D. I. Khomskii

Multiferroics: Different ways to combine magnetism and ferroelectricity

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 Fe3O4, is discussed.

Multiferroicity due to charge ordering

In this contribution to the special issue on multiferroics we focus on multiferroicity driven by different forms of charge ordering. We will present the generic mechanisms by which charge ordering can induce ferroelectricity in magnetic systems. There is a number of specific classes of materials for which this is relevant. We will discuss in some detail (i) perovskite manganites of the type (PrCa)MnO3, (ii) the complex and interesting situation in magnetite Fe3O4, (iii) strongly ferroelectric frustrated LuFe2O4 and (iv) an example of a quasi-one-dimensional organic system. All these are ‘type-I’ multiferroics, in which ferroelectricity and magnetism have different origins and occur at different temperatures. In the second part of this article we discuss ‘type-II’ multiferroics, in which ferroelectricity is completely due to magnetism, but with charge ordering playing an important role, such as (v) the newly discovered multiferroic Ca3CoMnO6, (vi) possible ferroelectricity in rare earth perovskite nickelates of the type RNiO3, (vii) multiferroic properties of manganites of the type RMn2O5, (viii) perovskite manganites with magnetic E-type ordering and (ix) bilayer manganites. (Some figures in this article are in colour only in the electronic version)

Orbital-Assisted Metal-Insulator Transition in VO2

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO2] hexagonal layer. Both cobaltites exhibit large thermopower values (S-300 K similar to 140 muV K-1), low resistivity values (rho(300) (K) 40-60 mOmega cm) and small thermal conductivities (kappa(300) (K) similar to I W K-1 m(-1)). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = rho(H)-rho(H=0)/rho(H=0)), reaching, at 2.5 K, -85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.

Orbitally induced Peierls state in spinels

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.

Spin-State Transition in LaCoO3: Direct Neutron Spectroscopic Evidence of Excited Magnetic States

A gradual spin-state transition occurs in LaCoO3 around T approximately 80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at approximately 0.6 meV appears, whose intensity increases with temperature, following the bulk magnetization. Within a model including crystal-field interaction and spin-orbit coupling, we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (t(2g)(5)e(g)(1), S=1) versus high-spin (t(2g)(4)e(g)(2), S=2) states. Since the g factor obtained from the field dependence of the inelastic neutron scattering is g approximately 3, the second interpretation is definitely favored.

Ising Magnetism and Ferroelectricity in Ca3CoMnO6

The origin of both the Ising chain magnetism and ferroelectricity in Ca3CoMnO6 is studied by ab initio electronic structure calculations and x-ray absorption spectroscopy. We find that Ca3CoMnO6 has alternate trigonal prismatic Co2+ and octahedral Mn4+ sites in the spin chain. Both the Co2+ and Mn4+ are in the high-spin state. In addition, the Co2+ has a huge orbital moment of 1.7micro_{B} which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed upward arrow upward arrow downward arrow downward arrow antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.

Interplay between orbital ordering and lattice distortions in LaMnO3, YVO3, and YTiO3

We have studied the interplay between orbital ordering, Jahn-Teller and

Pyroxenes : a new class of multiferroics

{\mathrm{GdFeO}}_{3}

Orbitally Driven Spin-Singlet Dimerization in S=1 La4Ru2O10

-type lattice distortions in perovskite-type transition-metal oxides using model Hartree-Fock calculations. It has been found that the covalency between A-site cations and oxygens causes interaction between the Jahn-Teller and