arXiv subjects

N. V. Ter-Oganessian

Publications and source records attributed to N. V. Ter-Oganessian.

11 recordsLinked to original sources

arXiv2023

Observation of linear magnetoelectric effect in a Dirac magnon antiferromagnet Cu$_3$TeO$_6$

Cu$_3$TeO$_6$, a three-dimensional antiferromagnet forming a unique spin-web lattice of spin-1/2 Cu2+ ions below the Neel temperature T$_N$ = 62 K, has recently been found to exhibit topological Dirac or nodal magnon dispersion. In this study, we report the discovery of the linear magnetoelectric (ME) effects in Cu$_3$TeO$_6$ below TN. Our pyroelectric current measurements at a constant magnetic field (H) reveal a linear increase of electric polarization (P) with H for both P // H and P $\perp$ H configurations; a maximum P // [110] = 20 $μ$C/m$^2$ is obtained at u0H // [1-10] = 14 T, corresponding to a linear ME coefficient 1.8 ps/m. Magnetic point group analysis and Monte-Carlo simulations confirm that finite linear ME coefficients are allowed in the off-diagonal and diagonal ME tensor components, consistent with the magnetic point group of $\bar{3}'$. As the parity-time symmetry can be broken in the presence of H or electric field E in the linear ME materials, we envisage that Cu$_3$TeO$_6$ should exhibit a H- or E-induced transformation in the topological magnon dispersion from a Dirac point/nodal line type into two Weyl point types.

cond-mat.str-el↗

arXiv2021

Phase transition between two different orientations of the Q phase in the NaNbO$_3$ thin film

Temperature evolution of dielectric response, atomic structure, and lattice dynamics in thin film of sodium niobate in the epitaxial NaNbO$_3$/SrRuO$_3$/(001)MgO heterostructure is studied by dielectric measurements, x-ray diffraction, and Raman spectroscopy. It is found that at room temperature NaNbO$_3$ is in ferroelectric state, whereas the temperature-dependent dielectric constant experiences a broad maximum at 440~K on cooling and at 500~K on heating and reveals a diffuse phase transition. Reciprocal space mapping shows the presence of both anti-phase and in-phase tilting of oxygen octahedra. The temperature dependence of the M-point reflections suggests reorientation of the in-phase octahedra tilting axis from being parallel to the substrate at room temperature to perpendicular orientation at high temperatures. The temperature evolution of the shape of the Raman spectra reveal the decrease of the number of constituting peaks on heating. These results are interpreted as indicating a temperature-driven transition between two different orientations of the bulk ferroelectric Q phase with respect to the interface, namely between the state with electric polarization pointing at $\approx45^{\rm o}$ to the normal at room temperature to the state with polarization parallel to the interface above the transition. Transitions of this kind can be anticipated from theoretical considerations, while the experimental evidences of such are yet scarce.

cond-mat.mtrl-sci↗

arXiv2019

Effect of pressure on the order-disorder phase transitions of $B$-cations in $AB'_{1/2}B''_{1/2}$O$_3$ perovskites

Perovskite-like oxides $AB'_{1/2}B''_{1/2}$O$_3$ may experience different ordering degrees of $B$-cations, that can be varied by suitable synthesis conditions or post-synthesis treatment. In this work the earlier proposed statistical model of order-disorder phase transitions of $B$-cations is extended to account for the effect of pressure. Depending on composition, pressure is found to either increase or decrease the order-disorder phase transition temperature. The change of transition temperature due to pressure in many cases reaches several hundreds of kelvin at pressures accessible in laboratory, which may significantly change the atomic ordering degree. The work is intended to help determining how pressure influences the degree of atomic ordering and stimulate research of the effect of pressure on atomic order-disorder phase transitions in perovskites.

cond-mat.mtrl-sci↗

arXiv2019

Linear magnetoelectric effect in the antiferromagnetic Sm2BaCuO5

We report the discovery of linear magnetoelectric effect in the well-known green phase compound, Sm2BaCuO5, which crystallizes in the centrosymmetric orthorhombic (Pnma) structure. Magnetization and specific heat measurements reveal the long-range antiferromagnetic ordering of Cu2+ and Sm3+-ions moments at TN1 = 23 K and TN2 = 5 K, respectively. Applied magnetic field induces dielectric anomaly at TN1 whose magnitude increases with field, which results in significant (1.7%) magnetocapacitance effect. On the other hand, the dielectric anomaly observed in zero-applied magnetic field at TN2 shows a small (0.4%) magnetocapacitance effect. Interestingly, applied magnetic field induces an electric polarization below TN1 and the polarization varies linearly up to the maximum applied field of 9 T with the magnetoelectric coefficient α ~ 4.4 ps/m, demonstrating high magnetoelectric coupling. Below TN2, the electric polarization decreases from 35 to 29 μC/m2 at 2 K and 9 T due to ordering of Sm-sublattice. The observed linear magnetoelectricity in Sm2BaCuO5 is explained using symmetry analysis.

cond-mat.str-el↗

arXiv2019

Magnetic and magnetoelectric properties of $A$FeF$_5$ ($A$ = Ca, Sr) spin-chain compounds

Fluorides in general are characterized by big variety of crystal structures, whereas those containing transition metals also often show sizable magnetic properties. The tendency of fluorine to form linear chain structures in many cases results in low-dimensional magnetism. Despite the plethora of magnetic phenomena in fluorides, their magnetoelectric properties are less studied than those of oxides. In the present work we theoretically study the magnetic and magnetoelectric properties of spin-chain compounds CaFeF$_5$ and SrFeF$_5$. The density functional theory is employed for determination of magnetic exchange constants, which are then used in Monte Carlo calculations. The symmetry analysis reveals that CaFeF$_5$ does not show magnetoelectric properties, whereas SrFeF$_5$ is a multiferroic.

