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Nickel oxide encapsulated gadolinium oxide (Gd2O3/NiO) core–shell nanoparticles (Nps) has been synthesized by polyol method and analyzed by transmission electron microscopy. The particle size is found to vary from 25 to 35 nm. Raman spectra show the gadolinium oxide core material peaks are found to be diminished by the mass effect of the NiO shell material. The dynamics of the magnetic moment has been studied with the help of temperature dependent electron paramagnetic resonance (EPR) spectroscopy. Peak-to-peak line width (ΔH pp) value of the EPR spectra increases with decreasing temperature. Temperature dependence of line width shows the dominance of spin–lattice relaxation in these systems, and it can be used as T 1 contrast agent. Spin number estimated from EPR studies increases with the decrease of the temperature. The interfacial exchange coupling between the core and shell region is found to be existing at very low temperatures as determined by EPR studies in this non-magnetic/antiferromagnetic (NM/AFM) core/shell nanosystem. The effects of the temperature on the magnetic properties of the Gd2O3/NiO core/shell NPs and the underlying mechanism have been discussed.

We report the hydrostatic pressure dependence of martensitic, magnetic and magnetocaloric effect in the Ni 48 Mn 39 Sn 13 − x Si x ( x = 1 and 4) Heusler alloys. The martensitic transition temperature increased with respect to pressure at the rate of 2.37 K/kbar and 0.7 K/kbar for x = 1 and 4 alloys respectively and decreases with the application of magnetic field at the rate of 1.2 K/T and 0.8 K/T for x = 1 and 4 alloys respectively. The Curie temperature of austenite phase changes marginally under pressure. The maximum magnetic entropy change (Δ S M ) of 5.13 J kg −1 K −1 at 0 kbar and 24.5 J kg −1 K −1 at 9.7 kbar were observed for x = 1 alloy. However the magnetic entropy change for x = 4 alloy seems to decrease as a function of pressure.

Role of Si content on the structural, thermal and soft magnetic properties of melt spun and annealed Fe 87− x Si x B 9 Nb 3 Cu 1 ( x = 11, 13, 18, 20, 25 and 30 at.%) alloys has been investigated. Processing of ribbons at a higher wheel speed (47 m/s) during melt spinning results in the formation of amorphous phase in all the alloys expect for 30 at.% Si whereas at lower wheel speed (34 m/s), amorphous phase has been observed only in 18 at.% Si alloy ribbon. Detailed thermal analyses revealed that higher glass forming ability of 18 at.% Si alloy is due to the near eutectic point in these alloy series and higher ratio between the crystallization and liquidus temperature ( T x / T L ). On annealing, nanocrystalline Fe 3 Si phase was found to be precipitated out in amorphous matrix in alloys up to 20 at.% Si ribbons whereas paramagnetic Fe 4.9 Si 2 B phase along with Fe 3 Si phase was observed in 25% Si ribbon. Detailed Mossbauer spectroscopy of the annealed ribbons revealed that the composition of the nanocrystalline Fe 3 Si phase increases from Fe-21 at.% Si towards the stoichiometric composition (Fe-25 at.% Si) with Si content. The results are also corroborated with precision lattice parameter measurements. The coercivity of the annealed ribbons initially decreases, attains a minimum at 18 at.% Si and then increases abruptly with Si content.

We report the role of film thickness on the structure, microstructure and magnetic properties of Co2FeGa thin films. The grain size was observed to increase with increase in film thickness. An in-plane magnetic anisotropy was observed for all the films. A gradual increase in saturation magnetization (Ms) and a nearly constant coercivity (Hc) was observed in the films with thickness >10 nm at room temperature. Curie temperature (TC) was found to increase gradually with increase in film thickness which is attributed to the increase in grain size with the film thickness. Thermal stability of the films was studied from the temperature dependent magnetic hysteresis loops which exhibit nearly a constant value of Ms till 873 K for all the films. This evidences the high temperature stability of Co2FeGa thin films. The increase in Hc at higher temperatures is attributed to the diffusion of Si into the film.

Structure, microstructure and magnetocaloric effect were investigated in Gd1-xGax (0.45 ≤ x ≤ 0.55) alloys. Phase identification and compositional analysis revealed that all these alloys consist of orthorhombic GdGa major phase with small fraction of tetragonal Gd3Ga2 and hexagonal GdGa2 secondary phases for Ga-lean and Ga-rich compositions respectively. The magnetic transition temperatures (TC) of the alloys were determined from AC susceptibility curves and it was found to be in the range of 181–186 K. Among these alloys, a maximum values of magnetic entropy change (ΔSM)max of 5.1 J kg−1-K, adiabatic temperature change (ΔTmax) of 4.6 K and refrigerant capacity (RC) of 644 J kg−1 were obtained in the x = 0.50 alloy.

