Multiferroics

Magnetoelectric coupling in helical multiferroics allows to steer spin order with electric fields. Here we show theoretically that in a helical multiferroic chain quantum information processing as well as quantum phases are highly sensitive to electric (E) field. Applying E-field, the quantum state transfer fidelity can be increased and made directionally dependent. We also show that E field transforms the spin-density-wave/nematic or multipolar phases of frustrated ferromagnetic spin− 1 2 chain in chiral phase with a strong magnetoelectric coupling. We find sharp reorganization of the entanglement spectrum as well as a large enhancement of fidelity susceptibility at Ising quantum phase transition from nematic to chiral states driven by electric field. These findings point to a new tool for quantum information with low power consumption.

This paper explores a class of non-linear constitutive relations for materials with memory in the framework of covariant macroscopic Maxwell theory. Based on earlier models for the response of hysteretic ferromagnetic materials to prescribed slowly varying magnetic background fields, generalized models are explored that are applicable to accelerating hysteretic magneto-electric substances coupled selfconsistently to Maxwell fields. Using a parameterized model consistent with experimental data for a particular material that exhibits purely ferroelectric hysteresis when at rest in a slowly varying electric field, a constitutive model is constructed that permits a numerical analysis of its response to a driven harmonic electromagnetic field in a rectangular cavity. This response is then contrasted with its predicted response when set in uniform rotary motion in the cavity.

Multiferroic bismuth ferrite, BiFeO 3 , was synthesized via conventional solid-state reaction method using Bi 2 O 3 , Fe 2 O 3 as starting materials. Effects of Bi 2 O 3 /Fe 2 O 3 molar ratio and calcination temperature on the phase composition, morphology and magnetic properties of produced powders were systematically studied using XRD, FESEM/EDS and VSM techniques, respectively. The results revealed that BiFeO 3 phase with rhombohedral R3c structure with a mean particle size of 40 nm was formed in the sample processed with a Bi 2 O 3 /Fe 2 O 3 molar ratio of 1:1 after calcination at 800 °C. Rietveld analysis which was applied to the x-ray diffraction data via MAUD software indicated high purity of 95%wt for the above sample. Deviation from the stoichiometric molar ratio (Bi 2 O 3 /Fe 2 O 3 : 0.9, 1.1, 1.2) yielded higher content of the intermediate phases of Bi 2 Fe 4 O 9 and Bi 25 FeO 40 . FESEM studies showed that the mean particle size was increased from 40 to 62 nm by increasing calcination temperature from 800 to 850 °C. VSM results for 1:1 molar ratio samples indicated that increasing the calcination temperature from 800 to 850 °C increased saturation magnetization (M s ) from 0.087 to 0.116 emu/g and also coercive field (H c ) from 60 to 100 Oe.

The iron potassium fluorides with the general formula Kx FeII x FeIII 5−x F15 (2 􏰕 x 􏰕 3) are interesting multiferroic materials, for which the coexistence of all the three ferroic orders (elastic, electric, and magnetic) is simultaneously observed below the magnetic transition. Although the phase diagram has been previously defined, in particular for the potassium-rich K3Fe5F15 phase, its complex magnetic behavior has not been completely understood. In this paper, the information obtained by magnetization measurements carried out on an oriented single crystal, revealing high anisotropy with coercive field reaching 14 kOe on the easy axis, are combined with Mo ̈ssbauer spectroscopy and powder neutron diffraction data to define the magnetic structure. The ferrimagnetic behavior of K3Fe5F15 arises from an antiferromagnetic triangular spin system, determined by neutron diffraction, frustrated by the difference in the magnetic contributions of adjacent structural layers, originated by F...

The synthesis of YFe 2 O 4 (YFO) is challenging because of high oxygen stoichiometry, it is a possible origin to realize the series of different phases at low temperature, balance of competing interaction between ions (Fe 2+ and Fe 3+ ) gives multiphase. The ionic radius of octahedral Fe-O which include, octahedral Fe-O inter atomic distances determined are for from the one expected for Fe 2+ and Fe 3+ octahedral ions and weak magnetic behavior and the transition temperature (250 K) also needs some improvement. These are the some difficulties, which were found during the synthesis of both room temperature multiferroics, which could be resolve by taking some major steps involves. The synthesis of multiferroic material can be improved by reducing its particle size, changing morphology by substitutional doping etc.

In the present work, it was reported for the first time the new synthesis of Sm 2 NiMnO 6 double perovskite oxides by solegel auto-combustion method. The Rietveld analysis of the X-ray ceramics diffraction pattern recorded at room temperature for Sm 2 NiMnO 6 ceramics sintered at 1000 C/5 min from powders obtained at 700 C/7 h confirm the formation of the double perovskite with a monoclinic structure and the space group P2 1 /n. The HRTEM analysis shows clear lattice fringes that confirm a high crystallinity level of the material corresponding to the monoclinic structure. The non-linear dielectric character was checked for the first time in Sm 2 NiMnO 6 double perovskite and the results reveals a strong nonlinearity and a small hysteretic behaviour. The present structural, magnetic and dielectric data make the Sm 2 NiMnO 6 system due to its multiferroic character a promising candidate to different modern electronic devices applications.

