Search results for "Multiferroics"
showing 10 items of 45 documents
Magnetic Interactions in BiFeO$_3$: a First-Principles Study
2018
First-principles calculations, in combination with the four-state energy mapping method, are performed to extract the magnetic interaction parameters of multiferroic ${\mathrm{BiFeO}}_{3}$. Such parameters include the symmetric exchange (SE) couplings and the Dzyaloshinskii-Moriya (DM) interactions up to second-nearest neighbors, as well as the single-ion anisotropy (SIA). All magnetic parameters are obtained not only for the $R3c$ structural ground state, but also for the $R3m$ and $R\overline{3}c$ phases in order to determine the effects of ferroelectricity and antiferrodistortion distortions, respectively, on these magnetic parameters. In particular, two different second-nearest-neighbor…
On the theory of thermodynamic properties of geometrically confined disordered ferroelectrics
2013
Abstract We propose a theoretical approach to calculate the thermodynamic properties of thin films fabricated from disordered ferroelectrics. To calculate the above thermodynamic properties, we use so-called random field method, modified for the case of thin films. The essence of modification is the altering of the interaction between impurity dipoles by geometrical confinement. We show that in thin films the ferroelectric phase transition is inhibited as compared to the case of bulk samples. Our theory is generalizable to ferroelectrics of other shapes as well as magnets and multiferroics.
Revisiting spin cycloids in multiferroic BiFeO3
2018
We revisit the inverse spin current model that has been previously used to explain the existence of magnetic cycloids in bulk multiferroic ${\mathrm{BiFeO}}_{3}$. Using a first-principles-based effective Hamiltonian method, and in combination with Monte Carlo simulations, we predict a magnetic phase diagram as a function of first- and second-nearest-neighbor interaction strength in the spin current model and show that, in contrast with previous understanding, both first and second nearest neighbors have to be taken into account to be in accordance with experimental findings, including the existence of type-1 and type-2 cycloids with, respectively, $[1\overline{1}0]$ and $[11\overline{2}]$ p…
Multiferroics by Rational Design: Implementing Ferroelectricity in Molecule-Based Magnets
2012
Multiferroics (MF) are materials that exhibit simultaneouslyseveral ferroic order parameters. Among the multiferroicmaterials, those combining antiferro- or ferroelectricity (FE)and antiferro-, ferri-, or ferromagnetism (FM) within thesame material are highly desirable: the coexistence of thepolar and magnetic orders paves the way towards four-levelmemories while their interactions through the magnetoelec-tric effect makes it possible to control the magnetization byelectric fields and hence to develop electronically tuneablemagnetic devices, which are an essential feature for spin-tronics.
Magnetoelectric effect in antiferromagnetic multiferroic Pb(Fe1/2Nb1/2)O3 and its solid solutions with PbTiO3
2017
Antiferromagnets (AFMs) are presently considered as promising materials for applications in spintronics and random access memories due to the robustness of information stored in the AFM state against perturbing magnetic fields. In this respect, AFM multiferroics may be attractive alternatives for conventional AFMs as the coupling of magnetism with ferroelectricity (magnetoelectric effect) offers an elegant possibility of electric-field control and switching of AFM domains. Here we report the results of comprehensive experimental and theoretical investigations of the quadratic magnetoelectric (ME) effect in single crystals and highly resistive ceramics of $\mathrm{Pb}(\mathrm{F}{\mathrm{e}}_…
Molecular analog of multiferroics: Electric and magnetic field effects in many-electron mixed-valence dimers
2012
We show here that mixed-valence (MV) magnetic molecules with a significant electron delocalization are extremely sensitive to an external electric field. In particular, we focus on the symmetric many-electron MV binuclear complexes that are on the borderline between Robin and Day classes II and III. In these molecules, the double-exchange, which has been shown to lead to the ferromagnetic ground spin state, competes with the electric field, which tends to localize the spin, thus creating an electric dipole and stabilizing the spin states with lower multiplicities. This provides an efficient and easy way to control the ground spin state of the molecule through the double-exchange mechanism. …
Superspin glass phase and hierarchy of interactions in multiferroic PbFe1/2Sb1/2O3: an analog of ferroelectric relaxors?
2014
We have fabricated new perovskite multiferroic PbFeSbO3 with a high degree (up to 0.9) of chemical ordering and unexpectedly high-temperature magnetic relaxor properties, which can barely be described within concepts of conventional spin glass physics. Notably, we found that the field-temperature phase diagram of this material, in the extremely wide temperature interval, contains the de Almeida–Thouless-type critical line, which has been the subject of long debates regarding its possible experimental realization. We explain our findings by the creation, at high temperatures of not less than 250 K, of giant superspins (SSs), owing, curiously enough, to the antiferromagnetic superexchange int…
Structural and Magnetic Dynamics in the Magnetic Shape Memory Alloy Ni$_2$MnGa
2014
Magnetic shape memory Heusler alloys are multiferroics stabilized by the correlations between electronic, magnetic and structural order. To study these correlations we use time resolved x-ray diffraction and magneto-optical Kerr effect experiments to measure the laser induced dynamics in a Heusler alloy Ni$_2$MnGa film and reveal a set of timescales intrinsic to the system. We observe a coherent phonon which we identify as the amplitudon of the modulated structure and an ultrafast phase transition leading to a quenching of the incommensurate modulation within 300~fs with a recovery time of a few ps. The thermally driven martensitic transition to the high temperature cubic phase proceeds via…
Free Charge Carriers in Mixed Valency Oxides as Possible Mediators of Magnetoelectric Effect
2009
We show, that in a substance with mixed-valence ions, which can supply free carriers and have electric dipole and spin moments, three types of long-range order (ferromagnetic, ferroelectric and magnetoelectric (ME)) can occur at low temperatures. The origin of the effect is that free charge carriers can mediate the multiferroic behavior via spin – spin (RKKY), dipole-dipole and dipole - spin interactions. We show that there is an optimal carrier concentration, at which the strength of ME interaction is maximal and comparable to that of RKKY interaction. Our analysis shows that disorder and finite conductivity in the above substances does not suppress multiferroic effects.
Magnetic susceptibility of multiferroics and chemical ordering
2017
Magnetic susceptibility of two types of perovskite compounds A(Fe0.5M0.5)O3 A=Ba,Sr with M=Nb and Sb differing in the degree of chemical ordering was studied between 2 and 380 K. In the low temperature region, these compounds exhibit antiferromagnetic and spin-glass transition at T = 16−30 K. For the ordered compounds (M = Sb) the inverse susceptibility depends on T linearly up to the highest temperatures used. This enables the determination of the Weiss temperature θ and effective moment of the Fe3+ ion expressed by a number of Bohr magnetons, peff. In contrast to it, the compounds with M = Nb are characterized by a higher degree of the glassines and by a concave character of the inverse s…