Effect of the Si/B ratio on the magnetic anisotropy distribution of Fe73.5Si22.52xBxCu1Nb3 „x57,9,16… alloys along nanocrystallization

The effect of the Si/B ratio on the magnetic anisotropy distribution of Fe73.5Si22.52xBxCu1Nb3 (x57,9,16) alloys has been studied. The influence of isochronal annealing on the hysteresis loop of the three studied alloys has been analyzed. They present two minima in coercivity: the first one can be ascribed to structural relaxation, and the second one is related to the averaging of the magnetocrystalline anisotropy, as predicted by the random anisotropy model. The relative importance of both minima changes with Si content: the lower the Si content, the more effective the structural relaxation and the less important the second minimum are. The mean value of the magnetic anisotropy distribution presents a similar behavior, evidencing the growing importance of the magnetoelastic anisotropy for the relaxed amorphous samples as the Si content is increased. From the evolution of the magnetic anisotropy distribution along nanocrystallization and the microstructural information obtained from transmission electron microscopy images, the behavior of the coercivity minima with changes in Si content can be ascribed to a different degree in compensation of magnetoelastic anisotropy due to the contributions of different signs coming from the nanocrystals and the amorphous matrixCICYT. Gobierno Español-MAT95-0961-CO2-0

X-ray Structure of a (o-ga/acto-Pentaacetoxypentyl)pyrazoline

1-(3,5-Dimethyl-3-nitro-1-pyrazolin-4-yl)­ penta-0-acetyl-o-galacto-pentitol, e20H29N 3012, Mr= 503·5, orthorhombic, P21212¡, a= 14·471 (9), b = 14·518 (3), e= 12·028 (2) A, V= 2527·0 (1·7) A3, z = 4, Dx = 1·323 Mg m- 3, .A(Mo Ka)= 0·7107 A, J.L = 0·1O mm- 1, F(OOO) = 1064, room temperature, final wR = 0·035 for 1836 observed reflexions. Bond distances and ang1es are all within the expected ranges. The pyrazoline ring exhibits approximate mirror symmetry and a puckering amplitude of 0·244 (6) A. Torsion angles of the acetoxy groups with the sugar chain range from 33 to 72°. Crystal cohesion is mainly due to van der Waals interactions but there are two intermolecular hydrogen bonds o108-2701/89/1o1563-03$03.00 linking molecules related by screw axes along [100] and [001]

Microstructure and Magnetic Properties of Mo Containing Nanoperm-Type Alloys

The influence of composition changes on the devitrification process, microstructure and magnetic properties of (Fe,Co)MoB(Cu) alloys was investigated. Amorphous and nanocrystalline samples were studied by differential scanning calorimetry, thermomagnetic gravimetry, X-ray diffraction, transmission electron microscopy and vibrating sample magnetometry and obtained results are correlated. Cu addition increases the thermal stability and the crystalline fraction of the nanocrystalline microstructure and improves the soft magnetic properties of the alloy. As B content increases the thermal stability of the nanocrystalline microstructure decreases and for the 20 at.% B alloy boride crystals are formed during the first crystallization stage. Saturation magnetization decreases and Curie temperature of the amorphous phase increases with B content. Co addition lowers the volume fraction of nanocrystals and as Co content increases in the alloy nanocrystals are enriched by Co, Curie temperature of the amorphous phase increases, linearly with Co content, and saturation magnetization and coercive field of nanocrystalline alloys increase as well.Comisión Interministerial de Ciencia y Tecnología de España (CICYT) Project MAT 2004-04618Junta de Andalucía. Procesos Asistenciales Integrados (PAI

Cellular automata simulations on nanocrystallization processes: From instantaneous growth approximation to limited growth

Cellular automata simulations have been performed to simulate the crystallization process under a limited growth approximation. This approximation resembles several characteristics exhibited by nanocrystalline microstructures and nanocrystallization kinetics. Avrami exponent decreases from a value n = 4 indicating interface controlled growth and constant nucleation rate to a value n ~ 1 indicating absence of growth. A continuous change of the growth contribution to the Avrami exponent from zero to 3 is observed as the composition of the amorphous phase becomes richer in the element present in the crystalline phase.Ministerio de Ciencia e Innovación de España (MICINN) y Unión Europea FEDER. MAT2010-20537Junta de Andalucía. Procesos Asistenciales Integrados (PAI) FQM-646

