1,721,026 research outputs found
Nucleation-and-growth process in Fe80B20amorphous alloys
No full textA nucleation-and-growth process in Fe80B20 amorphous alloy ribbons has been studied using differential scanning calorimetry using an isothermal analysis mode. Measurements have been performed in the temperature range between the glass transition and the onset of crystallization. Experimental data have been fitted using the theoretical Johnson-Mehl-Avrami equation. Kinetic parameters as well as the activation energy of crystallization have been determinated. It is shown that the mechanism, which governs the nucleation-and-growth process is independent of annealing temperature; while the nucleation frequency and growth rate are dependent on temperature value. Finally it is shown that the whole devitrification of the samples cannot be completely described using only the nucleation-and-growth process
Grain-growth process during crystallization of Fe80B20 amorphous alloys
No full textIt is shown that the full isothermal crystallization analysis of Fe80B20 amorphous alloy must include not only a crystal nucleation-and-growth process but also a grain-growth process and that these two processes are separated in time during isothermal annealing. The parameters characterizing the grain-growth process have been found by the fitting calorimetrical signal with the one expected from the normal grain-growth theory. Transmission electron microscopy and selected area electron diffraction seem to confirm our results since they show that there is a good agreement between the average grain radius of crystallization products seen in the micrograph and the one calculated from the obtained parameters
Oxidation-State and Short-Range Order at the Surfaces of Amorphous Fe40Ni40P14B6 Ribbons
No full textConversion electron Mossbauer spectroscopy (CEMS) was used to study the oxidation state at the surfaces of amorphous Fe40Ni40P14B6 ribbons obtained by melt-spinning in air and vacuum heated at 583 K. Different concentrations of ferric and ferrous ions depend on the different behaviour of phosphorous diffusion towards the two surfaces during low annealing temperature
The complex DSC analysis of the first crystallization peak of Fe80Si10B10 metallic glass
No full textThe crystallization of Fe-80 Si-10 B-10 glass was studied both by linear heating and isothermal differential scanning calorimetry (DSC). Two well separated crystallization peaks were always observed. On the basis of the classical isothermal Johnson-Mehl-Avrami (JMA) procedure both transient nucleation and transient growth kinetics with the complex exponent n similar to 2.8 and mean activation energy E(JMA)* = 323 kJ mol(-1) were determined for the first crystallization peak. Because for the degree of conversion alpha > 0.55 E *(alpha) dependence was observed, the deconvolution of the isothermal peak into the subsequent JMA nucleation-and-growth and grain-growth effects was used to fit the measured data. Simple JMA kinetics could not be determined for the linear heating first crystallization peak. All these results correlate with structural analysis studies
Bulk and surface magnetic study of Fe80B20 crystallization
No full textWe report a study relating the evolution of coercivity H-c, both in the bulk and in the surface region, to the microstructural changes occurring during the first stage of the crystallization process of Fe80B20 amorphous samples. The coercive field H-c has been studied with a magneto-optical Kerr ellipsometer in the surface and with a vibrating sample magnetometer in the bulk. The observed behavior of H-c can be accounted for by considering its dependence on the number density of grains and gives information on the domain wall dynamic indicating the occurrence of a two dimensional bending of the domain walls. In addition, the evolution of H-c is substantially insensitive to the details of the internal structure of the grains. This study reveals that the surface crystallization occurs via the same processes as in the bulk but it takes place at lower temperature with respect to the bulk. We also measured the Curie temperature of the surface region of the amorphous ribbon and we obtained a lower value with respect to the bulk. (C) 1997 American Institute of Physics
Short-Range Order at the Surfaces of Annealed Amorphous Fe40Ni40P14B6 Ribbons
No full textConversion electron Mossbauer spectroscopy (CEMS) and gamma transmission Mossbauer spectroscopy were used to measure the effects of annealing at 583 K in vacuum into about 200 nm thick layer below the two surfaces and on the bulk of the Fe40Ni40P14B6 amorphous ribbons prepared by means of melt-spinning technique. The results show a large distribution of hyperfine magnetic fields on the bulk and in the surfaces of the samples. By means of selective analysis of hyperfine magnetic field distribution, we have evalueded the correlation between the different degree of short range orders at the surfaces and in the bulk of the samples, and the phosphorus segregation associated with mechanical enbrittlement induced at low annealing temperature
Spin-dependent conductivity of nanosized magnetic inhomogeneities
No full textSpin-dependent scattering (SDS) originates at the interface between magnetic (M) and non-magnetic (NM) regions, and gives rise to the giant magnetoresistive effect (GMR), that is observed when M and NM regions are interleaved at the nanoscale level. The GMR intensity, i.e. the change in resistivity observed due to the application of an external magnetic field, is affected by the SDS strength that, in its turn, is inversely proportional to the lateral size of the magnetic regions [1]. Such a lateral scale is the effective size of the magnetic regions [2], Deff, that is the result of the effect of magnetic interactions on the real size of the magnetic regions, D. Similarly, if the magnetic morphology of the system is not uniform, the SDS strength changes with the lateral scale of the magnetization inhomogeneities [3].
