Multiferroic System Research Articles

Phase separation and electrical switching between two isosymmetric multiferroic phases in tensile strainedBiFeO3thin films

Phase separation of two different isosymmetric phases can occur as a result of their similar ground state energies. We have found such phase separation in a multiferroic system at a tensile-strain-driven morphotropic phase boundary of rhombohedral and orthorhombic bismuth ferrite. We utilize the emergent phase competition in order to implement three-step electric switching of ferroelectric polarization as well as rotation of the magnetic easy axis using electric fields. This observation provides a pathway to phase-competition-driven magnetoelectric devices.

Effect of Pr- and Nd- doping on structural, dielectric, and magnetic properties of multiferroic Bi0.8La0.2Fe0.9Mn0.1O3

Bi0.8La0.15RE0.05Fe0.9Mn0.1O3 (where RE = Pr and Nd) have been prepared via conventional solid state route. The Rietveld refinement of X-ray diffraction patterns shows that both systems crystallize in orthorhombic Pnma space group. Raman modes observed for these two systems indicate that both systems are very close to orthorhombic Pnma structure. Appearance of prominent A1-3 and weak E-2 modes in Bi0.8La0.15Nd0.05Fe0.9Mn0.1O3 indicate the presence of chemically more active Bi-O covalent bonds (which favors stereochemical activity of Bi lone pair electrons) in comparison to Bi0.8La0.15Pr0.05Fe0.9Mn0.1O3. Moreover, Bi0.8La0.15Nd0.05Fe0.9Mn0.1O3 system shows higher dielectric constant, low dielectric loss, and higher magnetization value in comparison to Bi0.8La0.15Pr0.05Fe0.9Mn0.1O3 system. Ferroelectric transition temperature decreases to 460 °C for both systems in comparison to 710 °C of Bi0.8La0.2Fe0.9Mn0.1O3. The improved dielectric and magnetic response suggests Bi0.8La0.15Nd0.05Fe0.9Mn0.1O3 a better multiferroic system than Bi0.8La0.15Pr0.05Fe0.9Mn0.1O3.

Magnetoelectric Coupling Phenomena Based on the Changes of Magnetic Properties in Multiferroic Nanocomposite BaTiO<sub>3</sub>-BaFe<sub>12</sub>O<sub>19</sub>

Magnetoelectric (ME) coupling effects in multiferroic materials have attracted much attention in recent times because of the intriguing science underpinning this phenomena and being currently intense interest in the implementation of this coupling in an electronic devices. A new multiferroic system comprising of BaTiO3 (BTO) and BaFe12O19 (BHF) has been synthesized as a bulk nanocomposite system in variation of weight fraction of BTO : BHF =1:1, 1:2 and 1:3 and the second sinter temperature was 925°C for 5, 10 and 15 hours. The presence of both phases were confirmed by X-Ray Diffraction (XRD) studies and MPS Magnet Physik EP3 Permagraph L was used to characterize magnetic properties.The morphology and particle size of nanocomposite was characterized by using Transmission Electron Microscope (TEM). No residual phases were identified in the XRD analysis for all parameters confirming the formation of a BTO-BHF composite system. The TEM images show that all samples have particle in nanosize.For weight fraction of BHF until 2 parts there is an increase of intrinsic coersive and magnetization saturation value. If the weight fraction of BHF exceeds from 2 parts, the coersivity and saturation values decrease. Meanwhile in compound of polyvinyl acetate (PVA) and BHF as a compare material, the magnetic properties increase with increasing the content of BHF until 3 parts. From the above results, it presumes that the nanocomposites with weight fraction of BTO : BHF = 1:3 for all time of sintering have ME coupling interaction showing a multiferroic nature. To give evidence of this phenomena, it needs a measurement of ME coupling coefficients for all parameters.