The microstructure and magnetic properties of Ni2MnGa base alloys with “Fe” substitution in place of “Mn” are studied. The processing technique used is melt spinning at wheel speeds of 20 m/s and 30 m/s followed by annealing at 1273 K (1000 °C) for 1 hour. Fe content is varied from 2 at. pct to 11 at. pct for alloys of Ni50Mn(25−x)Fe x Ga25 with Heusler stoichiometry. Austenite with B2 partial atomic ordering and premartensitic tweed structures were found at room temperature for all the alloys at different wheel speeds. After annealing at 1273 K (1000 °C) for 1 hour, austenite phase with L21 Heusler atomic ordering is stabilized in samples of all the processing conditions. Saturation magnetization, martensitic transformation temperature, and Curie temperature are measured. Martensite temperature and Curie temperature increase in proportion to iron content in the alloy. Saturation magnetization is sensitive to the phase content and compositional inhomogeneities.

A solvothermal process was used to prepare highly monodispersed 3D hierarchical Fe3O4 microspheres with spinel structure by exploring triethylene glycol as solvent. The shape and size of the magnetic nanoparticles were controlled by adjusting the concentrations of precursor with invariable base medium. The resulting Fe3O4 samples were phase pure and of hierarchical microspheres consisting of the nanospheres. On the basis of the characterization results, the research proposes a possible process for the growth of hierarchical microspheres. Moreover, the magnetic properties confirm its superparamagnetic behavior. These 3D hierarchical Fe3O4 microspheres can be promising for targeted drug delivery (bio-magnetic) applications.

Nanocrystalline ɛ-Fe 3 N and Co 4 N nitride phases are synthesized first time by using tris(1,2-diaminoethane)iron(II) chloride and tris(1,2-diaminoethane)cobalt(III) chloride precursors, respectively. To prepare ɛ-Fe 3 N and Co 4 N nitride phases, the synthesized precursors were mixed with urea in 1:12 ratio and heat treated at various temperatures in the range of 450–900 °C under the ultrapure nitrogen gas atmosphere. The precursors are confirmed by FT-IR study. The ɛ-Fe 3 N phase crystallizes in hexagonal structure with unit cell parameters, a = 4.76 A and c = 4.41 A. The Co 4 N phase crystallizes in face centred cubic (fcc) structure with unit cell parameters, a = 3.55 A. The estimated crystallite size for ɛ-Fe 3 N and Co 4 N phases are 29 nm and 22 nm, respectively. The scanning electron microscopy (SEM) studies confirm the nanocrystalline nature of the materials. The values of saturation magnetization for ɛ-Fe 3 N and Co 4 N phases are found to be 28.1 emu/g and 123.6 emu/g, respectively. The reduction of magnetic moments in ultrafine materials compared to bulk materials have been explained by spin pairing effect, lattice expansion, superparamagnetic behaviour and canted spin structures at the surface of the particles.

Fe–Ni–Co alloys of various compositions (Fe x Ni 80− x Co 20, x =20–50) were synthesized by using a sodium borohydride reduction route. The phase purity and crystallite size was ascertained by using powder X-ray diffraction (XRD). The alloys crystallize in the face centered cubic (fcc) structure with lattice parameters, a =3.546–3.558 A. The XRD line broadening indicates the fine particle nature of the materials. The estimated crystallite sizes were found to be 27.5, 27, 24, and 22.8 nm for x =20, 30, 40, and 50; alloys respectively. Scanning electron micrograph studies indicates particle sizes to be in the range of 83–60 nm for Fe–Ni–Co alloys. The values of saturation magnetization for Fe x Ni 80− x Co 20 are found to be in the range of 54.3–41.2 emu/g and are significantly lower than the bulk values (175–180 emu/g). The coercivity decreases from 170 to 122 Oe with decrease in Fe content. The observed magnetic behavior has been explained on the basis of size, surface effects, spin canting and the presence of superparamagnetic fractions in the ultrafine materials.

Small crystallites of a metastable phase Co 0.5 Pt 0.5 are precipitated by heating a rheological liquid precursor of cobalt–hydrazine complex and platinum chloride H 2 PtCl 6 · x H 2 O in polymer molecules of poly(vinylpyrrolidone) (PVP) in ethylene glycol. The hydrazine co-reduces nascent atoms from the Co 2+ and Pt 4+ that recombine and grow as Co 0.5 Pt 0.5 . The PVP molecules cap a growing Co 0.5 Pt 0.5 as it achieves a critical size so that it stops growing further in given conditions. X-ray diffraction pattern of a recovered powder reveals a crystalline Co 0.5 Pt 0.5 phase (average crystallite size D ∼8 nm) of a well-known Fm3m-fcc crystal structure with the lattice parameter a =0.3916 nm (density ρ =14.09 g/cm 3 ). A more ordered L1 0 phase ( ρ =15.91 g/cm 3 ) transforms ( D ≥25 nm) upon annealing the powder at temperature lesser than 700 °C (in vacuum). At room temperature, the virgin crystallites bear only a small saturation magnetization M s =5.54 emu/g ( D =8 nm) of a soft magnet and it hardly grows on bigger sizes ( D ≤31 nm) in a canted ferromagnetic structure. A rectangular hysteresis loop is markedly expanded on an optimally annealed L1 0 phase at 800 °C for 60 min, showing a surface modified coercivity H c =7.781 kOe with remnant ratio M r / M s =0.5564, and M s =39.75 emu/g. Crystallites self-assembled in an acicular shape tailor large H c from ideal single domains and high magnetocrystalline anisotropy of a hard magnet L1 0 phase.

