Theoretical study of mutual control mechanism between magnetization and polarization in multiferroic materials

The mutual control mechanism between magnetization and polarization in multiferroic materials is studied. The system contains a ferromagnetic sublattice and a ferroelectric sublattice. To describe the magneto–electric coupling, we propose a linear coupling Hamiltonian between ferromagnetism and ferroelectricity without microscopic derivation. This coupling enables one to retrieve the hysteresis loops measured experimentally. The thermodynamic properties of the system are calculated, such as the temperature dependences of the magnetization, polarization, internal energy and free energy.The ferromagnetic and ferroelectric hysteresis loops driven by either a magnetic or an electric field are calculated, and the magnetic spin and pseudo-spin are always flipped synchronously under the external magnetic and electric field. Our theoretical results are in agreement with the experiments.

Studies on multiferroic materials in high magnetic fields

In this article, studies on the magnetoelectric effects of multiferroic materials in high magnetic fields, particularly pulsed magnetic fields, are discussed and results for some representative ma- terials are presented. In the discussions on representative materials, the relationship between the crystallographic symmetry and the linear magnetoelectric effect in Cr203 is introduced. Then dras- tic changes in polarization caused by magnetic transitions are discussed through a case study of manganites with a perovskite4ype structure. In addition, high field studies on the magnetoelectric effects in BiFeO3, which is an exceptional multiferroic material, are presented and discussed in the framework of the Landau-Ginzburg theory.

Study on superstructure in ion co-doped BiFeO_3 by using transmission electron microscopy

La3＋ and V5＋ co-doped BiFeO3 ceramics are synthesized by rapid liquid sintering technique. The modulated structure in Bi0.85La0.15Fe0.97V0.03O3 is investigated by using transmission electron microscopy （TEM）. Two kinds of superstructures are observed in the samples. One is the component modulated superstructure and twin-domain, which is generated by La3＋ ordered substitution for Bi3＋ and frequently appears. The chemical composition of the superstructure is explored by x-ray energy dispersive spectroscopy （EDS）. The model of the ordered structure is proposed. Simulation based on the model is conducted. The second is the fluorite-type δ-Bi2O3 related superstructure. The relation between the ferroelectric property and the microstructure of the sample is also discussed.

Enhanced ferromagnetism and magnetoelectric response in quenched BiFeO3-based ceramics

The piezoelectric,ferromagnetism,and magnetoelectric response of BiFeO3-BaTiO3 ceramics with the compositions around the morphotropic phase boundary(MPB)of the solid solution are systematically investigated after the ceramics have been quenched from a high temperature.We find that the ferromagnetism of the quenched ceramics is greatly enhanced.An enhanced piezoelectric response d33 larger than 200 pC/N,which could be sustained up to 350℃,is measured.As a result of enhanced ferromagnetism and piezoelectric response,a large magnetoelectric response^1.3 V/cm·Oe(1 Oe=79.5775 A·m^-1)is obtained near the mechanical resonance frequency of the quenched ceramic samples.Our research also shows that in addition to the ferromagnetism and piezoelectric response,the mechanical quality factor is another important parameter to achieve high magnetoelectric response because the physical effects are coupled through mechanical interaction in BiFeO3-based materials.Our work suggests that quenching is an effective approach to enhancing the magnetoelectric response of BiFeO3-based materials and the materials belong to single-phase multiferroic materials with high magnetoelectric response.

Emergent ferroelectricity in disordered tri-color multilayer structure comprised of ferromagnetic manganites

Multiferroic materials,showing the coexistence and coupling of ferroelectric and magnetic orders,are of great technological and fundamental importance.However,the limitation of single phase multiferroics with robust magnetization and polarization hinders the magnetoelectric effect from being applied practically.Magnetic frustration,which can induce ferroelectricity,gives rise to multiferroic behavior.In this paper,we attempt to construct an artificial magnetically frustrated structure comprised of manganites to induce ferroelectricity.A disordered stacking of manganites is expected to result in frustration at interfaces.We report here that a tri-color multilayer structure comprised of non-ferroelectric La;Ca;MnO;（A）/Pr;Ca;MnO;（B）/Pr;Sr;MnO;（C） layers with the disordered arrangement of ABC-ACBCAB-CBA-BAC-BCA is prepared to form magnetoelectric multiferroics.The multilayer film exhibits evidence of ferroelectricity at room temperature,thus presenting a candidate for multiferroics.

