Room-temperature Multiferroics Research Articles

Multiferroicity and magnetoelectric coupling in doped ZnO

In this paper, we report the synthesis of pure ZnO, Li, Ni, and Mg doped (5wt%) ZnO samples via sol–gel method and studied their structural, optical, ferroelectric and ferromagnetic properties. X-ray diffraction (XRD) analysis indicates the successful formation of ZnO phase. FESEM results show that the grain size of synthesized samples varies from 60 to 85nm for pure and doped samples. It is observed that the value of dielectric constant and dielectric loss is maximum for pure ZnO which decreases for doped samples. Polarization versus electric field (P–E) loops of pure ZnO shows loosy capacitor behavior which tends towards non-linear ferroelectric behavior through the doping of Ni, Li and Mg in ZnO. Furthermore, the room temperature magnetization versus magnetic field (M–H) loops indicates the existence of ferromagnetism in pure, Ni, and Mg doped samples. However, Li doped ZnO sample shows an unusual behavior indicating the presence of both ferromagnetic and diamagnetic phases in the sample. The coexistence of both ferroelectricity and ferromagnetism simultaneously in Ni and Mg doped ZnO makes it a possible candidate for multiferroic devices. The coupling between ferroelectricity and ferromagnetism is measured through magnetoelectric (ME) coupling measurements which shows that the average value of second order magnetoelectric coefficient (β) increases from 5.23×10−6 to 3.08×10−4μV/cm-Oe2 for pure to Ni doped ZnO samples, respectively. The room temperature multiferroicity with magnetoelectric coupling in Ni and Mg doped ZnO is not reported earlier as per our knowledge.

Oxygen Diffusion and Nonstoichiometry in BiFeO3

Leakage current is a serious problem for the use of ferroelectricity in room-temperature multiferroics BiFeO3, and oxygen nonstoichiometry is considered as one of its principal origins. In order to establish a method to control oxygen content in the compound, we investigated the annealing process of stoichiometric BiFeO3 grains in air and revealed that oxygen diffusion occurs in two steps: (1) the weight of the sample decreases in a short time, which originates from the generation of oxygen deficiency near the surface of the grains; and then (2) it increases gradually and slowly, which originates from oxygen diffusion toward equilibrium in the inner part of the grains, introducing excess oxygen there. Step 1 causes the leakage current, and step 2 tends to cause inhomogeneity of oxygen content as well as the leakage current. Steps 1 and 2 are related to oxygen deficiency and excess oxygen often observed in thin films and bulk crystals, respectively. For the synthesis of homogeneous and highly insulating bulk sample, it is important to avoid these annealing processes, and it is a good way to grow a crystal with stoichiometric oxygen content by the control of atmospheric oxygen partial pressure and taking out its inner part.

ChemInform Abstract: Hexagonal Ferrites: A Review of the Synthesis, Properties and Applications of Hexaferrite Ceramics

AbstractReview: 560 refs.

Room-temperature multiferroic hexagonal LuFeO3 films.

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO(3) films are room-temperature multiferroics.

Possibility of room-temperature multiferroism in Mg-doped ZnO

Room-temperature multiferroic properties in Mg-doped ZnO samples are reported wherein Mg replaces Zn in the ZnO matrix and retains hexagonal wurtzite structure. The saturation magnetisation is increased from ∼2×10−4 emu/g to 3×10−4 emu/g for the dilute doping of 2 % Mg in pure ZnO and the ferroelectricity is also increased. Higher concentration of Mg does not lead to a significant enhancement in the magnetisation but improves the ferroelectric properties. An X-ray absorption spectroscopic study shows an enhancement in O vacancies with dilute doping of Mg. The origin of the multiferroic behaviour is understood based on their crystal and electronic structures.