The dependence of GMR on the external field is the counterpart of Deff, so direct indications about D are not easily accessed. However, if we resort to the GMR efficiency, gamma, [4], i.e. the change in GMR for a unit change of squared magnetization, the comparison between the values of gamma measured at low and at high applied field as a function of Deff enables one to evidence D with respect to Deff. Indeed, if Deff is larger than D, as soon as the external applied field overcomes the effect of magnetic interactions, the efficiency of the magnetic structure is expected to change, as the large effective magnetic volumes break into smaller parts. As a consequence of that, the lateral scale of the system decreases, and gamma is expected to increase accordingly.
In this work, we study different FexAg1-x nanogranular systems, where x is the relative Fe atomic concentration, 0.1 < x < 0.5. Under equilibrium conditions, Fe and Ag are not miscible, so using an out-of-equilibrium technique, in our case dc-magnetron sputtering, we obtain a deep intermixing of the two species. In this way, as a function of x, different samples with a different average magnetic length scale, namely with a different Deff, can be produced [3]. The evolution of the magnetic morphology of the systems was followed with zero-field-cooled and field-cooled magnetization measurements. Magnetization and GMR loops were recorded at two different temperatures, at 300 K and at 4 K. Indeed they represent two conditions were the contribution of interparticle interactions to systems dynamics is expected to be different. In this way, the effect of Deff can be better appreciated. We present the gamma dependence on x, measured both at low and at high applied field, gamma_L and gamma_H, respectively. For low values of x, gamma_L and gamma_H display the same dependence as a function of x, whilst for higher values gamma_L shows a broad maximum whilst gamma_L has a monotonic dependence that eventually approaches saturation. These data are presented and discussed and compared to magnetization loops and diffraction data in order to give an estimation of the Deff of the different samples.
[1] S. Zhang and P. M. Levy, J. Appl. Phys. 73, 5315, 1993.
[2] P. Allia, M. Coisson, F. Spizzo, P. Tiberto, F. Vinai, Phys. Rev. B 73 (2006) 054409
[3] P. Vavassori, E. Angeli, D. Bisero, F. Spizzo, F. Ronconi, J. Magn. Magn. Mater. 262 (2003) 52
[4] M. Tamisari, F. Spizzo, F. Ronconi, M. Sacerdoti, G. Battaglin, submitted to Journal of Applied Physics
Surface crystallization and magnetic properties of amorphous Fe80B20 alloy
No full textWe have studied the effects of surface crystallization on the magnetic properties of Fe80B20 amorphous alloys. The surface magnetic properties have been studied with magneto-optic Ken: measurements, while those of bulk with a vibrating sample magnetometer. This study reveals that surface crystallization is similar to the bulk process but occurs at a lower temperature. At variance with previous results on other iron-based amorphous alloys the surface crystalline layer does not induce bulk magnetic hardening. Furthermore, both the remanence to saturation ratio and the bulk magnetic anisotropy do not show appreciable variations after the formation of the surface crystalline layer. The Curie temperature of the surface layer is lower with respect to the bulk of the sample. These effects can be explained by a lower boron concentration in the surface region of the as-cast amorphous alloy. Measurements of the chemical composition confirm a reduction of boron concentration in the surface region. (C) 1997 American Institute of Physics
Evolution of coercivity during the early stages of Fe80B20 crystallization
No full textWe report a study of the evolution of coercivity (H-c) in partially crystallized Fe80B20 samples with an increasing number of crystallites obtained in a controlled way into ferromagnetic amorphous matrix. These samples allow to test existing models for interaction of domain walls with defects since it is possible to change only the number density of crystallites without modifying their dimensions. We observe that the evolution of H-c is mainly related to the nucleation frequency of grains and quite independent from their internal structure. The observed behavior of H-c can be accounted for by considering the dependence of coercivity on the number density of pinning sites and by assuming a two-dimensional bending of the domain walls during their motion. Moreover our results indicate that the changes occurring to the intergranular phase during annealing do not appreciably affect coercivity in the partially crystallized samples
Crystallization of amorphous Fe78Si9B13 alloy
No full textCrystallization kinetics of the amorphous Fe78Si9B13 alloy has been investigated by differential scanning calorimetry in both continuous heating and isothermal annealing modes. The morphology and the growth of different phases during the devitrification process have been studied by transmission electron microscopy and X-ray diffraction. The crystallization of this glass proceedes in two separate stages which can be observed during continous heating as well as during isothermal annealing. It is shown that the lack of complete information about the preparation conditions of the amorphous precursor makes impossible to foresee the stability of the amorphous phase, as well as the phase content, the morphology and the dimensions of the grains during the crystallization process and to compare the results obtained in different investigations
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