Fracture toughness of multiferroic composite materials

Fracture toughness of unidirectional ferromagnetic fiber reinforced ferroelectric matrix composite was studied based on the energy approach in a view of large scale. A half space edge crack with crack face perpendicular to the external fields was considered. The energy release rate was derived explicitly considering the magnetoelectric coupling under combined mechanical, electric and magnetic loading. Because of the magnetoelectric coupling through the interface, the fracture toughness is highly dependent on the polarization properties of the ferroelectric and ferromagnetic portions besides the volume fraction and the elastic properties of each composite.     

Electric-field control of local ferromagnetism using a magnetoelectric multiferroic

Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO(3). The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO(3) film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co(0.9)Fe(0.1)) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.     

多铁性BaTiO3/La2/3Sr1/3MnO3复合薄膜的制备及其强磁电效应的研究

使用脉冲激光沉积技术,在(001)取向的LaAlO3(LAO)单晶基片上外延生长了BaTiO3/La2/3Sr1/3MnO3(BTO/LSMO)双层复合薄膜.电学和磁学性能的研究显示复合薄膜具有较低的相对介电常数(εr=263),优良的铁电和铁磁性能以及高于室温的铁磁居里温度(Tc=317 K).复合薄膜的磁电电压系数(αE)为176 mV/A,高于同类结构磁电系统一个数量级,相应的界面耦合系数k值为0.68,表明铁磁层和铁电层界面之间存在较大程度的耦合.     

Controllable phase connectivity and magnetoelectric coupling behavior in CoFe(2)O(4)-Pb(Zr,Ti)O(3) nanostructured films

Magnetoelectric CoFe(2)O(4)-Pb(Zr,Ti)O(3) nanostructured films with various phase connectivity patterns were prepared by a pulsed laser deposition method. It was found that the microstructure as well as the phase connectivity pattern of the film varied remarkably with the variation of phase content ratio. All composite nanofilms exhibit evident ferromagnetic and ferroelectric characteristics, as well as distinct magnetoelectric coupling behavior upon increasing the magnetic field. The correlation between the phase connectivity pattern and magnetoelectric coupling behavior for the composite nanofilm was revealed. This work provides an efficient avenue to modulate the magnetoelectric coupling behavior for the ferroelectric-ferromagnetic composite nanofilm.     

弛豫多铁性材料研究进展

多铁性材料同时具有多种铁性(铁电性、铁磁性或铁弹性)的有序, 可实现电磁信号的相互控制, 成为近年来研究热点。在具有成分无序的复杂体系中, 长程铁性有序有可能被打破, 材料将表现出弛豫特性。我们将至少存在一种铁性弛豫特性的多铁性材料称之为弛豫多铁性材料。这类多铁性材料的极化强度(或磁化强度)在外加电场(或外加磁场)作用下响应更加灵敏, 其磁电耦合机制与长程有序的多铁性材料不同。本文结合国内外最新研究成果, 首先介绍了和弛豫铁性有序相关的物理概念, 重点阐述了多铁性材料在铁电和铁磁双弛豫态下的磁电耦合机制; 然后, 详细介绍了钙钛矿结构(包括PbB₁B₂O₃基和BiFeO₃基材料)和非钙钛矿结构(包括层状Bi结构和非正常铁电体)弛豫多铁性材料的研究进展; 最后, 对该领域亟待解决的问题进行了展望。     

Breakdown of Migdal–Eliashberg Theory via Catastrophic Vertex Divergence at Low Phonon Frequency

We investigate the applicability of Migdal–Eliashberg (ME) theory by revisiting Migdal’s analysis within the dynamical mean-field theory framework. First, we compute spectral functions, the quasi-particle weight, the self energy, renormalised phonon frequency and resistivity curves of the half-filled Holstein model. We demonstrate how ME theory has a phase-transition-like instability at intermediate coupling, and how the Engelsberg–Schrieffer (ES) picture is complicated by low-energy excitations from higher order diagrams (demonstrating that ES theory is a very weak coupling approach). Through consideration of the lowest-order vertex correction, we analyse the applicability of ME theory close to this transition. We find a breakdown of the theory in the intermediate coupling adiabatic limit due to a divergence in the vertex function. The region of applicability is mapped out, and it is found that ME theory is only reliable in the weak coupling adiabatic limit, raising questions about the accuracy of recent analyses of cuprate superconductors which do not include vertex corrections.      

Nonlocal effective parameters of a coated sphere medium

An analytical theory has been developed to find the general effective parameters of a nonlocal medium. The medium is nonlocal due to presence of spatial dispersion. The proposed theory is based upon the dipolar scattering model of the inclusions comprising the medium. The bianisotropy stemming from the magnetoelectric coupling at the inclusion and lattice level has been discussed. The developed theory is then applied to a medium which consists of coated spheres with realistic materials. Effects of different values of wavevector upon the effective permittivity, effective permeability, and magnetoelectric coefficient stemming from lattice effects have been studied for a coated sphere medium. It is shown that a coated sphere medium with a weak spatial dispersion gives rise to a broader range of frequencies, where real parts of the effective permittivity and permeability are negative. On the other hand, this range of frequencies becomes smaller when the spatial dispersion of the medium is not weak.     

