R. Govindaraj

Point defects aspect of magnetite nano particles as studied by Mossbauer spectroscopy

Mossbauer spectroscopy measurements have been carried out in nano particles of Fe3O4 to understand in a detailed manner about the point defects that are present and compared with the results of the bulk magnetite. Mossbauer measurements have been carried out in the as formed magnetite nano particles and subsequent to annealing in vacuum at different temperatures. The present study elucidates the implication of point defects on the local structural and magnetic properties of magnetites in terms of deduction of hyperfine parameters of Fe ions such as valence, quadrupole splitting and magnetic hyperfine field.

Dislocation driven chromium precipitation in Fe-9Cr binary alloy: a positron lifetime study

The influence of initial heat treatment on anomalous Cr precipitation within high temperature solubility region of the Fe–9Cr alloy has been investigated using positron lifetime studies. Air-quenched samples with pre-existing dislocations exhibited a distinct annealing stage in positron lifetime between 800 and 1100 K corresponding to Cr-precipitation. During this stage, Transmission Electron Microscopy showed fine precipitates of average size 4 nm, dispersed throughout the sample and from Energy-dispersive X-ray spectroscopy (EDS) analysis they are found to be Cr-enriched. The presence of dislocations is found to be responsible for Cr precipitation.

Mossbauer spectroscopic studies on NiCoFe

Mossbauer studies have been carried out at different temperatures across the ferromagnetic to paramagnetic transition in NiCoFe alloy. Based on the evolution of hyperfine field with temperature it could be deduced that the ferromagnetic to paramagnetic transition in this alloy occurs around 890 K. The critical exponent of the magnetic transition is deduced to be around 0.35 implying that the observed ferro magnetic ordering is due to t hree dimensional Heisenberg exchange interactions.

Partial inversion in nano zinc ferrite as studied using Mossbauer spectroscopy

Bulk and nano Zinc ferrite (ZnFe2O4) were prepared by solid state reaction and co-precipitation methods respectively. Mean size of the bulk and nano Zinc ferrite particles have been deduced to be around 49 and 8 nm respectively based on XRD measurements. Although the Mossbauer spectra corresponding to both the nano and bulk ZnFe2O4 are doublets exhibiting paramagnetic ordering of spins at room temperature, about 0.4 fraction of Fe atoms are associated with defects in the nano crystalline ZnFe2O4. Mossbauer spectrum obtained at 87 K on the nano ZnFe2O4 shows that close to 0.6 fraction of Fe atoms are associated with a weakly magnetic ordered component understood to be due to partial inversion of the population of Zn and Fe.

Annealing effects in Eurofer-97 steel as studied by Mossbauer spectroscopy

Eurofer-97 steel has been subjected to isochronal annealing treatments and studied by Mossbauer spectroscopy to understand the evolution of the ferromagnetic phases and to look for the possible retention of austenitic phase at room temperature. This Mossbauer study elucidates the retention of austenitic phase in addition to predominantly occurring ferritic phases due to quenching subsequent to annealing treatments beyond 1273 K.

Positron Annihilation Studies in Search of Fine Precipitates in Fe‐9Cr alloys

Positron annihilation lifetime studies were carried out on cold worked pure Fe and Fe‐9Cr alloy subjected to isochronal annealing in the temperature range from 300 to 1323 K. The measured lifetimes of Fe‐9Cr alloy showed three distinct annealing stages as compared to pure Fe viz., initial annealing of defects, a plateau between 623 K and 873 K and noticeable increase beyond 1123 K. The second annealing stage is likely due to the formation of chromium rich nanoclusters. Third annealing stage beyond 1123 K is attributed to highly defected martensitic phase formation during cooling from y‐phase.

Study of vacancy defects and their thermal stability in MeV Fe ion irradiated RAFM steel using positron beam Doppler broadening spectroscopy

ABSTRACT Indian reduced activation ferritic-martensitic steel was irradiated with 1.1 MeV Fe ions to various doses from 1 to100 dpa at room temperature. The depth profiling of irradiation-induced vacancy-type defects and the defect-recovery under post-irradiation annealing was studied using variable low-energy positron beam Doppler broadening spectroscopy. The influence of irradiation-induced defects on the microstructural properties was studied by glancing incidence x-ray diffraction (GIXRD) and nanoindentation technique. Positron annihilation study showed the signatures of reduced vacancy concentration at the peak damage region due to injected interstitial effect from 30 to 100 dpa and the widening of vacancy-interstitial recombination-rich region towards the end of ion range with the increase in dose. The GIXRD results were analysed by modified Williamson–Hall plot method, and the variation of coherent domain size and micro-strain with irradiation dose was studied. Irradiation-induced hardening was observed in the nanoindentation study. The features observed in the GIXRD and nanoindentation study are correlated with the depth-resolved defect distribution observed in the positron annihilation study.

Magnetic and structural studies in pristine and La doped BiFeO3 using Mössbauer spectroscopy

This work reports the bulk structural and magnetic properties of Bi1−xLaxFeO3 with x= 0, 0.1 and 0.2 using XRD and magnetization studies. Magnetic hyperfine fields and quadrupole splitting at Fe sites associated with FeO6 have been discussed in a detailed manner corresponding to pristine and La doped BiFeO3 using Mossbauer spectroscopy.

Studies of defects in Bi2Fe4O9 using Mössbauer spectroscopy

Effect of oxygen vacancies on the stability and magnetic properties of Bi2Fe4O9 has been addressed in a detailed manner using Mossbauer spectroscopy along with magnetization studies. This is studied mainly based on the variations in Mossbauer hyperfine parameters due to the changes in the local structure and magnetic properties at 57Fe atoms in Bi2Fe4O9 which are observed to be significantly influenced due to vacuum annealing. Oxygen vacancies concomitantly result in the formation of iron associated antiferromagnetic phase preferably at the boundaries of the grains of Bi2Fe4O9. Growth of these phases is observed to be strongly dependent upon subsequent air annealing treatments of this system.

Nanoparticles of lead hexaferrite as studied using Mössbauer spectroscopy

Nanoparticles of M-type Lead hexaferrite (PbFe12O19) (PFO) has been prepared using coprecipitation method. As prepared particles are deduced to be superparamagnetic based on the results of Mossbauer studies. Changes in the local structure and magnetic properties have been deduced using Mossbauer results obtained on the sample subsequent to controlled annealing treatments. The structural and magnetic properties of as prepared and annealed sample studied using XRD and magnetization respectively are consistent with those obtained using Mossbauer spectroscopy.