Synthesis and characterization of new Bi2FeNiO6 material using a citric acid assisted gel combustion technique

Abstract Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties–and in turn promise new device applications–as a result of the coupling between their dual order parameters. BiFeO3 is currently considered the most promising candidate material for device applications of room temperature multiferroics. However, its G-type antiferromagnetic behavior and high Néel temperature restrict the use of this material in potential applications. We report the synthesis and characterization of a novel Bi2FeNiO6 material for the first time using a citric acid assisted gel combustion technique. Magnetization studies reveal that it exhibits ferrimagnetism with Néel temperature around 500 K. Differential scanning calorimetry study also reveals a sharp phase transition at 500 K. These materials also exhibit good ferroelectric behavior with square type hysteresis with a remanent polarization (P r) of 1.28 μC cm−2, saturation polarization (P s) of 18 μC cm−2 and coercive fields 20 kV and–40 kV respectively. The results are convincing to make a big step towards developing devices that run on spin.

Symmetries and multiferroic properties of novel room-temperature magnetoelectrics: Lead iron tantalate – lead zirconate titanate (PFT/PZT)

Mixing 60-70% lead zirconate titanate with 40-30% lead iron tantalate produces a single-phase, low-loss, room-temperature multiferroic with magnetoelectric coupling: (PbZr0.53Ti0.47O3) (1-x)- (PbFe0.5Ta0.5O3)x. The present study combines x-ray scattering, magnetic and polarization hysteresis in both phases, plus a second-order dielectric divergence (to epsilon = 6000 at 475 K for 0.4 PFT; to 4000 at 520 K for 0.3 PFT) for an unambiguous assignment as a C2v-C4v (Pmm2-P4mm) transition. The material exhibits square saturated magnetic hysteresis loops with 0.1 emu/g at 295 K and saturation polarization Pr = 25 μC/cm2, which actually increases (to 40 μC/cm2) in the high-T tetragonal phase, representing an exciting new room temperature oxide multiferroic to compete with BiFeO3. Additional transitions at high temperatures (cubic at T>1300 K) and low temperatures (rhombohedral or monoclinic at T<250 K) are found. These are the lowest-loss room-temperature multiferroics known, which is a great advantage for magnetoelectric devices.

Interface-induced room-temperature multiferroicity in BaTiO3

Multiferroic materials possess two or more ferroic orders but have not been exploited in devices owing to the scarcity of room-temperature examples. Those that are ferromagnetic and ferroelectric have potential applications in multi-state data storage if the ferroic orders switch independently, or in electric-field controlled spintronics if the magnetoelectric coupling is strong. Future applications could also exploit toroidal moments and optical effects that arise from the simultaneous breaking of time-reversal and space-inversion symmetries. Here, we use soft X-ray resonant magnetic scattering and piezoresponse force microscopy to reveal that, at the interface with Fe or Co, ultrathin films of the archetypal ferroelectric BaTiO₃ simultaneously possess a magnetization and a polarization that are both spontaneous and hysteretic at room temperature. Ab initio calculations of realistic interface structures provide insight into the origin of the induced moments and bring support to this new approach for creating room-temperature multiferroics.

Transition metal modified bulk BiFeO 3 with improved magnetization and linear magneto-electric coupling

At present BiFeO 3 (BFO) is the most attractive and sole example, which possesses low magnetization value, high leakage current and low polarization in ceramic form. Single-phase room temperature multiferroics are rare in nature. This paper deals with the improved magnetic and observed linear magneto-electric coupling in Co and Sm co-doped BiFeO 3 ceramics synthesized by sol–gel process at low temperature ∼600 °C. As synthesized Bi 0.9Sm 0.10Fe 0.95Co 0.05O 3 (BSFCO) showed high impurities phases (20%) over wide range of calcination temperatures. Impurity phases reduced drastically from 20% to 5% after leaching with nitric acid. However the electrical and the magnetic properties were almost the same for both phases. Well-defined magnetic hysteresis with high magnetic moment was found at room temperature. Ferroelectric polarization studies demonstrated similar values and shape as reported in literature for the pure bulk BFO. Linear magneto-electric (ME) coupling and weak ME coefficient ( α) ∼ 0.6 e−10 s m −1 were observed in the co-doped BFO. The origin of the strong ferromagnetic property in our samples may be due to the presence of rare earth and transition metal ions at the lattice sites of BFO or due to impurity phase, since we have not seen any change in magnetization with reduction of impurity phase the later effect is more unlikely.