BiFeO 3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO 3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically. . MERAM based on exchange-bias coupling between a multiferroic that is ferroelectric and antiferromagnetic (FE-AFM, green layer), and a thin ferromagnetic electrode (blue). A tunneling barrier layer between the two top ferromagnetic layers provides the two resistive states. Interestingly, BiFeO 3 could act not only as the magnetoelectric active layer, but also as the tunneling barrier. Reproduced with permission from .

Multiferroic perovskite oxides have multiple ferroic order parameters simultaneously in the same phase in a certain temperature range, exhibiting an exciting way of coupling between the ferroic order parameters. This provides a possibility for constructing new types of multifunctional devices. Multiferroic domain structures in these materials are considered to be important factors to improve the efficiency and performance of future multiferroic devices. Recent developments in domain characterization techniques, particularly the aberration-corrected transmission electron microscopy (TEM) have enabled one to determine the domain structures at sub-angstrom scale, and the recent development of in-situ TEM techniques allows one to study the dynamic behavior of multiferroic domains under applied fields or stress while the atomic structures are imaged directly. This paper provides a comprehensive review of the recent advances on the structural characterizations of domain structures in ferroic and multiferroic perovskite oxides, which have been achieved by the notable advancement of aberration-corrected TEM.

The effects of milling energy induced during intermediate mechanical activation of precursors on the synthesis of nano-structured BiFeO3 powders have been systematically investigated. X-ray diffractometer, laser particles size analyzer, field emission scanning electron microscope, vibrating sample magnetometer and electrical evaluation techniques were used to study phase composition, particles size distribution, morphology, magnetic properties and ferroelectric properties of the products, respectively. Applying a total energy of 171.18kJ/g during milling led to formation of an amorphous structure which resulted in decreasing the formation temperature of bismuth ferrite phase by about 100˚C, although small amounts of secondary phases were detected. This sample shows the mean particles size of 170nm and the mean crystallite size of 40nm, when calcined at 750˚C. Saturation magnetization (MS) increased from 0.054 to 0.071A.m2/kg and coercive field (HC) decreased from 32.63 to 6.37kA/m by increasing the milling energy from 13.48 to 171.18kJ/g. In addition, electrical hysteresis loops demonstrated a decrease in the current leakage by increasing the milling energy and lowering the calcination temperature.

CuFe 1.6 Cr 0.4 O 4 -BaTiO 3 composites were prepared using conventional ceramic double sintering process. The presence of both the phases in composite was confirmed by X-ray diffraction (XRD). The variation of dielectric constant (ε ) with frequency in the range 100 Hz to 1 MHz and also with temperature at a fixed frequency of 10 KHz was studied. AC conductivity was derived from dielectric constant (ε ) and loss tangent (tan δ) measurements. The nature of conduction is discussed in the light of polaron hopping model. The static value of magnetoelectric conversion factor, i.e. DC(ME) H has been studied as a function of intensity of magnetic field. The maximum value of DC(ME) H was found to be 95.6 V cm −1 Oe.

The rare earth orthochromites are extremely interesting due to the richness of their optical, dielectric, and magnetic properties as well as due to their multiferroic properties which make them suitable materials to study in the nanoregime. However, the wet-chemical synthesis of these materials in nanosize is nontrivial. Here, we report for the first time, the detailed Raman spectra as well as magnetic and dielectric properties of chemically synthesized GdCrO3 nanoparticles of size ranging from 40 to 60 nm. The magnetic properties are dictated by competing Cr3+–Cr3+, Gd3+–Cr3+, and Gd3+–Gd3+ superexchange interactions in different temperature regions, resulting into an antiferromagnetic ordering at 167 K due to the Cr3+–Cr3+ followed by weak ferromagnetic ordering due to the onset of Cr3+–Gd3+ interactions. At lower temperature, it shows weak antiferromagnetic ordering due to Gd3+–Gd3+ interaction. Below 95 K, GdCrO3 nanoparticles showed the presence of negative magnetization due to Gd3+ and Cr3+ interactions resulting into weak ferromagnetic coupling. The Raman spectroscopy shows the characteristic Raman shifts indicating that below 450 cm−1, Gd3+ ions play a dominant role in determining the phonon frequencies of GdCrO3, and above 450 cm−1, the Cr+3 ions dominate. We also present for the first time the low temperature dielectric constant and loss tangent data for GdCrO3 in a broad temperature and frequency range. The dielectric constant shows a decrease in comparison to the bulk values due to the size dependent effects. It also shows a peak centered at around 320 K above which it shows a sharp decrease. The dielectric loss value in GdCrO3 nanoparticles is quite small and shows an interesting frequency dependent anomaly at lower temperature which might be due to the coupling between magnetic and dielectric order parameters.