Magnetic anomalies corresponding to the Verwey transition and reorientation of anisotropic vacancies are observed at 151 K and 306 K, respectively, in NiCoFe2O4 nanoparticles (NPs) synthesized by a modified-solvothermal method followed by annealing. Cationic disorder and spherical shape induced non-stoichiometry suppress the Verwey transition in the as-synthesized NPs. On the other hand, reorientation of anisotropic vacancies is quite robust. XRD and electron microscopy investigations confirm a single phase spinel structure and the surface morphology of the as-synthesized NPs changes from spherical to octahedral upon annealing. Rietveld analysis reveals that the Ni(2+) ions migrate from tetrahedral (A) to octahedral (B) sites upon annealing. The Mossbauer results show canted spins in both the NPs and the strength of superexchange is stronger in Co-O-Fe than Ni-O-Fe. Magnetic force images show that the as-synthesised NPs are single-domain whereas the annealed NPs are multi-domain octahedral particles. The FMR study reveals that both the NPs have a broad FMR line-width; and resonance properties are consistent with the random anisotropy model. The broad inhomogeneous FMR line-width, observation of the Verwey transition, tuning of the magnetic domain structure as well as the magnetic properties suggest that the NiCoFe2O4 ferrite NPs may be promising for future generation spintronics, magneto-electronics, and ultra-high-density recording media as well as for radar absorbing applications.

The partial substitution of Si with B on the structural and magnetic properties of Co2FeSi1-xBx (x = 0-0.5) alloys was systematically investigated. X-ray and microstructural investigations show the presence of second phase at the grain boundaries which increases with increasing boron content. From thermal analysis studies, it was observed that L21-B2 ordering temperature remain constant whereas the melting point decreases with increase in boron addition and merges with ordering temperature at x = 0.5. The increase in TC for the alloys x ≥ 0.25 was attributed to the increase in second phase due to boron.

Polycrystalline Ni-Mn-Ga thin films have been deposited on Si (100) substrate with different film thickness. The influence of film thickness on the phase structure and magnetic domain of the films has been examined by scanning electron microscope, atomic force microscopy and magnetic force microscopy. Analysis of structural parameters indicates that the film at lower thickness exhibits the coexistence of both austenite and martensite phase, whereas at higher thickness L12 cubic non magnetic phase is noticed. The grains size and the surface roughness increase along with the film thickness and attain the maximum of 45 nm and 34.96 nm, respectively. At lower film thickness, the magnetic stripe domain is found like maze pattern with dark and bright images, while at higher thickness the absence of stripe domains is observed. The magnetic results reveal that the films strongly depend on their phase structure and microstructure which influence by the film thickness.

50 nm Co2FeSi thin films were deposited on Si (111) by ultra high vaccum dc magnetron sputtering at different substrate temperatures 300°C, 450°C, 550°C and 600°C. The films are characterized by grazing incidence xrd, magnetic and ferromagnetic resonance measurements. The film deposited at 550°C has B2 crystal structure with high magnetization and low coercivity. Ferromagnetic resonance and magnetic measurements are used as a tool to investigate the homogeneity of Co2FeSi thin films.

Co/Cu core/shell nanocomposites (CSNCs) with varying shell thickness were synthesized through a modified polyol process in conjunction with a transmetallation reaction. Tuning of shell thickness was achieved by subtle change in concentration of copper acetate infused in different experiments against a fixed cobalt precursor concentration. Formation of Cu nanoparticles (NPs) was observed from UV–vis spectra, which showed a typical surface plasmon resonance peak at 560 nm. Precise control over particle size was obtained by the use of PVP as stabilizer, which was confirmed by a red shift of 20 cm −1 in the C O stretching frequency of various PVP coated Co/Cu CSNCs. Powder X-ray diffraction patterns elucidate the co-existence of distinctive fundamental peaks for both Co and Cu indicating that they have not formed an alloy during the reaction. TEM images corroborate with the above result by revealing the existence of lighter shaded Cu shells coated over dark inner cores of Co. Coalescence of adjacent Cu shells gives a matrix like appearance in case of Co 21.2 Cu 78.8 NPs; whereas Co 90.7 Cu 9.3 NPs were distinctly separated from each other. Magnetic hysteresis loops of all the Co/Cu CSNCs showed ferromagnetic behavior and an increase in magnetization value with decrease in shell thickness was observed.

