arXiv subjects
Publications and source records attributed to M. D. Glinchuk.
The review covers the theoretical and experimental results obtained in the recent years by the scientists with the help of comprehensive investigation of nanoferroics and multiferroics. The main attention will be paid to spontaneous flexoeffects and reentrant phase in nanoferroics as well as to a recently discovered giant magnetoelectric effect in multiferroics.
In the framework of Landau--Ginzburg--Devonshire (LGD) phenomenological theory, the influence of misfit strains, surface energy, and finite-size effects on phase diagrams, polar properties, and hysteresis loops has been calculated for multilayered thin films of the type ferroelectric/virtual ferroelectric. The influence of elastic deformations that arise at the interface thin film--substrate owing to a mismatch between the lattice constants in the film and the substrate on the phase diagrams of multilayered thin films virtual ferroelectric SrTiO$_{3}$/ferroelectric BaTiO$_{3}$ has been studied for the first time. In contrast to bulk BaTiO$_{3}$, in which only four phases (cubic, tetragonal, orthorhombic, and rhombohedral) can exist, it turned out that six thermodynamically stable BaTiO$_{3}$ phases (paraelectric phase and tetragonal (FEc), two monoclinic (FEaac and FEac), and two orthorhombic (FEa and FEaa) ferroelectric phases) can exist in multilayered SrTiO$_{3}$/BaTiO$_{3}$ films. The main polar properties of hysteresis loops (shape, coercive field, and spontaneous polarization) in thin multilayered SrTiO$_{3}$/BaTiO$_{3}$ films are calculated. It is shown that the system demonstrates a strong dependence of its polar properties on the thickness of SrTiO$_{3}$ and BaTiO$_{3}$ layers, as well as on the elastic misfit strains, with SrTiO$_{3}$ playing the role of dielectric layer: the thicker the layer, the stronger is the depolarization field, which, in its turn, reduces the spontaneous polarization in the BaTiO$_{3}$ film.
Using the symmetry theory we analyze of the flexomagnetic effect in all 90 magnetic classes and showed that 69 of them are flexomagnetic. Then we explore how the symmetry breaking, inevitably present in the vicinity of the surface, changes the local symmetry and thus the form of the flexomagnetic tensors. All possible surface magnetic classes (in the number of 19) were obtained from the 90 bulk magnetic classes for the surface cuts 001, 010 and 100 types. It appeared that all 90 bulk magnetic classes become flexomagnetic, piezomagnetic and piezoelectric in the vicinity of surface. Using the free energy approach, we show that the flexomagnetic effect leads to a new type of flexo-magnetoelectric (FME) coupling in nanosized and bulk materials, in all spatial regions, where the polarization and (anti)magnetization vectors are spatially inhomogeneous due to external or internal forces. The linear FME coupling, proportional to the product of the gradients of (anti)magnetization and polarization, flexoelectric and flexomagnetic tensors, is significant in nanosized ferroelectrics-(anti)ferromagnetics, where gradients of the polarization and magnetization obligatory exist. The spontaneous FME coupling induced by the spatial confinement give rise to the size-dependent linear magnetoelectric coupling in nanosized ferroelectrics-(anti)ferromagnetics. We show that the flexomagnetic effect may lead to improper ferroelectricity in bulk (anti)ferromagnetics via the linear and nonlinear FME coupling. Inhomogeneous spontaneous polarization is induced by the (anti)magnetization gradient, which exists in all spatial regions, where polarization varies and (anti)magnetization vector changes its direction. The gradient can be induced by the surface influence as well as by external strain via e.g. the sample bending.
Based on our analytical results [http://arxiv.org/abs/1006.3670], we predict that undoped nanoparticles (size <10-100nm) of incipient ferroelectrics without any magnetic ions can become ferromagnetic even at room temperatures due to the inherent presence of a new type of magnetic defects with spin S=1, namely oxygen vacancies, where the magnetic triplet state is the ground state in the vicinity of the surface (magnetic shell), while the nonmagnetic singlet is the ground state in the bulk material (nonmagnetic core). Consideration of randomly distributed magnetic spins (S=1) had shown that magnetic properties of incipient ferroelectric nanoparticles are strongly size and temperature dependent due to the size and temperature dependence of their dielectric permittivity and the effective Bohr radius proportional to permittivity. The phase diagrams in coordinates temperature - particle radius are considered. In particular, for particle radii less that the critical radius ferromagnetic long-range order appears in a shell region of thickness 5 - 50 nm once the concentration of magnetic defects exceeds the magnetic percolation threshold. The critical radius is calculated in the mean field theory from the condition of the magnetic defects exchange energy equality to thermal energy. For particle radii higher than critical value only the paramagnetic phase is possible. The conditions of the super-paramagnetic state appearance in the assembly of nanoparticles with narrow distribution function of their sizes are discussed also.
