arXiv subjects

D. D. Khalyavin

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

At least 73 records · Page 4Linked to original sources

arXiv2014

Symmetry of re-entrant tetragonal phase in Ba1-xNaxFe2As2: Magnetic versus orbital ordering mechanism

Magneto-structural phase transitions in Ba1-xAxFe2As2 (A = K, Na) materials are discussed for both magnetically and orbitally driven mechanisms, using a symmetry analysis formulated within the Landau theory of phase transitions. Both mechanisms predict identical orthorhombic space-group symmetries for the nematic and magnetic phases observed over much of the phase diagram, but they predict different tetragonal space-group symmetries for the newly discovered re-entrant tetragonal phase in Ba1-xNaxFe2As2 (x ~ 0.24-0.28). In a magnetic scenario, magnetic order with moments along the c-axis, as found experimentally, does not allow any type of orbital order, but in an orbital scenario, we have determined two possible orbital patterns, specified by P4/mnc1' and I4221' space groups, which do not require atomic displacements relative to the parent I4/mmm1' symmetry and, in consequence, are indistinguishable in conventional diffraction experiments. We demonstrate that the three possible space groups are however, distinct in resonant X-ray Bragg diffraction patterns created by Templeton & Templeton scattering. This provides an experimental method of distinguishing between magnetic and orbital models.

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arXiv2014

Polar and antipolar polymorphs of metastable perovskite BiFe0.5Sc0.5O3

A metastable perovskite BiFe0.5Sc0.5O3 synthesized under high-pressure (6 GPa) and high- temperature (1500 K) conditions was obtained in two different polymorphs, antipolar Pnma and polar Ima2, through an irreversible behaviour under a heating/cooling thermal cycling. The Ima2 phase represents an original type of a canted ferroelectric structure where Bi3+ cations exhibit both polar and antipolar displacements along the orthogonal [110]p and [1-10]p pseudocubic directions, respectively, and are combined with antiphase octahedral tilting about the polar axis. Both the Pnma and Ima2 structural modifications exhibit a long-range antiferromagnetic ordering with a weak-ferromagnetic component below TN ~ 220 K. Analysis of the coupling between the dipole, magnetic and elastic order parameters based on a general phenomenological approach revealed that the weak-ferromagnetism in both phases is mainly caused by the presence of the antiphase octahe- dral tilting whose axial nature directly represents the relevant part of Dzyaloshinskii vector. The magnetoelectric contribution to the spontaneous magnetization allowed in the polar Ima2 phase is described by a fifth-degree free-energy invariant and is expected to be small.

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arXiv2013

Magnetic ordering with reduced cerium moments in hole-doped CeOs2Al10

The lightly hole-doped system CeOs1.94Re0.06Al10 has been studied by muon spin relaxation and neutron diffraction measurements. A long-range antiferromagnetic ordering of the Ce-sublattice with substantially reduced value of the magnetic moment 0.18(1) μ_B has been found below T_N = 21 K. Similar to the undoped parent compound, the magnetic ground state of CeOs1.94Re0.06Al10 preserves the anomalous direction of the ordered moments along the c-axis. The obtained result reveals the crucial difference between electron- and hole-doping effects on the magnetic ordering in CeOs2Al10. The former suppresses the anisotropic c-f hybridization and promotes localized Ce moments. On the contrary, the latter increases the hybridization and shifts the system towards delocalized non-magnetic state.

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arXiv2013

Change of magnetic ground state by light electron-doping in CeOs2Al10

The effect of Ir substitution for Os in CeOs2Al10, with an unusually high Neel temperature of T~28.5K, has been studied by high-resolution neutron diffraction and magnetization measurements. A small amount of Ir (~ 8%) results in a pronounced change of the magnetic structure of the Ce-sublattice. The new magnetic ground state is controlled by the single ion anisotropy and implies antiferromagnetic arrangement of the Ce-moments along the a-axis, as expected from the anisotropy of the paramagnetic susceptibility. The value of the ordered moments, 0.92(1) mu_B, is substantially bigger than in the undoped compound, whereas the transition temperature is reduced down to 21K. A comparison of the observed phenomena with the recently studied CeRu1.9Rh0.1Al10 system, exhibiting similar behaviour [A. Kondo et al., J. Phys. Soc. Jpn. 82, 054709 (2013)], strongly suggests the electron doping as the main origin of the ground state changes. This provides a new way of exploring the anomalous magnetic properties of the Ce(Ru/Os)2Al10 compounds.

