Department of Physics

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Rapid solidification route by melt spinning has been adopted for preparation of a Ni52.5Mn24.5Ga23 (at %) ferromagnetic shape memory alloy in the form of ribbons. In the as-spun state, the ribbon revealed a predominant austenitic L21 structure in combination with martensitic feature as observed from x-ray diffraction studies. Transmission electron microscopic (TEM) evaluation showed these features in the form of martensitic plates. At low temperature, martensite to austenite transformation was exhibited by an increase in magnetization during heating cycle. The reverse effect was observed during cooling cycle. Annealing temperature and magnetising field was also found to effect this transformation.

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Advanced Materials Research

Development of Ni-Mn-Ga based ferromagnetic shape memory alloy by rapid solidification technique

A ferromagnetic shape memory alloy with nomial composition Ni52.5Mn24.5Ga23 (at%) was developed by a melt spinning technique. The as-spun ribbon showed dominant L21 austenitic (cubic) structure with a splitting of the primary peak in the X-ray diffractogram indicating the existence of a martensitic feature. The quenched-in martensitic plates were revealed in transmission electron microscopy. An increase of magnetization at low temperature indicated a martensite to austenite transformation and its reverse with a drop in magnetization during the cooling cycle. Higher magnetic fields propel martensite-austenite transformation spontaneously. © 2008 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials

Magnetic transitions and structure of a NiMnGa ferromagnetic shape memory alloy prepared by melt spinning technique

The ferromagnetic shape memory alloy with nominal composition of Ni52.5Mn24.5Ga23(at%) was developed by the melt-spinning technique. The as-spun ribbon showed dominant L21 austenitic (cubic) structure with splitting of primary peak in the X-ray diffractogram indicating existence of a martensitic feature. The quenched-in martensitic plates were revealed from Transmission electron microscopy (TEM). Increase of magnetisation at low-temperature rise indicates martensite to austenite transformation and its reverse with a drop in magnetisation during cooling cycle. The martensite to austenite transformation can be made spontaneous at higher magnetic field. © 2008 Elsevier B.V. All rights reserved.

Development of NiMnGa-based ferromagnetic shape memory alloy by rapid solidification route

Highly stable and optically transparent colloidal ZnO QDs have been synthesized at room temperature without using any surface capping agent. The as synthesized QDs show phase singularity of ZnO particles having wurzite (hexagonal) structure. TEM as well as AFM studies indicate that the average crystalline size of QDs is ∼7nm. Further, high resolution TEM image shows formation of hexagonal shape particles having lattice fringes along (001) plane of wurzite phase. Photoluminescence (PL) studies show bright luminescence with peak maximum at 530nm due to oxygen vacancy centers (Vo) present in QDs. The optical transmission spectrum of colloidal QDs of ZnO shows sharp absorption at 3.48eV which is blue shifted as compared to bulk ZnO (3.36eV) due to the quantum confinement effect. The band gap energy observed in QDs of ZnO is consistent with the energy calculated by using effective mass approximation model of Brus.

Journal of Optoelectronics and Advanced Materials

Optically transparent colloidal suspensions of single crystalline ZnO quantum dots prepared by simple wetchemistry

Recent discoveries indicate that when the materials are brought down to sizes in the range 1-100 nm, these exhibit unique electrical, optical, magnetic, chemical, and mechanical properties. Methods have now been established to obtain the monodisperse nanocrystals of various metallic and semiconducting materials, single-walled and multi-walled nanotubes of carbon and other metallic and non-metallic materials together with organic nanomaterials such as supra-molecular nanostructures, dendrimers, hybrid composites with tailored functionalities. The high surface-to-volume ratio with an added element of porosity makes these highly potential candidates for chemical and biological sensor applications with higher degree of sensitivity and selectivity as compared to their bulk counterparts. The paper reviews the recent developments and applications of chemical and biological sensors based on nanomaterials of various structural forms. © 2008, DESIDOC.

Defence Science Journal

Nanotechnology applications for chemical and biological sensors

We study the consequences of CPT and lepton number violation in the neutrino sector. For CPT violation we take gravity with which neutrino and antineutrino couple differently. Gravity mixes neutrino and antineutrino in an unequal ratio to give two mass eigenstates. Lepton number violation interaction together with CPT violation gives rise to neutrino-antineutrino oscillation. Subsequently, we study the neutrino flavor mixing and oscillation under the influence of gravity. It is found that gravity changes flavor oscillation significantly which influences the relative abundance of different flavors in present universe. We show that the neutrinoless double beta decay rate is modified due to the presence of gravity-the origin of CPT violation, as the mass of the flavor state is modified. © 2008 The American Physical Society.