Using the quantum-mechanical approach combined with the image charge method we calculated the lowest energy levels of the impurities and neutral vacancies with two electrons or holes located in the vicinity of flat surface of different solids. We obtained that the magnetic triplet state is the ground state of the impurities and neutral vacancies in the vicinity of surface, while the nonmagnetic singlet is the ground state in the bulk for e.g. He atom, Li+, Be++, etc. ions. The energy difference between the lowest triplet and singlet states strongly depends on the electron (hole) effective mass, dielectric permittivity of the solid and the distance from the surface. Pair interaction of the identical surface defects (two doubly charged impurities or vacancies with two electrons or holes) reveals the ferromagnetic spin state with the maximal exchange energy at the definite distance between the defects (~5-25 nm). We obtained that the nonmagnetic singlet state is the lowest one for a molecule with two electrons formed by a pair of identical surface impurities (like surface hydrogen), while its next state with deep enough negative energy minimum is the magnetic triplet. The metastable magnetic triplet state appeared for such molecule at the surface indicates the possibility of metastable orto-states of the hydrogen-like molecules, while they are absent in the bulk of material. We hope that obtained results could provide an alternative mechanism of the room temperature ferromagnetism observed in TiO2, HfO2, and In2O3 thin films with contribution of the oxygen vacancies.
The symmetry breaking inevitably present in the vicinity of any surface, namely an inversion center disappears in surface normal direction and only axes and planes normal to the surface conserve, gives rise to the spontaneous piezomagnetic, piezoelectric and magnetoelectric effects in nanosystems, while the effects can be absent in a bulk material. All these phenomena are inherent to nanos made of materials belonging to all ninety bulk magnetic classes. Therefore the new linear magnetoelectrics should appear among nanomaterials, nonpiezomagnetic and nonpiezoelectric in the bulk. To demonstrate this we consider the typical cases of ultra-thin films, nanowires and nanospheres. Coupled with a surface stress for nanoparticles and a mismatch strains for thin films on substrates the surface piezomagnetic and piezoelectric effects lead to the appearance of built-in magnetic and electric fields respectively. The built-in fields play an important role in the appearance of self-magnetization and self-polarization in the nanosystems paramagnetic and paraelectric in the bulk and can lead to the appearance of some other interesting properties absent in the bulk. We obtained analytical dependencies on sizes for the built-in fields and magnetoelectric coupling coefficients. The values of the built-in fields increase with the decrease of film thickness h or nanoparticles radii R as ~1/h or 1/R2 respectively, while the magnetoelectric coupling is inversely proportional to the sizes in both cases. This shows the strong influence of sizes on the considered properties of nanos and so opens the ways to govern the properties by the choice of the sizes and to create new multifunctional nanomaterials.
In the paper we present analytical calculations of the profiles and average values correlation radius of polarization fluctuations and generalized susceptibility in thin ferroelectric films of thickness L with in-plane (a-films) and out-of-plane (c-films) polarization orientation. The contribution of polarization gradient, surface energy and depolarization field (if any) were taken into account. For the second order ferroelectrics the correlation radius and generalized susceptibility diverge at the film critical thickness Lcr as anticipated. In the case of a-films where depolarization field is absent, the surface energy and polarization gradient govern the size effects such as thickness-induced phase transition, corresponding correlation radius and generalize susceptibility divergence in the phase transition point. In the case of c-films with strong depolarization field, the surface energy, polarization gradient, intrinsic depolarization effect related with polarization inhomogeneous distribution and incomplete screening in the electrodes determine the correlation radius and generalized susceptibility size effect. At that contribution of all the factors are comparable at reasonable material parameters, but could be easily tuned by varying of surface energy coefficients, polarization gradient contribution and screening conditions.
We show that misfit strain originated from the film-substrate lattice mismatch strongly increases the value of the quadratic magnetoelectric coupling. The giant magnetoelectric coupling, size effects and misfit strain cause strong changes of ferroic films phase diagrams at zero external magnetic and electric fields, in particular, the transformation of antiferromagnetic phase into ferromagnetic or ferrimagnetic ones for compressive or tensile misfit strains correspondingly as well as thickness induced paramagnetic or/and paraelectric phases appearance. Ferromagnetism appearance and magnetoelectric coupling increase in thin ferroelectric-antiferromagnetic films is in agreement with available experimental data and opens the way for tailoring of ferroic films magnetic and electric properties.