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arXiv2013

Highly frustrated magnetism in SrHo2O4: Coexistence of two types of short-range order

SrHo2O4 is a geometrically frustrated magnet in which the magnetic Ho3+ ions form honeycomb layers connected through a network of zigzag chains. At low-temperature two distinct types of short-range magnetic order can be inferred from single crystal diffraction data, collected using both polarized and unpolarized neutrons. In the (hk0) plane the diffuse scattering is most noticeable around the k = 0 positions and its intensity rapidly increases at temperatures below 0.7 K. In addition, planes of diffuse scattering at Q = (hk +/- l/2) are visible at temperatures as high as 4.5 K. These planes coexist with the broad peaks of diffuse scattering in the (hk0) plane at low-temperatures. Correlation lengths associated with the broad peaks are L ~ 150 Å in the a-b plane and L ~ 190 Å along c axis, while the correlation length associated with the diffuse scattering planes is L ~ 230 Å along the c axis at the lowest temperature. Both types of diffuse scattering are elastic in nature. The highly unusual coexistance of the two types of diffuse scattering in SrHo2O4 is likely to be the result of the presence of two crystallographically inequivalent sites for Ho3+ in the unit cell.

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arXiv2013

Field-induced long-range magnetic order in the spin-singlet ground state system YbAl3C3: Neutron diffraction study

The $4f$-electron system YbAl$_3$C$_3$ with a non-magnetic spin-dimer ground state has been studied by neutron diffraction in an applied magnetic field. A long-range magnetic order involving both ferromagnetic and antiferromagnetic components has been revealed above the critical field H$_C\sim $ 6T at temperature T=0.05K. The magnetic structure indicates that the geometrical frustration of the prototype hexagonal lattice is not fully relieved in the low-temperature orthorhombic phase. The suppression of magnetic ordering by the remanent frustration is the key factor stabilizing the non-magnetic singlet ground state in zero field. Temperature dependent measurements in the applied field H=12T revealed that the long-range ordering persists up to temperatures significantly higher than the spin gap indicating that this phase is not directly related to the singlet-triplet excitation. Combining our neutron diffraction results with the previously published phase diagram, we support the existence of an intermediate disordered phase as the first excitation from the non-magnetic singlet ground state. Based on our results, we propose YbAl$_3$C$_3$ as a new material for studying the quantum phase transitions of heavy-fermion metals under the influence of geometrical frustration.

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arXiv2012

A possible chiral spin-liquid phase in non-centrosymmetric $R$BaCo$_4$O$_7$

Based on a symmetry approach, we propose a possible explanation of the weak ferromagnetic component recently observed in YBaCo$_3$FeO$_7$ (Valldor et al. Phys Rev B, $\bf {84}$ 224426 (2011)) and other isostructural compounds in the high-temperature spin-liquid phase. Due to the polar nature of their crystal structure, a coupling between time-odd scalar spin chirality which we suggest as the primary order parameter and macroscopic magnetization is possible as follows from the general form of the appropriate free-energy invariant. The deduced pseudoproper coupling between both physical quantities provides a unique possibility to study the critical behaviour of the chiral order parameter.

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arXiv2011

Giant improper ferroelectricity in the ferroaxial magnet CaMn7O12

In rhombohedral CaMn7O12, an improper ferroelectric polarization of magnitude 2870 μC m-2 is induced by an incommensurate helical magnetic structure that evolves below TN1 = 90 K. The electric polarization was found to be constrained to the high symmetry three-fold rotation axis of the crystal structure, perpendicular to the in-plane rotation of the magnetic moments. The multiferroicity is explained by the ferroaxial coupling mechanism, which in CaMn7O12 gives rise to the largest magnetically induced, electric polarization measured to date.

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arXiv2011

The magnetic structures of the anisotropic inter-metallic compounds Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$

Two members of the iso-structural R$_2$CoGa$_8$ inter-metallic series, Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed Er$_2$CoGa$_8$ to adopt a 3D universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium and thulium based compounds. The corresponding magnetic space groups were determined to be $P_{2a}mmm'$ and $P_Cmmm$. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.