We report the temperature-dependent Raman and dielectric spectroscopy of chemically synthesized BiFeO3 nanoparticles (average size 50−60 nm). The Raman spectra (90−700 K) show two sets of transitions in the lowest Raman E mode, associated with Bi−O bond motion situated in close proximity to the spin reorientation transitions reported for BiFeO3, thereby indicating the existence of possible coupling between magnons and phonons for particle size below the helical order parameter (62 nm). These transitions are slightly shifted in temperature in comparison to the bulk single crystals. We also observe a step-like behavior in Raman peak position around the Neel temperature, suggesting that the phonons are influenced by the magnetic ordering in nanosized BiFeO3. The heat-flow measurements show two sharp endothermic peaks at 1094 and 1223 K representing rhombohedral to orthorhombic or monoclinic transition followed by transition into the cubic phase above 1200 K. The low temperature (20−325 K), frequency-dependent (1−106 Hz) dielectric constant and loss tangent measurements show that the loss tangent (10−3) and ac conductivity values (10−8 Ohm−1-cm−1) are orders of magnitude lower than the reported values for BiFeO3 ceramics, indicating high levels of ionic purity of our samples. The real part of the permittivity shows a slight reduction in its value (30) in comparison to the bulk single crystals. Similar to the Stokes Raman shift, its temperature-dependent dielectric constant also shows four weak anomalies at 85, 168, 205, and 230 K situated in close proximity to the spin reorientation transitions, indicating magnetoelectric coupling.

In the present attempt, we report modified features of structural, dielectric, magnetic and ferroelectric behaviour of BiFeO 3 (BFO) by perovskite-spinel composite approach. ZnFe 2 O 4 (ZFO) is used as spinel phase. The structural measurement of composite show anisotropically compression in the BiFeO 3 lattice with ZFO compositions and stimulates the variation in bond length, bond angle, tilting angle, electron density and resultant polarization. This affects on magnetic and dielectric behaviour of BFO. Room temperature magnetic measurement revealed enhancement of magnetization of BFO in composite, attributed to spin restructuring due to change in magnetic anisotropy, exchange energy and stress energy at interface with ZFO composition. There is around 7 times enhancement in magnetization as compared to pure BFO phase. Dielectric profile of composites shows decrease in dielectric constant as well as dielectric loss as compared to single phase BFO. P-E loop exhibits leaky ferroelectric behaviour of composite system with drop down in leakage current by 2 order of magnitude than pure BFO phase. Magnetic contributions of individual phases in composite are determined by Vegard Law while dielectric contributions are modelled by Maxwell-Garnett (MG) equation. The present work demonstrates that BFO-ZFO: perovskite-spinel composite approach to modify magnetic, dielectric and ferroelectric behaviour and to facilitate BFO as room temperature multiferroic system.

The influence of samarium (Sm) doping on the crystal structure, topographical and magnetic properties of BiFeO 3 (BFO) thin films was investigated. BFO-based films of pure and Sm:BFO [Bi 1-x Sm x FeO 3 (x = 0.00, 0.05 and 0.1): Sm:BFO] at various compositions were deposited on the glass substrates by spin coating method. Crystallization of Sm:BFO was obtained by calcinations of the film and thus single phase nature was obtained. X-Ray diffraction (XRD) carried out on the Sm:BFO films calcinated at 500°C reveals the single phase formation of Sm:BFO films in rhombohedral system which belongs to R3c space group. The presence of samarium was confirmed through EDAX spectrum and thus its increase in concentration is reflected as the doublet peak in the XRD which shows a trend to coalescence to form a broad single peak. The structural analysis and the influence of samarium which is reflected in the optical and vibrational analyses are discussed in detail. The room temperature magnetic hysteresis loops were found to be unsaturated and a relative increase in the magnetic moment with the substitution of Sm concentration was significantly observed.

The effect of Nd doping on Bi1−xNdxFeO3 (x = 0.0, 0.175, 0.20) multiferroics synthesized by chemical co-precipitation method has been investigated by Rietveld analysis of X-ray powder diffraction (XRD) data. The formations of the single-phase compounds were confirmed by XRD. X-ray diffraction along with the Rietveld-refinement showed a gradual change in crystal structure from rhombohedral to triclinic with increasing Nd doping concentration. The bond distances along with bond angles between atoms for all the compounds were calculated which supports the structural results. Raman spectroscopy also recommends a structural change and is accompanied by the weakening of long-range ferroelectric order with increasing doping concentration (x). The results of Raman spectra for BiFeO3 (BFO) match well with the earlier reported bulk ceramic and epitaxially grown thin film of BFO. The ferroelectric-paraelectric transition in 20% Nd BFO substituted was explained according to the change of Bi–O covalent bond as a result of decline of stereochemical activity of Bi lone pair electron and is further confirmed through ferroelectric polarization (P–E) hysteresis loop.

Polycrystalline Bi1−xBaxFe1−yMyO3 (M = Co and Mn; x = 0.1, y = 0.1) were synthesized by solid-state route method to study the compositional driven structural transformations in multiferroics. Room temperature X-ray diffraction patterns confirmed the formation of perovskite structure. Rietveld-refined crystal structure parameters revealed the existence of rhombohedral R3c symmetry for both the samples. The samples were found to be nearly free from any other secondary phases. Raman analysis reveals that Ba atom substitutes Bi site and Mn and Co atom substitutes Fe site into the BiFeO3 with the shifting of phonon modes. The red shift is attributed to Co or Mn doping where as blue shift occurs from Ba doping. The differential scanning calorimetry reveals the corresponding Neel temperature 370 °C and 326 °C for Co and Mn doped samples. Ba and Co substitution with x = 0.1 and y = 0.1 has not affected the Neel temperature of the parent BiFeO3 as well of Ba and Mn substitution. The variation of frequency dispersion in permittivity and loss pattern due to A-site and B-site substitution in BiFeO3 was observed in the dielectric response curve.

