Magnetic ordering of PbMnTeO6 crystallizing in the chiral noncentrosymmetric P312 space group has been experimentally investigated by high-resolution neutron powder diffraction (NPD) and theoretically by high-temperature expansion, density matrix renormalization group, and linear spin-wave theory approaches. Processing of low-temperature NPD data showed that, to describe the rather complex spin ordering in PbMnTeO6, two incommensurate modulations determined by the propagation vectors k1 and k2 should be used. The observed ground state with k1 = (31, 31, 0.4077) is a noncollinear 120 degrees spin structure, which is helically ordered along the layer stacking direction with a characteristic spiral period of similar to 13.2 & Aring;. Additionally, the second incommensurate modulation for magnetic structure with the propagation vector k2 = (31 + 81, 31 + 81, 82) was also observed. An effective spin model for explaining the observed magnetic characteristics in PbMnTeO6 was also constructed from numerical calculations. The dominant nearest-neighbor interaction was estimated as J1 = 12 +/- 2 K by fits of magnetic susceptibility, the magnetization curve, and the magnetic part of the specific heat between experimental and numerical results. It is shown that spiral ordering along the layers stacking direction can be attributed to frustrated interlayer couplings or the presence of an interlayer Dzyaloshinskii-Moriya interaction. Also, the incommensurate modulation of the propagation vector in the ab plane can be reproduced by introducing a subtle anisotropic distortion of triangular lattice.

Using neutron powder diffraction at a temperature of 35 K, slightly above the Neel temperature, the features of the crystal structure of two samples of layered honeycomb oxides of the same stoichiometric composition Li2Ni2TeO6 synthesized from precursors, Na2Ni2TeO6 and K2Ni2TeO6, were determined. They have a similar crystal structure, hexagonal space group P63/mcm, structure type P2, but there is a significant difference in the distances between the layers. Both Li2Ni2TeO6 samples crystallize into the orthorhombic space group Cmca, with minor differences in the unit lattice parameters. If Li2Ni2TeO6 from the potassium precursor is single-phase, then the compound from the sodium precursor contains 16 wt % of an additional phase with the same Li2Ni2TeO6 stoichiometry, but more deformed with monoclinic distortions described by the С2/m space group.

Rare-earth orthoferrites (RFeO3) provide a flexible playground for magnetic materials design, combining the magnetic properties arising from complex interactions between R3+ and Fe3+ cations within the robust framework of the perovskite structure. The most important magnetic property common to most orthoferrites is a spin reorientation transition in which the magnetic moments of Fe3+ cations rotate with respect to a crystallographic axis. SmFeO3 is unique among orthoferrites due to its high-temperature spin reorientation. It is possible to tune the spin reorientation transition to occur at room temperature by replacing Sm with Tm in the Sm0.70Tm0.30FeO3 perovskite. In this study, we show how small changes in composition in the Sm1-xTmxFeO3 (x = 0.30-0.50 ) series provide a high degree of control over the magnetic properties. This work also offers a rather unusual look into the magnetic structure of a samarium-based perovskite by means of neutron powder diffraction, which was made possible by using 152Sm. The combination of these results and magnetization measurements allowed the construction of the magnetic phase diagram of the series.

The trigonal layered quaternary tellurate Na2MnTeO6 has been studied by means of various techniques to clarify its magnetic properties. The crystal structure of this compound is based on the triangular arrangement of all cations in the parallel layers with the space group P (3) over bar 1c. By using symmetry analysis of the magnetic neutron scattering data, we have found that the solution for the magnetic structure corresponds to the magnetic Shubnikov group R (3) over bar 'c' (No. 167.4.1337). Mn4+ ions in an octahedral environment form a triangular network where all spins are directed from the center of each triangle. Overall magnetic structure in Na2MnTeO6 is commensurate 120 degrees spin helix with propagation vector k = (1/3, 1/3, 1/3) in variance with planar spin structure in structurally equivalent Li2MnTeO6 with magnetic propagation vector k = (1/3, 1/3, 0). The magnetization measurements show that Na2MnTeO6 experiences an antiferromagnetic order at T-N = 5.5 K. NMR, electron spin resonance, and thermodynamics experiments demonstrate the extended temperature region of 2D short-range correlations well above the ordering temperature.