Magnetic-field-induced spontaneous polarization reversal in multiferroicMn0.85Co0.15WO4

The magnetic and ferroelectric properties of the multiferroic system ${\text{Mn}}_{1\ensuremath{-}x}$${\text{Co}}_{x}$${\text{WO}}_{4}$ ($x=0.135$, 0.15, and 0.17) are studied in magnetic fields ${H}_{c}$ oriented along the monoclinic $c$ axis. ${\text{Mn}}_{0.85}$${\text{Co}}_{0.15}$${\text{WO}}_{4}$, which is right at the phase boundary between two helical spin structures, exhibits a spontaneous sign change of the ferroelectric polarization when cooled in fields ${H}_{c}>$ 25 kOe. The origin of the ferroelectric polarization is studied and two magnetic exchange interactions contributing to the polarization are identified. In ${\text{Mn}}_{0.85}$${\text{Co}}_{0.15}$${\text{WO}}_{4}$, domains of the characteristic helical spin structures, known for $x<0.15$ and $x>0.15$, coexist and form domain boundaries. The contributions of the different domains to the global polarization are determined. The polarization reversal in ${\text{Mn}}_{0.85}$${\text{Co}}_{0.15}$${\text{WO}}_{4}$ can be explained by a combination of various contributions to the polarization and a strong correlation between magnetic domains of different helical spin orders resulting in a smooth transition across the domain walls, which preserves the chirality of the spin spiral.

Two dimensional growth of ultrathin Fe films on BaTiO3 with sharp chemical interface

The Fe/BaTiO3 interface is a prototypical artificial multiferroic system displaying purely electronic magnetoelectric effects at room temperature. As magneto-electric coupling is essentially localized at the interface, the properties of the very first Fe layers in contact with BaTiO3 play a major role. In this paper, we investigate, by using X-ray photoemission spectroscopy and photoelectron diffraction, the in-situ growth, by molecular beam epitaxy, of ultrathin Fe films (∼7 monolayers) on a BaTiO3/SrTiO3(001) template. We found that growing the Fe films above room temperature (373 K) is essential in order to avoid island growth and obtain a continuous film. Post-annealing up to 473 K improves the film crystallinity but prevents chemical interdiffusion and roughening. Just an interfacial monolayer of oxidized iron is detected in these conditions, which appears as an unavoidable consequence of the Fe/BaTiO3 chemical interaction. Its active role in magnetoelectric coupling must be carefully taken into account to correlate theoretical predictions and experiments.

Reentrant spin-glass state in a geometrical frustrated multiferroic system: Role of disorder

We investigated the effect of magnetic (Mn) and nonmagnetic (Ga) doping at the Cr site of the layered geometrically frustrated antiferromagnetic compound LiCrO2. 10% Ga doping at the Cr site does not invoke any metastability typical of a glassy magnetic state. However, similar amount of Mn doping drives the system to a spin glass (SG) state which is particularly evident from the magnetic memory and heat capacity studies. The onset of glassy state in 10% Mn doped sample is of reentrant type developing out of higher temperature antiferromagnetic state. The SG state in the Mn-doped sample shows a true reentry with the complete disappearance of the antiferromagnetic phase below the SG transition. The lack of SG state on Ga doping indicates the importance of random ferromagnetic/antiferromagnetic bonds for the glassy ground state in LiCrO2.

Multiferroism and magnetoelectric coupling in (PbZr0.65Ti0.35O3)0.97–(BaFe12O19)0.03 ceramic composites

The ferroelectric, magnetic, and magneto-electric properties of a PZT/Ba-hexaferrite (PbZr0.65Ti0.35O3/BaFe12O19) ceramic composite are reported. The expected rhombohedral-PZT and Ba-hexaferrite phases have been confirmed from structural and Rietveld refinement analyses, without additional undesired phases. Well-saturated ferroelectric and magnetic hysteresis loops confirmed the ferroelectric and magnetic nature of the study system, revealing enhanced characteristics when compared to those reported for typical multiferroics. The existence and enhancement of the magneto-electric response have been demonstrated by measuring the effect of the external DC magnetic field on the dielectric permittivity. The obtained results suggest an excellent candidate for room temperature multiferroic system with enhanced properties.

Comparative study of magnetoelectric composite system Ba0.95Sr0.05TiO3–Ni0.8Co0.2Fe2O4 with ferrite prepared by different methods