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arXiv2011

Complex room temperature ferrimagnetism induced by zigzag oxygen-vacancy stripes in Sr3YCo4O10.72

The high temperature ferromagnetism in Sr3YCo4O10+delta perovskite, whose origin has been the subject of a considerable debate, has been studied by neutron powder diffraction and synchrotron X-ray diffraction measurements. Oxygen vacancy ordering creates a complex pattern of zigzag stripes in the oxygen-deficient CoO4+delta layers, where the Co ions are found in three distinct coordinations. The symmetry of this unprecedented structural modulation, in conjunction with the existence of different Co spin states, provide a straightforward explanation for the appearance of ferrimagnetism. A model for the magnetic structure compatible with these structural features is proposed, based on the refinement of powder neutron data. The macroscopic moment as a function of temperature that can be calculated from the values of the ordered spins extracted from refinements, is in excellent agreement with bulk magnetization. Unlike previous models, a collinear G-type magnetic structure with uncompensated moments due to distinct spin-states of Co imposed by different coordination is found.

cond-mat.mtrl-sci↗

arXiv2010

Spin Correlations in the Geometrically Frustrated RBaCo4O7 Antiferromagnets: Mean-Field Approach and Monte-Carlo Simulations

Spin correlations in the geometrically frustrated RBaCo4O7 compounds, usually described as an alternating stacking of Kagome and triangular layers on a hexagonal lattice, have been studied by mean-field approach and by Monte-Carlo simulations. The behavior of the system was modelled with an isotropic Heisenberg Hamiltonian as a function of the relevant parameter Jout/Jin, representing the ratio between exchange integrals inside the Kagome layers, Jin, and between Kagome and triangular layers, Jout. This ratio can be varied in real systems by appropriate chemical substitutions. At the mean-field level, long-range magnetic order with the wave vector at the K-point of symmetry (k=a*/3+b*/3) has been found for Jout/Jin>0.7. Below this value, the dominant Fourier modes are completely degenerate in the entire Brillouin zone. The Monte-Carlo simulations revealed that the long-range ordered configuration found in the mean-field calculations becomes the ground state of the system for Jout/Jin>1.5. Below this critical ratio, quasi one-dimensional magnetic ordering along the c-axis, involving spins of the triangular sublattice was observed. The correlations in the (ab) plane were found to have a short-range 120o character with correlation length dependent on Jout/Jin.

cond-mat.mtrl-sci↗

arXiv2010

Spin-ordering and magnetoelastic coupling in the extended Kagome system YBaCo4O7

Low temperature magnetic and structural behavior of the extended Kagome system YBaCo4O7 has been studied by single crystal neutron diffraction and high-resolution powder X-ray diffraction. Long-range magnetic ordering associated with a structural transition from orthorhombic Pbn2_1 to monoclinic P2_1 symmetry has been found at T_1 ~ 100 K. The interplay between the structural and magnetic degrees of freedom testifies that the degeneracy of the magnetic ground state, present in the orthorhombic phase, is lifted through a strong magnetoelastic coupling, as observed in other frustrated systems. At T_2 ~ 60 K, an additional magnetic transition is observed, though iso-symmetric. Models for the magnetic structures below T_1 and T_2 are presented, based on refinements using a large number of independent reflections. The results obtained are compared with previous single crystal and powder diffraction studies on this and related compositions.

cond-mat.mtrl-sci↗

arXiv2010

Long-range magnetic order in CeRu2Al10 evidenced by muSR and neutron diffraction

The low temperature state of CeRu2Al10 has been studied by neutron powder diffraction and muon spin relaxation (muSR). By combining both techniques, we prove that the transition occurring below T*~27K, which has been the subject of considerable debate, is unambiguously magnetic due to the ordering of the Ce sublattice. The magnetic structure with propagation vector k=(1,0,0) involves collinear antiferromagnetic alignment of the Ce moments along the c-axis of the Cmcm space group with a reduced moment of 0.34(2)mu_B. No structural changes within the resolution limit have been detected below the transition temperature. However, the temperature dependence of the magnetic Bragg peaks and the muon precession frequency show an anomaly around T2~12 K indicating a possible second transition.

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arXiv2006

Spin-state ordering and magnetic structures in YBaCo2O5.5/5.44

The antiferromagnetic - ferromagnetic phase transition in YBaCo2O5.50 and YBaCo2O5.44 cobaltites with different types of oxygen-ion ordering in the [YO0.5/0.44] layers has been studied by neutron powder diffraction. Using the magnetic symmetry arguments in combination with group-theoretical analysis, the crystal and magnetic structures above and below the phase transformation temperature, Ti, were determined and successfully refined. In both cases, the proposed models involve a spin-state ordering between diamagnetic (t2g6eg0, S = 0) and paramagnetic (t2g4eg2, S = 2) Co3+ ions with octahedral coordination. Electronic ordering results in a nonzero spontaneous magnetic moment in the high-temperature magnetic phases with isotropic negative exchange interactions. In the case of YBaCo2O5.5, the phase transformation does not change Pmma symmetry of the crystal structure. The wave vectors of magnetic structures above and below Ti are k = 0 and k = c*/2, respectively. In the case of YBaCo2O5.44, a crossover P4/nmm - I4/mmm was involved to solve the low-temperature magnetic structure. The wave vectors in both high-temperature and low-temperature magnetic phases are k = 0. Mechanisms of the phase transformation in both compositions are discussed in the light of obtained magnetic structures. The proposed spin configurations were compared with other models reported in literature.