The phase transformation in Ni48Mn39In13 melt-spun ribbons has been studied by employing in-situ transmission electron microscopy (TEM) techniques. At room temperature, the investigations showed the martensite phase consisting of plates with internal stacking faults. This phase exhibited the presence of 7M and 5M modulations. In addition, a small volume fraction of the austenite phase was observed. Upon in-situ heating from room temperature to 95 degrees C, the martensite phase transformed to austenite phase. However, in the subsequent cooling-heating cycles, the martensite phase was retained at high temperature. We discuss our experimental observations and the possible mechanisms for the stabilization of the martensite phase due to thermal cycling. (c) 2012 Elsevier Ltd. All rights reserved.

Melt spun Ni50-xMn37+xIn13 (2

The magnetic structure of the seven layered (7 M) modulated martensite phases in Mn-rich Ni–Mn–Ga alloys was studied using Mossbauer spectroscopy. The Mossbauer results clearly demonstrate that in contrast to the non-modulated tetragonal structure two new magnetic sublattices exists for the 7 M orthorhombic martensite phase. Based on the unit cell symmetry and atomic coordination, the additional magnetic sublattices have been assigned to the Ni site. The variation in the magnetic properties of the martensite phases has been related to the underlying magnetic structure.

The residual stress instituted in Ni–Mn–Ga thin films during deposition is a key parameter influencing their shape memory applications by affecting its structural and magnetic properties. A series of Ni–Mn– Ga thin films were prepared by dc magnetron sputtering on Si(1 0 0) and glass substrates at four different sputtering powers of 25, 45, 75 and 100 W for systematic investigation of the residual stress and its effect on structure and magnetic properties. The residual stresses in thin films were characterized by a laser scanning technique. The as-deposited films were annealed at 600 1C for 1 h in vacuum for structural and magnetic ordering. The compressive stresses observed in as-deposited films transformed into tensile stresses upon annealing. The annealed films were found to be crystalline and possess mixed phases of both austenite and martensite, exhibiting good soft magnetic properties. It was found that the increase of sputtering power induced coarsening in thin films. Typical saturation magnetization and coercivity values were found to be 330 emu/cm 3 and 215 Oe, respectively. The films

Melt-spun ribbons of Co 69 Fe 7 Si 14− x Nb x B 10 alloys with x =0, 2 and 4 have been prepared and characterized for structure and soft magnetic properties. Ribbons with x =0 and x =2 are found to be completely amorphous whereas the ribbon with x =4 contains irregular shaped faulted Co 2 Si orthorhombic phase with grain size of about 100 nm. Nb addition is found to decrease the degree of amorphicity and induce perpendicular anisotropy, deteriorating the soft magnetic and magnetoimpedance properties.

A novel modified polyol process has been developed for the synthesis of hollow copper nanopots. Cobalt hydrazine complex was employed for the preparation of cobalt nanoparticles (NPs), which in turn acted as sacrificial seeds for the production of Cu nanopots and nanocups. The reduction of Cu2+ to Cu0 was observed by the colour change from bluish green to dark brownish red. The reaction was followed with the help of a UV–Vis spectrometer. A surface plasmon band at 558 nm indicated the formation of Cu NPs. Chemical characterisation was carried out using inductively coupled plasma optical emission spectrometer and Leco gas analysers, CS-444 and TC-600. Crystal structure was obtained by carrying out X-ray diffraction studies and it showed a face centred cubic structure. Particle size and morphology were obtained from scanning electron microscope. On average, copper nanopots of size 72 nm could be obtained using this method.

In this article, we report a simple and fast route for the synthesis of cobalt in the shape of nanorods (Co-NRs). It involves a modified polyol process in which a cobalt hydrazine complex was taken as a precursor with poly(vinylpyrrolidone) (PVP) molecules as a stabiliser in ethylene glycol (solvent) in a stainless steel bomb digestion vessel with Teflon inner lining. The solution was heated in the bomb at 200°C for three different reaction times 30, 60 and 90 min in performing the reaction. After the scheduled reaction, which resulted in a blackish colour turned-up from an initially pink-coloured sample, the samples were cooled down to room temperature. Recovered powders by repeated washing in water were studied for further characterisations. Reactions were analysed with Fourier transform–infrared spectroscopy and the XRD patterns of the samples reveal that the PVP molecules promptly control the formation of surface modified Co-NRs of a primarily face-centred cubic phase. These samples exhibit a ferromag...

The structure, microstructure, magneto-structural transition and magnetocaloric effect have been investigated in series of (Gd 5− x Zr x )Si 2 Ge 2 alloys with 0≤ x ≥0.20. X-ray powder diffraction analysis revealed the presence of orthorhombic structure for Zr containing alloys at room temperature in contrast to the monoclinic structure observed in the parent Gd 5 Si 2 Ge 2 alloy. The microstructural studies reveal that, low Zr addition ( x ≤0.1) resulted in low volume fraction of detrimental Gd 5 Si 3 -type secondary phase compared to that present in the parent alloy. All the Zr containing alloys have shown the presence of only second order magnetic transition unlike the parent alloy showing both first order structural and second order magnetic transition. A moderate (Δ S ) M value of −5.5 J/kg K was obtained for the x =0.05 alloy at an enhanced operating temperature of 292 K compared to −7.8 J/kg K at 274 K of the parent alloy for an applied field of 2 T. The interesting feature of Zr ( x =0.05) containing alloy is the wide operating temperature range of ∼25 K than that of ∼10–12 K for the parent, which resulted in enhanced net refrigerant capacity of 103 J/kg compared to that of 53 J/kg for the parent alloy.