Preparation and magnetic properties of Bi m + 1 Fe m − 3 Ti 3O 3 m + 3 thin films with magnetic order above room temperature

Thin films of Bi m + 1 Fe m − 3 Ti 3O 3 m + 3 (BFTO) with m ≦ 9 have been successfully grown on (100) SrTiO 3 by chemical solution deposition. These films had the c axis normal to the film plane. The conversion electron Mössbauer spectoroscopy (CEMS) showed that the spectra of BFTO thin films exhibit an asymmetric quadrupole doublet for m = 8 at 300 K, indicative of being paramagnetic, while, for m = 9, clearly show six hyperfine lines indicating presence of magnetic order at 300 K. From the intensity ratio of asymmetric peaks, the polarization axis of BFTO films with m ≥ 8 was deduced to be likely along <101> of the perovskite-like unit. On the other hand, it was found from the spectral fitting that the BFTO thin film with m = 9 has the Néel temperature around 310 K and the spin axis making an angle of about 60° to the c-axis. These indicate that the BFTO ( m = 9) thin film is a promising candidate for room-temperature multiferroics.

Room Temperature Multiferroicity in Zn0.98Cu0.02O Film Prepared in N Plasma

Zn0.98Cu0.02O films have been prepared under different conditions by pulsed laser deposition. Ferromagnetism with saturate magnetization of about 7 emu/cm3 at 300 K has been observed in Zn0.98Cu0.02O film prepared in N plasma at room temperature. The concentration of oxygen vacancies was increased by N substituting O in N plasma, and the ferromagnetism originated from the aligned magnetic moments of more Cu ions mediated by oxygen vacancies. Furthermore, ferroelectricity has been confirmed by the observation of the electrical field dependent converse piezoelectric coefficient d 33 loop at room temperature, indicating the potential multiferroic applications.

Twisting and turning towards new multiferroics

Controlling the rotation and tilt of oxygen octahedra in perovskite structures provides a new route towards room-temperature multiferroics.

Room temperature multiferroicity in Bi rich Fe deficient Gd doped Bi 1.2Gd 0.1Fe 0.8O 3

A clean signature of room temperature multiferroicity in Bi 1.2Gd 0.1Fe 0.8O 3 ceramics is observed. Samples were prepared by slow step sintering technique at 850 °C. Impurity peaks normally present in original BiFeO 3 are completely suppressed with the increase of Bi concentration. Apart from this, structural transformation took place from R3 C to Pn2 1 a space group and a clear orthorhombic grain growth habit is seen by SEM microstructure. Suppression of impurity phases favors the reduction of mobile oxygen vacancies and hence reduces leakage current for which ferroelectric properties of Bi 1.2Gd 0.1Fe 0.8O 3 are enhanced. We argue that the addition of Gd is likely to suppress the spiral spin modulation and at the same time increase the canting angle which favors the enhanced ferromagnetism. Excess Bi is expected to act as point defects and occupy interstitial positions which in turn surrounded by oxygen vacancies and is likely to promote defect driven ferromagnetism with enhanced spin and electric polarization. This finding encourages further revaluation of long-discounted BiFeO 3 for multiferroics research.