The complex crystal and magnetic structure of strontium-doped ytterbium manganites using high-resolution neutron diffraction at low temperature starting from 2.6 K has been studied. Yb0.6Sr0.4MnO3 was crystallized with the mixed phases of crystal structure: orthorhombic with Pbnm space group enclosing Jahn–Teller Mn3+ ions, and hexagonal crystal system of P63cm space group describing JT ions free phase over the whole temperature range. From the refinement of the neutron diffraction patterns, a distinguished difference in the behaviors of the two phases was observed with a variation in temperature where opposite lattice—temperature-dependent behavior was found. The magnetic ordering in hexagonal symmetry was frustrated and was having a canted spin of Mn magnetic moments in the plane of the Г2-type symmetry. Mn ions were arranged together in the well-separated triangular layers parallel to (ab) plane as a result of the antiferromagnetic exchange interaction between the spins of atoms in the most nearest neighbors, which make the Mn spin subsystem of low-dimensional and frustrated at TN≈150 K. The other phase of the orthorhombic crystal system showed the C-type antiferromagnetic at TN≈70 K. The contribution of Yb atom in the magnetic ordering down to T = 2.6 K has not been registered. The magnetization–temperature dependence of Yb0.6Sr0.4MnO3 at different applied magnetic field was calculated using Monte Carlo simulation based on Ising model. Theoretically calculated model and experimental data were in good agreement. The complex magnetic ordering of the Yb0.6Sr0.4MnO3 for the mixed crystal structure was well described by Ising model.

The neutron diffraction studies of powder samples from the family of A2MnTeO6 tellurates (А = Ag, Tl) were performed at room and low temperatures. The specific features of their crystal structure were investigated, and the partial substitution of Mn and Te atoms was revealed. The spin structure of Ag2MnTeO6 was determined in an ordered state at Т = 1.6 K by symmetry and full-profile analysis. It represented a non-collinear 120° triangular structure in the ab plane and a spin helicoid along the c-axis with the propagation vector k = (1/3 1/3 1/3). The neutron diffraction measurements of Tl2MnTeO6 did not reveal any additional reflections associated with the organization of long-range magnetic order up to Т = 1.6 K. The studied compounds were compared with the powders belonging to the same family and the earlier published results, which served to reveal a similarity between their crystal and magnetic structures and Na2MnTeO6 and a radically different type of their magnetic ordering as compared to Li2MnTeO6.

This study is devoted to magnetic neutron diffraction analysis (elastic neutron scattering yielding information about spin ordering in magnetic materials). Experimental approaches to magnetism are intensely developed mainly due to the discovery of new functional materials with unusual properties. Nevertheless, magnetic neutron diffraction analysis is a unique method, which makes it possible to directly determine magnetic structures (i.e., values and mutual orientations of magnetic moments and their orientations relative to the crystallographic axes). Neutron diffraction proved to be a good instrument for studying both simple commensurate magnetic structures in bulk samples and more complex noncollinear spiral magnets. Magnetic neutron diffraction is widely applied for studying magnetic structures in nanoscale multilayer film materials. However, bulk samples with a layered crystal structure, which can be considered as natural quasi-two-dimensional objects due to the weakness of interlayer couplings, are of greater interest for physics of low-dimensional magnetism. Our neutron diffraction studies on quasi-two-dimensional magnets, transition-metal oxides A3M2SbO6 (A = Li or Na, M = Co or Ni) and rare-earth intermetallic compounds La1–xTbx(Ni1–xMnx)2Si2, are briefly considered.

