Multiferroic Compound Research Articles

A comparative Raman study between PrMnO3, NdMnO3, TbMnO3 and DyMnO3

In this paper, we present a detailed Raman study of the non-multiferroic compounds PrMnO3 and NdMnO3 and the multiferroic compounds TbMnO3 and DyMnO3 as a function of temperature and magnetic field. All studied systems show anomalous phonon shifts close to the Néel transition TN. In PrMnO3 and NdMnO3, the frequency softenings are partly attributed to an orbital-spin-phonon coupling whereas in TbMnO3 and DyMnO3, the relatively weak frequency shifts are rather attributed to an expansion of the Mn−O bond lengths. On the other hand, the frequencies of TbMnO3 phonons are shifted as a function of magnetic field, while those of PrMnO3 remain unaffected. These frequency shifts are interpreted in terms of local oxygen rearrangements under magnetic field that could play an important role in the multiferroicity of TbMnO3 and DyMnO3.

Relaxation dynamics of modulated magnetic phases in the skyrmion host GaV4S8 : An ac magnetic susceptibility study

We report on the slow magnetization dynamics observed upon the magnetic phase transitions of GaV4S8, a multiferroic compound featuring a long-ranged cycloidal magnetic order and a N\'eel-type skyrmion lattice in a relatively broad temperature range below its Curie temperature. The fundamental difference between GaV4S8 and the chiral helimagnets, wherein the skyrmion phase was first discovered, lies within the polar symmetry of GaV4S8, promoting a cycloidal instead of a helical magnetic order and rendering the magnetic phase diagram essentially different from that in the cubic helimagnets. Our ac magnetic susceptibility study reveals slow relaxation dynamics at the field-driven phase transitions between the cycloidal, skyrmion lattice and field-polarized states. At each phase boundary, the characteristic relaxation times were found to exhibit a strong temperature dependence, starting from the minute range at low temperatures, decreasing to the micro- to millisecond range at higher temperatures.

Enhanced magnetization in morphologically and magnetically distinct BiFeO3 and La0.7Sr0.3MnO3 composites

Nanomaterials exhibit properties different from those of their bulk counterparts. The modified magnetic characteristics of manganite nanoparticles were exploited to improve magnetization in multiferroic BiFeO3 compound. We studied the composite of two morphologically and magnetically distinct compounds BiFeO3 (BFO) and La0.7Sr0.3MnO3 (LSMO). The microcrystalline BiFeO3 sample was prepared by solid state reaction method and the nanocrystalline La0.7Sr0.3MnO3 by sol-gel method. Composites with nominal compositions (1−x)BiFeO3–(x)La0.7Sr0.3MnO3 were prepared by modified solid state reaction method. The phase purity and crystal structures were checked by using X-ray diffraction. The formation of composites with phase separated BFO and LSMO was confirmed using Raman and Fourier Transform Infrared spectroscopy studies. The composite samples showed relatively high value of magnetization with finite coercivity. This improvement in magnetic behavior is ascribed to the coexistence of multiple magnetic orderings in composite samples. We scrutinized the possibility of oxygen vacancy or Fe mixed valency formation in the samples using X-ray photoelectron spectroscopy technique.

Electrical, optical and magneto-electric characteristics of BiBaFeCeO6 electronic system

Double perovskite multiferroic compound BiBaFeCO6 was synthesized by a high-temperature solid-state reaction method. We mainly report X-rays structural and dielectric properties of polycrystalline BiBaFeCO6. The micrograph of the sample, recorded at room temperature, exhibits the homogeneous distribution of grains on the surface of the sample. Analysis of optical spectrum obtained using UV spectroscopy provides the optical band gap of the material. Studies of frequency and temperature dependence of imaginary and real components of complex impedance and modulus have provided the contributions of bulk (grains), grain boundary and interface in electrical and structural properties. The occurrence of above phase transitions (ferroelectricity) in the material has been confirmed by the appearance of a field dependence of polarization loop and anomalies in differential scanning calorimetry (DSC) curve. The frequency-temperature dependence of electrical conductivity of BiBaFeCeO6 obeys Jonscher’s universal power law. The magneto-electric effect of the material has also been studied.