Continuum theory and phase-field simulation of magnetoelectric effects in multiferroic bismuth ferrite

Multiferroic materials possess electric and magnetic orderings simultaneously, making it possible to manipulate the electric state of a multiferroic by magnetic field, or vice versa. Among all single-phase multiferroics, BiFeO 3 is particularly exciting, for its room temperature multiferroicity, excellent ferroelectric properties, and recently demonstrated electric control of antiferromagnetic domains, which opens door for its applications in spintronics. In this paper, we report a systematic theoretical and computational study on the structure and evolution of magnetoelectric domains in multiferroic BiFeO 3. A continuum description is developed for antiferromagnetic ordering first, which is then incorporated into an unconventional phase field method that couples ferroelastic, ferroelectric, and antiferromagnetic orderings through the characteristic function of variants. The internal elastic, electric, and magnetic fields are carefully analyzed, taking into account both bulk and thin film geometries and boundary conditions. The theory is implemented into numerical simulations, where we not only observe the coupled ferroelectric and antiferromagnetic domains, and demonstrate the electric control of antiferromagnetic ordering, but also reveal the switching of antiferromagnetic domains by mechanical stress that is yet to be reported in experiment. Our study offers deep insight into the microstructural evolution and macroscopic properties of BiFeO 3, and provides a powerful tool to study a wide range of multiferroic materials with magnetoelectric coupling.

Enhanced Magnetization and Modulated Orbital Hybridization in Epitaxially Constrained BiFeO3 Thin Films with Rhombohedral Symmetry

BiFeO3 (BFO) is currently considered to be the most promising candidate material for device applications of room-temperature multiferroics. However, there exist some controversial arguments on the origin of the enhanced magnetization and polarization observed in the epitaxially constrained BFO thin film heterostructures. More specifically, the issue can be addressed by the following question: Can the epitaxial strain enhance the magnetization and the ferroelectric polarization in BFO? To clarify this controversial issue, we have systematically examined the magnetization characteristics of the rhombohedral BFO films epitaxially grown on (111)-oriented SrTiO3 (STO) in terms of the in-plane misfit strain between the BFO layer and the STO substrate. The increase in the saturation magnetization with decreasing film thickness was found to be closely related with the misfit strain. By carefully examining synchrotron X-ray absorption spectra, we have further correlated the enhanced magnetization in a highly strai...

In-situ formation of barium ferrite in iron-doped “tetragonal tungsten bronze”: Elaboration of room temperature multiferroic composites

Recent studies of ceramics of formula Ba 2LnFeNb 4O 15 (Ln=rare earth) with the “tetragonal tungsten bronze” (TTB) structure have correlated their room temperature multiferroics properties to the occurrence of barium ferrite parasitic phases. This work presents the elaboration of Ba 2LaFeNb 4O 15 and Ba 2EuFeNb 4O 15 composite samples with an excess of hematite in the TTB nominal composition. The influence of crystal-chemistry on the phase content and properties of Ba 2LnFeNb 4O 15 TTB composites is discussed. A particular focus on the mechanisms related to the in-situ formation of barium ferrite is given. We show that we can control the spurious ferrite phase in TTB multiferroic composites and thus modulate their magnetic response.

Comparison of room-temperature multiferroics in Bi 4Fe 2TiO 12 film and bulk

It was reported that both dielectricity and magnetism at room temperature were appreciably improved in Bi 4Fe 2TiO 12 film compared with Bi 4Fe 2TiO 12 bulk. X-ray diffraction profiles reveal similar crystalline nature and random orientation of the two, but X-ray photoelectron spectroscopy (XPS) experiments indicate that it is 1.4 eV lower binding energy of core-state O1s in the film relative to that of the bulk, so the improvement of multiferroics in the film is attributed to oxygen vacancies and high fraction of interface. The results have promising applications in multifunctional integrated devices.

Phase Transitions in Multiferroic BiFeO3 Crystals, Thin‐Layers, and Ceramics. Enduring Potential for a Single Phase, Room‐Temperature Magnetoelectric ′Holy Grail′

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Phase transitions in multiferroic BiFeO3 crystals, thin-layers, and ceramics: enduring potential for a single phase, room-temperature magnetoelectric ‘holy grail’

Magnetic phase transitions in multiferroic bismuth ferrite (BiFeO3) induced by magnetic field, epitaxial strain, and composition modification are considered. These transitions from a spatially modulated spin spiral state to a homogenous antiferromagnetic one are accompanied by tghe release of latent magnetization and a linear magnetoelectric effect that makes BiFeO3-based materials efficient room-temperature single phase multiferroics.