Fulltext

Physical Review D - Particles, Fields, Gravitation and Cosmology

CPT and lepton number violation in the neutrino sector: Modified mass matrix and oscillation due to gravity

Composite nanoparticles of aerosol OT-alginate hydrogel loaded with Fe 3 O4 and rhodamine 6G (R6G) in average diameter between 25 nm and 50 nm were synthesized by emulsification-cross-linking process and characterized by transmission electron microscopy (TEM) and SQUID magnetometery measurements. TEM measurements show that the Fe3 O4 particles are uniformly distributed within AOT-alginate nanoparticles. The Ca 2+ cross-linked AOT-alginate nanoparticles loaded with Fe 3O4 and rhodamine showed superparamagnetic behavior at room temperature with saturation magnetization of ∼50 emu/g of Fe 3O4. Similar behavior was exhibited by Fe 2+ crosslinked AOT-alginate nanoparticles loaded with Fe3O4 and rhodamine except it showed twice the magnetization of Ca 2+ crosslinked counterpart. A decrease in the amount of rhodamine loading for the composite nanoparticle systems as compared to the bare AOT-alginate nanoparticles was found. © 2008 IEEE.

IEEE Transactions on Magnetics

Fe3O4 incorporated AOT-alginate nanoparticles for drug delivery

We predict the existence of a ferromagnetic shape memory alloy Ga2MnNi using density-functional theory. The martensitic start temperature (TM) is found to be approximately proportional to the stabilization energy of the martensitic phase (δEtot) for different shape memory alloys. Experimental studies performed to verify the theoretical results show that Ga2MnNi is ferromagnetic at room temperature and TM and TC are 780 and 330 K, respectively. Both from theory and experiment, the martensitic transition is found to be volume conserving that is indicative of shape memory behavior. ©2008 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Theoretical prediction and experimental study of a ferromagnetic shape memory alloy: Ga2MnNi

The investigation addresses the effect of in-water quenching process parameters on the magnetic properties of Fe and Co based amorphous wires. The in-water quenching process parameters such as water drum speed, crucible nozzle orifice diameter, crucible nozzle to water level separation and gas pressure for melt ejection were found to be critically important to obtain continuous and ductile wires. The thinner wires with a diameter of ∼ 124 μm revealed superior soft magnetic properties with coercivities as low as ∼5·63 A m-1 (70·8 mOe) which was attributed to their high amorphous nature and consequent isotropy. This was also manifested from their surface morphology as observed by scanning electron microscopy. The Fe 77.5Si7.5B15 and Co72.5Si 12.5B15 (at.-%) wires in their as prepared state revealed high fracture strength, above 3000 MPa, and high microhardness ∼ 1200 HV (Vickers pyramid number). © 2008 Institute of Materials, Minerals and Mining.

Materials Science and Technology

Preparation of Fe and Co based amorphous wires by in-water quenching process

Pyramidal ZnO nanorods with hexagonal structure having c-axis preferred orientation are grown over large area silica substrates by a simple aqueous solution growth technique. The as-grown nanorods were studied using XRD, SEM and UV-vis photoluminescence (PL) spectroscopy for their structural, morphological and optical properties, respectively. Further, the samples have also been annealed under different atmospheric conditions (air, O2, N2 and Zn) to study the defect formation in nanorods. The PL spectra of the as-grown nanorods show narrow-band excitonic emission at 3.03 eV and a broad-band deep-level emission (DLE) related to the defect centers at 2.24 eV. After some mild air annealing at 200 °C, fine structures with peaks having energy separation of ∼100 meV were observed in the DLE band and the same have been attributed to the longitudinal optical (LO) phonon-assisted transitions. However, the annealing of the samples under mild reducing atmospheres of N2 or zinc at 550 °C resulted in significant modifications in the DLE band wherein high intensity green emission with two closely spaced peaks with maxima at 2.5 and 2.7 eV were observed which have been attributed to the VO and Zni defect centers, respectively. The V-I characteristic of the ZnO:Zn nanorods shows enhancement in n-type conductivity compared to other samples. The studies thus suggest that the green emitting ZnO:Zn nanorods can be used as low voltage field emission display (FED) phosphors with nanometer scale resolution. © 2007 Elsevier B.V. All rights reserved.