A. Srinivasan

Oxygen octahedra distortion induced structural and magnetic phase transitions in Bi1−xCaxFe1−xMnxO3 ceramics

The co-doping of Ca and Mn in respective Bi and Fe-sites of BiFeO3 lattice leads to structural transition from rhombohedral (R3c space group) to orthorhombic (Pbnm space group) crystal symmetry. The tilt angle for anti-phase rotation of the oxygen octahedra of BiFeO3 at room temperature is observed to be ∼13.8°. It decreases with the increase in the co-doping percentage which suggests the composition-driven structural phase transition. The remnant magnetization for sample with 15% of co-doping becomes about 16 times that of BiFeO3. It may be attributed to the suppression of cycloid spin structure and uncompensated spins at the surface of nanocrystallites. Further increase in co-doping percentage results in the sharp reduction of remnant magnetization due to the dominant contribution from the collinear antiferromagnetic ordering in the Pbnm space group. The Arrott plot analysis clearly indicates the composition-driven crossover from the antiferromagnetic to weak ferromagnetic ordering and vice versa. Electron spin resonance results provide the evidence for the composition-driven phase transitions from an incommensurate spin cycloidal modulated state to one with nearly homogeneous spin order. The band gap (2.17 eV) of BiFeO3 measured using UV-Vis spectra was supported by the resonance Raman spectra.

Effect of Co and Cu substitution on the magnetic entropy change in Ni46Mn43Sn11 alloy

We report the observation of magnetic entropy change due to martensitic phase transformations of bulk off-stoichiometric Heusler alloys with compositions Ni46Mn43Sn11, Ni44Mn43Co2Sn11, and Ni44Mn43Cu2Sn11, prepared by arc melting method. The martensitic transition of the parent ternary alloy (Ni46Mn43Sn11) shifts to lower temperatures upon Co and Cu substitution. Inverse magnetocaloric effect was observed in these alloys near the martensitic transformation temperature when subjected to an applied magnetic field. Ni46Mn43Sn11, Ni44Mn43Co2Sn11, and Ni44Mn43 Cu2Sn11 alloys exhibited maximum positive magnetic entropy change of 7.9, 11.3, and 18.8 J kg−1 K−1, respectively, under an applied magnetic field of 1.8 T.

Low Temperature dc Electrical Conductivity of V2O5–SnO–TeO2 Glasses Exhibiting Majority Charge Carrier Reversal

The dc electrical conduction in xV 2 O 5 20SnO (80 - x)TeO 2 (18 mol% < x < 50 mol%) glasses has been studied in the temperature range from 100 to 480 K. Glasses with x < 20 exhibited p-type conduction whereas the rest show n-type conduction. All the glass compositions exhibited a crossover from variable range hopping (VRH) to small polaron hopping (SPH) conduction at a characteristic temperature well below room temperature. The low temperature dc conduction mechanism in these glasses has been analysed using Motts approach. Mott parameter analysis gave values for the density of states at the Fermi level N(E F ) between 1.42 × 10 26 m -3 eV -1 and 15.0 x 10 26 m -3 eV -1 at 230 K. The disorder energy W d was found to be varying between 0.02 and 0.03 eV. N(E F ), the average hopping distance R VRH and the VRH-SPH transition temperature T R exhibit an interesting composition dependence, which is interpreted in terms of the majority charge carrier reversal (MCCR) phenomenon occurring in these glasses.

Novel polyvinyl alcohol-bioglass 45S5 based composite nanofibrous membranes as bone scaffolds.

Composite nanofibrous membranes based on sol-gel derived 45SiO2 24.5CaO 24.5 Na2O 6 P2O5 (bioglass, BG) and 43SiO2 24.5CaO 24.5 Na2O 6 P2O5 2Fe2O3 (magnetic bioglass, MBG) blended with polyvinyl alcohol (PVA) have been electrospun. These low cost membranes were mostly amorphous in structure with minor crystalline (sodium calcium phosphate) precipitates. All membranes were biodegradable. Among these, the composites exhibited higher tensile strength, better proliferation of human osteosarcoma MG63 cells and higher alkaline phosphatase enzyme activity than the bare PVA membrane, indicating their potential in bone tissue engineering. The magnetic PVA-MBG scaffold was also found to be a promising candidate for magnetic hyperthermia application.

Electrical and optical studies on Pb-modified amorphous Ge–Se–Te films

PbxGe42−xSe48Te10 (3 ≤ x ≤ 15) and Pb20GexSe70−xTe10 (17 ≤ x ≤ 24) amorphous thin films were prepared by thermal evaporation of corresponding bulk glasses on glass substrates. A p-type to n-type conduction change was observed at the compositions with x = 9 at wt% Pb and x = 21 at wt% Ge in PbxGe42−xSe48Te10 (3 ≤ x ≤ 15) and Pb20GexSe70−xTe10 (17 ≤ x ≤ 24) films, respectively. The optical bandgap of these thin film samples was determined from optical absorption data. The composition dependence of optical bandgap of the thin films is interpreted on the basis of the variation of the average bond energy in these amorphous semiconductors with change in composition. The direct current electrical resistivity of these samples was measured in the temperature range of 310–400 K. The sheet resistance and the activation energy for electrical conduction of the films decreased with increasing Pb content in the PbxGe42−xSe48Te10 (3 ≤ x ≤ 15) series, whereas the sheet resistance and the activation energy for electrical conduction exhibited a maximum at 21 at wt% of Ge in the Pb20GexSe70−xTe10 (17 ≤ x ≤ 24) series. The electronic conduction in the Pb–Ge–Se–Te films in the 310–400 K regime has been attributed to the band transport mechanism.