Effect of Ba Substitution on the Structural and Magnetic Properties of BiFeO3

Bi1-xBaxFeO3 (0 ≤ x ≤ 0.3) nanopowders were synthesized using sol-gel technique. The structural and magnetic properties were investigated using X-ray diffraction, SEM and VSM. As Ba2+ doping concentration was increased, the structure of the samples changed from rhombohedral to tetragonal or monoclinic. The structural change might be an important factor for achieving the ferroelectric properties in this material. The lattice parameters were observed to increase with increase in Ba2+ concentration. All the M-H loops showed the ferromagnetic behavior. Magnetization was observed to enhance with increase in Ba concentration. The enhancement in the magnetization due to Ba2+ doping may be due to the replacement of Bi3+ ions by Ba2+ which might have resulted in the suppression of spiral spin structure.

磁有序CoH_(2)SeO_(4)薄片中的室温铁电性

本文通过第一性原理计算和实验表征证明了二维CoH_(2)SeO_(4)薄膜的磁有序和滑移铁电性.首先,实验结果证实了粉末CoH_(2)SeO_(4)样品的反铁磁序.同时,第一性原理计算表明单层CoH_(2)SeO_(4)具有反铁磁(AFM-I)基态(TN≈75 K),且预测了二维CoH_(2)SeO_(4)薄膜具有以不对称的三重势阱态为特征的滑移铁电性,并在实验中测得了180°压电滞回线、可反转铁电畴和二次谐波信号.此外,在基于二维CoH_(2)SeO_(4)薄膜的电容器中获得铁电材料所特有的电滞回线和蝴蝶状的电容曲线.介电温谱测试表明二维CoH_(2)SeO_(4)薄膜的铁电转变温度约为370 K.CoH_(2)SeO_(4)中滑移铁电性和反铁磁性的出现为获得低维多铁材料指出了一条新的途径.

Nonreciprocal directional dichroism in multiferroics

Nonreciprocal directional dichroism in multiferroics,namely magnetoelectric coupling in the dynamic regime,is endowed with rich physics and promising applications,which are entangled with fundamental physical components,such as spin,orbital,lattice,charge,and topology.Such a linear nonreciprocal response behavior in the GHz-THz frequency range,represented by optical magnetoelectric effect and magnetochiral dichroism,occurs ubiquitously in material systems with the spontaneous breaking of space-time symmetry,and is subject to Onsager’s reciprocal theorem in the thermodynamic limit.Microscopically,these nonreciprocal responses are usually encoded by toroidization(chirality)and electromagnon(quasiparticle),thus establishing a comprehensive understanding of magnetoelectric coupling and irreversible dynamics.Herein,the basic mechanisms and emergent nonreciprocal directional dichroism in single-phase multiferroics are summarized.We expect that the present review will stimulate diverse possibilities toward nonreciprocal directional dichroism within and beyond multiferroics.

High-entropy enhanced room-temperature ferroelectricity in rare-earth orthoferrites

Single-phase multiferroic materials of rare-earth orthoferrites with magnetism and ferroelectricity are of great technological importance in storage devices.However,the polarization(P)of these materials is generally weak(0.01μC-cm^(-2)),and the ferroelectricity is reported to exist below room temperature(25℃).Here,(Bi_(0.2)La_(0.2)Y_(0.2)Dy_(0.2)Tb_(0.2))FeO_(3)(BLYDTFO)high-entropy oxides that exhibit a saturation P of 5.3μC.cm^(-2)at the electric field(E)of 45 kV·cm^(-1)at room temperature was designed and fabricated by the conventional solid-phase method.The results show that configurational entropy introduces atomic disorder and a larger tilt of BO6 octahedron,which facilitates noncentrosymmetric distortion and ferroelectricity at room temperature compared with other single components(LaFeO_(3),YFeO_(3),DyFeO_(3),and TbFeO_(3)).This high-entropy approach expands the compositional window of the rare-earth orthoferrites to enhance the ferroelectricity in multiferroic applications.

Sol-gel preparation of La-doped bismuth ferrite thin film and its low-temperature ferromagnetic and ferroelectric properties

Bi0.85La0.15FeO3 thin film was prepared on ATO glass substrates by sol-gel technique. The effect of La doping on phase structure, film surface quality, ion valence, and ferroelectric/magnetic properties of Bio.85La0.15FeO3 film were investigated. La doping suppressed the formation of impurity phases and the transition of Fe3＋ to Fe2＋ ions at room temperature. Compared with the un-doped BiFeO3, La-doping also increased the average grain size and the film density, which resulted in the decrease of film leakage current density. The remanent polarization and saturation magnetization were enhanced significantly by La doping. The remanent polariza- tion of Bi0.85La0.15FeO3 films gradually decreased while saturation magnetization increased with the decrease of measuring tempera- ture within a range from 50 to 300 K.