G. Amarendra

Atomic scale study of thermal reduction of nano goethite coexisting with magnetite

Evolution of the local structure and magnetic properties of nano particles of goethite having magnetite as a composite due to controlled annealing treatments in vacuum has been studied using Mossbauer spectroscopy. Importance of size, defect associated with structural OH- for the observed structural and magnetic properties of goethite has been emphasized in this study. Present Mossbauer results show that thermal annealing at low temperatures (420-550 K) lead to a partial conversion / reduction of orthorhombic goethite to cubic spinel oxides such as maghemite and off-stochiometric magnetite. This study further establishes that annealing treatments beyond 650 K predominantly results in topotactic conversion of goethite to haematite. Underlying physics of the transitions of goethite to iron oxides and the important role of desorbed hydrogen for the orthorhombic to cubic structural transitions has been elucidated in this study.

Local structure and magnetic properties of cubic iron oxide nanoparticles formed in zeolite as deduced using Mössbauer spectroscopy

Local structure and magnetic properties of cubic iron oxide nanoparticles as prepared in zeolite matrix have been studied in a detailed manner using Mossbauer spectroscopy. Superparamagnetic particles of cubic iron oxide are observed to be strongly bound in zeolite matrix. Annealing around 870 K is understood to result in detrapping of the bound nanoparticles. This leads to the onset of coarsening of the nanoparticles thus resulting in an appearence of a broad sextet characterised by a low hyperfine field. On the otherhand, the occurrence of the sextet, due to the onset of coarsening of the bare cubic iron oxide superparamagnetic nanoparticles, has been observed subsequent to annealing around 520 K. Linewidth and quadrupole splitting corresponding to the fraction of 57Fe absorbers associated with superparamagnetic particles are shown to be sensitively dependent upon the annealing temperature with respect to detrapping and aggregation of nanoparticles formed in zeolite.

Atomic scale understanding of magnetic properties in Ni50Fe35Co15

Mossbauer spectroscopic studies have been carried out at different temperatures across ferromagnetic to paramagnetic transition in Ni50Fe35Co15 and the evolution of hyperfine parameters such as centre shift and magnetic hyperfine fields with temperature has been studied. Mossbauer spectrum obtained at 300 K in Ni50Fe35Co15 exhibiting fcc crystal structure is a six line pattern with the mean value of the hyperfine field close to 33 Tesla. Ferromagnetic to paramagnetic transition has been observed to occur in this system around 895 K matching with that of magnetization results. Debye temperature of this nickel rich alloy is deduced to be around 470 K matching with that of Ni. Effect of prolonged annealing at 750 K on the magnetic property is also investigated with respect to the thermal stability of the alloy .

Mössbauer Studies of Local Environments of 57Fe in Iron Oxide Particles Dispersed Fe Matrix

Mixture of iron with 25 wt% of hematite has been subjected to high energy ball milling and Mossbauer measurements have been carried out at room temperature in the as ball milled condition and subsequent to annealing at different elevated temperatures at a pressure of 10−6 mbar. Mossbauer results imply the dispersion of α‐Fe2O3 particles in metallic iron matrix in the as ball milled sample. Evolution of α‐Fe2O3 in to γ‐Fe2O3, Fe3O4 and FeO by means of reducing the valence of Fe ion of the oxide due to vacuum annealing in the range of 600–1000 K in this ball milled mixture has been elucidated in the present study.

Defect-induced magnetism in liquid sodium-exposed stainless steel as studied by Mössbauer and positron annihilation spectroscopic techniques

Detailed Mossbauer studies carried out in liquid sodium (Na)-exposed austenitic stainless steel (SS-316) show that there is a partial formation of ferromagnetically (FM)-ordered ferritic zones in the paramagnetic austenitic matrix. Results of low energy positron beam-based Doppler broadening studies imply the occurrence of vacancy kind of defects in the liquid Na-exposed SS-316. Correlating these results, the partial occurrence of FM-ordered zones in the liquid Na-exposed SS-316 is understood to be due to open volume defects, predominantly that of Ni vacancies occurring at the surface and upto a certain depth of liquid Na-exposed stainless steel. These results are elucidated in terms of hyperfine parameters associated with ferritic zones.

Mossbauer spectroscopic studies in ferroboron

Mossbauer spectroscopic studies have been carried out in a detailed manner on ferroboron in order to understand the local structure and magnetic properties of the system. Evolution of the local structure and magnetic properties of the amorphous and crystalline phases and their thermal stability have been addressed in a detailed manner in this study. Role of bonding between Fe 4s and/or 4p electrons with valence electrons of boron (2s,2p) in influencing the stability and magnetic properties of Fe-B system is elucidated.

Atomic scale study of ball milled Ni-Fe2O3 using Mössbauer spectroscopy

Evolution of hyperfine fields at Fe atoms has been studied in a detailed manner in a mixture of Ni and α-Fe2O3 subjected to high energy ball milling using Mossbauer spectroscopy. Mossbauer results indicate the dispersion of α-Fe2O3 particles in Ni matrix in the as ball milled condition. Evolution of α-Fe2O3 due to ball milling, reduction of the valence of associated Fe and possible interaction between the oxide particles with Ni in the matrix due to annealing treatments has been elucidated in the present study.