The scaling of the magnetic heat capacity in the two manganites La0.85Ag0.15MnO3 and Sm0.55Sr0.45MnO has given the critical exponents α = –0.23 and ν = 0.7433 of the heat capacity and correlation radius of the magnetic order parameter, respectively, which do not belong to any known universality class. These results cannot be attributed to chemical inhomogeneities and/or structural imperfections because the samples are of a high quality. Thus, unusual critical exponents can be associated not only with the chemical disorder and/or structural defects but also with the collective behavior of the lattice. An analogy has been revealed between the effects of the magnetic field and doping on ternary oxides of transition metals: the magnetic field affecting lattice distortions through the orientation of t2g orbitals acts as chemical doping. It seems that scaling relations are more stable than critical exponents in them. The synchronism of lattice distortions and ferromagnetism leads to a novel criticality, but their desynchronization induced by magnetostructural disorder results in the violation of scaling relations between isothermal and isomagnetic exponents. Although double-exchange systems demonstrate novel criticality, they satisfy scaling relations until the magnetic behavior is synchronized with the coherent lattice behavior in the form of cooperative Jahn–Teller distortions. Breaking of double exchange bonds leads to the formation of metamagnetic clusters with magnetic dipole–dipole interaction between them, which desynchronizes lattice distortions and ferromagnetism, resulting in the violation of scaling relations. The proposed new universality class includes diverse materials such as manganites, cobaltites, crystalline Fe–Pt and amorphous Fe–Mn alloys, and high-Tc superconductors. Unusual criticality in double-exchange systems is due to an unusual semiclassical nature of double-exchange ferromagnetism caused by real exchange, i.e., electron current through Mn3+–O–Mn4+ chains with the conservation of the spin rather than by virtual exchange as in a usual ferromagnet. Double-exchange ferromagnetism arises only because to freely itinerate, electrons orient the magnetic moments of Mn cations in a single direction.

The magnetic structure of quasi-two-dimensional (2D) honeycomb lattice Na2Ni2TeO6 has been determined by low-temperature neutron diffraction and the crystal structure fine details at room temperature have been established by a combination of synchrotron and neutron powder diffraction. The atomic structure is described by the P6(3)/mcm space group, but the strong presence of stacking faults defects in layered ordering is found for the first time. Both magnetization and specific heat data indicates an establishment of a long-range antiferromagnetic order with T-N = 25 +/- 1 K, preceded by a short-range order on 2D honeycomb lattice at about 34 K. Determined effective magnetic moment mu(eff) = 4.35 mu(B) is in excellent agreement with numerical estimation using effective g-factor g = 2.19 directly measured by electron spin resonance. The ground magnetic state at T = 1.5 K is represented by the commensurate zigzag-type magnetic order. Magnetic moments of Ni are almost perpendicular to the honeycomb layers, which antiferromagnetically coupled along the c-direction. Besides, the coherent magnetic scattering area has a disk shape that is homogeneous over the ab plane and compressed along the c-axis that indicates 2D nature of magnetic correlations in the compound. The magnetic diffuse neutron scattering related to the presence of strong short-range spin-spin correlations above T-N was observed. Based on our experimental measurements we propose magnetic phase diagram for Na2Ni2TeO6. (C) 2019 Elsevier B.V. All rights reserved.

MnSnTeO6, a new chiral antiferromagnet, was prepared both by topotactic transformation of the metastable rosiaite-type polymorph and by direct synthesis from coprecipitated hydroxides. Its structure and its static and dynamic magnetic properties were studied comprehensively both experimentally (through X-ray and neutron powder diffraction, magnetization, specific heat, dielectric permittivity, and ESR techniques) and theoretically (by means of ab initio density functional theory (DFT) calculations within the spin-polarized generalized gradient approximation). MnSnTeO6 is isostructural with MnSb2O6 (space group P321) and does not show any structural transition between 3 and 300 K. The magnetic susceptibility and specific heat exhibit an antiferromagnetic ordering at TN ≈ 9.8 K, which is confirmed by low-temperature neutron data. At the same time, the thermodynamic parameters demonstrate an additional anomaly on the temperature dependences of magnetic susceptibility χ(T), specific heat Cp(T) and dielectric permittivity ε(T) at T* ≈ 4.9 K, which is characterized by significant temperature hysteresis. Clear enhancement of the dielectric permittivity at T* is most likely to reflect the coupling of dielectric and magnetic subsystems leading to development of electric polarization. It was established that the ground state of MnSnTeO6 is stabilized by seven exchange parameters, and neutron diffraction revealed incommensurate magnetic structure with propagation vector k = (0, 0, 0.183) analogous to that of MnSb2O6. Ab initio DFT calculations demonstrate that the strongest exchange coupling occurs between planes along diagonals. All exchange parameters are antiferromagnetic and reveal moderate frustration.