Multiferroic xBa0.95Sr0.05TiO3-(1‒x)BiFe0.9Gd0.1O3 [xBST-(1‒x)BFGO], where x=0.00-0.40, have been synthesized by the conventional solid-state reaction method. The crystalline phase, microstructure, relaxor behavior, ac conductivity, impedance spectroscopy, dc magnetic properties, complex initial permeability and magnetoelectric coefficient of these solid solutions have been investigated. The crystal structure is found to change from rhombohedral in BFGO rich compositions to cubic when x≥0.30. Room temperature dielectric properties are investigated within the frequency range of 1 kHz -1 MHz and found to increase with BST content. The frequency dependence of high temperature dielectric measurements indicated that the composites with x≥ 0.20, exhibit relaxor ferroelectric behavior. The ac conductivity obeys the Jonscher’s universal power law and BST helps to enhance the electrical conductivity of the composites. Studies of impedance spectroscopy suggest that only grains have the contribution to the conductivity mechanism in this material. Magnetizations as a function of applied magnetic field measurements show weak ferromagnetism for
0.10≤x≤ 0.30 composites. The maximum value of remnant magnetization is found to be 0.565×103 A/m (=0.08 emu/g) for x =0.25 which is better than previously reported BaTiO3 - BiFeO3 systems. The complex initial permeability is found to improve with the increase in BST concentration due to the reduction of oxygen vacancies. In addition, an enhanced magnetoelectric (ME) coupling is also observed and determined by the ME coefficient. The maximum value of ME coefficient is found to be 21.71×10-4 V/A (=1.67 mV/cm Oe) for the x =0.25 composition. The BST-BFGO solid solutions show high-performance multiferroic properties and can be selected for further investigation.

Barium (Ba) doping at Bismuth (Bi) site is reported to enhance magnetic properties of Bismuth Ferrite (BiFeO 3) while Yttrium (Y) doping at the same is found to improve ferroelectric and dielectric properties. To investigate the combined effect of Ba and Y co-doping, Ba 0.1 Bi 0.9-x Y x FeO 3 (x ¼ 0.0, 0.1 and 0.2) are synthesized using chemical synthesis route and their magnetoelectric and dielectric properties are studied in detail. The X-Ray Diffraction study confirms the single phase nature of the synthesized samples. The ferroelectric property is enhanced for x ¼ 0.1 while the magnetic studies show enhanced magnetisation for both co-doped samples with highest enhancement in x ¼ 0.1. The doped samples exhibit a first order field induced metamagnetic transition. The dielectric constant also increases with reduced tangent loss and conductivity for x ¼ 0.1. Therefore, co-doping with Ba and Y improves the magnetoelectric and dielectric property of BiFeO 3 .

Mixing 60-70% lead zirconate titanate with 40-30% lead iron tantalate produces a single-phase, low-loss, room-temperature multiferroic with magnetoelectric coupling: (PbZr 0.53 Ti 0.47 O 3 ) (1-x) -(PbFe 0.5 Ta 0.5 O 3 ) 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 C 2v -C 4v (Pmm2-P4mm) transition. The material exhibits square saturated magnetic hysteresis loops with 0.1 emu/g at 295 K and saturation polarization P r = 25 μC/cm 2 , which actually increases (to 40 μC/cm 2 ) in the high-T tetragonal phase, representing an exciting new room temperature oxide multiferroic to compete with BiFeO 3 . 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.

The effect of Sr2+ doping on Bi1-xSrxFeO3 (x = 0.0, 0.15, 0.175, 0.25) multiferroic ceramics synthesized by citrate sol-gel method has been investigated by Rietveld analysis of X-ray powder diffraction data, Raman spectroscopy and dielectric measurement. X-ray diffraction along with the Rietveld–refinement showed a gradual change in crystal structure from rhombohedral (R3c) to pseudotetragonal (P4/mmm) with enhanced divalent Sr2+ ion concentration. All the 13 Raman modes predicted by group theory (ΓR3c = 4 A1 + 9 E) for R3c structure of Bi1-xSrxFeO3 (x = 0.0, 0.15, 0.175, 0.25) were observed in the present study. The A1-2 and E-4 modes are completely suppressed, while to that A1-3, E-8 mode in Bi1-xSrxFeO3 (x = 0.175, 0.25) and E-2, E-5, and E-8 modes (x = 0.25) disappear completely as compared to parent BFO. The structural phase transition and weakening of long-range ferroelectric order with increasing doping concentration are thus further confirmed from Raman scattering spectra. The dielectric anomaly has been occurred in dielectric constant and dielectric loss near 325 °C, 305 °C, 270 °C and 250 °C (f = 10 kHz) in BiFeO3, Bi0.85Sr0.15FeO3, Bi0.825Sr0.175FeO3 and Bi0.75Sr0.25FeO3, respectively.