Terahertz dielectric analysis and spin-phonon coupling in multiferroic GeV4S8

We present an investigation of the multiferroic lacunar spinel compound GeV$_4$S$_8$ using time-domain terahertz spectroscopy. We find three absorptions which either appear or shift at the antiferromagnetic transition temperature, T$_N = 17$ K, as S=1 magnetic moments develop on vanadium tetrahedra. Two of these absorptions are coupled to the magnetic state and one only appears below the N\'{e}el temperature, and is interpreted as a magnon. We also observe isosbestic points in the dielectric constant in both the temperature and frequency domains. Further, we perform analysis on the isosbestic features to reveal an interesting collapse into a single curve as a function of both frequency and temperature, behavior which exists throughout the phase transitions. This analysis suggests the importance of spectral changes in the terahertz range which are linear in frequency and temperature.

Rich magnetoelectric phase diagrams of multiferroic single-crystal α−NaFeO2

The magnetic and dielectric properties of the multiferroic triangular lattice magnet compound alpha-NaFeO2 were studied by magnetization, specific heat, dielectric permittivity, and pyroelectric current measurements and by neutron diffraction experiments using single crystals grown by a hydrothermal synthesis method. This work produced magnetic field (in the monoclinic ab-plane, B_ab, and along the c*-axis, B_c) versus temperature magnetic phase diagrams, including five and six magnetically ordered phases in B_ab and along B_c, respectively. Comparing the polarization direction to the magnetic structures in the different ferroelectric phases, we conclude that the extended inverse Dzyaloshinskii-Moriya mechanism expressed by the orthogonal components p1 ~ rij x (Si x Sj ) and p2 ~ Si x Sj can explain the polarization directions. Based on calculations incorporating exchange interactions up to fourth-nearest-neighbor (NN) couplings, we infer that competition among antiferromagnetic second NN interactions in the triangular lattice plane, as well as weak interplane antiferromagnetic interactions, are responsible for the rich phase diagrams of alpha-NaFeO2.

Flux growth of multiferroic Cu3Nb2O8 single crystals

Cu3Nb2O8 belongs to a new multiferroic family with coexisting structure and magnetic chiralities. In this work, we report on the flux growth of Cu3Nb2O8 single crystals using a V2O5-K2MoO4 mixture as the flux with a 5:1 ratio. The obtained crystals have an average size of 3×3×2mm3 and high quality determined from X-ray diffraction, specific heat and susceptibility measurements. The experimental results on the single crystals reveal robust anomalies at 26.2K and 24.5K in specific heat and magnetic anisotropy along different crystallographic axes, which provide a better understanding of this fascinating multiferroic compound.

Topochemical Reduction of YMnO3 into a Composite Structure.

Topochemical modification methods for solids have shown great potential in generating metastable structures inaccessible through classical synthetic routes. Here, we present the enhanced topotactic reduction of the multiferroic compound YMnO3. At moderate temperature in ammonia flow, the most reduced YMnO3-δ (δ = 0.5) phase could be stabilized. XRD, PND, and HREM results show that phase separation occurs into two intimately intergrown layered sublattices with nominal compositions ∞[YMn2+O2+x](1-2x)+ and ∞[YMn2+O3-x](1-2x)- containing versatile Mn2+ coordinations. The former sublattice shows original AA stacking between Mn layers, while AB stacking in the latter results from oxygen removal from the parent YMnO3 crystal structure.

Investigation of structural and multiferroic properties of three phases of BiFeO3 using modified Becke Johnson potential technique

Electronic band structure, ferroelectric and ferromagnetic properties of Cubic, Tetragonal and Rhombohedral (hexagonal axis) phases of multiferroic BiFeO3 compound has been investigated using first-principles calculations under the generalized gradient (GGA) and TB-mBJ semi local (Tran-Blaha modified Becke-Johnson) potential approximations using WIEN2k code. For this purpose, the total energies were calculated as a function of reduced volumes and the data were fitted to Brich Murnaghan equation. The estimated ground state parameters are found to be comparable with those of experimental ones. The semiconducting behavior of the material was obtained using TB-mBJ method in the spin polarized mode. Analysis of the density of states indicates that the valence band consists of Fe-d and O-p states, while the conduction band is composed of Fe-d and Bi-p states. The analysis of electron localization function shows that stereochemically active lone-pair electrons are present at Bi sites of Rhombohedral and Tetragonal phases and are responsible for the displacements of Bi atoms from the centro-symmetric to the non-centrosymmetric structure leading to the exhibition of ferroelectricity. Further, it has been concluded that the “lone pair” may have been formed due to the hybridization of 6s and 6p atomic orbitals with 6s2 electrons filling one of the resulting orbitals in Bi. The Polarization and the magnetic properties including susceptibility were obtained. The calculated magnetic moments at the iron sites are not integer values, since Fe electrons have a hybridization interaction with the neighboring O ions.

