Influence of microstructure in the martensitic transformation and in the physical and magnetic properties in metamagnetic shape memory alloys

Resumen

Ferromagnetic shape memory alloys have raised substantial interest from both fundamental and applied points of view, due to the unique properties they show related to the occurrence of a first-order structural transformation – the martensitic transformation – between magnetically ordered phases. In particular, in Ni-Mn-Z (Z=In, Sn, and Sb) Heusler alloys, the so-called metamagnetic shape memory alloys, the transformation takes place between a ferromagnetic austenite and a weakly magnetic martensitic phase, in such a way that a large magnetization drop occurs at the martensitic transformation. This allows the induction of the martensitic transformation by an applied magnetic field, thus giving rise to multifunctional properties (namely giant magnetoresistance, magnetic shape memory effect and large inverse magnetocaloric effect) of great technical interest for practical applications, like sensing or magnetic refrigeration. The transformation temperatures, the magnetization of the different structural phases, the entropy change associated to the martensitic transformation and, in general, all the magnetostructural features giving rise to those functional properties depend on the composition, the structure and the microstructure of the alloys. The compositional dependence has been deeply studied in ferromagnetic shape memory alloys, being directly related to the electronic concentration. Variations in the composition like doping with different magnetic atoms (cobalt doping, in particular) have shown a strong influence in the magnetism of these materials. Also, the effect of the atomic order has been thoroughly analyzed in several previous works and shown to be a parameter of great influence to tune the magnetostructural properties of this kind of systems, although some metamagnetic shape memory alloys present an extraordinary high stability of the atomic order. The role of the microstructure, in turn, has been less investigated, in spite of its potential for tuning the magnetostructural properties of these systems, in particular as a suitable alternative in cases with highly stable atomic order. Therefore, in this work we have focused in the study of the influence of the microstructure in metamagnetic shape memory alloys (Ni-Mn-In and Ni-Mn-Sn systems) together with the effect of Co-doping, in order to gain insight into these effects and to control and improve the properties of these materials. Ni-Mn-In and Ni-Mn-Sn ternary alloys, and the corresponding quaternary (Co-doped) alloys have been prepared and subsequently subjected to thermo-mechanical treatments (hand milling, ball milling, and thermal annealing at different temperatures) in order to modify their microstructure in a controlled way. The alloys have been characterized macroscopically by calorimetric and magnetic measurements, and studied at the microscopic level mainly through X-ray and neutron diffraction, which provided us information about the crystal structures, the atomic order in the alloys, the microstructural parameters as grain size and internal strains and the magnetic structures. The combination of the results obtained in the microscopic study with the calorimetric and magnetic macroscopic characterization allowed us to gain a better understanding of the role of the microstructure and cobalt doping in the magnetostructural properties of these metamagnetic shape memory alloys and identify routes to obtain improved functional properties. Starting with the ternary Ni-Mn-Sn system, we have stablished the correlation between microstructural parameters and magnetostructural properties in these alloys. We have characterized the different microstructural states induced by thermo-mechanical treatments and correlated them with the properties of the alloys (in particular, the magneto-caloric effect). First, we have used laboratory X-rays diffraction to obtain the microstructural parameters of a Ni50Mn35Sn15 alloy subjected to different thermal treatments after hand-milling, and we have related the defects on the sample with the internal strains state induced by the milling and annealing processes. Then, powder neutron diffraction has been used to determine the crystal and magnetic structures of the alloys, allowing relating the magnetocaloric effect with the magnetic coupling. Finally, the magnetic characterization of the set of samples was completed by Mössbauer spectroscopy, showing also the suitability of this technique for microstructural characterization. The cobalt-doped quaternary system, Ni-Co-Mn-Sn-Co has been the next subject of study. We have determined the magnetostructural properties in milled and in annealed samples of the in Ni45Co5Mn35Sn15 alloy, showing an improvement of the magnetostructural properties by Co addition, related with a change in the magnetic coupling of the Mn atoms, and an unconventional improved magnetocaloric effect in a soft-milled alloy. Therefore, the presence of a small amount of defects affects in a different way to the ternary and quaternary alloys: while in the Ni-Mn-Sn system the introduction of defects such as vacancies or antiphase boundaries reduces the entropy change associated to the martensitic transformation, the effect is opposite in Ni-Co-Mn-Sn. This unusual result incited further investigation of the effect of milling, with a systematic study on the microstructure by synchrotron X-rays powder diffraction in ball-milled samples, showing the evolution of the microstructural parameters and the magnetostructural properties with the milling time. The increase of the milling time produces amorphization of the material and induces the martensitic phase, reducing in this way the amount of austenite with martensitic transformation and decreasing the enthalpy associated to the transition. Moreover, neutron powder diffraction has shown that the magnetic structure in austenite remains ferromagnetic upon milling but with a significant decrease of the ordered magnetic moments. With the aim of obtaining further understanding of the effect of Cobalt doping the magnetic coupling, we have selected the Ni-Mn-In system as case study. Ni50Mn34In16 and Ni45Co5Mn37In13 alloys with two different thermal treatments were macroscopically characterized by magnetometry and calorimetry measurements and the atomic order and magnetic structures studied by neutron diffraction. It has been shown the variation in the Curie and martensitic transformation temperatures due to the different thermal treatments and Co doping and, more relevantly, the increase produced in the saturation magnetization of the austenite phase, reaching ca. 60% when Co-doping and thermal treatment by slow cooling are combined. As shown by neutron diffraction in austenite phase, slow cooling thermal treatments produce a higher degree of atomic order, together with a reduction of strains and defects, which cause an increase in the total ordered magnetic moment and a slight enhancement of the ferromagnetic coupling, while cobalt doping has a stronger effect in increasing the ferromagnetic coupling, which explains the noticeable effect in the magnetization. The spin density maps obtained from polarized neutron diffraction have revealed the magnetic interaction pathways responsible for this coupling scheme. Since in the systematic study of the effect of milling we have observed that long milling times lead to amorphous states, we have extended our study to the recrystallization processes in Ni-Co-Mn-Sn and Ni-Co-Mn-In alloys, with the analysis of the evolution of the different phases, the cell parameters and microstrutural parameters as grain size, micro and macrostrains.