In this paper, effect of addition of Ni0.8Co0.2Fe2O4 (NCF), prepared by three different methods namely solid state reaction method, sol–gel and co-precipitation, has been studied on microstructural, magnetic, dielectric, ferroelectric and magnetoelectric properties of multiferroic composite system 0.95 Ba0.95Sr0.05TiO3–0.05 Ni0.8Co0.2Fe2O4. Ba0.95Sr0.05TiO3 (BST) has been prepared by solid state reaction method. Titular representation of NCF samples prepared by sol–gel, co-precipitation, solid state reaction method is N–SG, N–CP, N–SS respectively and that of corresponding magnetoelectric composite is C–SG, C–CP, C–SS. X–ray diffraction analysis of the composite samples (C–SG, C–SS, C–CP) indicated the presence of both NCF and BST phases. Rietveld analysis of XRD pattern further confirmed the proper phase formation in the composite samples. Sol–gel and co-precipitation processes result in finer NCF particles as confirmed by Transmission electron microscopy (TEM). Sample N–SG possesses uniform particle size and shape. Magnetization versus magnetic field (M–H) loops of samples C–SS and C–CP possess respectively highest value of remanant magnetization and magnetic coercive field. Dielectric properties of BST, NCF and composite samples have been measured in the frequency range of 20Hz–1MHz and temperature range of 50–170°C. The composite sample C–CP results in highest value of dielectric constant in comparison to samples C–SS and C–SG. Polarization versus electric field (P – E) measurements pointed that composite sample C–SG possesses highest value of remanant and saturation polarization and C–SS possesses lowest value of coercive electric field. The composite sample C–SS possesses highest value of magnetoelectric coefficient.

Competing antiferromagnetism and local magnetic order in the bulk ceramic PZT–PFW multiferroic system: searching for the most promising ratio between PZT and PFW

Bulk ceramic compounds of formula xPZT + (1 − x)PFW, where PZT = Pb(Zr0.575Ti0.425)O3 and PFW = Pb(Fe2/3W1/3)O3, for x = 0.15; 0.20; 0.25; 0.37; 0.50; 0.60; 0.65; 0.70 and 0.80, were synthesized. In these single phase perovskites a change from rhombohedral to cubic structure is observed as the proportion of PFW increases. Magnetic investigations including zero-field cooled and field-cooled magnetization versus temperature measurements, magnetic hysteresis loops measurement and ac susceptibility measurements were performed. At high temperatures a clear antiferromagnetic (AFM)-like transition was observed for high PFW concentrations. The low temperature behaviour indicated spin freezing and thermal activation of magnetic moments and measurements confirmed the existence of a barrier maximum for x = 0.37. Competition between the long-range AFM order and short-range magnetic ordering of small nano-sized regions with finite magnetic moments was observed in the studied system.

Structural control of magnetic anisotropy in a strain-driven multiferroic EuTiO3thin film

Octahedral distortion plays a key role in engineering the physical properties of heterostructures composed of perovskite oxides. We observe a strong in-plane uniaxial magnetic anisotropy in a strain-enabled multiferroic EuTiO3 thin film epitaxially grown on a (110)o DyScO3 substrate. First principles calculations show that the magnetic anisotropy is closely correlated with the uniaxial TiO6 octahedral tilting and the ferroelectric polarization of the film, indicating potential strong magnetoelectric coupling in the strain-engineered multiferroic system.

Fabrication and characterization of tunable multiferroic Bi0.7Dy0.3FeO3 based on-chip micro-inductor

An optical lithographic process is developed to realize Si compatible on-chip micro-inductor using single phase room temperature multiferroic system like Bi0.7Dy0.3FeO3 (BDFO) as a core material. On-chip inductor is characterized for inductance and Q factor in the frequency range of 1kHz-1MHz. The properties are studied at different values of current passing through the coil and also bias applied directly across the BDFO core. Tunability goes on increasing with current and reaches as high as ~90% at 0.07A of DC current passing through the coil whereas ~30% tunability is obtained at 10V of DC voltage applied across the core. It indicates that substantial change in permeability occurs even at low fields due to presence of magnetoelectric (ME) coupling in BDFO system. The use of the single layer multiferroic BDFO core simplifies the on-chip fabrication process. In addition it helps to avoid the formation of defects that are normally formed at the interface of hetero-structures. Consequently it makes the process more reliable. The integration of the on-chip multiferroic micro-inductor would be useful in miniaturizing the inductive components in electronic device circuit.

