R.S. de Biasi

Influence of iron concentration and particle size on the ESR linewidth of Al2O3: Fe3+ powders

The linewidth of electron spin resonance (ESR) absorption in polycrystalline samples of chromium-doped alumina (Al2O3: Cr3+) has been investigated as a function of chromium concentration and particle size. The linewidth is found to be directly proportional to chromium concentration and inversely proportional to particle size. The experimental results are consistent with the mechanism of dipolar broadening in diluted solid solutions; the influence of particle size may be attributed to size-dependent fluctuations of the zero-field parameter, D.

Influence of aging on the thermal and structural properties of amorphous selenium prepared by ball milling

The influence of aging on thermal and structural properties of ball-milled amorphous selenium has been investigated. Differential scanning calorimetry (DSC) spectra suggest that, with aging, part of the amorphous structure formed by chain molecules transforms into another structure consisting of Se8 rings, which is also found in amorphous selenium prepared by rapid quenching or vapor deposition. An attempt to obtain detailed information about this mixed amorphous structure was done by using radial distribution function RDF(r) analysis.

Influence of manganese concentration on the ESR spectrum of Mn2+ in SrO

Electron spin resonance spectra of manganese-doped strontium oxide have been studied at room temperature for Mn concentrations between 0.10 and 0.40 mol.%. The results suggest that the range of the exchange interaction between Mn2+ ions is about 0.63 nm.

Structural study of TM60Ti40 (TM=Fe, Co, Ni) amorphous alloys produced by mechanical alloying

Abstract This paper describes the determination of the local atomic structure of amorphous TM60Ti40 (TM=Fe, Co, Ni) alloys produced by mechanical alloying (MA) technique. The interatomic distances and the coordination numbers for the TM–TM, TM–Ti and Ti–Ti first neighbors were obtained by making the deconvolution of the first atomic shells of the radial distribution functions, which were calculated from the measured X-ray scattering patterns for the three alloys. The deconvolution was possible because the local atomic structures of these alloys can be accurately predicted by the additive hard-sphere (AHS) model. Thus, the number of TM–TM and TM–Ti first neighbors given by the AHS were used in the deconvolution to find the number of minority Ti–Ti pairs. Our results show complete amorphization for the Ni60Ti40 and Co60Ti40 alloys and partial amorphization in the case of the Fe60Ti40 alloy. These results are consistent with the free enthalpies of mixing for these alloys. A chemical short-range order, greater than expected, was found in the Fe60Ti40 alloy and attributed to partial amorphization of the Fe–Ti system by MA.

Ferromagnetic resonance study of structural relaxation and crystallization of the Metglas 2826A amorphous alloy

The ferromagnetic resonance technique was used to study the ageing behaviour of the metallic glass Fe32Ni36Cr14P12B6 (Metglas 2826A). The peak-to-peak linewidth was measured for several isothermal annealing times in the temperature range 350 to 375° C. After an initial decrease, attributed to structural relaxation, the linewidth increases linearly with the transformed fraction of the first crystallization phase. For 0.05 <f< 0.50, the transformed fraction,f, as a function of time,t, satisfies the Avrami equation with the exponentn being between 1.58 and 1.71. The activation energy for the first crystallization phase is estimated from the times to reach 5 to 40% transformation to be 306 kJ mol−1. The experimental results suggest that the second phase of crystallization begins when the transformed fraction of the first phase is of the order of 50%.

Crystallization of the amorphous alloy Fe40Ni40B20: a ferromagnetic resonance study

The ferromagnetic resonance (FMR) technique was used to study the crystallization of the metallic glass Fe40Ni40B20 (Vitrovac 0040). The line intensity of the amorphous phase was measured for several isothermal annealing times in the temperature range 360 to 375° C. The transformed fraction, as derived from FMR data, satisfies the Johnson-Mehl-Avrami equation with the exponentn between 1.49 and 1.70. The activation energy for crystallization is estimated from the times to 25% to 75% transformation to be 364 kJ mol−1.

Structural, thermal, optical, and photoacoustic study of nanocrystalline Bi2Te3 produced by mechanical alloying

Nanocrystalline Bi2Te3 was produced by mechanical alloying and its properties were investigated by differential scanning calorimetry (DSC) x-ray diffraction (XRD), Raman spectroscopy (RS), and photoacoustic spectroscopy (PAS). Combining the XRD and RS results, the volume fraction of the interfacial component in as-milled and annealed samples was estimated. The PAS results suggest that the contribution of the interfacial component to the thermal diffusivity of nanostructured Bi2Te3 is very significant.

Temperature dependence of the ESR linewidth in KFeS2

Abstract The ESR linewidth in KFeS 2 has been measured in the range 80–340 K. The linewidth is found to decrease steadily as the temperature is increased, without any discontinuity or singularity at the transition temperature T N =245 K. In the paramagnetic state, the region of enhanced linewidth extends further above T N than for two-and three-dimensional compounds such as K 2 MnF 4 and MnF 2 .

Crystallization of Fe81B13.5Si3.5C2 amorphous alloy observed by ferromagnetic resonance and differential scanning calorimetry

Abstract The crystallization kinetics of the amorphous metallic alloy Fe 81 B 13.5 Si 3.5 C 2 (Metglas 2605SC) was studied using ferromagnetic resonance (FMR) measurements on samples annealed at 693 K for different times, and differential scanning calorimetry (DSC) with several heating rates between 2 and 30 K s −1 . The increase of the FMR linewidth with annealing time is attributed to crystallization. Assuming a linear relation between the linewidth increase and the transformed fraction, a value of 1.20±0.10 was obtained for the Avrami exponent, n . The DSC measurements showed three exothermal peaks, suggesting a crystallization process involving more than one phase. The lower temperature peak, which appears at about 750 K for a heating rate of 2 K min −1 , increases with heating rate and becomes dominant for heating rates greater than about 10 K min −1 . The Avrami exponent calculated from the DSC results for this peak, n =1.24±0.10, agrees with FMR results within experimental error and is consistent with a diffusion-controlled process with a nucleation rate close to zero.

ESR investigation of Fe3+ diffusion in rutile

Abstract The electron spin resonance (ESR) technique was used to investigate the diffusion of Fe 3+ in rutile (TiO 2 ). ESR absorption intensity was measured for several annealing times and three different temperatures of isothermal annealing: 1273, 1323 and 1373 K. The activation energy for diffusion, calculated from the experimental data using a theoretical model based on the Fick equation, was found to be E A = 215 kJ mol −1 . This result is consistent with experimental data using the radioactive-tracer sectioning technique and represents an independent measurement of the activation energy for diffusion of trivalent iron in rutile, which is difficult to achieve by other means in this mixed-valence system.