Differential scanning calorimeter studies on Pb modified Ge-Se-Te glasses

Bulk glasses PbxGe42 − xSe48Te10(0 ≤ x ≤ 15) and Pb20GexSe70 − xTe10(17 ≤ x ≤ 24) have been prepared by quenching the melt. The non-isothermal properties of these glasses have been determined using a Differential scanning calorimeter (DSC). The composition dependence of the glass transition temperature, crystallisation temperature, excess heat capacity at glass transition, glass forming tendency and activation energy of glass transition and crystallisation show anomalous features near the composition at which the p- to n-type transition occurs. These observations reflect the changes occurring in the chemical bonding and the nature of the glassy network in these glasses. The results have been interpreted in terms of existing models and the majority charge carrier reversal phenomenon occurring in these glasses.

Critical behavior and magnetic entropy change at magnetic phase transitions in Ni50Mn35In14Si1 ferromagnetic shape memory alloy

We have investigated critical behaviour and magnetocaloric effect in polycrystalline Ni50 Mn35 In14 Si1 alloy near the second-order ferromagnetic phase transitions (SOPT) at austenite Curie temperature and at martensite Curie temperature by determining the critical exponents, β , γ and δ corresponding to the temperature dependence of spontaneous magnetization, initial susceptibility and isothermal magnetization, respectively. The field dependence of the maximum value of the magnetic entropy change at the two T C s was estimated using the Maxwell relation as well as from the values of the critical exponents. Values of obtained by these two methods at both the SOPT are in remarkable agreement with each other. The critical exponents have been determined by analysing isothermal magnetization data using two different methods, viz. , the modified Arrott plot method and the Widom scaling relation. The scaling plots depicted on linear as well as logarithmic scales confirm the reliability of the values of critical exponents obtained. The values of the critical exponents of polycrystalline Ni50 Mn35 In14 Si1 alloy at both the T C s are close to those predicted by mean-field theory confirming the presence of long-range magnetic ordering in the investigated alloy.

Enhanced soft magnetic properties in magnetic field annealed amorphous Fe(Co)–Zr–B alloys

We report the effects of longitudinal magnetic field annealing (MFA) on the microstructure, magnetic domain structure, and magnetic softness of Co substituted amorphous Fe89−x−yCoyBxZr11 alloys. A two-phase structure characterized by bcc Fe(Co) nanocrystals (size 100 Oe) due to the formation of Fe(Co)–Zr compounds. Large MS (173 emu/g) and low HC (0.4 Oe) were obtained for an Fe69Co10B10Zr11 alloy annealed at 823 K. Lorentz microscopy results reveal that the average size of the domains decreases with increasing Co content and magnetic ripple structures are observed in s...

Evaluation of sol–gel based magnetic 45S5 bioglass and bioglass–ceramics containing iron oxide

Multicomponent oxide powders with nominal compositions of (45-x)·SiO2·24.5CaO·24.5Na2O·6P2O5xFe2O3 (in wt.%) were prepared by a modified sol-gel procedure. X-ray diffraction (XRD) patterns and high resolution transmission electron microscope images of the sol-gel products show fully amorphous structure for Fe2O3 substitutions up to 2 wt.%. Sol-gel derived 43SiO2·24.5CaO·24.5Na2O·6P2O5·2Fe2O3 glass (or bioglass 45S5 with SiO2 substituted with 2 wt.% Fe2O3), exhibited magnetic behavior with a coercive field of 21 Oe, hysteresis loop area of 33.25 erg/g and saturation magnetization of 0.66 emu/g at an applied field of 15 kOe at room temperature. XRD pattern of this glass annealed at 850 °C for 1h revealed the formation of a glass-ceramic containing sodium calcium silicate and magnetite phases in nanocrystalline form. Temperature dependent magnetization and room temperature electron spin resonance data have been used to obtain information on the magnetic phase and distribution of iron ions in the sol-gel glass and glass-ceramic samples. Sol-gel derived glass and glass-ceramic exhibit in-vitro bioactivity by forming a hydroxyapatite surface layer under simulated physiological conditions and their bio-response is superior to their melt quenched bulk counterparts. This new form of magnetic bioglass and bioglass ceramics opens up new and more effective biomedical applications.

Influence of phosphate precursors on the structure, crystallization behaviour and bioactivity of sol–gel derived 45S5 bioglass

Obtaining bioglass composition 45S5® in completely amorphous form without crystalline inclusion by the sol–gel route has remained a challenge so far. Here, we demonstrate that with appropriate phosphate precursor and by controlling the polymerization and gelation reactions by adjusting the pH of the reactants, one can overcome this challenge. As-synthesized glass was heat treated to understand the devitrification behaviour of the sol–gel derived glass and to obtain bioglass–ceramics. As-synthesized and heat treated glasses were characterized using powder X-ray diffraction, transmission electron microscope, differential scanning calorimeter and differential thermal analyzer. For assessing the in vitro bioactivity of sol–gel derived glass and glass–ceramic powders, we tested the apatite forming ability on their surface upon immersion in simulated body fluid. Properties of sol-derived 45S5 glass are then compared with the bulk counterpart obtained by conventional melt quenching method. This study reveals a procedure to prepare completely amorphous sol–gel derived 45S5 glass which can be used as a bioactive material for bone implant, tooth coating, bone tissue engineering and drug delivery applications.