The manganese tellurate Li2MnTeO6 consists of trigonal spin lattices made up of Mn4+ (d3, S = 3/2) ions. The magnetic properties of this compound were characterized by several experimental techniques, which include magnetic susceptibility, specific heat, dielectric permittivity, electron spin resonance (ESR), nuclear magnetic resonance (NMR) and neutron powder diffraction (NPD) measurements, and by density functional calculations (DFT). The magnetic susceptibility chi(T) demonstrates very unusual behavior. It isdescribed by the Curie-Weiss law at high temperature with Curie-Weiss temperature of Theta = -74 K, exhibits no obvious anomaly indicative of a long-range magnetic ordering at low magnetic fields. At high magnetic fields, however, the character of chi(T) changes showing a maximum at about 9 K. That this maximum of chi(T) reflects the onset of an antiferromagnetic order was confirmed by specific heat measurements, which exhibit a clear lambda-type anomaly at TN around 8.5 K even at zero magnetic field, and by 7Li NMR and dielectric permittivity measurements. The magnetic structure of Li2MnTeO6, determined by neutron powder diffraction measurements at 1.6 K, is described by the 120-degree non-collinear spin structure with the propagation vector k = (1/3, 1/3, 0). Consistent with this finding, the spin exchange interactions evaluated for Li2MnTeO6 by density functional calculations are dominated by the nearest-neighbor antiferromagnetic exchange within each triangular spin lattice. This spin lattice is strongly spin frustrated with f = |Theta|/TN around 8 and exhibits a two-dimensional magnetic character in a broad temperature range above TN.

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li2MnGeO4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2(1) space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T-N approximate to 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C(a)c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 mu(B)/Mn2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T-N. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li2MnGeO4, which is symmetry consistent on a small scale with the long-range magnetic state below T-N. The reconstructed radial spin-pair correlation function S(0)S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r approximate to 12 angstrom at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T-N. As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li2MnGeO4 is presented.

Crystallographic and magnetic properties of new layered honeycomb-lattice Li3Co2SbO6 antimonate were studied and compared with its sodium precursor Na3Co2SbO6.

The structural and magnetic properties of YbMnO3 and Yb0.82Sr0.18MnO3 multiferroics were studied by neutron powder diffraction (NPD), magnetometry and electron spin resonance (ESR) technics in a wide temperature range. Neutron diffraction measurements showed that the substitution of ytterbium ions with strontium ions in hexagonal h - YbMnO3 (space group P6(3)cm) leads to the destabilization of the crystal structure of the last compound and appearance of the mixture of three phases with different structure: hexagonal phase h - Yb0.95Sr0.05MnO3 (space group P6(3)cm), orthorhombic phase o - Yb0.69Sr0.31MnO3 (space group Pbnm), hexagonal phase SrMnO3 (space group P6(3)cm). This fact was proved by the ESR measurements in which a several signals due to the phases of different structure were observed. NPD measurements showed that the magnetic structure of h - Yb0.95Sr0.05MnO3 phase is similar to the magnetic structure of the pure h - YbMnO3 and demonstrate the presence of the antiferromagnetic ordering in the samples. ESR and magnetization measurements of h - YbMnO3 sample proved the presence of the antiferromagnetic correlations and also they showed the appearance of the ferromagnetically correlated nanoregions.