Anisotropy of hexagonal ferrites with M, W and Y structure containing Fe 3+ and Fe 2+ as magnetic ions

Since their discovery in the 1950s there has been an increasing degree of interest in the hexagonal ferrites, also know as hexaferrites, which is still growing exponentially today. These have become massively important materials commercially and technologically, accounting for the bulk of the total magnetic materials manufactured globally, and they have a multitude of uses and applications. As well as their use as permanent magnets, common applications are as magnetic recording and data storage materials, and as components in electrical devices, particularly those operating at microwave/GHz frequencies. The important members of the hexaferrite family are shown below, where Me = a small 2+ ion such as cobalt, nickel or zinc, and Ba can be substituted by Sr:•M-type ferrites, such as BaFe12O19 (BaM or barium ferrite), SrFe12O19 (SrM or strontium ferrite), and cobalt–titanium substituted M ferrite, Sr- or BaFe12−2xCoxTixO19 (CoTiM).•Z-type ferrites (Ba3Me2Fe24O41) such as Ba3Co2Fe24O41, or Co2Z.•Y-type ferrites (Ba2Me2Fe12O22), such as Ba2Co2Fe12O22, or Co2Y.•W-type ferrites (BaMe2Fe16O27), such as BaCo2Fe16O27, or Co2W.•X-type ferrites (Ba2Me2Fe28O46), such as Ba2Co2Fe28O46, or Co2X.•U-type ferrites (Ba4Me2Fe36O60), such as Ba4Co2Fe36O60, or Co2U .The best known hexagonal ferrites are those containing barium and cobalt as divalent cations, but many variations of these and hexaferrites containing other cations (substituted or doped) will also be discussed, especially M, W, Z and Y ferrites containing strontium, zinc, nickel and magnesium. The hexagonal ferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures. They all have a magnetocrystalline anisotropy (MCA), that is the induced magnetisation has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetisation, the uniaxial hexaferrites, and those with an easy plane (or cone) of magnetisation, known as the ferroxplana or hexaplana ferrites. The structure, synthesis, solid state chemistry and magnetic properties of the ferrites shall be discussed here. This review will focus on the synthesis and properties of bulk ceramic ferrites. This is because the depth of research into thin film hexaferrites is enough for a review of its own.There has been an explosion of interest in hexaferrites in the last decade for more exotic applications. This is particularly true as electronic components for mobile and wireless communications at microwave/GHz frequencies, electromagnetic wave absorbers for EMC, RAM and stealth technologies (especially the X and U ferrites), and as composite materials. There is also a clear recent interest in nanotechnology, the development of nanofibres and fibre orientation and alignment effects in hexaferrite fibres, and composites with carbon nanotubes (CNT). One of the most exciting developments has been the discovery of single phase magnetoelectric/multiferroic hexaferrites, firstly Ba2Mg2Fe12O22 Y ferrite at cryogenic temperatures, and now Sr3Co2Fe24O41 Z ferrite at room temperature. Several M, Y, Z and U ferrites have now been characterised as room temperature multiferroics, and are discussed here. Current developments in all these key areas will be discussed in detail in Sections 7 The microwave properties of hexagonal ferrites, 8 Magnetoelectric (ME), multiferroic (MF) and dielectric properties of hexaferrites, 9 Hexaferrite composites, 10 Hexagonal ferrite fibres, 11 Nanoscale hexagonal ferrite particles, ceramics and powders of this review, and for this reason now is the appropriate time for a fresh and critical appraisal of the synthesis, properties and applications of hexagonal ferrites.