Bulk samples of spinel ferrites with the compositional formula, Co 1 À x Ni x Fe 2 O 4 (x ¼0.0, 0.5, 1.0) were synthesized by a solid-state reaction route to discuss doping effect on the structural, vibrational and dielectric properties. The crystal structure and cell parameters were refined by the Rietveld analysis, infers that ceramics crystallized in single-phase cubic structure with Fd-3m space group. The variation in cell parameters with Ni substitution confirmed the partial substitution of the Co 3 þ by Ni 3 þ into the spinel CoFe 2 O 4 structure. A blue shift in Ni doped CoFe 2 O 4 ferrite has been observed as compared to parent CoFe 2 O 4 from Raman scattering measurements. The dielectric constant (ε 0 ) and loss tangent (tan δ) have been investigated as a function of composition and frequency in the frequency range 10 Hz-1 MHz. Porous ceramics are useful for microelectronics with lower value of dielectric constant.

The structural, vibrational, magnetic and dielectric properties of polycrystalline BiFeO3 and Bi0.95Pr0.05FeO3 are investigated by combining X-ray diffraction, Raman scattering spectra, magnetometry and dielectric measurements. Structural symmetry with rhombohedral R3c phase is revealed for both parent and 5% Pr substitution at Bi site, serving no chemical pressure and causes no structural transition from R3c to any other phase is identified from x-ray diffraction patterns and Raman scattering spectra. The shifting of phonon modes towards higher frequency side is attributed to lower atomic mass of Pr ion as compared to Bi ion. The magnetic measurements at room temperature indicate that Pr substitution induces ferromagnetism and discerns large and non-zero remnant magnetization as compare to pristine BiFeO3. Both dielectric permittivity and loss factor of Bi0.95Pr0.05FeO3 strongly decreases with increased frequency. Significant role of hopping of oxygen ion vacancies in Bi0.95Pr0.05FeO3 is inferred from modulus spectra and ac conductivity analysis.

A 0.9BiFeO3–0.1Ba0.8Sr0.2TiO3 ceramic is synthesized by conventional solid-state reaction method and investigated by structural, dielectric, thermal, Raman spectra and magnetization properties. The 0.9BiFeO3–0.1Ba0.8Sr0.2TiO3 is crystallized in rhombohedral distorted perovskite structure with space group R3c. The dielectric loss is investigated over wide range of temperature at 100 kHz. During its evolution, an anomaly is observed at 300 C which corresponds to TN (antiferromagnetic transition temperature). Besides, it is confirmed by DSC measurement. A magnetic property is confirmed by the temperature dependence of magnetization (M–T) under an applied magnetic field of 0.05 T. That revealed an antiferromagnetic transition of 0.9BiFeO3–0.1Ba0.8Sr0.2TiO3 ceramic. The latter confirms which was
previously mentioned. Furthermore, these results reveal considerable spectral changes in the vicinity of the Neel temperature TN. Finally, this shift is discussed through structural, dielectric, thermal, vibrational and magnetic combination near the TN phase.

Polycrystalline Bi 1−x La x Fe 1−x Ti x O 3 (x = 0.000-0.250) ceramics were synthesized by the tartaric acid modified sol-gel technique. It was observed that the co-substitution of La and Ti at Bi and Fe sites in BiFeO 3 suppresses the impurity phase formation which is a common problem in the bismuth ferrite ceramics. The quantitative crystallographic phase analysis of x-ray diffraction pattern by employing Rietveld technique was performed with the help of FULLPROF program which suggests the existence of compositional driven crystal structure transition from rhombohedral (space group R3c) to the orthorhombic (space group Pbnm) symmetry. The oxygen octahedral tilt angle was found to be ∼13.82°for BiFeO 3 (space group R3c) and decreases with the increase in co-substitution percentage. The structural transition breaks the spin cycloid structure in co-substituted BiFeO 3 nanocrystallites and leads to canting of the antiferromagnetic spin structure. Hence, the remnant magnetization increases up to 10% of cosubstitution and becomes 22 times that of BiFeO 3 . However, it decreases for higher cosubstitution percentage due to significant contribution from the collinear antiferromagnetic ordering in the orthorhombic crystal symmetry. The dielectric constant attains a maximum for 10% of co-substitution.

The Z ferrite Sr3Co2Fe24O41, produced from a stoichiometric aqueous inorganic sol-gel precursor, is reported and its magnetic hysteresis loop characterized for the first time. The precursor was an iron(III)hydroxide sol stabilised with NO3 - counterions and doped with stoichiometric nitrate salts, which had a particle size similar to halide stabilised Ba3Co2Fe24O41 Z ferrite precursor sols investigated previously. When fired the amorphous gel formed a mixture of α-Fe2O3, BaM and CoFe2O4 from 600°C, but these products then converted directly to the pure Z phase at 1200°C, without first forming the Y ferrite (Ba2Co2Fe12O22) phase always observed prior to Z ferrite crystallization in Ba3Co2Fe24O41 systems. The resulting material was a very soft ferrite, with a very low coercivity of 5.6 kA m-1 and a magnetisation of 48.5 emu g-1 at an applied field of 5 T.