Effect of Ca doping on enhancement of ferroelectricity and magnetism in HoMnO3 multiferroic system

This paper reports an alternative approach to achieve orthorhombic HoMnO3 (HMO) structure by Ca doping at Holmium site. It is observed that pure HMO undergoes hexagonal (P6 3 cm) to orthorhombic (Pbnm) structural transformation when Ca is substituted at Ho site through high temperature sintering at 1,450 °C for 10 h. With the substitution of Ca in HMO (Ho1−xCaxMnO3, x = 0.05), hexagonal unit cell volume has been contracted significantly. However with further increase of Ca dopant concentration (x = 0.1, 0.2), unit cell volume for each composition are marginally enhanced. Spontaneous polarization (Ps), remanent polarization (Pr) values are observed to be enhanced up to the limiting doping value for x = 0.1. Further substitution of Ca in HMO (x > 0.2) reduces the Ps and Pr value marginally. Similarly dielectric constant (er) and dielectric loss values are increased up to x = 0.1 and then decreased up to x = 0.2. These observations are expected to be linked with the structural and microstructural changes with the variation of x. At x = 0.1, larger grains are developed, with enhanced value of polarization and dielectric constant. In addition to above mentioned enhanced ferroelectric polarization and dielectric behaviour, we have observed peculiar magnetic property from M to H curve. In all Ho1−xCaxMnO3 (x = 0–0.2) samples, a switching behaviour in low field was observed in field dependence of magnetization at room temperature and the behaviour found to be enhanced with Ca concentration up to 0.1 and then decreases.

Intermixing and Magnetoelectric Coupling in Ferroelectric/Multiferroic Superlattices

We develop a simple model of superlattices consisting of alternate ferroelectric and multiferroic layers with interface intermixing based on the Landau-Ginzburg theory. Multiferroic system is described using a Landau-Ginzburg potential with a positive biquadratic coupling between magnetization and polarization. Intermixing at interfaces forms intermixed layers with electrical properties different from its constituent layers. Effects of intermixing and magnetoelectric coupling on multiferroic phase transitions in the ferroelectric/multiferroic superlattices is calculated and discussed.

Multiferroic properties of BiFeO3-(K0.5Bi0.5)TiO3 ceramics

Ceramics of xBiFeO3-(1−x)(K0.5Bi0.5)TiO3 (where x=0.6, 0.7 and 0.8) have been produced, and characterized using neutron diffraction, revealing a G-type antiferromagnetic structure. The ferroelectric Curie temperature was >430°C, and the Néel temperature was in the range 228–254°C, increasing with x. Ferroelectricity was confirmed using strain and polarization-field measurements, for the composition x=0.6, with a Ps=26μC/cm2 and a coercive field=3.85kV/mm. Piezoelectricity was observed for this material with a low-field d33 of 37pm/V, and a high-field d33⁎ of 80pm/V where x=0.6. The reduction in coercive field in this system is shown to correlate with the oxygen octahedral tilt angle. The solid solution xBiFeO3-(1−x)(K0.5Bi0.5)TiO3 has potential as a high temperature, piezoelectric, multiferroic system.

Repulsive magneto-structural interaction in the ferromagnetic shape memory alloys Ni2Mn1+xIn1−x

A phenomenological theory is presented to study the multiferroic system in the shape memory alloys Ni2Mn1+xIn1−x with the Heusler-type structure, where the phase transitions are characterized by the two order parameters, i.e., the martensitic distortion e3 and the magnetization M. The Landau free energy is expanded in powers of e3 and M with including the Zeeman energy and the energy increase by the uniaxial force on e3. The resultant free energy is applied to analyze the experimental results, such as the phase diagram in the temperature–concentration plane and the reentrant ferromagnetism by increasing temperature. The magnetic fields are shown to cause the ferromagnetic–ferromagnetic or metamagnetic transitions as observed. Further, it is predicted that the uniaxial force can dominate the appearance of the ferromagnetism in some alloys. It is found that these exotic behaviors of this alloy system are all ascribed to the repulsive interaction between e3 and M, which originates from their biquadratic term. Through the present analyses, it is verified that this repulsive interaction is expected in other alloy systems Ni2Mn1+xX1−x (X=Sn, Sb) and makes a contrast to the attractive interaction realized in another group of the alloy systems including Ni2+xMn1−xGa and Ni2Mn1−xCuxGa.

