arXiv subjects

I. S. Beloborodov

Publications and source records attributed to I. S. Beloborodov.

At least 19 recordsLinked to original sources

arXiv2019

Electronic ferroelectricity and magneto-electric effect in phase separated magnetic oxidesElectronic ferroelectricity and magneto-electric effect in phase separated magnetic oxides

We consider phase separated states in magnetic oxides (MO) thin films. We show that these states have a non-zero electric polarization. Moreover, the polarization is intimately related to a spatial distribution of magnetization in the film. Polarized states with opposite polarization and opposite magnetic configuration are degenerate. An external electric field removes the degeneracy and allows to switch between the two states. So, one can control electric polarization and magnetic configuration of the phase separated MO thin film with the external electric field.

cond-mat.str-el↗

arXiv2019

Stripe structures in phase separated magnetic oxides

We investigate the phase separated inhomogeneous charge and spin states in magnetic oxides. In particular, we study one dimensional harmonic waves and stripe structures. We show that harmonic spin charge waves are unstable and inevitably transform into two or three dimensional structures, while the stripe structures can be stable for certain parameters. Such stripe structures may allow the control of magnetic state with electric field in a magnetic oxide thin film.

cond-mat.str-el↗

arXiv2018

Studying of the interlayer interaction in magnetic multilayers (FM/I/FM) measuring the FMR peak asymmetry

We experimentally study the interlayer interaction in a magnetic multilayer system ferromagnet/insulator/ferromagnet with different spacer thickness. We demonstrate that the sign and the magnitude of the interaction can be deduced from the FMR peak shape rather than from the FMR peak shift. The proposed technique allows studying the interlayer interaction using a single sample (without a reference sample for comparison).

cond-mat.mtrl-sci↗

arXiv2018

Magneto-electric effect in doped magnetic ferroelectrics

We propose a model of magneto-electric effect in doped magnetic ferroelectrics. This magneto-electric effect does not involve the spin-orbit coupling and is based purely on the Coulomb interaction. We calculate magnetic phase diagram of doped magnetic ferroelectrics. We show that magneto-electric coupling is pronounced only for ferroelectrics with low dielectric constant. We find that magneto-electric coupling leads to modification of magnetization temperature dependence in the vicinity of ferroelectric phase transition. A peak of magnetization appears. We find that magnetization of doped magnetic ferroelectrics strongly depends on applied electric field.

cond-mat.mtrl-sci↗

arXiv2018

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

We study interlayer exchange interaction in magnetic tunnel junctions with ferroelectric barrier. We focus on the influence of image forces on the voltage dependence of the interlayer magnetic interaction (magneto-electric effect). The influence of the image forces is twofold: 1) they significantly enforce magneto-electric effect occurring due to the surface charges at the interface between ferroelectric and ferromagnets; 2) in combination with voltage dependent dielectric constant of the ferroelectric barrier image forces cause an additional contribution to the magneto-electric effect in magnetic tunnel junctions. This contribution can exceed the one coming from surface charges. We compare the interlayer exchange coupling voltage variation with spin transfer torque effect and show that for half-metallic electrodes the interlayer exchange coupling variation is dominant and defines the magnetic state and dynamics of magnetization in the tunnel junction.

cond-mat.mes-hall↗

arXiv2017

Influence of the Coulomb interaction on the exchange coupling in granular magnets

We develop a theory of the exchange interaction between ferromagnetic (FM) metallic grains embedded into insulating matrix by taking into account the Coulomb blockade effects. For bulk ferromagnets separated by the insulating layer the exchange interaction strongly depends on the height and thickness of the tunneling barrier created by the insulator. We show that for FM grains embedded into insulating matrix the exchange coupling additionally depends on the dielectric properties of this matrix due to the Coulomb blockade effects. In particular, the FM coupling decreases with decreasing the dielectric permittivity of insulating matrix. We find that the change in the exchange interaction due to the Coulomb blockade effects can be a few tens of percent. Also, we study dependence of the intergrain exchange interaction on the grain size and other parameters of the system.