BFO (Bismuth ferrite) or BiFeO3 multiferroics Nano-powders were synthesized by a Sol–gel Auto-combustion and Xerogel methods at 400°C to 800°C temperature range. The Sol-gel technique is a wet-chemical method used for synthesis of solid materials like glassy and ceramic from small molecules. Here, the sol or solution evolves gradually towards the formation of a gel, convert from liquid state to solid state. The sol was transparent and homogenous, the pH (potential of hydrogen) ranges 1 to 2 were controlled by the help of Ammonia Solution (NH4OH). Ethylene Glycol was the chelating agent of Fe3+ and Bi3+ cations. We were measured by various characterization techniques like X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM) and UV Spectrum measurements. BFO Nano-powders with different size in nm are synthesized. In this paper, we exertion to explain, two types of sol–gel techniques and finally shown that one of them is a better Sol–gel process to synthesis a pure BFO.

Single-phase BiFeO3, Bi0.825Pb0.175FeO3 and Bi0.725La0.1Pb0.175FeO3 ceramic samples were prepared via solid state reaction route to study the effects of co-doping (La/Pb) on their crystal structure and ferroelectromagnetic behavior. X-ray diffraction data accompanied by Rietveld refinement showed that BiFeO3 crystallize in rhombohedral distorted perovskite structure (space group-R3c) while to that BPFO and BLPFO crystallize in tetragonal (P4/mmm) symmetry. Presence of E (TO9) and E (LO9) modes in Raman scattering spectra for BiFeO3 and Bi0.725La0.1Pb0.175FeO3 infers ferroelectric nature. However, absence of both modes in Bi0.875Pb0.175FeO3 indicates the paraelectric nature. The room temperature dielectric constant (10 Hz–1 MHz) of BiFeO3, Bi0.825Pb0.175FeO3 and Bi0.725La0.1Pb0.175FeO3 is about 100, 60, and 200. Ferroelectromagnetic measurements revealed the existence of ferroelectricity with remnant polarization of 0.24 μC/cm2 in Bi0.725La0.1Pb0.175FeO3, paraelectricity in Bi0.875Pb0.175FeO3, and weak (strong) ferromagnetism with remnant magnetizations of 0.25 (1.57) emu/g in Bi0.725La0.1Pb0.175FeO3 (Bi0.825Pb0.175FeO3).

Synthesis of the precursors, using research grade materials and locally available facilities, is optimized in order to obtain a suitable sol for BiFeO 3. The optimally synthesized BiFeO 3 sol is then spun onto Cu and glass substrates to obtain films of thickness 100-300nm. Films are studied with the help of XRD, VSM, SEM, and Spectroscopic Ellipsometry. X-ray diffraction results show the formation of BiFeO 3 even in the as-deposited samples. However, some secondary phases were also present with high intensity. After annealing at 300 o C, secondary phases became less intense but could not be completely eliminated. Another BiFeO 3 sol was prepared by changing the solvents and by controlled addition of sodium oleate as ligand. Thin films, prepared using this sol, are single phased even in the as-deposited form. (110) preferred orientation of these BiFeO 3 thin films is achieved after annealing at 300 o C. Ferroelectric switching with a phase angle of 180 o is observed as a function of externally applied electric field. Ferromagnetic properties are also observed in these BiFeO 3 thin films. Dense nanostructure with uniformly distributed grains ~ 100 nm is observed by scanning electron microscope. Optical results show absorption in the blue and green region with a band gap ~ 2.7 eV. Dielectric constants of the films show dispersion in the range of 100Hz-1MHz. These results demonstrate that careful use of solvents and ligands can result in pure BiFeO 3 sol and then thin films; a promising candidate for memory and photovoltaic applications.

The Mo-doped BiFe 1−x Mo x O 3 , where x = 0 and 0.6, samples were synthesised by the solid-state reaction method. Admirable ferroelectric and piezoelectric properties are expected on substituting solid solutions of bulk BFO with other oxide perovskite compounds. It offers an alternative and is an environment-friendly candidate for lead-free ferroelectric and piezoelectric devices. The prepared samples were investigated by XRD, Fourier transform infrared spectroscopy (FTIR), current density (JE) measurement, PE loop tracer and field emission scanning electron microscopy to examine their crystal structure, bonding nature, current density, ferroelectric hysteresis loop and surface morphology. The Mo-doped BFO shows a change in structure, increased grain size, reduction in current density and enhancement in the PE loop on doping Mo at the Fe site.

Multiferroic bulk YMnO3 sample was prepared
through the solid state reaction method. After characterizing
the sample structurally, a systematic investigation of
magnetization and specific heat has been undertaken over a
temperature range 2–300 K under different magnetic fields.
Based on these studies, it has been found that the sample
exhibited a paramagnetic to ferrimagnetic phase transition
of spin glass type at *42 K that could be attributed to spin
cantering. The magnetic transition peak seen in the magnetic
entropy change versus temperature curves became
broader with increasing magnetic field. A large magnetic
entropy change of *1 J mol-1 K-1 was obtained under a
magnetic field change of 0–10 T.

Novel ferromagnetic-dielectric particulate composites of Ni 0.5 Zn 0.5 Fe 1.95 O 4−δ (NZF) and PbZr 0.52 Ti 0.48 O 3 (PZT) were prepared by conventional ceramic method. The presence of two phases in composites was confirmed by XRD technique. The variations of dielectric constant (ε) with frequency in the range of 100 kHz-1 MHz at room temperature and also with temperature at three different frequencies (50 kHz, 100 kHz, and 500 kHz) were studied. Detailed studies on the dielectric properties were done confirming that the magnetoelectric interaction between the constituent phases may result in various anomalies in the dielectric behaviour of the composites. It is proposed that interfaces play an important role in the dielectric properties, causing space charge effects and Maxwell-Wagner relaxation, particularly at low frequencies and high temperatures. The piezoelectric d 33 constant was studied at room temperature, and the d 33 constant value decreased with ferrite content. Magnetic properties like B-H loops traces were studied to understand the saturation magnetic (M s ) and magnetic moment (μ B ) of the present particulate composites. The magnetoelectric (ME) output was measured by varying dc bias magnetic field. A large ME output signal of 2780 mV/cm Oe was observed in the composite having 50% ferrite. The temperature variation of longitudinal modulus (L) and internal friction (Q −1 ) of these particulate composites at 104 kHz was studied in the temperature range 30 • C-420 • C by the composite oscillator technique. Longitudinal modulus showed a sharp minimum, and internal friction exhibits a sharp peak at ferroelectric-paraelectric phase transition. These ferroelectric-dielectric particulate composites were prepared with a view to using them as ME sensors and transducers.

Single phase Bi0.80La0.15A0.05FeO3-
δ (A = Ca, Sr,Ba) dense ceramics were synthesized via solid
state reaction method. Structural studies through X-ray diffraction shows that all prepared ceramics
crystallized in a rhombohedrally (R3C) distorted BiFeO3 structure with compressive lattice distortion
induced by the rare earth (La3+) ion and divalent co-doping at the Bi-site for the Raman study. Scanning
electron micrograph of the compounds showed the uniform distribution of grains on the sample surface
with high density. A large ferromagnetic hysteresis loop is observed for La/Ba co-doped BiFeO3 as
compared with BiFeO3 prepared under similar conditions, with saturation magnetization of 6.85 emu/g
and remnant magnetization of 2.72 emu/g at 300K. Clear ferromagnetic ground state was observed in
Bi0.80La0.15Ba0.05FeO3 and weak ferromagnetism in BLCFO and BLSFO samples. Dielectric constant and
dielectric loss were found to decrease with increase in frequency for all the compounds. These improved
properties of La/Ba co-doped BFO demonstrate the possibility of enhancing the magnetic applicability
and makes very promising for industrial applications such as new devices in information storage.

Thermoelectric Power studies of Ni-Mg ferrites having chemical formula Ni 1-x Mg x FeO 4 (x = 0.2, 0.4, 0.6 and 0.8) were investigated from room temperature to well beyond Curie temperature by the differential method. The Seebeck coefficient is negative for all the composition. It clearly speaks that all the considered ferrite compositions behave as n-type semiconductors. Plots of Seebeck coefficient (S) versus temperature shows maximum at Curie temperature. The values of the charge carrier concentration have been computed from the observed values of Seebeck coefficient. The electrical properties of the Ni-Mg mixed ferrites have been measured at room temperature by two-probe method. On the basis of these results an explanation for the conduction mechanism in Ni-Mg mixed ferrites is suggested.

As the limit of Moore’s law is reached, an alternative to conventional CMOS
technology must be soon realized. The field of Spintronics presents a possibility
of logic computation by switching of magnetic spins. However, the realization of
spintronic devices faces a major challenge with the lack of a scalable and energyefficient
transducer. A promising path for low-power and controlled magnetization
switching is by using the Magnetoelectric effect, which couples the electric field to
the intrinsic magnetization. The magnetoelectric transducer consists of a bi-layer of
piezoelectric-magnetostrictive thin-film materials, in which the coupling between the
electric and the spin domain occurs via strain. The strain induced in the piezoelectric
layer by the applied electric field is transferred to the magnetostrictive film that in
turn switches the intrinsic magnetization. For the application of these magnetoelectric
transducers in spintronic devices, a detailed understanding of the coupling mechanism
between the bi-layer as well as voltage-controlled strain generation must be developed
in order to improve the efficiency of these devices. This thesis aims to study
this magnetoelectric coupling in CoFeB/PZT composite multiferroic system using
Anisotropic Magnetoresistance. A change in magnetization is caused via strain by
applying a voltage to the gate electrode of the device. The switching of magnetization
affects the resistance of the waveguide along the direction of magnetization, which is
used to quantify the change in the magnetoelastic field per unit applied voltage. The
materials used in this work are feasible for fabricating in integrated devices and are
compatible for generating spin waves in coplanar waveguides. To this end, various
devices are developed at the nanometre scale that have different gating configurations
and device parameters. The objective is to identify the effect of non-uniform strain
on the coupling coefficient and study the gatability for different device configurations.
Magnetoelectric switching was observed in many devices and a strong coupling
coefficient was found in case of non-uniform strain configuration. This gives a proof
of concept for resistance based magnetoelectric characterization and motivates the use
of these materials in magnetoelectric transducer for spin wave generation. Although
the values of coupling coefficients were lower than those reported previously, at the
nanometre scale, less voltage is needed for switching of magnetization. This paves
way for low-power spintronic devices for scalable circuits. Finally, the detection of
magnetization switching is attempted by reversing the magnetoelectric transduction
and measuring the generated voltage. The thesis is concluded by addressing various
fabrication related challenges and delineates the goals for future work.