Multiferroic magnetoelectric coupling effect of three-layer multiferroic (Ba0.6Sr0.4TiO3–Ni0.6Zn0.4Fe2O4)3 heterojunction fabricated by sol–gel process

Multiferroic materials have been widely used in novel multifunctional smart devices for their excellent magnetoelectric coupling properties. In this work, a three-layer multiferroic (Ba_0.6Sr_0.4TiO₃–Ni_0.6Zn_0.4Fe₂O₄)₃ heterojunction was fabricated using a sol–gel process followed with a rapid thermal process. The phase composition, microtopography as well as magnetoelectric coupling properties of the heterojunction were investigated at room temperature. It demonstrated that such a three-layer heterostructure shows not only excellent ferroelectricity and ferromagnetism, but also good magnetoelectric coupling. The maximum value of the in-plane magnetoelectric coupling coefficient is ~?55 mV cm^?1 Oe^?1 at 630 Oe. Meanwhile, the macro-magnetoelectric coupling characteristics of the heterojunction are verified by the dynamic processes of the ferroelectric and ferromagnetic domain structure induced by the stress exerted on dipoles in an applied field, making it more attractive in the design of modern multifunctional devices.

     

Dynamical Mean-Field Theory for the Normal Phase of the Attractive Hubbard Model

We analyze the normal phase of the attractive Hubbard model within dynamical mean-field theory. We present results for the pair-density, the spinsusceptibility, the specific heat, the momentum distribution, and for the quasiparticle weight. At weak coupling the low-temperature behavior of all quantities is consistent with Fermi liquid theory. At strong coupling all electrons are bound in pairs, which leads to a spin gap and removes fermionic quasiparticle excitations. The transition between the Fermi liquid phase and the pair phase takes place at a critical coupling of the order of the band-width and is generally discontinuous at sufficiently low temperature.     

Effect of cobalt layer thickness on the magnetoelectric properties of Co/PbZr0.45Ti0.55O3/Co heterostructures

We have studied the magnetic properties and determined the optimal range of thicknesses of cobalt films grown on PbZr_0.45Ti_0.55O₃ (PZT) ceramic substrates with smoothed surfaces. Using an ion-beam deposition-sputtering process, we have produced Co/PZT/Co heterostructures with smooth, thermally stable PZT/Co interfaces. The structures exhibit a reproducible magnetoelectric effect comparable in magnitude to that in known layered structures fabricated by splicing ferromagnetic and ferroelectric layers. A ferromagnetic layer of optimal thickness makes the largest contribution to the magnetoelectric effect. The structures can be used as elements of more complex, multilayer structures. The use of the ion-beam deposition-sputtering process allows one to scale up such heterostructures by microelectronic means in producing magnetoelectric devices.     

Electrical transport and magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 composites

The results of the structure, electrical transport and magnetoresistance of a ferromagnet-ferroelectric-type La_0.67Ca_0.33MnO₃ (LCMO)/BaTiO₃ composites fabricated by the sol-gel method are presented. The structure and morphology characterization indicates no apparent variations in morphology and particle size in spite of the existence of BaTiO₃. The insulator-metal transition temperature (T_IM) is shifted to a higher temperature and resistivity decreases with the increase of low content BaTiO₃. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO₃. By calculating in terms of a ferromagnetic grain coupling model, we attribute these transport properties to the enhancement of the ferromagnetic coupling between the neighboring grains, which could be explained by the increase of the carrier concentration at the grain boundary due to the introduction of BaTiO₃ and the associated magnetoelectric coupling effect.     

Magnetocapacitance in BaTiO3-CoFe2O4 nanocomposites

We report on the magnetoelectric characterization of epitaxial CoFe₂O₄-BaTiO₃ nanocomposites grown by rf sputtering on Nb-doped SrTiO₃ (001). Multiferroicity of the material is corroborated with both, ferroelectric and ferromagnetic hysteresis loops. Magnetoelectric coupling is investigated via magnetocapacitance measurements; a change of the capacitance of about 2% is observed for an applied magnetic field of 8 T. The possible origins of this magnetocapacitance are discussed.     

A coupling phenomenon between the magnetization of two ferromagnetic thin films separated by a thin metallic film--Application to magnetic memories

A new type of magnetic interaction between two ferromagnetic thin films separated by a third metallic nonmagnetic thin film is studied. This coupling tends to bring the magnetization of the ferromagnetic materials into parallel alignment. The coupling is shown to be dependent upon the intermediate material and its thickness, and on the nature of the ferromagnetic media present. These interactions are observed for intermediate metal thicknesses up to 300 Å in the case of Palladium. The measured surface energy of this coupling is of the order of 0.1 erg/cm². A study of the variation of the coupling with temperature is given.     

