 K another structural transition takes place where the propagation vector suddenly drops down to a \emph{quasi}-commensurate value $\mathbf{k}_s=(0,0,2.0060(6))$. The \emph{quasi}-commensurate phase is stable in the temperature range of 40K - 110 K, and below 40 K the propagation vector jumps back to the incommensurate value $\mathbf{k}_s=(0,0,\sim2.06)$. Both low temperature structural transitions are strongly first order with large thermal hysteresis. The orbital density wave in the \emph{quasi}-commensurate phase has been found to be substantially suppressed in comparison with the incommensurate phases, which naturally explains unusual magnetic behaviour recently reported for this perovskite. Analysis of the refined structural parameters revealed that that the presence of the \emph{quasi}-commensurate phase is likely to be associated with a competition between the Pb$^{2+}$ lone electron pair and Mn$^{3+}$ Jahn-Teller instabilities.

cond-mat.str-el↗

arXiv2020

Spontaneous cycloidal order mediating a spin-reorientation transition in a polar metal

We show how complex modulated order can spontaneously emerge when magnetic interactions compete in a metal with polar lattice distortions. Combining neutron and resonant x-ray scattering with symmetry analysis, we reveal that the spin reorientation in Ca$_3$Ru$_2$O$_7$ is mediated by a magnetic cycloid whose eccentricity evolves smoothly but rapidly with temperature. We find the cycloid to be highly sensitive to magnetic fields, which appear to continuously generate higher harmonic modulations. Our results provide a unified picture of the rich magnetic phases of this correlated, multi-band polar metal.

cond-mat.str-el↗

arXiv2020

Diffraction by multipoles in a 5d2 rhenium double perovskite

A recent polarized neutron diffraction experiment on the 5d2 rhenium double perovskite Ba2YReO6 held at a low temperature uncovered weak magnetic diffraction peaks. Data analysis inferred a significantly reduced Re dipole moment, and long-range order compatible with an antiferromagnet, non-collinear motif. To interpret the experimental findings, we present a model wavefunction for Re ions derived from the crystal field potential, Coulomb interaction, and spin-orbit coupling that fully respects the symmetry of the low-temperature ordered state. It is used to calculate in analytic form all multipole moments visible in neutron and resonance enhanced x-ray diffraction. A minimal model consistent with available neutron diffraction data predicts significant multipolar moments up to the hexadecapole, and, in particular, a dominant charge-like quadrupole moment. Calculated diffraction patterns embrace single crystal x-ray diffraction at the Re L-edge, and renewed neutron diffraction, to probe the presumed underlying multipolar order.

cond-mat.str-el↗

arXiv2020

Multipole Orders in a Chain Ferrate Na2FeSe

Fundamental block and staggered orders of magnetic Fe multipoles in Na2FeSe2 are classified by their symmetry and magnetoelectric properties. Our structure model incorporates ferromagnetic or antiferromagnetic coupling between chains. The ferrate salt is valued in studies of highly correlated electrons as the only iron selenide known to possess chain-like structural units hosting ferrous cations. Axial and polar (Dirac) multipoles are compulsory in the electronic structure since Fe ions exhibit enantiomorphic symmetry in the parent K2ZnO2-type compound. Calculated Bragg diffraction patterns for neutrons and x-rays reveal specific contributions from both multipole types.

cond-mat.str-el↗

arXiv2020

Lone octupole and bulk magnetism in osmate 5d2 double perovskites

Cubic double perovskites that host heavy ions with total angular momentum J = 2 can exhibit a singular magnetic state epitomized by a lone octupole and bulk ferro-type magnetism. It exists in the Chen - Balents Hamiltonian with a quadrupole interaction and competing exchange forces between the ions. Our symmetry inspired analysis mirrors the Dzyaloshinskii - Man'ko theory of latent antiferromagnetic ordering, and a 3-k collinear structure. Experimental tests of the singular state include neutron and x-ray Bragg diffraction.

cond-mat.str-el↗

arXiv2020

Spontaneous Rotation of Ferrimagnetism Driven by Antiferromagnetic Spin Canting

Spin-reorientation phase transitions that involve the rotation of a crystal$'$s magnetization have been well characterized in distorted-perovskite oxides such as the orthoferrites. In these systems spin reorientation occurs due to competing rare-earth and transition metal anisotropies coupled via $f$-$d$ exchange. Here, we demonstrate an alternative paradigm for spin reorientation in distorted perovskites. We show that the $R_2\mathrm{CuMnMn_4O_{12}}$ (R = Y or Dy) triple A-site columnar-ordered quadruple perovskites have three ordered magnetic phases and up to two spin-reorientation phase transitions. Unlike the spin-reorientation phenomena in other distorted perovskites, these transitions are independent of rare-earth magnetism, but are instead driven by an instability towards antiferromagnetic spin canting likely originating in frustrated Heisenberg exchange interactions, and the competition between Dzyaloshinskii-Moriya and single-ion anisotropies.

cond-mat.str-el↗

arXiv2019

Fragile ground state and rigid field-induced structures in zigzag ladder compound BaDy2O4

We report on a sequence of field-induced transitions in the zigzag ladder compound BaDy2O4 studied with powder neutron diffraction and magnetisation measurements. In agreement with the previously published results, the low temperature zero-field structure is characterised by two half-integer propagation vectors, k1=[1/2 0 1/2] and k2= 1/2 1/2 1/2]. However, on application of an external magnetic field, the Bragg peaks corresponding to the zero-field structure lose their intensity rather rapidly and disappear completely in a field of 2.5 kOe. In the intermediate fields, 2.5 to 22.5 kOe, new peaks are observed characterised by the propagation vector k0=[0 0 1/3] corresponding to an up-up-down (uud) structure as well k=0 ferromagnetic peaks. This regime of fields corresponds to a pronounced plateau in the magnetisation curve. Remarkably, the uud structure survives heating to at least 1.4 K, three times higher temperature than the TN of 0.48 K for the zero-field structure. Above 22.5 kOe, the k0 peaks disappear while the k = 0 peaks gain significant intensity indicating an increase in the polarisation of the system. The analysis of the intensities of the field-induced reflections allows for a clear identification of the magnetic structures in both the intermediate and high field regimes.