An experimental study of long‐range magnetic order formation mechanisms in a layered structure with a honeycomb arrangement of the magnetic atoms Na2Ni2TeO6 is conducted. For the first time, the strong spin correlations are directly observed above the Neel temperature TN that is manifested in the presence of broad diffuse peaks on neutron diffraction patterns obtained with the XYZ polarization analysis. Due to the possibility of separating the magnetic, nuclear incoherent, and nuclear coherent contributions to the total neutron scattering cross section, it is unequivocally established that the observed diffuse scattering has magnetic nature. The spin‐pair correlation function is reconstructed by modeling diffuse neutron scattering on Na2Ni2TeO6 with reverse Monte Carlo method. The obtained results indicate 2D nature of the magnetic correlations, and moreover, the symmetry of short‐range magnetic state corresponds to long‐range zigzag‐type magnetic order in the honeycomb net, which is established earlier based on the theoretical calculations.

The effect of isovalent substitution of rare-earth cations on the phase separation in half-doped manganites was studied on the example of Sm-0.(32)Pr(0.18)Sr(0.5)MnO(3 )by high-resolution neutron powder diffraction, neutron beam depolarization, second-harmonic magnetic response, and magnetization and resistivity measurements from 4 K up to room temperature and higher. A structural phase transition from the orthorhombic Pbnm phase to a mixture of Pbnm and monoclinic P2(1)/m phases was observed upon cooling. The magnetic ground state was found to be phase-separated into three magnetic phases emerging at different temperatures, viz., ferromagnetic (FM) and antiferromagnetic A and charge-ordered CE types. FM clusters arise far above room temperature in the orthorhombic phase and coalesce upon cooling to produce the long-range FM order below 250 K and to arrive at the percolative FM phase below 120 K. The A- and CE-type orders form in the monoclinic phase at the temperatures 200 K and 120 K, respectively. The Sm/Pr isovalent substitution qualitatively changes the phase separation and significantly increases its temperature range compared to the parent compounds. The results obtained give us knowledge of phase separation occurring in systems with strong electron correlations and extend opportunities for fine-tuning of their properties.

Antiferromagnetic PbMnTeO6, also known as mineral kuranakhite, has been reported recently to have all three cations in trigonal prismatic coordination, which is extremely unusual for both Mn(4+) and Te(6+). In this work, the phase was reproduced with the same lattice parameters and Néel temperature TN = 20 K. However, powder neutron diffraction unambiguously determined octahedral (trigonal antiprismatic) coordination for all cations within the chiral space group P312. The same symmetry was proposed for SrMnTeO6 and PbGeTeO6, instead of the reported space groups P6[combining macron]2m and P31m, respectively. PbMnTeO6 was found to be a robust antiferromagnet with an assumingly substantial scale of exchange interactions since the Néel temperature did not show any changes in external magnetic fields up to 7 T. The determined effective magnetic moment μeff = 3.78μB was in excellent agreement with the numerical estimation using the effective g-factor g = 1.95 directly measured here by electron spin resonance (ESR). Both specific heat and ESR data indicated the two-dimensional character of magnetism in the compound under study. The combination of chirality with magnetic order makes PbMnTeO6 a promising material with possible multiferroic properties.

Mariia D. Kuchuguraa,b, Alexander I. Kurbakova,b, Elena A. Zverevac, Tatyana M. Vasilchikovac, Grigory V. Raganyanc, Alexander N. Vasilievc,d,e Victor A. Barchukf and Vladimir B. Nalbandyanf aNRC «Kurchatov Institute» PNPI, 188300 Gatchina, Russia bFaculty of Physics, St. Petersburg University, 199034, St. Petersburg, Russia cFaculty of Physics, M.V. Lomonosov Moscow State University, Moscow 119991, Russia dNational Research South Ural State University, Chelyabinsk 454080, Russia eNational University of Science and Technology "MISiS", Moscow 119049, Russia. fFaculty of Chemistry, Southern Federal University, 344090 Rostov-on-Don, Russia