The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd(5)Si(x)Ge(4-x) compounds with x = 1.7-2.3. Microstructural studies reveal the presence of Gd(5)(Si, Ge)(4)-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x = 1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x = 2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100 K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of -8 mu V/K was obtained at the magneto-structural transition for the x = 2 compound. (C) 2011 Elsevier B.V. All rights reserved.

Effects of 150 MeV Ni 11+ swift heavy ion (SHI) irradiation on copper ferrite nanoparticles have been studied at the fluences of 1 × 10 11 , 1 × 10 12 , 1 × 10 13 , 1 × 10 14 and 5 × 10 14 ions/cm 2 . The XRD pattern shows the irradiation fluence dependant preferential orientation. Scanning electron microscope analysis displays fine blocks of material for pristine while partial agglomeration on irradiation. Notably, a large number of holes are present at the fluence of 5 × 10 14 ions/cm 2 . The magnetization measurements performed in these samples exposes that the coercivity and remanence magnetization value increases due to the magnetocrystalline anisotropy up to the fluence of 1 × 10 13 ions/cm 2 . At 1 × 10 14 ions/cm 2 fluence, the induced thermal energy overcomes the magnetocrystalline anisotropy constant and causes a decrease in coercivity and remanence values. The saturation magnetization decreases up to the fluence of 1 × 10 13 ions/cm 2 and then it increases for further irradiation. The change of crystalline orientation observed from XRD, the creation of holes from SEM and the change in magnetic properties are discussed on the basis of electro-phonon coupling and it invokes the thermal spike theory.

In this paper we report the investigation of transition metal oxide compound, La0·67Ca0·25Sr0·04Ba0·04MnO3 (LCSBMO), along with La0·67Ca0·33MnO3 (LCMO), synthesized by sol–gel route under identical conditions. The effect of simultaneous low level substitution of large size ions such as Sr2+ and Ba2 + for Ca2 + ions on the electronic transport and magnetic susceptibility properties are analysed and compared apart from microstructure and lattice parameters. The temperature dependent electrical transport of the polycrystalline pellets of LCSBMO and LCMO when obeying the well studied law, \(\rho = \rho_{0} + \rho_{2} \;T^{2}\) for T TMI, the polaron activation energy is found to differ by about 11%. In addition, the temperature dependent a.c. magnetic susceptibility study of the compounds shows a shift of about 6% in the paramagnetic to ferromagnetic transition temperature (285 K for LCSBMO and 270 K for LCMO).

Lanthanum based mixed valence manganite system La 1− x Ca x −0.08 Sr 0.04 Ba 0.04 MnO 3 (LCSBMO; x =0.15, 0.24 and 0.33) synthesized through the sol–gel route is systematically investigated in this paper. The electronic transport and magnetic susceptibility properties are analyzed and compared, apart from the study of unit cell structure, microstructure and composition. Second order phase transition is observed in all the samples and significant difference is observed between the insulator to metal transition temperature ( T MI ) and paramagnetic (PM) to ferromagnetic (FM) transition temperature ( T C ). In contrast to the insulating FM behaviour usually observed in La 1− x Ca x MnO 3 (LCMO) for x =0.15, a clear insulator to metal transition is observed for LCSBMO for the same percentage of lanthanum. The temperature dependent resistivity of polycrystalline pellets, when obeying the well studied law ρ = ρ o + ρ 2 T 2 for T T MI , is observed to differ significantly in the values of ρ o and ρ 2 , with the electrical conductivity increasing with x . The variable range hopping model has been found to fit resistivity data better than the small polaron model for T > T MI . AC magnetic susceptibility study of the polycrystalline powders of the manganite system shows the highest PM to FM transition of 285 K for x =0.33.

Alloys of Fe 77 Ga 23 B x ( x =0, 0.025, 0.050, 0.075 and 0.1) were heat treated at 1000 °C/10 h and characterized for microstructural features and magnetostriction. The study indicates that the parent alloy consists of three phases viz. A2 as the major phase and L1 2 and DO 3 as minor phases. However, the volume fraction of L1 2 and DO 3 decreases with B addition and as a result magnetostriction improves. The decrease in volume fraction of these phases is attributed to B-segregation to the grain boundary, which seems to prevent the formation of these phases. The alloy with x =0.1, however, exhibits an additional Fe 2 B phase and consequently its presence leads to the re-emergence of the DO 3 phase, affecting the magnetostriction.