For the first time we predict the conditions of superparaelectric phase appearance in the ensemble of non-interacting spherical ferroelectric nanoparticles. The superparaelectricity in nanoparticle was defined by analogy with superparamagnetism, obtained earlier in small nanoparticles made of paramagnetic material. Calculations of correlation radius, energetic barriers of polarization reorientation and polarization response to external electric field, were performed within Landau-Ginzburg phenomenological approach for perovskites Pb(Zr,Ti)O3, BiFeO3 and uniaxial ferroelectrics rochelle salt and triglycine sulfate.
We show that the presence of free carriers in a substance can generate the multiferroic behavior. Namely, if the substance has mixed-valence ions, which can supply free carriers and have electric dipole and spin moments, all three types of long-range order (ferromagnetic, ferroelectric and magnetoelectric (ME)) can occur at low temperature. The physical origin of the effect is that charge carriers can mediate the multiferroic behavior via spin - spin (RKKY), dipole-dipole and dipole - spin interactions. Our estimate of the interaction magnitude shows that there exist an optimal carrier concentration, at which the strength of ME interaction is maximal and comparable to that of spin-spin RKKY interaction. This permits to conclude that in substances, where RKKY interaction between local spins is not small, a substantial value of free carriers mediated ME interaction can occur. Our analysis shows that disorder in the above substances does not suppress multiferroic effects.
The general approach for the consideration of the magnetoelectric effects in ferroic nanorods is proposed in the framework of the phenomenological theory. The intrinsic surface stress, magneto- and electrostriction as well as piezoelectric and piezomagnetic effects are included into the free energy. The intrinsic surface stress under the curved nanoparticle surface is shown to play an important role in the shift of ferroelectric and ferromagnetic transition temperatures and built-in magnetic and electric fields appearance, which are inversely proportional to the nanorod radius. We consider the case of quadratic and linear magnetoelectric coupling coefficients. The linear coupling coefficient is radius independent, whereas the quadratic ones include terms inversely proportional to the nanorod radius and thus strongly increase with decrease of the radius. The predicted giant relative dielectric tunability in the vicinity of ferromagnetic and ferroelectric phase transition points induced by quadratic magnetoelectric coupling increases by 2-50 times. The quadratic magnetoelectric coupling dramatically changes the phase diagrams of ferroic nanorods when the radius decreases. In particular the second order phase transition may become a first one, the triple point state characterized by continuous set of order parameters appears at zero external electric and magnetic fields and the tricritical points appear under external fields.
Using electron spin resonance, lattice position and dynamic properties of Mn2+ ions were studied in 0.5 and 2 % manganese doped SrTiO3 ceramics prepared by conventional mixed oxide method. The measurements showed that Mn2+ ions substitute preferably up to 97 % for Sr if the ceramics is prepared with a deficit of Sr ions. Motional narrowing of the Mn2+ ESR spectrum was observed when temperature increases from 120 K to 240-250 K that was explained as a manifestation of off-center position of this ion at the Sr site. From the analysis of the ESR spectra the activation energy Ea = 86 mV and frequency factor 1/?0 ? (2-10)x10^(-14) 1/s for jumping of the impurity between symmetrical off-center positions were determined. Both values are in agreement with those derived previously from dielectric relaxation. This proves the origin of dielectric anomalies in SrTiO3:Mn as those produced by the reorientation dynamics of Mn2+ dipoles.
Recently new incipient ferroelectrics with off-center ions SrTiO3 with Mn2+ substituted for Sr2+ was revealed (Tkach et. al PRB 73, 104113 (2006). Experimental investigation of Sr1-xMnxTiO3 properties, namely temperature, frequency and external electric field dependence of dielectric permittivity at different concentrations gave evidence about phase transitions induced by the impurities. However their nature was not found out up to now because of the absence of the theoretical analysis of the results. In this paper we performed such analysis by the theoretical description of the temperature of dielectric permittivity maxima dependence on Mn2+ concentration, the change of residual polarization with temperature, frequency and temperature dependence of the permittivity with Arrhenius and Vogel-Fulcher law at smaller and larger concentration respectively. The obtained results had shown that at x less than 0,03 and x between 0,15 and 0,03 the considered system Sr1-xMnxTiO3 is in dipole glass state and mixed ferroglass phase respectively.