A comparative study of hyperfine interactions in Aurivillius compounds prepared by mechanical activation and solid-state sintering

Abstract X-ray diffraction and Mössbauer spectroscopy techniques were used to study the structure and hyperfine interactions of multiferroic Aurivillius compounds Bim+1Ti3Fem-3O3m+3. Samples were synthesized by two methods, that is, the solid-state sintering at various temperatures and mechanical activation in a high-energy ball mill. The compounds were obtained from a mixture of three polycrystalline powder oxides, that is, TiO2, Fe2O3 and Bi2O3. At room temperature, the Aurivillius compounds are paramagnetic materials with orthorhombic crystal structure. The c lattice parameter of the unit cell depends linearly on the m − number of layers with perovskite-like structure. Based on the Mössbauer studies, it is concluded that the hyperfine interactions parameters do not change with m number.

Enhanced magnetization and reduced leakage current by Zr substitution in multiferroic ScMnO3

Despite numerous attempts of electron doping in different manganites (RMnO3, R=rare earth), successful reports are scarce in the literature till date. In this paper, we have synthesized a series of phase-pure electron doped multiferroic compound Sc1−xZrxMnO3 (x=0, 0.05, 0.1, and 0.2) and evaluated the effect of doping on structural properties, oxidation states of cations, DC magnetization, heat capacity, resistivity, dielectric behaviour and ferroelectricity in the material. The presence of Zr4+ and mixed valence state of Mn comprising of Mn2+ and Mn3+ ions are confirmed using X-ray photoelectron spectroscopy. All these samples exhibit antiferromagnetic ordering; as Zr4+ content increases, antiferromagnetic ordering gradually diminishes while shifting to low temperatures. Additionally, ferromagnetic-like interaction develops in doped systems which gives rise to hysteresis in isothermal magnetization loops with greatly enhanced magnetization in comparison to pure antiferromagnetic nature of x=0 i.e. ScMnO3. Interestingly, even with zero magnetic moment of Sc3+, Schottky-like anomaly is observed at 5K in heat capacity data of samples with x=0.1 and 0.2, a result that we attribute to the highly resistive nature of doped samples. Moreover, while measuring ferroelectric hysteresis loops, we observe a significant reduction of leakage current in doped sample (x=0.2) compared to pure ScMnO3. Additionally, the compound x=0.2 shows improved dielectric and ferroelectric behaviour. It is proposed that doping of Zr4+ compensates for the cation deficiency and consequently eliminates the inherent oxygen vacancies by charge compensation.

Magnetic and electric characterization of multiferroic organometallic compound [(CH3)2NH2]Mn0.5Ni0.5(HCOO)3

[(CH3)2NH2]Mn0.5Ni0.5(HCOO)3 is prepared via solvothermal method. Phase purity and crystal structures are determined at room temperature using powder X-ray diffraction. Temperature variation of magnetization and isothermal field variation of magnetization indicate antiferromagnetic ordering with weak ferromagnetism. This is possibly induced by spin canting. Thermally activated relaxation is observed which is attributed to ordering of dimethylamine cation. This is associated with first order improper antiferroelectric transition. Temperature dependence of polarization and its spontaneity has been confirmed from pyroelectric current measurements. Effect of electric ordering is clearly reflected in impedance spectroscopy data. Two regions are distinct above and below electric ordering temperature which are demarcated by the magnitude of resistance and capacitance of equivalent circuit model and activation energy. Co-existence of magnetic and electric ordering at low temperature is a signature of multiferroic phase.

Structural, dielectric and impedance characteristics of (Sm0.5Li0.5)(Fe0.5V0.5)O3 multiferroics

A single phase multiferroic magnetoelectric compound, (Sm0.5Li0.5)(Fe0.5V0.5)O3, is found at room temperature. This sample is fabricated using solid state reaction route. Analysis of the structural properties of the sample by X-ray diffraction method confirmed the fabrication of the desired compound with orthorhombic unit cell structure. Morphological property of the sample is recorded by field emission scanning electron microscopy (FE-SEM). Impedance spectroscopy is employed to determine the various electrical parameters such as dielectric constant, loss tangent, impedance, electric modulus, etc. at different temperature (RT–400 °C) within a frequency range of (1 kHz–1 MHz). Furthermore, the bulk resistance as function of temperature and J–E characteristic showed the semiconducting behavior of sample with negative temperature coefficient of resistance (NTCR) type nature of the compound. The M–H loop of this sample indicates the ferromagnetic nature with very low coercive field. A dedicated magnetoelectric set up is used to measure the ME coefficient as a function of magnetic field and found a significant ME coefficient of 2.99 mV Cm−1 Oe−1 at room temperature.