cond-mat.mes-hall↗

arXiv2017

Microscopic theory of the Coulomb based exchange coupling in magnetic tunnel junctions

We study interlayer exchange coupling (IEC) based on the many-body Coulomb interaction between conduction electrons in magnetic tunnel junction (MTJ). This mechanism complements the known IEC based on virtual electron hopping (or spin currents). We find that these two mechanisms have different behavior on system parameters. The Coulomb based IEC may exceed the hopping based exchange coupling. We show that the Coulomb based exchange coupling, in contrast to the hopping based coupling, depends strongly on the dielectric constant of the insulating layer. The dependence of the IEC on the dielectric properties of the insulating layer in MTJ is similar to magneto-electric (ME) effect where electric and magnetic degrees of freedom are coupled. We calculate the IEC as a function of temperature and electric field for MTJ with ferroelectric (FE) layer and show that IEC has a sharp decrease in the vicinity of the FE phase transition and varies strongly with external electric field.

cond-mat.mes-hall↗

arXiv2017

Influence of image forces on the electron transport in ferroelectric tunnel junctions

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.

cond-mat.mes-hall↗

arXiv2017

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

cond-mat.mes-hall↗

arXiv2015

Conductance of a SET with a retarded dielectric layer in the gate capacitor

We study conductance of a single electron transistor (SET) with a ferroelectric (or dielectric) layer placed in the gate capacitor. We assume that ferroelectric (FE) has a retarded response with arbitrary relaxation time. We show that in the case of "fast" but still retarded response of the FE (dielectric) layer an additional contribution to the Coulomb blockade effect appears leading to the suppression of the SET conductance. We take into account fluctuations of the FE (dielectric) polarization using Monte-Carlo simulations. For "fast" FE these fluctuations partially suppress the additional Coulomb blockade effect. Using Monte-Carlo simulations we study the transition from "fast" to "slow" FE. For high temperatures the peak value of the SET conductance is almost independent of the FE relaxation time. For temperatures close to the FE Curie temperature the conductance peak value non-monotonically depends on the FE relaxation time. A maximum appears when the FE relaxation time is of the order of the SET discharging time. Below the Curie point the conductance peak value decreases with increasing the FE relaxation time. The conductance shows the hysteresis behavior for any FE relaxation time at temperatures below the FE transition point. We show that conductance hysteresis is robust against FE internal fluctuations.

cond-mat.mes-hall↗

arXiv2015

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

We study the competition of magneto-dipole, anisotropy and exchange interactions in composite three dimensional multiferroics. Using Monte Carlo simulations we show that magneto-dipole interaction does not suppress the ferromagnetic state caused by the interaction of the ferroelectric matrix and magnetic subsystem. However, the presence of magneto-dipole interaction influences the order-disorder transition: depending on the strength of magneto-dipole interaction the transition from the ferromagnetic to the superparamagnetic state is accompanied either by creation of vortices or domains of opposite magnetization. We show that the temperature hysteresis loop occurs due to non-monotonic behavior of exchange interaction versus temperature. The origin of this hysteresis is related to the presence of stable magnetic domains which are robust against thermal fluctuations.

cond-mat.mes-hall↗

arXiv2015

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.

cond-mat.dis-nn↗

arXiv2015

Coupling of (ferro)electricity and magnetism through Coulomb blockade in Composite Multiferroics

Composite multiferroics are materials exhibiting the interplay of ferroelectricity, magnetism, and strong electron correlations. Typical example --- magnetic nano grains embedded in a ferroelectric matrix. Coupling of ferroelectric and ferromagnetic degrees of freedom in these materials is due to the influence of ferroelectric matrix on the exchange coupling constant via screening of the intragrain and intergrain Coulomb interaction. Cooling typical magnetic materials the ordered state appears at lower temperatures than the disordered state. We show that in composite multiferroics the ordered magnetic phase may appear at higher temperatures than the magnetically disordered phase. In non-magnetic materials such a behavior is known as inverse phase transition.

cond-mat.mes-hall↗

arXiv2015

Proximity coupling of granular film with ferroelectric substrate and giant electro-resistance effect

