Spin amplitude modulation driven magnetoelectric coupling in the new multiferroic FeTe2O5Br

Lone-pair self-containment in pyritohedron-shaped closed cavities: optimized hydrothermal synthesis, structure, magnetism and lattice thermal conductivity of Co15F2(TeO3)14

Co₁₅F₂(TeO₃)₁₄ features an extremely rare example of the Te(iv) lone pairs self-containment in pyritohedron-shaped [(TeO₃)₁₄]²⁸⁻ units, which allows Te atom to vibrate with large amplitude, leading to extremely low lattice thermal conductivity.

Transition Metal Selenite Halides: A Fascinating Family of Magnetic Compounds

The problem of searching for low-dimensional magnetic systems has been a topical subject and has attracted attention of the chemistry and physics community for the last decade. In low-dimensional magnetic systems, magnetic ions are distributed anisotopically and form different groups such as dimers, chains, ladders, or planes. In 3D frameworks, the distances between magnetic ions are equal in all directions while in low-dimensional systems the distances within groups are different from those between groups. The main approach of searching for desired systems is a priori crystal chemical design expecting the needed distribution of transition metal ions in the resulting structure. One of the main concepts of this structural design is the incorporation of the p-element ions with stereochemically active electron pairs and ions acting as spacers in the composition. Transition metal selenite halides, substances that combine SeO32− groups and halide ions in the structure, seem to be a promising object of investigation. Up to now, there are 33 compounds that are structurally described, magnetically characterized, and empirically tested on different levels. The presented review will summarize structural peculiarities and observed magnetic properties of the known transition metal selenite halides. In addition, the known compounds will be analyzed as possible low-dimensional magnetic systems.

Electronic and magnetic properties of low-dimensional system Co2TeO3Cl2

The electronic and magnetic properties of transition metal oxyhalide compound Co₂TeO₃Cl₂ are investigated using first principle calculations within the framework of density functional theory. To find the underlying spin-lattice of this compound, various hopping integrals and exchange interactions are calculated. The calculations reveal that the dominant inter-chain and intra-chain interactions are in the ab plane. The exchange path is visualized by Wannier function plotting. The nearest neighbor and next nearest neighbor exchange interactions are antiferromagnetic, making the system frustrated in low dimension. Calculations are also done with spin-orbit coupling (SOC) to find out the effect of SOC on this compound. Calculation of magnetocrystalline anisotropy suggests that the easy axis is along the crystallographic b direction.

High pressure and multiferroics materials: a happy marriage

The community of material scientists is strongly committed to the research area of multiferroic materials, both for the understanding of the complex mechanisms supporting the multiferroism and for the fabrication of new compounds, potentially suitable for technological applications. The use of high pressure is a powerful tool in synthesizing new multiferroic, in particular magneto-electric phases, where the pressure stabilization of otherwise unstable perovskite-based structural distortions may lead to promising novel metastable compounds. The in situ investigation of the high-pressure behavior of multiferroic materials has provided insight into the complex interplay between magnetic and electronic properties and the coupling to structural instabilities.

Multiferroics of spin origin

Multiferroics, compounds with both magnetic and ferroelectric orders, are believed to be a key material system to achieve cross-control between magnetism and electricity in a solid with minute energy dissipation. Such a colossal magnetoelectric (ME) effect has been an issue of keen interest for a long time in condensed matter physics as well as a most desired function in the emerging spin-related electronics. Here we begin with the basic mechanisms to realize multiferroicity or spin-driven ferroelectricity in magnetic materials, which have recently been clarified and proved both theoretically and experimentally. According to the proposed mechanisms, many families of multiferroics have been explored, found (re-discovered), and newly developed, realizing a variety of colossal ME controls. We overview versatile multiferroics from the viewpoints of their multiferroicity mechanisms and their fundamental ME characteristics on the basis of the recent advances in exploratory materials. One of the new directions in multiferroic science is the dynamical ME effect, namely the dynamical and/or fast cross-control between electric and magnetic dipoles in a solid. We argue here that the dynamics of multiferroic domain walls significantly contributes to the amplification of ME response, which has been revealed through the dielectric spectroscopy. Another related issue is the electric-dipole-active magnetic resonance, called electromagnons. The electromagnons can provide a new stage of ME optics via resonant coupling with the external electromagnetic wave (light). Finally, we give concluding remarks on multiferroics physics in the light of a broader perspective from the emergent electromagnetism in a solid as well as from the possible application toward future dissipationless electronics.

Magnetic, structural, and electronic properties of the multiferroic compound FeTe₂O₅Br with geometrical frustration

We report electron spin resonance (ESR), Raman scattering, and interband absorption measurements of the multiferroic FeTe₂O₅Br with two successive magnetic transitions at T(N1) = 11.0 K and T(N2) = 10.5 K. ESR measurements show all characteristics of a low-dimensional frustrated magnet: (i) the appearance of an antiferromagnetic resonance (AFMR) mode at 40 K, a much higher temperature than T(N1), and (ii) a weaker temperature dependence of the AFMR linewidth than in classical magnets, ΔH(pp)(T) ∝ T(n) with n = 2.2-2.3. Raman spectra at ambient pressure show a large variation of phonon intensities with temperature while there are no appreciable changes in phonon numbers and frequencies. This demonstrates the significant role of the polarizable Te⁴⁺ lone pairs in inducing multiferroicity. Under pressure at P = 2.12-3.04 GPa Raman spectra undergo drastic changes and absorption spectra exhibit an abrupt drop of a band gap. This evidences a pressure-induced structural transition related to changes of the electronic states at high pressures.

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 (IRs) 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, α-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 IRs 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.

The new nickel tellurite chloride compound Ni15Te12O34Cl10--synthesis, crystal structure and magnetic properties

A new nickel tellurite oxohalide, Ni(15)Te(12)O(34)Cl(10), has been prepared by chemical vapour transport reactions and the crystal structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the triclinic space group P1[combining macron] with the pseudomonoclinic cell parameters a = 10.3248(6) Å, b = 10.3249(6) Å, c = 11.6460(8) Å, α = 73.782(6)°, β = 73.782(6)°, γ = 63.51(2)°, Z = 1, R(1) = 0.0264. The Ni(2+) ions have octahedral [NiO(6)] and [NiO(4)Cl(2)] coordinations, the Te(4+) ions have one-sided [TeO(3)] and [TeO(4)] coordinations. The crystal structure can be described as consisting of nickel oxide ribbons extending along (001) that are connected by corner sharing [TeO(3)] and [TeO(4)] groups to build the open framework structure. The chlorine atoms and the Te-lone pairs are facing voids in the oxide framework. The new compound undergoes two successive antiferromagnetic ordering transitions at ∼50 K and ∼10 K. The Curie-Weiss temperature obtained from detailed evaluation of the high-temperature magnetic susceptibilities is positive indicating predominant ferromagnetic superexchange interactions between the Ni magnetic moments.

Persistent spin dynamics intrinsic to amplitude-modulated long-range magnetic order

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe(2)O(5)Br is surprisingly found to persist down to 53(3) mK (T/T(N)~1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T→0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence.