V. Raghavendra Reddy

High Coercivity of Oleic Acid Capped CoFe2O4 Nanoparticles at Room Temperature

High coercivity (9.47 kOe) has been obtained for oleic acid capped chemically synthesized CoFe(2)O(4) nanoparticles of crystallite size approximately 20 nm. X-ray diffraction analysis confirms the formation of spinel phase in these nanoparticles. Thermal annealing at various temperatures increases the particle size and ultimately shows bulk like properties at particle size approximately 56 nm. The nature of bonding of oleic acid with CoFe(2)O(4) nanoparticles and amount of oleic acid in the sample is determined by Fourier transform infrared spectroscopy and thermogrvimetric analysis, respectively. The Raman analysis suggests that the samples are under strain due to capping molecules. Cation distribution in the sample is studied using Mossbauer spectroscopy. Oleic acid concentration dependent studies show that the amount of capping molecules plays an important role in achieving such a high coercivity. On the basis of above observations, it has been proposed that very high coercivity (9.47 kOe) is the result of the magnetic anisotropy, strain, and disorder of the surface spins developed by covalently bonded oleic acid to the surface of CoFe(2)O(4) nanoparticles.

Multiferroic properties of polycrystalline Bi1−xCaxFeO3

It is shown that Ca2+ doping at Bi-site results in the release of weak ferromagnetism in BiFeO3. Structural transformation from rhombohedral to triclinic is observed with 10% Ca doping. Raman measurements show the presence of oxygen vacancies with Ca doping and no evidence of either intermediate valence or the tetravalence of iron is observed from Mossbauer measurements. No significant change in Neel temperature is observed with Ca doping. The observed weak ferromagnetism and ferroelectric nature at room temperature indicates the multiferroic nature of Bi1−xCaxFeO3 (x=5% and 10%) samples.

Study of the effect of Mn doping on the BiFeO3 system

In this work a Mn doped magnetoelectric BiFeO3 system is studied. X-ray diffraction (XRD), scanning electron microscopy, energy dispersive x-ray analysis (EDX), Mossbauer spectroscopy at room and high temperatures, differential scanning calorimetry (DSC), high temperature magnetization, dielectric constant measurements and x-ray photoelectron spectroscopy (XPS) are used to characterize the samples. The XRD result shows BiFeO3 as a major phase along with about 1–2% impurity phase. EDX shows the equi-atomic ratio of Bi and Fe site cations. Using DSC it is observed that the Neel temperature decreases with Mn doping. Using Mossbauer and XPS it is observed that Fe exists in the +3 oxidation state. The samples have an antiferromagnetic nature with Mn doping.

Study of weak ferromagnetism in polycrystalline multiferroic Eu doped bismuth ferrite

Room temperature spontaneous magnetization and dielectric anomaly at Neel temperature are observed in 15% Eu doped bismuth ferrite indicating the multiferroic nature of the sample. With 15% Eu doping structural transformation from rhombohedral to triclinic is observed. Fe3+ and Eu3+ oxidation states are observed from F57e and E151u Mossbauer measurements, respectively. The high field F57e Mossbauer spectrum in longitudinal geometry shows an enhancement in the intensity of lines corresponding to Δm=0 transitions, i.e., second and fifth lines in six line pattern. This observation suggests that the origin of spontaneous magnetization is due to weak ferromagnetism of Dzyaloshinskii–Moriya type.

Electro-caloric effect in lead-free Sn doped BaTiO3 ceramics at room temperature and low applied fields

Structural, dielectric, ferroelectric (FE), 119Sn Mossbauer, and specific heat measurements of polycrystalline BaTi1–xSnxO3 (x = 0% to 15%) ceramics are reported. Phase purity and homogeneous phase formation with Sn doping is confirmed from x-ray diffraction and 119Sn Mossbauer measurements. With Sn doping, the microstructure is found to change significantly. Better ferroelectric properties at room temperature, i.e., increased remnant polarization (38% more) and very low field switchability (225% less) are observed for x = 5% sample as compared to other samples and the results are explained in terms of grain size effects. With Sn doping, merging of all the phase transitions into a single one is observed for x ≥ 10% and for x = 5%, the tetragonal to orthorhombic transition temperature is found close to room temperature. As a consequence better electro-caloric effects are observed for x = 5% sample and therefore is expected to satisfy the requirements for non-toxic, low energy (field) and room temperature b...

Eu doping in multiferroic BiFeO3 ceramics studied by Mossbauer and EXAFS spectroscopy

Bismuth ferrite ceramics (BiFeO(3)) are multifunctional materials classified as multiferroics for their special magnetic and electric properties that can be modified by substitutional doping at the Bi and/or Fe sites. Understanding the relation between magnetoelectric response and structural/electronic modification upon doping is a relevant issue. In this work, the structure of Eu-doped multiferroic systems (Bi(1-x)Eu(x)FeO(3), x = 0, 0.5, 0.1, 0.15) as well as the valence state of Fe and Eu ions have been investigated combining Mossbauer and x-ray absorption fine structure (XAFS) spectroscopy techniques. The Eu(3+) doping at the Bi site results in better magnetic properties. High temperature (57)Fe Mossbauer data and Fe K-edge XAFS results show that FeO(6) octahedron distortions reduce with Eu(3+) doping. It is conclusively shown that the observed magnetic properties in BiFeO(3) with chemical substitution (Eu) are mainly due to the structural distortions and not due to Fe multiple valence. (151)Eu Mossbauer measurements show that the Eu(3+)(Bi(3+)) site is magnetically inactive in BiFeO(3).

Low temperature Raman and high field 57Fe Mossbauer study of polycrystalline GaFeO3.

The magnetic and phonon properties of polycrystalline magnetoelectric/multiferroic GaFeO(3) are studied. Using high field (57)Fe Mossbauer spectroscopy, occupation of Fe is observed at four cation sites. A Fe population of about 6% is observed at the tetrahedral Ga1 site, which explains the observed pinched-like M-H curve and initial sharp increase of the magnetization. The calculated net magnetization value from Mossbauer data suggests that the Fe moment at the Ga1 site is parallel to Fe1 and opposite to that of Fe2 and Ga2 sites, resulting in ferrimagnetism. From low temperature Raman data, anomalous temperature variation in frequency at T(C) is observed for the mode at ∼700 cm(-1).

Superconductivity in Ru substituted polycrystalline BaFe 2-x Ru x As 2

The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples of BaFe2-xRuxAs2 for nominal Ru content in the range of x=0.75 to 1.125. A systematic suppression of the spin density wave transition temperature (TSDW) precedes the appearance of superconductivity in the system. A phase diagram is proposed based on the measured TSDW and superconducting transition temperature (TC) variations as a function of Ru composition. Band structure calculations, indicate introduction of electron carriers in the system upon Ru substitutiom. The calculated magnetic moment on Fe shows a minimum at x=1.0, suggesting that the suppression of the magnetic moment is associated with the emergence of superconductivity. Results of low temperature and high field Mossbauer measurements are presented. These indicate weakening of magnetic interaction with Ru substitution

Study of site-disorder in epitaxial magneto-electric GaFeO3 thin films

Epitaxial thin films of GaFeO3 (GFO) with different thicknesses (22–170 nm) are prepared on (001) oriented yttria-stabilized zirconia substrate using pulsed laser deposition. The M-H data measured below Curie temperature (TC) mimic two phase composite magnetic system consisting of hard and soft magnetic phases. The results are explained in terms of Fe distribution among the available cation sites of GFO. Thermo-magnetic irreversibility and cusp in zero-field cooled magnetization are observed for all the films and are explained in terms of the magnetic anisotropy of the GFO.

Magnetic and 57Fe Mössbauer study of magneto-electric GaFeO3 prepared by the sol-gel route.

This work reports the preparation of magneto-electric GaFeO(3) by the sol-gel route and its characterization by x-ray diffraction, dc-magnetization, ac-susceptibility, low temperature and high field (57)Fe Mössbauer spectroscopy and dielectric constant measurements. The prepared samples are found to be single phase from x-ray diffraction studies. The crystallite sizes are found to be in the nano-regime for the samples sintered at low temperatures. From the temperature dependent dc-magnetization (M-T) measurements, bifurcation of the zero-field cooled (ZFC)-field cooled data and a cusp in the ZFC data are observed. With the help of low-field ac-susceptibility, (57)Fe Mössbauer and detailed dc-magnetic measurements these features are explained in terms of the magnetic anisotropy of the sample ruling out phenomena like spin-glass and super-paramagnetism as quoted in the literature for this compound. Apart from this, very interesting and different M-H behavior mimicking composite two-phase magnets is observed for the samples sintered at different temperatures. A symmetric M-H loop is observed for samples sintered at low temperatures and a pinched M-H loop is observed for samples sintered at high temperatures. The observed magnetic properties are explained by estimating the Fe cation distribution using high field (57)Fe Mössbauer spectroscopy measurements. An anomaly in the dielectric constant data at the Curie temperature indicates the ME coupling of the samples.