cond-mat.str-el↗

arXiv2018

Linear magnetoelectric effect as a signature of long-range collinear antiferromagnetic ordering in the frustrated spinel CoAl2O4

The ground state of the frustrated A-site magnetic spinel CoAl2O4 has been a controversial issue whether it is a collinear antiferromagnetic ordering or a spiral spin - liquid state, as the ratio of the two competing interactions, J2/J1 lies close to the boundary between these two ground states. Here, we address the magnetic ground state in CoAl2O4 with different amount of Co2+/Al3+ site disorder from the study of magnetoelectric effect and Monte Carlo simulations. CoAl2O4 with low site disorder exhibits linear magnetoelectric effect below the magnetic ordering temperature. With increasing disorder, the magnetoelectric effect is suppressed and the sample with 14% disorder exhibits a spin glass behavior without the magnetoelectric effect. Monte Carlo simulations support the experimental findings and suggest that the site disorder suppresses long - range antiferromagnetic order and induces a spin glass state. Since the linear magnetoelectric effect requires a long - range magnetic ordering, we suggest that the ground state of CoAl2O4 with low site disorder is a collinear antiferromagnet.

cond-mat.str-el↗

arXiv2017

Multiferroic rust: magnetoelectric effect in göthite, $α$-FeOOH

By means of symmetry analysis, density functional theory calculations, and Monte Carlo simulations we show that goethite, $α$-FeOOH, is a linear magnetoelectric below its Néel temperature $T_{\rm N}=400$ K. The experimentally observed magnetic field induced spin-flop phase transition results in either change of direction of electric polarization or its suppression. Calculated value of magnetoelectric coefficient is 0.24 $μ$C m$^{-2}$ T$^{-1}$. The abundance of goethite in nature makes it arguably the most widespread magnetoelectric material.

cond-mat.str-el↗

arXiv2015

Electric polarization of magnetic domain walls in magnetoelectrics

Two prominent magnetoelectrics MnWO$_4$ and CuO possess low-temperature commensurate paraelectric magnetically ordered phase. Here using Monte Carlo simulations we show that the walls between the domains of this phase are ferroelectric with the same electric polarization direction and value as that in the magnetoelectric phases of these compounds. We also suggest that experimental observation of electric polarization of domain walls in MnWO$_4$ should help to determine the macroscopic interactions responsible for its magnetoelectric properties.

cond-mat.str-el↗

arXiv2014

Cation-ordered A$'_{1/2}$A$''_{1/2}$B$_2$X$_4$ magnetic spinels as magnetoelectrics

We show that 1:1 ordering of A$'$ and A$''$ cations in A$'_{1/2}$A$''_{1/2}$B$_2$X$_4$ magnetic spinels results in appearance of magnetoelectric properties. Possible value of magnetically induced electric polarization is calculated using the recently proposed microscopic model, which takes into account spin-dependent electric dipole moments of magnetic ions located in noncentrosymmetric crystallographic positions. We build phenomenological models of magnetic phase transitions in cation-ordered spinels, which describe ferromagnetic and antiferromagnetic ordering patterns of B cation spins, and calculate the respective magnetoelectric responses. We find that magnetoelectric coefficients diverge at ferromagnetic or weak ferromagnetic phase transitions in ordered spinels.

cond-mat.str-el↗

arXiv2013

Interpretation of magnetoelectric phase states using the praphase concept and exchange symmetry

The majority of magnetoelectric crystals show complex temperature-magnetic field or temperature-pressure phase diagrams with alternating antiferromagnetic incommensurate, magnetoelectric, and commensurate phases. Such phase diagrams occur as a result of successive magnetic instabilities with respect to different order parameters, which usually transform according to different irreducible representations (IR) of the space group of the crystal. Therefore, in order to build a phenomenological theory of phase transitions in such magnetoelectrics one has to employ several order parameters and assume the proximity of various instabilities on the thermodynamic path. In this work we analyze the magnetoelectrics MnWO4, CuO, NaFeSi2O6, NaFeGe2O6, Cu3Nb2O8, alpha-CaCr2O4, and FeTe2O5Br using the praphase concept and the symmetry of the exchange Hamiltonian. We find that in all the considered cases the appearing magnetic structures are described by IR's entering into a single exchange multiplet, whereas in the cases of MnWO4 and CuO by a single IR of the space group of the praphase structure. Therefore, one can interpret the complex phase diagrams of magnetoelectrics as induced by a single IR either of the praphase or of the symmetry group of the exchange Hamiltonian. Detailed temperature-magnetic field phase diagrams of MnWO4 and CuO for certain field directions are obtained and the magnetic structures of the field-induced phases are determined.

cond-mat.str-el↗

arXiv2012

Magnetoelectric effect due to local noncentrosymmetry

Magnetoelectrics often possess ions located in noncentrosymmetric surroundings. Based on this fact we suggest a microscopic model of magnetoelectric interaction and show that the spin-orbit coupling leads to spin-dependent electric dipole moments of the electron orbitals of these ions, which results in non-vanishing polarization for certain spin configurations. The approach accounts for the macroscopic symmetry of the unit cell and is valid both for commensurate and complex incommensurate magnetic structures. The model is illustrated by the examples of MnWO4, MnPS3 and LiNiPO4. Application to other magnetoelectrics is discussed.

cond-mat.str-el↗