cond-mat.str-el↗

arXiv2019

The magnetic structure and spin-flop transition in the A-site columnar-ordered quadruple perovskite $\mathrm{TmMn_3O_6}$

We present the magnetic structure of $\mathrm{TmMn_3O_6}$, solved via neutron powder diffraction - the first such study of any $R\mathrm{Mn_3O_6}$ A-site columnar-ordered quadruple perovskite to be reported. We demonstrate that long range magnetic order develops below 74 K, and at 28 K a spin-flop transition occurs driven by $f$-$d$ exchange and rare earth single ion anisotropy. In both magnetic phases the magnetic structure may be described as a collinear ferrimagnet, contrary to conventional theories of magnetic order in the manganite perovskites. Instead, we show that these magnetic structures can be understood to arise due to ferro-orbital order, the A, A$'$ and A$''$ site point symmetry, $mm2$, and the dominance of A-B exchange over both A-A and B-B exchange, which together are unique to the $R\mathrm{Mn_3O_6}$ perovskites.

cond-mat.str-el↗

arXiv2018

Field-induced magnetic charge in a cubic Laves compound UAl2

Magnetic diffraction of polarized neutrons by the cubic Laves compound UAl2 in a magnetic field has unveiled weak Bragg spots that are nominally forbidden. On the one hand, they can be viewed as magnetic analogues of the basis-forbidden (2, 2, 2) reflection in diamond-type structures that has been painstakingly and frequently investigated over almost a century. Alternatively, the pattern of weak intensities can be assigned to Dirac multipoles imbedded in field-induced magnetic charge. To this end, a published diffraction pattern is successfully confronted with intensities calculated from the appropriate magnetic space-group that includes Dirac dipoles (anapoles) to describe the basis-forbidden magnetic reflections, and conventional (axial) dipole and octupole multipoles to describe basis-allowed magnetic reflections.

cond-mat.str-el↗

arXiv2018

Direct observation of anapoles by neutron diffraction

The scope of magnetic neutron scattering has been expanded by the observation of electronic Dirac dipoles (anapoles) that are polar (parity-odd) and magnetic (time-odd). A zero-magnetization ferromagnet Sm0.976Gd0.024Al2 with a diamond-type structure presents Dirac multipoles at basis-forbidden reflections that include the standard (2, 2, 2) reflection. Magnetic amplitudes measured at four such reflections are in full accord with a structure factor calculated from the appropriate magnetic space group.

cond-mat.mtrl-sci↗

arXiv2018

Direct observation of electron density reconstruction at the metal-insulator transition in NaOsO3

5d transition metal oxides offer new opportunities to test our understanding of the interplay of correlation effects and spin-orbit interactions in materials in the absence of a single dominant interaction. The subtle balance between solid-state interactions can result in new mechanisms that minimize the interaction energy, and in material properties of potential use for applications. We focus here on the 5d transition metal oxide NaOsO3, a strong candidate for the realization of a magnetically driven transition from a metallic to an insulating state exploiting the so-called Slater mechanism. Experimental results are derived from non-resonant and resonant x-ray single crystal diffraction at the Os L-edges. A change in the crystallographic symmetry does not accompany the metal-insulator transition in the Slater mechanism and, indeed, we find no evidence of such a change in NaOsO3. An equally important experimental observation is the emergence of the (300) Bragg peak in the resonant condition with the onset of magnetic order. The intensity of this space-group forbidden Bragg peak continuously increases with decreasing temperature in line with the square of intensity observed for an allowed magnetic Bragg peak. Our main experimental results, the absence of crystal symmetry breaking and the emergence of a space-group forbidden Bragg peak with developing magnetic order, support the use of the Slater mechanism to interpret the metal-insulator transition in NaOsO3. We successfully describe our experimental results with simulations of the electronic structure and, also, with an atomic model based on the established symmetry of the crystal and magnetic structure.

cond-mat.str-el↗

arXiv2018

The magnetic structures of rare-earth quadruple perovskite manganites $R$Mn$_7$O$_{12}$

We report a neutron powder diffraction study of $R$Mn$_7$O$_{12}$ quadruple perovskite manganites with $R$ = La, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the $A$ and $B$ manganese sublattices occurs on cooling below the N$\mathrm{\acute{e}}$el temperature. The respective magnetic structures are collinear, with one uncompensated Mn$^{3+}$ moment per formula unit as observed in bulk magnetisation measurements. We show that both LaMn$_7$O$_{12}$ and NdMn$_7$O$_{12}$ undergo a second magnetic phase transition at low temperature, which introduces a canting of the $B$ site sublattice moments that is commensurate in LaMn$_7$O$_{12}$ and incommensurate in NdMn$_7$O$_{12}$. This spin canting is consistent with a magnetic instability originating in the $B$ site orbital order. Furthermore, NdMn$_7$O$_{12}$ displays a third magnetic phase transition at which long range ordering of the Nd sublattice modifies the periodicity of the incommensurate spin canting. Our results demonstrate a rich interplay between transition metal magnetism, orbital order, and the crystal lattice, which may be fine tuned by cation substitution and rare earth magnetism.

cond-mat.str-el↗