Structural study in highly compressed BiFeO3 epitaxial thin films on YAlO3

We report a study on the thermodynamic stability and structure analysis of the epitaxial BiFeO3 (BFO) thin films grown on YAlO3 (YAO) substrate. First, we observe a phase transition of MC–MA–T occurs in thin sample (<60 nm) with an utter tetragonal-like phase (denoted as MII here) with a large c/a ratio (∼1.23). Specifically, MII phase transition process refers to the structural evolution from a monoclinic MC structure at room temperature to a monoclinic MA at higher temperature (150 °C) and eventually to a presence of nearly tetragonal structure above 275 °C. This phase transition is further confirmed by the piezoforce microscopy measurement, which shows the rotation of polarization axis during the phase transition. A systematic study on structural evolution with thickness to elucidate the impact of strain state is performed. We note that the YAO substrate can serve as a felicitous base for growing T-like BFO because this phase stably exists in very thick film. Thick BFO films grown on YAO substrate exhibit a typical “morphotropic-phase-boundary”-like feature with coexisting multiple phases (MII, MI, and R) and a periodic stripe-like topography. A discrepancy of arrayed stripe morphology in different direction on YAO substrate due to the anisotropic strain suggests a possibility to tune the MPB-like region. Our study provides more insights to understand the strain mediated phase co-existence in multiferroic BFO system.

Theory and the Experimental Confirmation of the Local Electronic Structure of the Multiferroic PbVO3, a New Member of PbTiO3Family, Studied by X-ray Near Edge Absorption Structure: I

Recently, an interesting multiferroic system PbVO 3 [A. A. Belik et al. : Chem. Mater. 17 (2005) 269] has been successfully prepared using a high-pressure and high-temperature technique. The crystallographic features of PbVO 3 were reported. In this note, we concentrate on the theoretical and the experimental X-ray near edge absorption structure (XANES) spectra by considering the K-edge of Vanadium. The tetragonality ( c / a =1.229 at 300 K) of PbVO 3 is the largest in the PbTiO 3 family of compounds. Thus, one is led naturally to examine the effects of the changes in the tetragonality and axial oxygen position on the electronic structure (i.e., XANES spectrum). We study these effects in two ways. At a given temperature, we vary the tetragonality and the axial oxygen position, and quantify the changes in terms of the XANES difference spectrum. Secondly, we compute the XANES spectra at three different temperatures, namely, 90, 300, and 530 K, and quantify the changes in terms of the difference spectrum. We note that in this compound the tetragonality increases almost monotonically with temperature from 12 to 570 K, without transition to the cubic phase under ambient pressure. A key objective of the current investigation is to gain an understanding of various absorption features in the vicinity of the K-edge of V, in terms of valence, local site symmetry, local coordination geometry, local bond distances, charge transfer, and local projected density of states. We consider both the polarized and unpolarized XANES spectra, theoretically. The experiment was performed on the polycrystalline material after the theoretical investigation. In short, we have performed a local electronic study, theoretical as well as experimental, which complements the crystallographic features reported recently for PbVO 3 . The local electronic study given here is supplemented and enhanced by the O-K edge results indicated in the accompanying Paper II, which gives both the experimental and theoretical analyses of the XES, XAS, and fundamental band gap of PbVO 3 , which is found to be 1.2 eV.

ChemInform Abstract: Recent Progress in Nanostructured Multiferroic Bi2FeCrO6 Thin Films.

AbstractReview: 47 refs.

Hybrid Multiferroic Nanostructure with Magnetic–Dielectric Coupling

The development of methods to economically synthesize single wire structured multiferroic systems with room temperature spin-charge coupling is expected to be important for building next-generation multifunctional devices with ultralow power consumption. We demonstrate the fabrication of a single nanowire multiferroic system, a new geometry, exhibiting room temperature magnetodielectric coupling. A coaxial nanotube/nanowire heterostructure of barium titanate (BaTiO(3), BTO) and cobalt (Co) has been synthesized using a template-assisted method. Room temperature ferromagnetism and ferroelectricity were exhibited by this coaxial system, indicating the coexistence of more than one ferroic interaction in this composite system.

First principles study of the electric polarization and of its switching in the multiferroic GaFeO3 system

The electric polarization in the multiferroic GaFeO3 system is determined from its electronic structure using first principles methods and the modern theory of polarization. By carefully following the electric polarization on a path connecting the polar and centrosymmetric structures, it is found to be − 25 μC cm−2, which is ten times larger than a previous estimation given in the literature a few years ago and two times smaller than the value obtained in a recent similar study. The switching of this electric polarization through a centrosymmetric structure is discussed in terms of the total energy barrier. It is exhibited that such a switching is particularly difficult to achieve in relation to the tetrahedral environment of half of the Ga atoms. The switching via domain wall motion is also discussed.