Electric field-modulated Hall resistivity and magnetization in magnetoelectric Ni-Mn-Co-Sn/PMN-PT laminate

The effects of electric field on the magnetization and Hall resistivity were investigated in a laminated composite consisting of Ni₄₃Mn₄₁Co₅Sn₁₁ alloy and Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ ferroelectric single crystal. Upon applying an electric field (3 kV/cm) on the single crystal, the change of Hall resistivity in the alloy is up to 45%. The co-action of magnetization change and the different carrier concentration between the martensitic and austenite phases of alloy, which result from the stress-induced martensitic transformation, are responsible for the electric field-modulated Hall resistivity.     

镍酸镧界面层对锆钛酸铅膜–钴铁氧体陶瓷复合磁电材料性能的影响

以化学溶液沉积法制得的LaNiO3薄膜作为Pb(Zr0.52Ti0.48)O3膜-CoFe2O4陶瓷复合材料的导电界面层,利用其优良的导电性保证磁电信号的有效输出。LaNiO3界面层的引入,使得复合材料体系的铁电性能、压电性能得到有效表达,其层面内磁电耦合系数达到58mV/(cm Oe)。继续优化制备工艺及界面层的参数,磁电耦合性能将进一步提高。     

Multiferroics with Spiral Spin Orders

Cross correlation between magnetism and electricity in a solid can host magnetoelectric effects, such as magnetic (electric) induction of polarization (magnetization). A key to attain the gigantic magnetoelectric response is to find the efficient magnetism–electricity coupling mechanisms. Among those, recently the emergence of spontaneous (ferroelectric) polarization in the insulating helimagnet or spiral‐spin structure was unraveled, as mediated by the spin‐exchange and spin–orbit interactions. The sign of the polarization depends on the helicity (spin rotation sense), while the polarization direction itself depends on further details of the mechanism and the underlying lattice symmetry. Here, we describe some prototypical examples of the spiral‐spin multiferroics, which enable some unconventional magnetoelectric control such as the magnetic‐field‐induced change of the polarization direction and magnitude as well as the electric‐field‐induced change of the spin helicity and magnetic domain.     

Sintering of ferrite-BaTiO3 bulk particulate composites

Particulate magnetoelectric ceramic composites (PMCC) have received much attention since the last decade. These composites have many technological applications and are usually composed by magnetostrictive and piezoelectric phases. Cobalt-based spinel ferrites are among the most studied magnetostrictive phases for ferrite-based PMCCs and BaTiO₃ is an interesting choice for the piezoelectric phase because it is a lead-free ceramic, unlike the traditional PZT. In this work, cobalt ferrite (FCO) and Ni–Co ferrite (FNICO) were produced by the ceramic method and mixed to BaTiO₃ (TB) in order to further obtain sintered ferrite-BaTiO₃ particulate ceramic composites with a composition of 15 mol% ferrite – 85 mol% BaTiO₃. The ferrites, the BaTiO₃, and the ferrite-BaTiO₃ mixtures were analyzed by dilatometry, thermogravimetry (TG), and calorimetry (DSC) in temperatures up to 1300–1400 °C, with the aim to analyze the sintering behavior and the interactions between both ferrites and the BaTiO₃ during sintering. Sintered TB-FNICO and TB-FCO composite samples were also produced and they were analyzed by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The dilatometry results evidenced that the densification of the ferrite-BaTiO₃ samples is impaired, when compared to the pure ferrite and BaTiO₃ samples. The DSC/TG results evidenced the occurrence of reactions between the ferrites and the BaTiO₃ when they are co-sintered in air or argon atmospheres. The XRD patterns of the sintered composite samples did not exhibit diffraction peaks attributed to a third phase, whilst the punctual EDS analysis showed evidence of diffusion between the ferrite and BaTiO₃ particles.     

Room temperature multiferroic properties of BiFeO3–MnFe2O4 nanocomposites

The novel functionalities of multiferroic magneto-electric nanocomposites have spawned substantial scope for fast-paced memory devices and sensor applications. Following this, herein we report the development of nanocomposites with soft ferromagnetic MnFe₂O₄ and ferroelectric BiFeO₃ to fabricate a system with engineered multiferroic properties. A modified sol-gel route called Pechini method is demonstrated for the preparation of the (1-x) BiFeO₃-x MnFe₂O₄ (x = 10%, 30%, 50%, 70%) nanocomposites. The crystallographic phase, structure, and morphology are characterized by XRD, FESEM, and HRTEM. The accurate crystallite size and lattice strain are determined by Williamson-Hall plot method and a comparative study with Scherer's equation is carried out. TEM image evidences the interface between BiFeO₃ and MnFe₂O₄ nanoparticles in the composite. The room temperature magnetic response reveals the strong dependence of magnetic saturation, remanent magnetization, and coercivity of the nanocomposites on MnFe₂O₄ addition. The dielectric response and impedance analysis of the prepared nanocomposites are observed. The electrical performance of the composite is affected by grain, grain boundaries, and oxygen vacancies. The unsaturated P-E loops exhibit the leaky ferroelectric behavior for the nanocomposite. The intrinsic magnetoelectric coupling between ferroelectric BiFeO₃ and ferromagnetic MnFe₂O₄ has been determined by varying H_dc/H_ac and its maximum coupling coefficient (α) is found to be 25.39 mV/cmOe for 70% BiFeO₃ -30% MnFe₂O₄ nanocomposite. These distinctive and achievable characteristics of the nanocomposite would enable the designing of magnetic field sensors, spintronic devices, and multiferroic memory devices.     

