Nanomagnetism in nanocrystalline multiferroic bismuth ferrite lead titanate films

Multiferroics conventionally refer to the materials exhibiting co-existing electric, magnetic, and structure order parameters. Interplay between ferroelectricity, magnetism, and ferroelasticity in a single phase makes multiferroics truly multifunctional providing control over magnetic and electric ordering by applying electric and magnetic fields, respectively. Incorporation of multiferroic-based components into nanoscale applications will enable additional degrees of freedom in manipulating with spin and charge not easily attainable otherwise. Multiferroic bismuth ferrite lead titanate has been chemically synthesized in form of nanocrystalline films. The morphology of the films revealed a single perovskite phase confined within crystalline grains of few tens of nm in size. The films were found to exhibit ferroelectricity and ferromagnetism with characteristic electric polarization and magnetization hysteresis loops, transformations associated with spin reorientation in an external magnetic field and the spin-glassy behavior well above the room temperature. High degree of magnetic frustration and disorder in the spin system spatially confined in the nanograins, distribution of the grains anisotropy axis, inter-grain interactions, and the effects of uncompensated spins on the large effective surface/interface favored by the nanocrystalline morphology were assumed to be responsible for the anomalous magnetic properties and glassy dynamics in the films.     

Giant uniaxial stress-permeability effect on electrical parameters of heterotypic MnZn ferrite devices and electromagnetic effect

The effects of uniaxial stress on permeability and electrical parameters of heterotypic manganite zinc (MnZn) ferrite devices have been investigated. Giant stress-permeability, stress-capacitance and stress-impedance that are independent of skin effects have been simultaneously observed to exist in a wide range of frequency at room temperature. All the uniaxial stress effects enhance with increasing the permeability of the ferrite. The stress-inductance is same as the stress-impedance and reverse to the stress-capacitance in phase. The stress effects under uniaxial pulling force are analogical with those under uniaxial pressing force. A composite of electrostrain/stress-permeability has been fabricated. Its electromagnetic effects have been observed to be homologous with the stress effects and can also exist in wide range of frequency but display some maximums. Analysis shows that both stress and electromagnetic effects originate from the variation of the magnetic domain structure in the ferrites caused by applied mechanical stress.     

Thin film bilayers of multiferroic bismuth ferrite on Pt–Si substrate

A multiferroics/multiferroics BiFeO₃/Bi_0.90La_0.10Fe_0.90Zn_0.10O₃ (BFO/BLFZO) bilayer was deposited on Pt/TiO₂/SiO₂/Si substrates by radio frequency sputtering. The BLFZO layer strongly affects the phase purity, orientation growth, and leakage current of BFO layer. The bilayered capacitor exhibits a high dielectric permittivity of ～162 and an improved magnetic behavior of 2M_s ～ 34.6 emu/cm³, together with an excellent fatigue endurance. A remanent polarization of 2P_r ～ 116.2 μC/cm² for the bilayered capacitor is better than those of reported BFO bilayers. The impedance study indicates that lower freely mobile charges are responsible for the improved electrical behavior of the BFO/BLFZO bilayer. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic phases of bismuth ferrite

We have recently shown that BiFeO₃ has at least four different magnetic phases, contrary to the conventional wisdom. Below room temperature it undergoes spin reorientation transitions at T₂=200 K and T₁=140 K analogous to those in orthoferrites; and above room temperature it undergoes a structural transition near 185°C first reported by Polomska et al. This may help explain the apparent linear magnetoelectric effect at 20°C reported by D. Lebeugle et al. [Phys. Rev. Lett. 100, 227602 (2008)] which is nominally forbidden due to the long wavelength cycloidal spin structure assumed. We also find evidence of an unusual acentric spin glass below ca. 200 K, related not to T_N but to T₁ and T₂.     

A systematic study on magnetic, dielectric and magnetocapacitance properties of Ni doped bismuth ferrite

In this paper we report the structural, magnetic, magnetocapacitance and dielectric properties of BiFe_1−xNi_xO₃ nanoceramics (with x=0, 0.1) prepared by the sol-gel method. XRD analysis showed formation of single phase nanoceramics (particle size ∼50 nm by TEM). Samples of BiFe_1−xNi_xO₃ were divided into two parts—one of them quenched in liquid nitrogen and another sintered in the normal way. We observed the enhancement in magnetic and dielectric properties of quenched sample. The splitting of zero field cool (ZFC) and field cool (FC) magnetization curves at low temperature reveals spin- glass behavior. Quenched sample showed the enhancement of blocking temperature.     