The effect of Fe and rare-earths (Tb&Dy) addition on the martensitic transformation, magnetic and mechanical properties of the Ni50Mn30Ga20 (at%) ferromagnetic shape memory alloy was investigated. The results show that the rare-earths exhibit negligible solubility in the Ni–Mn–Ga–Fe matrix phase and formed grain boundary phase. The grain boundary precipitate enhances the compressive ductility of the alloys. The martensite transformation temperature was found to be increased by the addition of rare-earths, while, Fe increased the Curie temperature of the Ni–Mn–Ga alloy. However, with increase in Fe content, an additional (Ni,Fe)3(MnGa) precipitate is formed, leading to decrease in martensite transformation temperature and compressive ductility.

Polycrystalline Gd 5 Si 2− x Ge 2− x Nb 2 x alloys (0 ≤ 2 x ≤ 0.2) were investigated for microstructure and magnetocaloric effect. The 2 x = 0.05 and 0.1 alloys were mainly composed of monoclinic Gd 5 Si 2 Ge 2 -type and orthorhombic Gd 5 Si 4 -type phases whereas other alloys exhibit monoclinic Gd 5 Si 2 Ge 2 -type phase at room temperature. The microstructural studies reveal that Nb addition results in the formation of Nb 5 (Si,Ge) 3 and α-Nb phases. The former promotes the nucleation of Gd 5 Si 3 -type phase while the latter suppresses it. At low Nb addition (2 x ≤ 0.1) the microstructural modification results in low volume fraction of detrimental Gd 5 Si 3 -type phase leading to enhanced magnetocaloric effect (Δ S ) M value compared to that of the parent compound. A maximum (Δ S ) M value of −9.6 J/kg K with refrigerant capacity of 98 J/kg was obtained for the 2 x = 0.1 alloy for a field change of 2 T near room temperature. The magneto-structural transitions shifted to higher temperatures with increase in Nb content up to 2 x = 0.1.

Fe-rich FePt nanoparticles with an average size of 5 nm were prepared using a reverse micelle route. As synthesized FePt powders were disordered face centered cubic in structure. DSC thermogram displays phase transformation at 575 °C, based on which heat treatments were carried in the temperature range of 500–600 °C. XRD patterns show that the degree of ordering increased with the increase in heat treatment temperature. An utmost coercivity of 3.2 kOe was observed for 600 °C heat-treated sample. The M–H loops were asymmetric in nature suggesting the presence of a second phase, Fe-oxide (Fe3O4) that is also observed from the XRD patterns. Mossbauer spectra reveals that on heat treatment the super-paramagnetic phase transforms into an ordered face centered tetragonal phase. The presence of 23% super-paramagnetic phase even after heat-treating at 600 °C reveals that the phase transformation is incomplete.

The Gd 5 Si x Ge 3.9− x Fe 0.1 alloys ( x = 1.8–2.1) alloys exhibited a first-order magnetic phase transition with hysteresis of 8–13 K upon heating and cooling. Microstructural studies revealed that the alloys were comprised of monoclinic Gd 5 Si 2 Ge 2 -type phase with small fraction of the Gd 5 Si 3 -type and FeSi-rich minor phases. The hysteresis losses are significantly reduced in the x = 2.1 alloy due to the absence of field-induced phase transition.

Ferroelectromagnetic, La 2 Bi 7 Fe 5 Ti 3 O 27 compound was synthesized using a solid-state reaction technique. Basic crystal data of the compound were obtained by XRD. Detailed studies of electrical properties (i.e., dielectric and impedance) of the material carried out in wide frequency (10 2 to 10 6 Hz) and temperatures (30 to 500 °C) ranges showed that these properties are strongly dependent on frequency and temperature. Magnetic measurements of the material provided a remanent magnetization of 0.011 emu/g at room temperature.

This study reports detailed microstructural and magnetic characterization of rapidly solidified Ni2(Mn,Fe)Ga heusler alloys processed using the melt spinning technique. Series of Ni50Mn(25-x)Fe(x= 2, 5, 8, 11)Ga25 alloys were prepared by vacuum arc melting and then melt spun at constant wheel speed of 20 m/sec to obtain samples in the form of ribbons. X-ray diffraction analysis of as-cast Ni2(Mn,Fe)Ga alloy with different ‘Fe’ concentrations revealed austenite phase with L21 Heusler atomic order at room temperature. Transmission electron microscopy of melt spun ribbons reveals a precursor tweed structures due to magnetic tweed contrast when the ‘Fe’ concentrations are 8% and 11%. In case of 11% ‘Fe’ substituted alloy martensite phase was found to from at the grain boundary triple junctions. Thermo magnetic measurements determine that, as the ‘Fe’ concentration increases from 2 to 11%; it enhances the magnetic transition temperature from 375 to 403 K.