In the present work, low pressure synthesis of BiMnO 3 (BMO) nanoparticles (NPs) by hydrothermal method is reported. Temperature-driven structural transitions of BMO are studied by X-ray diffraction (XRD) and exhibited that as-synthesized BMO is stabilized in monoclinic phase with C2 symmetry. However at 473 K, XRD pattern reveals abrupt changes in the intensity of peaks and lattice parameters. The origin of temperature-dependent symmetry transition is understood by sphenoid to prismatic transition by curtail of crystalline growth along (110) direction. The monoclinic structure with C2/c symmetry is found till 723 K. However, at 773 K and above XRD patterns are matched with Bi 2 Mn 4 O 10 and Bi 2 O 3 phases. The temperature profile of magnetic study exhibits antifer-romagnetic behaviour. M–H curve provides evidence of weak ferromagnetism. The present results suggest that NPs of BMO exhibit coexistence of antiferro-magnetism–ferromagnetism, however bulk BMO is ferromagnetic. X-ray pho-toelectron spectroscopy study reveals the presence of Mn in both ?3 and ?4 states in BMO NPs. Superexchange interaction between Mn 4? –O–Mn 3? induces AFM along with weak FM in BMO. This study signifies the low pressure synthesis of BMO and its anomalous structural and magnetic behaviour.

Tb-doped BiFeO 3 multiferroics nanoparticles fabricated via micro-emulsion route were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The fully characterized Tb x Bi 1 À x FeO 3 nanoparticles were then subjected to magnetic behavior evaluation for various technological applications. The thermogravimetric analysis (TGA) conducted in the range 25-1000 1C predicted the temperature ( $ 960 1C) for phase formation. XRD estimated the crystallite size 30-47 nm, while the particles size estimated by SEM was found (80-120 nm). The XRD data confirmed the rhombohedral (space group R3c) phase with average cell volume 182.66 Å 3 (for BiFeO 3 ). Various other physical parameters like bulk density, X-ray density and porosity were also determined from the XRD data and found in agreement with theoretical predictions. The magnetic studies showed that as Bi 3 þ was substituted by Tb 3 þ , all magnetic parameters were altered. The maximum saturation magnetization (M s ) (0.6691 emug À 1 ) was exhibited by Tb 0.02 Bi 0.98 FeO 3 while the Tb 0.00 Bi 1.00 Fe 1.00 O 3 showed the maximum (549 Oe) coercivity. The evaluated magnetic behavior categorized these materials as soft magnetic materials that may be useful for fabricating advanced technological applications.

Multiferroic BaTiO3–CoFe2O4–BaTiO3thin films were synthesized by spin coating of Co(NO3)2–Fe(NO3)3and Ba(OAc)2–Ti(i-OPr)4dissolved in a mixture of acetic acid and dimethylformamide. Stepwise thermaltreatment at 200, 400 and 800◦C led to the desired phases. The phase formation process was monitoredby thermoanalytic, XRD, IR and Raman measurements. SEM was used to characterize the structure andthickness of each layer. Two samples with different thicknesses of perovskite and spinel layers are dis-cussed. Sample 1 consists of 120 nm CoFe2O4between two 70 nm BaTiO3layers. For sample 2, the 90 nmBaTiO3top and bottom layers are enclosing 220 nm CoFe2O4. The surface qualities were determined byAFM indicating rms roughness values of about 4 nm. Magnetic investigations reveal slightly anisotropicbehavior. Polarization measurements show hysteresis loops (PS(max)= 29 µC cm−2; PR(max)= 17 µC cm−2)with switching peaks at the corresponding coercive fields

Multiferroic materials like Bismuth Iron Oxide (BiFeO3), YMnO3, BiMnO3, TbMnO3 has attracted worldwide attraction due its wide range of applications in data storage devices, spintronic devices, sensors and multiple stage memories. Among these materials BiFeO3 is a promising candidate as it exhibit room temperature antiferromagnetic and ferroelectric properties. However, BiFeO3 suffers from some drawbacks including large leakage current, inhomogeneity in spin structure and volatile nature of Bi2O3. In order to overcome these problems we here report Lanthanum (La)
doped Bi1-xLaxFeO3 (where, x=0.0-0.5) thin films prepared by sol-gel method. The effect of La substitution on structural, optical, magnetic and dielectric properties has been studied. Undoped BiFeO3 thin films show the presence of impurity phase of Bi2Fe4O9 however it is seen that La doping hinders the formation of secondary phases and gives pure rhombohedrally distorted perovskite structure. Moreover, with increasing La content transition from rhombohedral to orthorhombic symmetry is also observed. XRD peaks shift to low angles arises due to slight difference in ionic radii of La+3 (1.06Å ) and Bi+3 (1.03Å ). Band gap of undoped BiFeO3 is low as compared to La doped for x=0.1
due to the presence of impurity phase and with increase in La content the band gap decreases from 2.7-2.6eV. La+3 doping also affect the dielectric properties as dielectric constant increases with x. Ferromagnetic behavior, as opposed to antiferromagnetic, in
undoped and doped BiFeO3 is due to suppression of spiral spin structure. An enhancement in magnetization is observed with increasing La content.