Interplay between elasticity,ferroelectricity and magnetism at the domain walls of bismuth ferrite

Multiferroic domain walls have recently been proposed as the active element in devices related to spintronics, data storage, and magnetic logic. Among multiferroics, BiFeO₃ is by far the most studied material. Its domain walls have shown rich behaviours that include conductivity in an otherwise insulating crystal, and magnetotransport properties that are markedly different from those of the bulk. In this article we summarize the experimental evidence and the current models used to understand the interplay between elastic, electric, and magnetic properties of the domain walls. Starting from the considera‐ tion of antiferromagnetic domain structures on a background of ferroelectric domains, and emphasizing pinning effects, we proceed to discuss the microscopic behavior of ferroelectricity and magnetism at the walls. We describe how domain organization in BiFeO₃ is caused by structural transformations, and scrutinize modelling works pinpointing their characteristic features. Finally, we summarize the recent progress and list open questions for future study on BiFeO₃ domain structures. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Dielectric, magnetic and magnetoelectric properties of La and Nb codoped bismuth ferrite

Single-phase Bi(0.80)La(0.20)FeO(3) (BLFO) and Bi(0.80)La(0.20)Fe(1-x)Nb(x)O(3) (BLFNO) samples were prepared in order to study the dielectric, magnetic and magnetoelectric properties of La and Nb codoped BiFeO3. Rietveld refinement of x-ray diffraction patterns of La and Nb codoped samples has been performed using the R3c space group. Magnetic hysteresis loops revealed that codoping can effectively increase the spontaneous magnetization due to change in the bond angle of Fe-O-Fe as a result of distortion created by the Nb5+ doping. Magnetic field-induced relative change of the dielectric constant for BLFO and BLFNO samples is a signature of magnetoelectric coupling.     

Domain walls, magnetization, and magnetoelectric effect in bismuth ferrite films

The specific features of the antiferromagnetic domain structure, magnetization, and polarization induced by an inhomogeneous micromagnetic distribution in films of bismuth ferrite multiferroics have been investigated. It has been shown that the magnetic domain structure correlates with the ferroelectric domain structure, and the character of the rotation of the antiferromagnetic vector depends on the type of ferroelectric domain walls. An asymmetry in the distribution of the antiferromagnetic vector has been observed for the cases of 109° and 71° ferroelectric domain walls. It has been demonstrated that there are differences in the distributions of the polarization and magnetization in bismuth ferrite films with ferroelectric domains separated by 109° and 71° walls. The basic mechanisms responsible for the magnetization in domain walls in multiferroics have been considered.     

Effect of a planar magnetic field on the domain wall dynamics in a double-layer uniaxial magnetic film

Motion of an isolated domain wall in a double-layer uniaxial magnetic film, where the film layers differ in characteristic length, saturation magnetization and damping parameter, is investigated by solving the Slonczewski equation. A planar magnetic field is applied normal to the domain-wall plane. The dependences of the threshold field and limiting velocity of disruption of the steady-state motion of the domain wall on the planar magnetic field value are obtained. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 60–63, December, 2008.     

Properties of mechanically activated bismuth ferrite

Methods of mechanochemical treatment applied to eliminate the disadvantages of BiFeO₃ that impede its practical use are described. It is shown that the use of certain techniques for the mechanical activation of precursors allows us to enhance the structural and thermal stability of bismuth ferrite, to reduce its electric conductivity, and to improve its electrophysical properties.     

Some comments on magnetization reversal in uniaxial ferrite magnets

The interpretation of Hadjipanayis et al. of their experimental results on magnetization reversal in ferrite magnets is critically analyzed on the basis of the present knowledge of the mechanisms governing magnetization, magnetization reversal and coercivity, both in single and polydomain particles.     

Change in periodicity of the incommensurate magnetic order towards commensurate order in bismuth ferrite lead titanate

xBiFeO₃-(1−x)PbTiO₃ has been synthesised in bulk polycrystalline ceramic form and then self-disintegrated to form powder (P4mm) (x=0.7) or crushed (R3c) (x=0.75, 0.9) to relieve stress. High-resolution neutron powder diffraction has been employed to observe incommensurate antiferromagnetic ordering within the materials, and its dependence on phase at 7.5 K. It is shown that in the rhombohedral phase the period of the magnetic structure depends on PbTiO₃ addition, and increases from 790 Å (x=0.9) to 840 Å (x=0.75). The tetragonal phase restores the collinear antiferromagnetic order.     

