O. G. Udalov

Coupling of ferroelectricity and ferromagnetism through Coulomb blockade in composite multiferroics

Department of Theoretical Physics, Moscow Institute of Physics and Technology, 141700 Moscow, Russia(Dated: March 3, 2014)Composite multiferroics are materials exhibiting the interplay of ferroelectricity, magnetism, andstrong electron correlations. Typical example | magnetic nano grains embedded in a ferroelectricmatrix. Coupling of ferroelectric and ferromagnetic degrees of freedom in these materials is due tothe inuence of ferroelectric matrix on the exchange coupling constant via screening of the intragrainand intergrain Coulomb interaction. Cooling typical magnetic materials the ordered state appears atlower temperatures than the disordered state. We show that in composite multiferroics the orderedmagnetic phase may appear at higher temperatures than the magnetically disordered phase. Innon-magnetic materials such a behavior is known as inverse phase transition.

Interplay of Coulomb blockade and ferroelectricity in nanosized granular materials

We study electron transport properties of composite ferroelectrics --- materials consisting of metallic grains embedded in a ferroelectric matrix. In particular, we calculate the conductivity in a wide range of temperatures and electric fields, showing pronounced hysteretic behavior. In weak fields, electron cotunneling is the main transport mechanism. In this case, we show that the ferroelectric matrix strongly influences the transport properties through two effects: i) the dependence of the Coulomb gap on the dielectric permittivity of the ferroelectric matrix, which in turn is controlled by temperature and external field; and ii) the dependence of the tunneling matrix elements on the electric polarization of the ferroelectric matrix, which can be tuned by temperature and applied electric field as well. In the case of strong electric fields, the Coulomb gap is suppressed and only the second mechanism is important. Our results are important for i) thermometers for precise temperature measurements and ii) ferrroelectric memristors.

Electric field control of magnetic properties and magneto-transport in composite multiferroics.

We study magnetic state and electron transport properties of composite multiferroic system consisting of a granular ferromagnetic thin film placed above the ferroelectric substrate. Ferroelectricity and magnetism in this case are coupled by the long-range Coulomb interaction. We show that magnetic state and magneto-transport strongly depend on temperature, external electric field and electric polarization of the substrate. Ferromagnetic order exists at finite temperature range around ferroelectric Curie point. Outside the region the film is in the superparamagnetic state. We demonstrate that magnetic phase transition can be driven by an electric field and magneto-resistance effect has two maxima associated with two magnetic phase transitions appearing in the vicinity of the ferroelectric phase transition. We show that positions of these maxima can be shifted by the external electric field and that the magnitude of the magneto-resistance effect depends on the mutual orientation of external electric field and polarization of the substrate.

Competition of the Coulomb and hopping-based exchange interactions in granular magnets

We study exchange coupling due to the interelectron Coulomb interaction between two ferromagnetic grains embedded into insulating matrix. This contribution to the exchange interaction complements the contribution due to virtual electron hopping between the grains. We show that the Coulomb and the hopping based exchange interactions are comparable. However, for most system parameters these contributions have opposite signs and compete with each other. In contrast to the hopping based exchange interaction the Coulomb based exchange is inversely proportional to the dielectric constant of the insulating matrix

Electron transport properties of composite ferroelectrics

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Influence of image forces on electron transport in ferroelectric tunnel junctions

. The total intergrain exchange interaction has a complicated dependence on the dielectric permittivity of the insulating matrix. Increasing

Competition of magneto-dipole, anisotropy and exchange interactions in composite multiferroics.

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The Coulomb based magneto-electric coupling in multiferroic tunnel junctions and granular multiferroics

one can observe the ferromagnet-antiferromagnet (FM-AFM) and AFM-FM transitions. For certain parameters no transition is possible, however even in this case the exchange interaction has large variations, changing its value by three times with increasing the matrix dielectric constant.

Influence of image forces on the interlayer exchange interaction in magnetic tunnel junctions with a ferroelectric barrier

We study electron transport in composite ferroelectrics —materials consisting of metallic grains embedded in a ferroelectric matrix. Due to its complex tunable morphology the thermodynamic properties of these materials can be essentially different from bulk or thin-film ferroelectrics. We calculate the conductivity of composite ferroelectrics by taking into account the interplay between charge localization, multiple grain boundaries, strong Coulomb repulsion, and ferroelectric order parameter. We show that the ferroelectricity plays a crucial role on the temperature behavior of the conductivity in the vicinity of the ferroelectric-paraelectric transition.

Magnetoelectric effect in doped magnetic ferroelectrics

We study influence of image forces on conductance of ferroelectric tunnel junctions. We show that the influence of image forces is twofold: i) they enhance the electro-resistance effect due to polarization hysteresis in symmetric tunnel junctions at non-zero bias and ii) they produce the electro-resistance effect due to hysteresis of dielectric permittivity of ferroelectric barrier. We study dependence of ferroelectric tunnel junction conductance on temperature and show that image forces lead to strong conductance variation with temperature.