Martensitic transitions and the nature of ferromagnetism in the austenitic and martensitic states of Ni-Mn-Sn alloys

Structural and magnetic transformations in the Heusler-based system Ni0.50Mn0.50¿xSnx are studied by x-ray diffraction, optical microscopy, differential scanning calorimetry, and magnetization. The structural transformations are of austenitic-martensitic character. The austenite state has an L21 structure, whereas the structures of the martensite can be 10M , 14M , or L10 depending on the Sn composition. For samples that undergo martensitic transformations below and around room temperature, it is observed that the magnetic exchange in both parent and product phases is ferromagnetic, but the ferromagnetic exchange, characteristic of each phase, is found to be of different strength. This gives rise to different Curie temperatures for the austenitic and martensitic states.

Elastocaloric effect associated with the martensitic transition in shape-memory alloys

The elastocaloric effect in the vicinity of the martensitic transition of a Cu-Zn-Al single crystal has been studied by inducing the transition by strain or stress measurements. While transition trajectories show significant differences, the entropy change associated with the whole transformation (DeltaSt) is coincident in both kinds of experiments since entropy production is small compared to DeltaSt. The values agree with estimations based on the Clausius-Clapeyron equation. The possibility of using these materials for mechanical refrigeration is also discussed.

Crystallization behaviour of some melt spun Nd-Fe-B alloys

The kinetics of crystallization of four amorphous (or partially amorphous) melt spun Nd-Fe-B alloys induced by thermal treatment is studied by means of differential scanning calorimetry and scanning electron microscopy, In the range of temperatures explored experimentally, the crystallization process is thermally activated and generally proceeds in various stages. The Curie temperature and the crystallization behavior have been measured. The apparent activation energy of crystallization of most of the crystallization stages has been determined for each melt spun alloy. The explicit form of the kinetic equation that best describes the first stage of crystallization has been found. It follows in general the Johnson-Mehl-Avrami-Erofe'ev model, but clear deviations to that model occur for one alloy. Scanning electron microscopy demonstrates that preferentially hetereogeneous nucleation occurs at the ribbon surface which was in contact with the wheel. From crystallization kinetics results the lower part of the experimental time-temperature-transformation curves for all studied alloys are deduced and extrapolated to the high temperature limit of their range of validity, also deduced.

Accuracy in the experimental calorimetric study of the crystallization kinetics and predicitve transformation diagrams: Application to a Ga-Te amorphous alloy

The uncertainties inherent to experimental differential scanning calorimetric data are evaluated. A new procedure is developed to perform the kinetic analysis of continuous heating calorimetric data when the heat capacity of the sample changes during the crystallization. The accuracy of isothermal calorimetric data is analyzed in terms of the peak-to-peak noise of the calorimetric signal and base line drift typical of differential scanning calorimetry equipment. Their influence in the evaluation of the kinetic parameters is discussed. An empirical construction of the time-temperature and temperature heating rate transformation diagrams, grounded on the kinetic parameters, is presented. The method is applied to the kinetic study of the primary crystallization of Te in an amorphous alloy of nominal composition Ga20Te80, obtained by rapid solidification.

Magnetoelasticity and magnetoresistance in Cu-Al-Mn shape-memory alloys

We study the effect of a magnetic field on the martensitic transition of a Cu-Al-Mn shape-memory alloy. The martensitic transition has been studied through resistance measurements under applied magnetic fields ranging from 0 to 50 kOe. Negative magnetoresistance showing an almost linear dependence with the square of the magnetization has been observed. This magnetoresistive effect is associated with the existence of small ferromagnetic Mn-clusters. Its strength and thermal dependence is different in both phases. The martensitic transition temperature is slightly increased and its spread in temperature significantly reduced upon increasing the field. These results show the existence of magnetoelastic coupling, which favors the nucleation of those martensitic variants with the easy magnetization axis aligned with the field.

Coexisting ferro- and antiferromagnetism in Ni2MnAl Heusler alloys

The structural and magnetic properties of stoichiometric Ni2MnAl are studied to clarify the conditions for ferromagnetic and antiferromagnetic ordering claimed to occur in this compound. X-ray and magnetization measurements show that although a single phase B2 structure can be stabilized at room temperature, a single L21 phase is not readily stabilized, but rather a mixed L21+B2 state occurs. The mixed state incorporates ferromagnetic and antiferromagnetic parts for which close-lying Curie and a Néel temperatures can be identified from magnetization measurements.

Magnetic hardening mechanisms in Nd-Fe-B nanocrystalline material

Microstructural and magnetic measurements of the evolution by heat treatment of initially amorphous Nd16Fe76B8 alloys prepared by melt spinning are presented. Evidence of magnetic hardening above a threshold temperature induced by magnetic isolation of the Nd2Fe14B grains is provided. A thermodynamic and kinetic explanation of local melting of the intergranular nanostructured Nd¿rich eutectic phase at temperatures below 900 K based on capillary effects is presented. A subsequent Ostwald ripening process moves Nd to wet intimately the hard magnetic grains, becoming, on cooling, a real paramagnetic isolating thin film (~2.5 nm). By using a simple analogy, it is shown that the switching magnetization field in a single¿domain crystal can be drastically affected through the exchange coupling to neighboring grains with different orientation of the easy axis. This effect should be important enough to reinforce the coercive field of polycrystalline hard magnetic materials and explains the observed enhancement from 0.9 to 1.9 T.

Light induced defects in thermal annealed hydrogenated amorphous silicon

The metastable defects of a-Si:H samples annealed at temperatures in the 300-550°C range have been studied by photothermal deflection spectroscopy (PDS). The light-soaked samples show an increase in optical absorption in the 0.8 to 1.5 eV range. The metastable defect density decreases when the annealing temperature increases, while the defect density increases. This decrease in the metastable defect density shows an almost linear correlation with the decrease in the hydrogen content of the samples, determined by IR transmission spectroscopy and thermal desorption spectroscopy.

Analysis of the role of mobility-lifetime products in the performance of amorphous silicon p-i-n solar cells

An analytical model of an amorphous silicon p-i-n solar cell is presented to describe its photovoltaic behavior under short-circuit conditions. It has been developed from the analysis of numerical simulation results. These results reproduce the experimental illumination dependence of short-circuit resistance, which is the reciprocal slope of the I(V) curve at the short-circuit point. The recombination rate profiles show that recombination in the regions of charged defects near the p-i and i-n interfaces should not be overlooked. Based on the interpretation of the numerical solutions, we deduce analytical expressions for the recombination current and short-circuit resistance. These expressions are given as a function of an effective ¿¿ product, which depends on the intensity of illumination. We also study the effect of surface recombination with simple expressions that describe its influence on current loss and short-circuit resistance.

Magnetoelasticity and magnetoresistance in Cu-Al-Mn shape-memory alloys

We study the effect of a magnetic field on the martensitic transition of a Cu-Al-Mn shape-memory alloy. The martensitic transition has been studied through resistance measurements under applied magnetic fields ranging from 0 to 50 kOe. Negative magnetoresistance showing an almost linear dependence with the square of the magnetization has been observed. This magnetoresistive effect is associated with the existence of small ferromagnetic Mn-clusters. Its strength and thermal dependence is different in both phases. The martensitic transition temperature is slightly increased and its spread in temperature significantly reduced upon increasing the field. These results show the existence of magnetoelastic coupling, which favors the nucleation of those martensitic variants with the easy magnetization axis aligned with the field.