In this paper we study the size effects of the ferroelectric nanotube phase diagrams and polar properties allowing for effective surface tension and depolarization field influence. The approximate analytical expression for the paraelectric-ferroelectric transition temperature dependence on the radii of nanotube, polarization gradient coefficient, extrapolation length, surface tension and electrostriction coefficient was derived. It was shown that the transition temperature could be higher than the one of the bulk material for negative electrostriction coefficient. Therefore we predict conservation and enhancement of polarization in long ferroelectric nanotubes. Obtained results explain the observed ferroelectricity conservation and enhancement in Pb(Zr,Ti)O_3 and BaTiO_3 nanotubes.
In the paper we consider size effects of phase transitions and polar properties of the thin antiferroelectric films. We modified phenomenological approach proposed by Kittel. The Euler-Lagrange equations were solved by direct variational method. The free energy with renormalized coefficients depending on the film thickness has been derived. The approximate analytical expression for the coefficients dependence on film thickness, temperature, polarization gradient coefficient and extrapolation lengths were obtained. We have shown how the anti-ferroelectric double hysteresis loop transforms into the ferroelectric single one under the film thickness decrease. Proposed theoretical consideration explains the experimental results obtained in antiferroelectric PbZrO_3 thin films.
For the first time we proposed the model for the calculations of the relaxor ferroelectrics films properties in the framework of the random field theory. We took into account the misfit strain between film and substrate as well as surface piezoelectric effect that causes built-in electric field in the strained films. In the statistical theory framework we calculated random field distribution function with the electric dipoles and monopoles as the field sources. It was shown that with thickness decrease the mean field decreases, while the width of the distribution function increases. This leads to the additional smearing of the phase transition in the films in comparison to the bulk relaxors. As an example the dependence of the order parameter and dielectric susceptibility on the film thickness, temperature and random fields distribution function parameters was obtained. For free standing film the existence of critical thickness of relaxor state transformation into glassy state was predicted. Contrary to this the appearance of misfit strain induced ferroelectric phase appeared to be possible for some pairs film-substrate. We have shown that susceptibility temperature maximum shift with frequency in relaxor ferroelectric thin films obeys Vogel-Fulcher law with parameters dependent on film thickness. For the first time the analytical dependences of freezing temperature decreases and activation energy on the thickness was obtained, namely freezing temperature decreases and activation energy increases with film thickness decrease. Obtained results quantitatively agree with the available experimental data for PbMg1/3Nb2/3O3 relaxor thin films.
We calculated the influence of surface tension on the barriers of oxygen ionic conductivity in nanograin ceramics. Activation energy of oxygen ions diffusion via oxygen vacancies which were considered as the dilatational centers was calculated. This energy was shown to decrease with nanoparticle sizes decreasing. The distribution function of activation energy was derived on the basis of distribution of nanoparticle sizes. We obtained an analytical expressions of ionic conductivity dependence on the temperature and nanograin sizes. These formulas fitted pretty good the observed earlier behaviour of oxygen conductivity in nanograin ceramics of ZrO2:16% Y observed earlier. Therefore the consideration we carried out had shown that the surface tension in nanoparticles is physical mechanism responsible for the essential enhancement of the oxygen ionic conductivity observed in nanograin samples, the main contribution to the conductivity being related to the region in vicinity of the particle surface.
The theoretical and experimental investigation of ferroelectric nanopowders is performed. The manifestation in radiospectroscopy spectra of size driven ferroelectric-paraelectric phase transition at some critical particle average size was the main goal of the consideration. In theoretical part the size effect for the materials with ferroelectric tetragonal phase and cubic paraelectric phase was considered allowing for the spontaneous polarization inhomogeneity inside a particle and distribution of particle sizes. In ESR the transformation of the spectra from tetragonal symmetry to cubic symmetry with decreasing of nanoparticle sizes was calculated. Measurements of Fe3+ ESR spectra in nanopowder of BaTiO3 were carried out at room temperature. The decrease of intensity of tetragonal symmetry ESR lines of Fe3+ and appearance of cubic symmetry line with asymmetry of the shoulders was observed with the average sizes decrease with complete disappearance of tetragonal spectrum at average size less or equal 40 nm. The comparison of the theory with experiment was carried out. The value of critical size Rc = 40 nm was extracted from ESR data. The asymmetry and broadening of right hand side shoulder of ESR cubic symmetry line was shown to be related to contribution of paramagnetic centers in the vicinity of the particles surface. The deconvolution of the cubic line allowed to show, that this region size is about 3 nm.