Effect of Fe-substitution on the phase formation, partitioning behaviour of Fe in the co-existing phases, magneto-structural transition, magnetic entropy change and associated hysteresis losses has been investigated in Gd 5 Si 2 Ge 2 alloy. The virgin alloy crystallizes in monoclinic Gd 5 Si 2 Ge 2 -type phase, while Fe-substituted alloys form mixed monoclinic Gd 5 Si 2 Ge 2 -type and orthorhombic Gd 5 Si 4 -type phases. Electron probe microanalysis reveals that Fe does not dissolve in the matrix, but influences the magneto-structural transitions. Magneto-structural characterization of the Fe-containing alloys reveals that the Fe-substitution suppresses the structural transition observed at 273 K in virgin alloy. A maximum magnetic entropy change, Δ S M of 6.5 J/kg-K at 273 K was observed for a field change of 2 T in Gd 5 Si 2 Ge 2 alloy. The Fe-substituted alloys exhibit lower value of Δ S M but with reduced hysteresis losses.

With a view to understand the influence of nano size on various properties of cobalt-doped ZnO-based diluted magnetic semiconductors, a series of materials were prepared by the citrate gel route. The phase and morphology studies have been carried out by X-ray diffraction and transmission electron microscopy, respectively. All the samples of the present investigation are found to have hexagonal wurtzite structure and crystallite sizes are found to vary from 25 nm to 65 nm. From the optical absorption measurements it has been observed that upon doping with cobalt, the energy band gap is found to shift towards lower energy side (red shift) while it shifts towards higher energy side (blue shift) when the crystallite size is increased continuously. It has been observed from the XPS results that oxidation state of Cobalt is +2 and that the difference in binding energies of Co 2p 3/2 and Co 2p 1/2 is found to increase continuously with increasing crystallite size. Finally, all the samples are found to exhibit room temperature ferromagnetism and the specific magnetization decreases with increasing crystallite size.

Tb0.3Dy0.7Fe1.95 alloy was cast and directionally solidified by modified Bridgman technique at different solidification rates and investigated for microstructural features and magnetic properties. The phase relationship between the co-existing phases viz., (Tb,Dy)Fe2, (Tb,Dy)Fe3 and (Tb,Dy)-rich are studied as a function of solidification rate to evolve a structure–property correlation. Higher solidification rate improves grain texture and favour large magnetostriction. The magnetization and Mossbauer studies indicate no perceptible change in Fe magnetic moment, with a slight deviation in the 1:3 intensity ratio of the two sextets. Comparing the Mossbauer and microstructural results a relationship has been established between the intensity ratio and the volume fraction of the deleterious minor phase (Tb,Dy)Fe3 in (Tb,Dy)Fe2 microstructure.

Alloys of Dy0.7Tb0.3Fe1.95 − x Nb x (x = 0, 0.025, 0.05 and 0.075) were prepared in vacuum employing an induction furnace and investigated for the magnetic properties and microstructural features. A significant improvement in the magnetostriction has been obtained for the alloys containing Nb as compared to that of the parent alloy (x = 0). The Curie temperature and the Fe hyperfine field of the (Tb,Dy)Fe2 phase are nearly invariant despite Nb addition to the parent alloy. The microstructural features, on the other hand, indicated that another Laves phase, NbFe2 is formed as the primary phase at the expense of the undesired (Tb,Dy)Fe3 phase. From the Mossbauer studies also the formation of NbFe2 phase has been evidenced.

Alloys of Fe 100 − x Ga x [ x = 17–30] were prepared and their structural and magnetic properties investigated. The microstructure of all the alloys exhibit s A2 as the main phase, while an ordered L1 2 phase is observed for the alloys with x ⩾ 25. The hyperfine field distributions derived from Mossbauer studies show DO 3 type ordering for x > 20 in addition to the A2 and L1 2 structure. Higher magnetostriction is observed for the compositions x = 17 and 25.

Crystal structure of the phases, microstructure and magnetocaloric effect were investigated in Gd5Si2(Ge1−xGax)2 alloys with x = 0, 0.01, 0.02 and 0.03. The phase identification and compositional analysis were carried out by a combination of X-ray diffraction, scanning electron microscopy and electron probe micro-analysis. The Gd5Si2Ge2-type matrix phase having an orthorhombic structure co-exists with Gd5Ge3-type and GdGe-rich phases. Ga was found to have preferentially partitioned to the orthorhombic Gd5Si2Ge2-type matrix phase. Magneto-structural transition temperatures determined from differential scanning calorimetry and thermo-magnetic techniques revealed that Ga substituted alloys exhibit only magnetic transition unlike parent Gd5Si2Ge2 alloy which displays both structural and magnetic transitions. A maximum magnetic entropy change (ΔS)M of 7.0 J/kg K was obtained in x = 0 alloy for a magnetic field change of 2 T and its magnitude was lower in the Ga substituted alloys.