Sr and Pb co-doping effect on the crystal structure, dielectric and magnetic properties of BiFeO3 multiferroic compounds

Bi1−x(Sr1/2Pb1/2)xFeO3 (0.10 ≤ x ≤ 0.30) multiferroic compounds have been synthesized by a conventional solid-state reaction. The influences of A-site Sr/Pb co-doping on the structure, dielectric and magnetic properties of BiFeO3 are investigated systematically. X-ray diffraction reveals that the crystal structure of Bi1−x(Sr1/2Pb1/2)xFeO3 transforms from the rhombohedral symmetry (space group R3c) to the cubic symmetry (space group Pm3¯ m) with Sr/Pb concentration increasing. Meanwhile, the intensities of BiO bond vibrations continuously decrease and finally disappear. The RT polarization versus electric field (P-E) curves confim the ferroelectric nature for all the samples. Furthermore, it is found that the dielectric constant and dielectric loss tangent of the samples measured in the frequency range of 100–107 Hz decrease drastically at room temperature. A considerable reduction in the leakage current is observed for BFO with Sr/Pb doping content increasing. The ferromagnetic property enhances with the Sr/Pb substitution increasing, which is ascribed to the cooperation of the Fe2+OFe3+ double exchange interaction and the effective suppression of antiferromagnetic cycloidal spin structure caused by the crystalline structure transformation. All results indicate that the Sr and Pb co-doping can effectively improve the magnetic and high-frequency dielectric properties of the multiferroic BiFeO3 compounds.

MOCVD Growth of Perovskite Multiferroic BiFeO3 Films: The Effect of Doping at the A and/or B Sites on the Structural, Morphological and Ferroelectric Properties

Bismuth ferrite (BiFeO3) materials have been the subject of intense research activity in the last two decades. The great interest arises from the BiFeO3 being one of the rare multiferroic compounds in which ferroelectricity and magnetism coexist at room temperature. To improve these properties several studies have been reported on the doping at the A and/or B sites of the BiFeO3 perovskite structure. In this short review, the attention is focused to the synthesis of BiFeO3 and BiFeO3 doped with Ba or Dy at the A site and Ti at the B site through Metal Organic Chemical Vapor Deposition (MOCVD). The applied MOCVD process consists of an in situ one step approach using a multi‐metal source precursor mixture containing the Bi(phenyl)3 and Fe(tmhd)3 (phenyl = ‐C6H5; H‐tmhd = 2,2,6,6‐tetramethyl‐3,5‐heptandione) as source of Bi and Fe ions. This study evidences the effect of doping on the structural, morphological and piezo/ferroelectric properties of BiFeO3 and doped systems. In summary, this mini‐review illustrates the possibility to apply a simple MOCVD approach to produce good quality pure and doped BiFeO3 films.

Combined First-Principles Calculations and Experimental Study of the Phonon Modes in the Multiferroic Compound GeV4S8

The lattice dynamics of the GeV4S8 compound has been investigated using both density functional calculations and Raman/infrared (IR) measurements. While the accordance between the computed and the experimental data is very good in the low-temperature, ferroelectric phase (25K, Imm2), this is not the case for the high-temperature, paraelectric one within the F43m group. Using group theory and first-principles calculations, we show that the IR/Raman phonon modes are, however, compatible with the I4m2 space group. Analysis of the different modes at the ferroelectric transition shows that simultaneous weakening/strengthening of two symmetry-related bonds within the V4S4 cluster is a direct consequence of the orbital-order instability driving the ferroelectric transition. The softening of modes associated with such distortions call for strong orbital occupation fluctuations above Tc. These fluctuations, associated with the discrepancy between the X-ray scattering F43m group and the lattice dynamics I4m2 gr...