cond-mat.mtrl-sci↗

arXiv2006

Singularities of Magnetic and Elastic Characteristics of La2/3Ba1/3MnO3: Analysis of Martensitic Kinetics

A coordinated temperature behavior of magnetic susceptibility and internal friction has been observed in the La2/3Ba1/3MnO3 manganite in the temperature region of the crystal phase separation 5-340 K. Stepwise temperature behavior of the susceptibility of the single crystal sample and corresponding singular behavior of the internal friction in the polycrystalline manganite have been found. These small-scale features of the temperature dependences of the susceptibility and the internal friction are considered to be a reflection of martensitic kinetics of the structural phase transformation R3c-Imma in the 200 K temperature region.

cond-mat.mtrl-sci↗

arXiv2005

Structural phase separation in La2/3Ba1/3MnO3: macroscopic and microscopic aspects

The structural phase transformation in the La2/3Ba1/3MnO3 perovskite manganite was studied by ultrasound, neutron diffraction and electron microscopy techniques. Minima of the velocity and maxima of the decrement of the ultrasound point out the structural transition temperature Ts=200 K. The hysteretic temperature behavior and giant alteration of the ultrasound velocity and decrement present an evidence of martensitic character of the phase transition at a macroscopic level. The martensitic transformation in La2/3Ba1/3MnO3 is tightly connected with electronic transitions in both coexisting high symmetry R3c and low symmetry Imma crystal phases. Neutron diffraction data show that the electronic structure of the compound exhibits anomalous behavior in the vicinity of Ts: temperature dependences of the Mn-O-Mn bond angles and Mn-O bond lengths have singularities. Parameters of the coexisting crystalline phases studied have different character of temperature correlation with parameters of the magnetic subsystem of the compound.

cond-mat.mtrl-sci↗

arXiv2005

Structure-dependent microwave dielectric properties of (1-x)La(Mg1/2Ti1/2)O3 - xLa2/3TiO3 ceramics

Structure and dielectric characterization were performed on the (1-x)La(Mg1/2Ti1/2)O3 - xLa2/3TiO3 perovskite ceramics with the actual composition 0<=x<=0.52. The sequence of structure transformations was detected as x is increased: P21/n (0<=x<0.3) - Pnma (0.3<=x<0.37) - Imma (0.37<=x<0.4) - I2/a (0.4<=x<=0.49) - R-3c (0.49<x<=0.52). The structure evolution from the tilt configuration a+b-b- to a-a-a- are considered in terms of competing the repulsive energy and the Madelung energy. Relative permittivity of the ceramics was measured as a function of temperature and LT content. Temperature coefficient of the resonant frequency was found to pass zero-value at the narrow compositional range between x=0.49 and 0.52, where the discontinuous I2/a - R-3c crossover occurs. Temperature and compositional variation of these fundamental microwave dielectric parameters are discussed in respect to type of the phase transitions (continuous/discontinuous) between the structures.

cond-mat.mtrl-sci↗

arXiv2005

Neutron diffraction, magnetization and ESR studies of pseudocubic Nd(0.75)Ba(0.25)MnO3 and its unusual critical behavior above Tc

Results of structural neutron diffraction study, magnetization and ESR measure-ments are presented for insulating Nd0.75Ba0.25MnO3, Tc = 129 K. The crystal structure is refined in the range 4.2-300 K. The compound is found to exhibit the Jahn-Teller (JT) transition at 250 K. The field cooled (FC) magnetization data are in a reasonable agreement with the predictions for a 3D isotropic ferromagnet above Tc. However, these measurements reveal a difference between the FC and zero FC data in the paramagnetic region. ESR results are also in a correspondence with behavior of a cubic ferromagnet above T* = 143 K. It is shown that an anisotropic exchange coupling of the Mn and Nd magnetic moments can give a substantial contribution in ESR linewidth masking its critical enhan-cement. The different temperature treatments of the sample reveal a temperature hysteresis of the ESR spectra below T* indicating an anomalous response in the paramagnetic region. The study of phase transition in this manganite suggests change in its character from the second to first order at T*. The conventional free energy including the magnetization and magnetic field is not found to describe the first order transition. This suggests that the charge, orbital and JT phonon degrees of freedom, in addition to magnetization, may be the critical variables, the unusual character of the transition being determined by their coupling. Unconventional critical behavior is attributed to orbital liquid metallic phase that coexists with the initial orbital ordered phase below T*.

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