Alloys of Y1?-?x Gd x Fe2B y (x = 0, 0.25, 0.5, 0.75 and 1; y = 0, 0.1, 0.15 and 0.2) have been prepared and investigated for structural and magnetic properties. The compounds with x = 0 and 1 are found to form in single phase with C15-type cubic Laves phase structure, while those with x = 0.25, 0.5 and 0.75 are observed to form with small quantities of secondary (Y,Gd)Fe3 phase. The lattice parameters, Curie temperature and the average Fe hyperfine field are found to increase with increasing x. The Gd-Gd and Gd-Fe interactions are attributed to be the main reason for the enhancement of magnetic properties. Boron was found to stabilize the (Y,Gd)Fe2 phase without affecting the magnetic properties. � 2008 Springer Science+Business Media B.V.

The SmCo5/α-Fe nanocomposite powders were prepared by high energy ball milling and the inter-diffusion reaction between the SmCo5 and α-Fe magnetic phases were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and 57Fe Mossbauer spectroscopy. While structural and magnetic measurements could reveal only the presence of SmCo5 and α-Fe phases, Mossbauer studies could clearly specify the extent of alloying between Fe and Co atoms in terms of evolution of α-Fe(Co) phase as a function of milling time. It has been found that the fractional volume of α-Fe(Co) solid solution tends to increase at the expense of the initial α-Fe phase upon progressive milling.

The magnetic properties of Nd 4.5 Fe 77− x Mn x B 18.5 ( x =0, 1 and 2) nanocomposites prepared by the crystallization of amorphous precursors were investigated. Addition of Mn is found to decrease the crystallization temperature of the amorphous ribbons. The intrinsic coercivity i H c and maximum energy product ( BH ) max increase from 2.6 kOe and 9.1 MGOe for x =0 to 3.1 kOe and 10.3 MGOe for x =1, respectively, and the remanence ratio M r / M s increases from 0.70 to 0.72. The effect of Mn on Curie temperature T C and the thermal stability of M r and i H c were also studied. 57 Fe Mossbauer spectra have been recorded for x =0, 1 and 2 ribbons at room temperature and site preference of the Mn atoms in Fe 3 B and Nd 2 Fe 14 B phases is discussed using the Mossbauer spectroscopy.

Evolution of structure and magnetocaloric properties in ball-milled Gd 5 Si 2 Ge 2 and Gd 5 Si 2 Ge 2 /0.1 wt% Fe nanostructured powders were investigated. The high-energy ball-milled powders were composed of very fine grains (70–80 nm). Magnetization decreased with milling time due to decrease in the grain size and randomization of the magnetic moments at the surface. The magnetic entropy change (Δ S M ) was calculated from the isothermal magnetization curves and a maximum value of 0.45 J/kg K was obtained for 32 h milled Gd 5 Si 2 Ge 2 alloy powder for a magnetic field change of 2 T while it was still low in Fe-contained alloy powders. The thermo-magnetic measurements revealed that the milled powders display distribution of magnetic transitions, which is desirable for practical magnetic refrigerant to cover a wide temperature span.

A magnetic field-assisted ball milling has been employed for the preparation of SmCo5 + 10 wt% Fe nanocomposite powders in the presence of oleic acid as surfactant. Milling experiments were also carried out without using surfactant and the nanocomposite powders so obtained, with and without surfactant, were investigated for their structural and magnetic properties using SEM, XRD, VSM and Mossbauer spectrometry. The field-milled SmCo5/Fe nanopowders in the presence of surfactant display a possible grain orientation and possess relatively high coercivity as compared with that of SmCo5/Fe powders obtained with field-milling or conventional milling. Mossbauer studies revealed that the formation of α-Fe(Co) (soft magnetic phase) is more pronounced for the powders milled without surfactant.

SmCo 5 + x wt% Fe ( x = 0 , 5 and 10) nanocomposite powders were synthesized by mechanical milling and were consolidated into bulk shape by spark plasma sintering (SPS) technique. The evolution of structure and magnetic properties were systematically investigated in milled powders as well as in SPS samples. A maximum coercivity of 8.9 kOe was achieved in spark plasma sintered SmCo 5 +5 wt% Fe sample. The exchange spring interaction between the hard and soft magnetic phases was evaluated using δ M – H measurements and the analysis revealed that the SPS sample containing 5 wt% Fe had a stronger exchange coupling between the magnetic phases than that of the sample with10 wt% Fe.

Ni-rich ferromagnetic Ni–Mn–Ga Heusler alloys were investigated for structural and magneto-caloric effect. The structural transition from austenite to martensite near room temperature was observed with the increase of Ni content. The saturation magnetization measured at room temperature decreased with the increase of Ni content. A large change in magnetization (32%) was observed even for a small increase in Ni concentration (12%). The martensitic transition temperature (TM) and Curie temperature (TC) of all the samples were studied using thermo-magnetic as well as differential scanning calorimetry (DSC) techniques. The martensite and the magnetic transitions were found to be sensitive to the composition and were tuned through alloy composition to occur simultaneously at near room temperature. A change in magnetic entropy, (ΔS)M of −0.68 J/kg K was determined in Ni55Mn20Ga25 for which TM and TC were close to room temperature.