A Self-consistent Approach Applied to the Ferro and Antiferromagnetism of Nanotubes

We employ a self-consistent simulation approach based on mean field theory to investigate the physical properties of both ferromagnetic and antiferromagnetic nanotubes. At the beginning, dipole-dipole interaction is neglected to facilitate theoretical analysis. Due to the geometric shape of the nanotubes and the magnetic uniaxial anisotropy, the spins are found always ordering parallel to the longitudinal axis of the nanotubes, no matter the external magnetic field is absent or applied along the axis, showing a typical ferromagnetic and antiferromagnetic characters. This peculiar feature allows us to build the one-dimensional magnetic chain models. Consequently, the results obtained with our theoretical models and numerical approach are exactly identical, manifesting the correctness of the new simulation approach. Finally, the dipole-dipole interaction is taken into account. It is very interesting to find that a very weak dipole-dipole interaction is able to make the spins align mainly along the longitudinal axis, though they tilt inwards or outwards slightly and symmetrically.     

Carrier-mediated magnetoelectricity in complex oxide heterostructures

Increasing demands for high-density, stable nanoscale memory elements, as well as fundamental discoveries in the field of spintronics, have led to renewed interest in exploring the coupling between magnetism and electric fields. Although conventional magnetoelectric routes often result in weak responses, there is considerable current research activity focused on identifying new mechanisms for magnetoelectric coupling. Here we demonstrate a linear magnetoelectric effect that arises from a carrier-mediated mechanism, and is a universal feature of the interface between a dielectric and a spin-polarized metal. Using first-principles density functional calculations, we illustrate this effect at the SrRuO3/SrTiO3 interface and describe its origin. To formally quantify the magnetic response of such an interface to an applied electric field, we introduce and define the concept of spin capacitance. In addition to its magnetoelectric and spin capacitive behaviour, the interface displays a spatial coexistence of magnetism and dielectric polarization, suggesting a route to a new type of interfacial multiferroic.     

Magnetoelectric effect of the multilayered CoFe2O4/BaTiO3 composites fabricated by tape casting

This paper presents the structural, ferroelectric, ferromagnetic, resonance and magnetoelectric (ME) properties of multilayered ME composites fabricated using tape casting method. The compositions corresponding to CoFe₂O₄ (CFO) with particle size of ~150?nm and BaTiO₃ (BTO) with particle size of ~100?nm were chosen as ferromagnetic and ferroelectric phases, respectively. Delamination was found at the interface between CFO and BTO layers, which was related to the residual stress due to the difference in thermal expansion coefficient between the two layers. The largest direct magnetoelectric and converse magnetoelectric coefficients of the multilayered ME composite were, respectively, 36?μV/cm?Oe at a bias magnetic field of 2,800?Oe and 1.16?×?10^?3?G/V at a frequency of 30?kHz. In addition, the corresponding interfacial coupling coefficient was calculated to be 3.2?×?10^?5. For the multilayered ME composite, a resonance frequency of 4.96?MHz and a bandwidth of 40?kHz were obtained using capacitance-frequency spectrum method.     

Theory of magnetoelectric effect in ferromagnetic-piezoelectric bilayer structures

A theory of the magnetoelectric (ME) effect in a ferromagnetic-piezoelectric bilayer structure is presented. An expression for the ME voltage coefficient in terms of the parameters of the ferromagnetic and piezoelectric phases is obtained, and the dependence of this coefficient on the frequency and the ratio of the ferromagnetic and piezoelectric layer thicknesses is analyzed. The results of numerical calculations of the ME voltage coefficient for a permendur-PZT bilayer system are in satisfactory agreement with experimental data.     

Magnetoelectric charge trap memory

It is demonstrated that a charge-trapping layer placed in proximity to a ferromagnetic metal enables efficient electrical and optical control of the metal's magnetic properties. Retention of charge trapped inside the charge-trapping layer provides nonvolatility to the magnetoelectric effect and enhances its efficiency by an order of magnitude. As such, an engineered charge-trapping layer can be used to realize the magnetoelectric equivalent to today's pervasive charge trap flash memory technology. Moreover, by supplying trapped charges optically instead of electrically, a focused laser beam can be used to imprint the magnetic state into a continuous metal film.     

一种基于压力泊松方程的流体结构紧耦合算法

从流固耦合系统的整体控制方程出发,推导出与流体控制方程一致的耦合等价方程,并得到基于耦合方程的压力泊松方程,通过求解耦合系统压力泊松方程和一致的等价方程就能获得耦合系统的解,而不需要直接求解整体耦合系统的控制方程,有利于降低求解自由度。预估-校正多步迭代格式用于耦合系统的时间推进,克服了传统迭代耦合方法由于时间不同步而产生较大数值误差的不足。应用该方法对附带局部突起的主动脉弓动脉瘤进行流固耦合分析,验证数值方法的可行性。