Nanocrystalline Fe-Nb-(B,Ge) alloys from ball milling: microstructure, thermal stability and magnetic properties

Fe75B20Nb5, Fe75Ge10B10Nb5 and Fe75Ge20Nb5 alloys were prepared by ball milling from pure powders and their microstructure and magnetic properties were studied. A nanocrystalline solid solution of-Fe type is the main phase formed, although traces of some intermetallics were found in the Fe-B-Nb alloy. The local arrangements of Fe atoms in Ge containing alloys continuosly evolves with milling time. The obtained powders are thermally stable even heating up to 773 K. After heating up to 1073 K, intermetallic compounds are detected. The best soft magnetic properties are achieved after heating up to 773 K, due to stress relaxation of the nanocrystalline microstructure (for Fe-Ge-Nb alloy, coercivity ~600 A/m

Influence of Milling Time on the Homogeneity and Magnetism of a Fe70Zr30 Partially Amorphous Alloy: Distribution of Curie Temperatures

In this work, the mechanically alloyed Fe70Zr30 (at. %) composition has been used to study the influence of milling time on its homogeneity and magnetic properties. The microstructure and Fe environment results show the formation of an almost fully amorphous alloy after 50 h of milling in a mixture of pure 70 at. % Fe and 30 at. % Zr. The soft magnetic behavior of the samples enhances with the increase of the milling time, which is ascribed to the averaging out of the magnetocrystalline anisotropy as the crystal size decreases and the amorphous fraction increases. The formation of a non-perfectly homogenous system leads to a certain compositional heterogeneity, motivating the existence of a distribution of Curie temperatures. The parameters of the distribution (the average Curie temperature, T C ¯ , and the broadening of the distribution, ∆ T C ) have been obtained using a recently reported procedure, based on the analysis of the approach towards the saturation curves and the magnetocaloric effect. The decrease of ∆ T C and the increase of T C ¯ with the milling time are in agreement with the microstructural results. As the remaining α-Fe phase decreases, the amorphous matrix is enriched in Fe atoms, enhancing its magnetic response.AEI/FEDER-UE Project MAT 2016-77265-

On the isothermal kinetics analysis of transformations in metastable systems. A combined use of isothermal and non-isothermal calorimetry

A procedure to optimize the isothermal calorimetric data of very slow transformation processes of metastable systems is proposed. The method uses an experimental baseline to identify the transitory effects due to the equipment. Moreover, the combined use of isothermal and non-isothermal results is shown to be effective to overcome the intrinsic problems of low signal and signal drift for such processes. The procedure has been applied to the analysis of the nanocrystallization kinetics of the Fe60Co18Nb6B16 alloy at different devitrification stages. Based on microstructural observations, an instantaneous growth approach was assumed and a phenomenological expression of the dependence of the nucleation frequency with both the transformed fraction and the temperature was obtaine

Relationship between coercivity and magnetic moment of superparamagnetic particles with dipolar interaction

The temperature dependence of the hysteresis loops of Nanoperm-type alloys has been studied. In the high-temperature region above the coercivity maximum, the response of the system can be modeled as that of dipolar-interaction superparamagnetic particles, considering a mean interaction field. Special attention has been paid to the influence of the particle size distribution on the applicability of the mean-field model. The two main effects of the dipolar interaction (coercivity and distortion of the thermal dependence of the apparent magnetic moment) have been correlated

On the use of JMAK theory to describe mechanical amorphization: a comparison between experiments, numerical solutions and simulations

The kinetics of amorphization during ball milling is generally analyzed using two different approaches: the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory and Delogu and Cocco’s model for which a region deterministically transforms after it reaches a certain number of collisions. The application of JMAK analysis to the latter model predicts Avrami exponents to be higher than the experimental ones (typically close to one). We develop simulations based on the probabilistic character of the nucleation phenomenon and concave growth of the amorphous phase in the core of a nanocrystal. The predictions of our simulations are in good agreement with the low Avrami exponents and with the size evolution of the remaining crystallites found experimentally. From these values, the parameters involved in the simulated model (growth rate and probability of nucleation) can be estimated.AEI/FEDER-UE (Project MAT-2016-77265-R)Junta de Andalucía (Grupo PAI