Multiferroics — materials which are simultaneously (ferro)magnetic and ferroelectric, and often also ferroelastic — attract now considerable attention, both because of the interesting physics ...involved and as they promise important practical applications. In this paper, I give a survey of microscopic factors determining the coexistence of these properties, and discuss different possible routes to combine them in one material. In particular, the role of the occupation of d-states in transition metal perovskites is discussed, possible role of spiral magnetic structures is stressed, and the novel mechanism of ferroelectricity in magnetic systems due to combination of site-centred and bond-centred charge ordering is presented. Microscopic nature of multiferroic behaviour in several particular materials, including magnetite Fe
3O
4, is discussed.
Spin ice systems display a variety of very nontrivial properties, the most striking being the existence in them of magnetic monopoles. Such monopole states can also have nontrivial electric ...properties: there exist electric dipoles attached to each monopole. A novel situation is encountered in the moment fragmentation (MF) state, in which monopoles and antimonopoles are perfectly ordered, whereas spins themselves remain disordered. We show that such partial ordering strongly modifies the electric activity of such systems: the electric dipoles, which are usually random and dynamic, become paired in the MF state in (d, -d) pairs, thus strongly reducing their electric activity. The electric currents existing in systems with noncoplanar spins are also strongly influenced by MF. We also consider modifications in dipole and current patterns in magnetic textures (domain walls, local defects) and at excitations with nontrivial dynamics in a MF state, which show very rich behaviour and which could in principle allow to control them by electric field.
We consider the superstructures, which can be formed in spinels containing on B sites the transition-metal ions with partially filled t(2g) levels. We show that, when such systems are close to the ...itinerant state (e.g., have an insulator-metal transition), there may appear in them an orbitally driven Peierls state. We explain by this mechanism the very unusual superstructures observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest that a similar phenomenon should be observed in NaTiO2 and possibly in some other systems.
Two-dimensional (2D) magnetic materials are of great current interest for their promising applications in spintronics. Here we propose the van der Waals (vdW) material VI3 to be a 2D Ising ...ferromagnet (FM), using density functional calculations, crystal field level diagrams, superexchange model analyses, and Monte Carlo simulations. The a1ge′−11 ground state in the trigonal crystal field gives rise to the 2D Ising FM due to a significant single ion anisotropy (SIA) and enhanced FM superexchange both associated with the Sz=1 and Lz=−1 states of V3+ ions. We find that a tensile strain on the VI3 monolayer further stabilizes the a1ge′−11 ground state, and its Curie temperature (TC) would increase from 70 to 90–110 K under a 2.5%–5% tensile strain. Moreover, we suggest a group of spin-orbital states with a strong SIA which may help to search more 2D Ising magnets.
The close connection of electricity and magnetism is one of the cornerstones of modern physics. This connection has a crucial role from a fundamental point of view and in practical applications, ...including spintronics and multiferroic materials. A breakthrough was a recent proposal that in magnetic materials called spin ice the elementary excitations have a magnetic charge and behave as magnetic monopoles. I show that, besides magnetic charge, there should be an electric dipole attached to each magnetic monopole. This opens new possibilities to study and control such monopoles using an electric field. Thus, the electric-magnetic analogy goes even further than usually assumed: whereas electrons have electric charge and magnetic dipole (spin), magnetic monopoles in spin ice, while having magnetic charge, also have an electric dipole.
The material class of rare earth nickelates with high Ni3+ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance ...of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO3 thin films has also triggered intensive research efforts. LaNiO3 seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO3 single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni2+ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.
This article presents a survey of many nontrivial effects connected with the coupling of electric and magnetic degrees of freedom in solids—the field initiated by I.E. Dzyaloshinskii in 1959. I ...briefly consider the main physics of multiferroic materials, and concentrate on different effects “beyond multiferroics”, based on the same physical mechanisms which operate in multiferroics. In particular they lead to nontrivial electric properties of different magnetic textures—such as the appearance of dipoles on magnetic monopoles in spin ice, dipoles on some domain walls in the usual ferromagnets, on skyrmions etc. The inverse effect, the appearance of magnetic monopoles on electric charges in magnetoelectrics, is also discusses. This nontrivial electric activity of different magnetic textures has manifestations in many experimental properties of these materials, and it can potentially lead to novel applications.
Specific features of orbital and spin structure of transition-metal compounds in the case of the face-sharing MO sub(6) octahedra are analyzed. In this geometry, we consider the form of the ...spin-orbital Hamiltonian for transition-metal ions with double (e super()sigma sub(g)) or triple (t sub(2g)) orbital degeneracy. Trigonal distortions typical of the structures with face-sharing octahedra lead to splitting of t sub(2g) orbitals into an a sub(1)g singlet and e super(pi) sub(g) doublet. For both doublets (e super()sigma sub(g) and e super(pi) sub(g)), in the case of one electron or hole per site, we arrive at a symmetric model with the orbital and spin interaction of the Heisenberg type and the Hamiltonian of unexpectedly high symmetry: SU(4). Thus, many real materials with this geometry can serve as a testing ground for checking the prediction of this interesting theoretical model. We also compare general trends in the spin-orbital ("KugelKhomskii") exchange interaction for three typical situations: those of MO sub(6) octahedra with common corner, common edge, and the present case of common face, which has not been considered yet.
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