We study electron transport in granular film placed above the ferroelectric substrate. We show that the conductivity of granular film strongly depends on the ferroelectric state due to screening effects which modify the Coulomb blockade in granular film. In particular, the electric current in granular film is controlled by the direction of ferroelectric polarization. We show that the ferroelectric/granular film system has a large electro-resistance effect. This effect can be utilized in memory and electric field sensor applications.

cond-mat.dis-nn↗

arXiv2015

Single electron tunneling with "slow" insulators

Usual paradigm in the theory of electron transport is related to the fact that the dielectric permittivity of the insulator is assumed to be constant, no time dispersion. We take into account the "slow" polarization dynamics of the dielectric layers in the tunnel barriers in the fluctuating electric fields induced by single-electron tunneling events and study transport in the single electron transistor (SET). Here "slow" dielectric implies slow compared to the characteristic time scales of the SET charging-discharging effects. We show that for strong enough polarizability, such that the induced charge on the island is comparable with the elementary charge, the transport properties of the SET substantially deviate from the known results of transport theory of SET. In particular, the coulomb blockade is more pronounced at finite temperature, the conductance peaks change their shape and the current-voltage characteristics show the memory-effect (hysteresis). However, in contrast to SETs with ferroelectric tunnel junctions, here the periodicity of the conductance in the gate voltage is not broken, instead the period strongly depends on the polarizability of the gate-dielectric. We uncover the fine structure of the hysteresis-effect where the "large" hysteresis loop may include a number of "smaller" loops. Also we predict the memory effect in the current-voltage characteristics $I(V)$, with $I(V)\neq -I(-V)$.

cond-mat.str-el↗

arXiv2014

Memory effect in ferroelectric single electron transistor: violation of conductance periodicity in the gate voltage

The fundamental property of most single-electron devices with quasicontinuous quasiparticle spectrum on the island is the periodicity of their transport characteristics in the gate voltage. This property is robust even with respect to placing the ferroelectric insulators in the source and drain tunnel junctions. We show that placing the ferroelectric inside the gate capacitance breaks this periodicity. The current-voltage characteristics of this SET strongly depends on the ferroelectric polarization and shows the giant memory-effect even for negligible ferroelectric hysteresis making this device promising for memory applications.

cond-mat.mes-hall↗

arXiv2014

Phenomenological theory of magneto-electric coupling in granular multiferroics

We study coupling between the ferroelectric polarization and magnetization of granular ferromagnetic film using a phenomenological model of combined multiferroic system consisting of granular ferromagnetic film placed above the ferroelectric (FE) layer. The coupling is due to screening of Coulomb interaction in the granular film by the FE layer. Below the FE Curie temperature the magnetization has hysteresis as a function of electric field. Below the magnetic ordering temperature the polarization has hysteresis as a function of magnetic field. We study the magneto-electric coupling for weak and strong spatial dispersion of the FE layer. The effect of mutual influence decreases with increasing the spatial dispersion of the FE layer. For weak dispersion the strongest coupling occurs in the vicinity of the ferroelectric-paraelectric phase transition. For strong dispersion the situation is the opposite. We study the magneto-electric coupling as a function of distance between the FE layer and the granular film. For large distances the coupling decays exponentially due to the exponential decrease of electric field produced by the oscillating charges in the granular ferromagnetic film.

cond-mat.mes-hall↗

arXiv2014

Interplay of ferroelectricity and single electron tunneling

We investigate the interplay of ferroelectricity and quantum electron transport at the nanoscale in the regime of Coulomb blockade. Ferroelectric polarization in this case is no longer the external parameter but should be self-consistently calculated along with electron hopping probabilities leading to new physical transport phenomena studying in this paper. These phenomena appear mostly due to effective screening of a grain electric field by ferroelectric environment rather than due to polarization dependent tunneling probabilities. At small bias voltages polarization can be switched by a single excess electron in the grain. In this case transport properties of SET exhibit the instability (memory effect).

cond-mat.mes-hall↗