Specific features of magnetic order in a multiferroic compound CuCrO2 determined using NMR and NQR data for 63, 65Cu nuclei

Results of studying the paramagnetic and ordered phases of a CuCrO2 single crystal using nuclear magnetic and nuclear quadrupole resonances on 63,65Cu nuclei are presented. The measurements have been carried out in wide ranges of temperature (T = 4.2–300 K) and magnetic-field strength (Н = 0–94 kOe), with the magnetic fields being directed along a and c axes of the crystal. The components of the electric-field gradient tensor and the magnetic-shift tensor (Ka,c) have been determined. The temperature dependences Ka(H || a) and Kc(H || c) for the paramagnetic phase are described by the Curie–Weiss law and reproduce the behavior of the magnetic susceptibility (χa,c). The hyperfine field on a copper nucleus has been determined, which is equal to hhfa,c= 33 kOe/μB. Below the temperature ТN = 23.6 K, nuclear magnetic resonance and nuclear quadrupole resonance spectra for 63,65Cu nuclei have been recorded typical of helical magnetic structures, which are incommensurable with the lattice period.

Structural, electrical and magnetic characteristics of Ni/Ti modified BiFeO3 lead free multiferroic material

Recently, scientists and researcher are focussing on multiferroic materials which are widely used in various multifunctional devices. In this communication, synthesis and characterisation of nickel and titanium modified bismuth ferrite have been reported. A lead-free multiferroic compound, Bi(Ni0.40Ti0.40Fe0.20)O3, has been synthesized using solvent-free solid-state reaction route at 1073 K in an air atmosphere. The formation of a single-phase with orthorhombic symmetry and the substitution/concentration of Ni/Ti at the Fe-site of BiFeO3 were confirmed by X-ray diffraction and energy dispersive X-ray microanalysis spectroscopy techniques respectively. Based on X-ray reflection profiles, the average particle size was estimated to be around 30 nm. Study of surface morphology of the compound by field emission scanning electron microscope has shown nearly uniform distribution of grains of different dimension with some voids. The density (measured by Archimedes method) of as-synthesized pellets was found to be nearly 92.8% of the theoretical density. A significant effect of substitution of multiple elements at the Fe site on dielectric constant and tangent loss of BiFeO3 has been observed. Detailed analysis of dielectric and impedance data, collected in a wide range of frequency (1–1000 kHz) and temperature (298–773 K), has provided many important results on structure-properties relationship and dielectric relaxation of modified bismuth ferrite. Magnetic field dependent magnetisation, measured by vibrating sample magnetometer (VSM at room temperature), shows a significant enhancement in the value of remnant magnetization of Ni/Ti modified bismuth ferrite.

Doping effects on trimerization and magnetoelectric coupling of single crystal multiferroic (Y,Lu)MnO3

Hexagonal RMnO3 is a multiferroic compound with a giant spin–lattice coupling at an antiferromagnetic transition temperature, Lee et al (2008 Nature 451 805). Despite extensive studies over the past two decades, the origin and underlying microscopic mechanism of strong spin–lattice coupling remain very much elusive. In this study, we have tried to address this problem by measuring the thermal expansion and dielectric constant of doped single crystals Y1−xLuxMnO3 where x = 0, 0.25, 0.5, 0.75, and 1.0. From these measurements, we confirm that there is a progressive change in the physical properties with doping. At the same time, all our samples exhibit clear anomalies at TN, even in the samples where x = 0.5 and 0.75. This is opposed to some earlier ideas, which suggests an unusual doping dependence of the anomaly. Our work reveals yet another interesting facet of the spin–lattice coupling issue in hexagonal RMnO3.

Optical absorption spectrum and electronic structure of multiferroic hexagonal YMnO3 compound

Optical absorption (OA) spectrum and electronic structure of the hexagonal YMnO3 compound have been investigated by employment of the first-principles calculations based on density functional theory. The calculations were performed upon the ferroelectric structure of the YMnO3, by testing various approximations of the exchange-correlation effects between the Mn d-electrons and considering two types of magnetic ordering of the Mn sub-lattice: (1) collinear anti-ferromagnetic order of the G-type and (2) non-collinear antiferromagnetic order that correspond to magnetic space group P63. The results demonstrate that satisfactory agreement between the theoretical and the experimental OA spectrum can be achieved only if both non-collinear anti-ferromagnetic order of the Mn spins and strong correlations between the Mn d-electrons are taken into account. The latter is found to be best described by effective Hubbard parameter Ueff = 2.55 eV. The principal features of the OA spectrum are interpreted in terms of calculated electronic structure. It is found that the most important, threshold 1.6 eV OA peak is generated by electron transitions from strongly hybridized occupied Mn d- and its neighboring in-plane O p-states to unoccupied Mn d-states. It is also concluded that the electronic gap (calculated as ∼1.1 eV) should be smaller than the optical one (∼1.6 eV).