Poling magnetic field dependence and memory effect of magnetoelectric coupling in honeycomb antiferromagnet cobalt niobate

We have firstly studied the poling magnetic field dependence of magnetoelectric (ME) coupling in a honeycomb antiferromagnet Co₄Nb₂O₉, and found that the ME susceptibilities do not increase monotonously with increasing the poling magnetic field strength (H_Pole) as the usual case, but reach maxima around 10 kGs. This phenomenon results from opposite H_Pole dependences of domain wall thickness and energitically preferred domain's proportion. More interestingly, if the sample returns to paramagnetic phase after being poled and then reenters antiferromagnetic phase without being poled, ME coupling still exists. This memory effect is attributed to the presence of ME domain nucleation centers in paramagnetic phase.     

In situ study of electric-field-induced ferroelectric and antiferromagnetic domain switching in polycrystalline BiFeO3

Antiferromagnetic domain switching induced by ferroelectric polarization switching has previously been observed in situ in both multiferroic BiFeO₃ single crystals and thin films. Despite a number of reports on macroscopic magnetoelectric measurements on polycrystalline BiFeO₃, direct in situ observation of electric-field-induced antiferromagnetic domain switching in this material has not been addressed due to the lack of high-quality samples capable of electrical poling. Here, the electric field control of antiferromagnetic domain texture is identified in polycrystalline BiFeO₃ using in situ neutron diffraction, showing the resultant magnetic domain reorientation induced by an electric field. An antiferromagnetic domain reorientation to a value of 2.2-2.5 multiples of a random distribution (MRD) is found to be induced by an electric field that provides a non-180° ferroelectric-ferroelastic domain texture of 2.2-2.5 MRD along the field direction. The current results show well-controlled coupling of multiferroic domain texturing in single-phase polycrystalline BiFeO₃.     

Lead-free BLTO/NMFO magnetoelectric composite films prepared by the sol-gel method

The lead-free ferroelectric films of Bi_4?xLa_xTi₃O₁₂(BLTO) and ferromagnetic films of Ni_1?xMn_xFe₂O₄(NMFO) were prepared on Pt/Ti/SiO₂/Si substrate by means of the sol-gel and spin-coating method. The lead-free magnetoelectric composite films with the structure of Bi_3.4La_0.6Ti₃O₁₂/Ni_0.7Mn_0.3Fe₂O₄/substrate (BN) and Ni_0.7Mn_0.3Fe₂O₄/Bi_3.4La_0.6Ti₃O₁₂/ substrate (NB) were also deposited on Pt/Ti/SiO₂/Si substrate. The X-ray diffraction results show that two composite films possess BLTO and NMFO phases without any intermediate phase. The SEM images show that two composite films exhibit layered structure, clear interface and no transition layer between BLTO and NMFO films. Two composite films exhibit both good ferromagnetic and ferroelectric properties, as well as magnetoelectric coupling effect. The deposition sequence of ferroelectric and ferromagnetic films in the composite films has significant influence on the ferroelectric, ferromagnetic and magnetoelectric coupling properties of the composite films. The values of magnetoelectric voltage coefficient of the BN composite films are higher than those of the NB composite films at any fixed H_bias.     

Ceramic processing and multiferroic properties of the perovskite YMnO3-BiFeO3 binary system

The perovskite (1−x)YMnO₃−xBiFeO₃ binary system is very promising because of its multiferroic end members. Nanocrystalline phases have been recently obtained by mechanosynthesis across the system, and the perovskite structural evolution has been described. Two continuous solid solutions with orthorhombic Pnma and rhombohedral R3c symmetries were found, which coexist within a broad compositional interval of 0.5 ≤ x ≤ 0.9. This might be a polar-nonpolar morphotropic phase boundary region, at which strong phase-change magnetoelectric responses can be expected. A major issue is phase decomposition at moderate temperatures that highly complicates ceramic processing. This is required for carrying out a sound electrical characterization and also for their use in devices. We present here the application of Spark Plasma Sintering to the ceramic processing of YMnO₃-BiFeO₃ phases. This advanced technique, when combined with nanocrystalline powders, allowed densifying phases at reduced processing temperatures and times, so that perovskite decomposition was avoided. Electrical measurements were accomplished, and the response was shown to be mostly dominated by conduction. Nonetheless, the intrinsic dielectric permittivity was obtained, and a distinctive enhancement in the phase coexistence region was revealed. Besides, Rayleigh-type behavior characteristic of ferroelectrics was also demonstrated for all rhombohedral phases. Magnetic characterization was performed in this region, and antiferromagnetism was shown.