Rearrangement of the domain structure of single-crystal ferrite films in rotating magnetic fields

The behavior of the microband domain structure of films having pronounced crystallographic anisotropy is Investigated in rotating magnetic fields. In small fields, passing through the difficult axis, discontinuities are observed on the curves of mechanical moments which are associated with the reorientation of the magnetic moment by 90°. At the same time a new metastable transient domain phase appears in the form of wide bands stretched along the difficult axis.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 3#x2013;6, June, 1985.     

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 materials are presented. In the discussions on representative materials, the relationship between the crystallographic symmetry and the linear magnetoelectric effect in Cr₂O₃ is introduced. Then drastic changes in polarization caused by magnetic transitions are discussed through a case study of manganites with a perovskite-type 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.     

Thermostimulated surface autosegregation in bismuth ferrite single crystals

The chemical and phase composition and morphological features of the surface nanostructure have been studied by the methods of high-resolution scanning electron microscopy, X-ray microprobe analysis, and atomic force microscopy in bismuth ferrite single crystals. This structure was formed as a result of the thermostimulated surface segregation after annealing in air or vacuum at the pressure of 10⁻⁴ Pa. It has been experimentally found that, at temperatures less than 500°C, Bi₂F₄O₉ nanoparticles were formed due to the selective diffusion of iron atoms to the surface. Starting from 300°C in vacuum and 450°C in air, the segregating atom type changed and nanophases with high bismuth concentration (sillenites Bi_{26 − x} Fe _x O₃₉ and Bi₂O₃ appeared in some regions. The partial orientation of new phases has been observed in some surface regions. A probable mechanism of the described phenomenon that represents a combination of selective intrinsic mass-transport of atoms from the bulk to the surface and their thermal evaporation has been discussed.     

Gigantic uniaxial pressure effect in single crystals of iron-based superconductors

In order to elucidate the anisotropic pressure effect on superconductivity in an iron-based superconductor, magnetization measurements have been performed in Ba(Fe_0.92Co_0.08)₂As₂ single crystals under uniaxial pressures applied along the c-axis. Gigantic T_c suppression, dT_c/dP_//c = −15 K/GPa, was observed when the anisotropic deformation with the a-expansion and c-compression was induced by the c-pressure, which should be compared with dT_c/dP < +1 K/GPa in the isotropic pressure case. This suggests that the a-axis (c-axis) compression has a positive (negative) contribution to T_c.     

Giant magnetorefractive effect in magnetic granular CoFe-MgF alloys

This paper reports on an IR study (5–20 µm) of the dispersion and field dependences of the magnetorefractive effect in granular CoFe-MgF (metal-insulator) systems, which exhibit, in compositions close to the percolation threshold, a tunneling magnetoresistance of 7.5% in a field of 1700 Oe. The change in the reflectivity of p-polarized light under magnetization in the IR spectral region 5–7 µm is of the order of 0.1%, while in the 10-to 12-µm region, this change, reaches record-high values of 1.2–1.5%, which exceed the usual odd and even magnetooptic reflectivities in the IR range by two orders of magnitude. The magnetorefractive effect in magnetic systems with tunneling conduction originates from spin-dependent high-frequency tunneling.     

Giant magnetoresistance effect in a magnetic barrier nanostructure

The giant magnetoresistance (MR) effect is theoretically studied in a magnetically modulated two-dimensional electron gas. We find that the significant transmission difference for electron tunneling through parallel and antiparallel magnetization configurations results in a considerable MR effect. We also find that the MR ratio strongly depends on the magnetic strength and the distance between the left edges of two ferromagnetic strips as well as the temperature.     

Classical dependence of the domain wall velocity on the magnetic field in garnet ferrite films with orthorhombic magnetic anisotropy

The dependence of the domain wall velocity V on the acting magnetic field H is investigated for bismuth-containing single-crystal garnet ferrite films with orthorhombic magnetic anisotropy. It is shown that this dependence includes both the initial linear portion and a saturation portion and exhibits a complex behavior. This behavior is explained within the model of domain wall motion with spin wave radiation.