Tesis doctoral de Nazmunnahar Mst —
Nowadays magnetic refrigeration which is based on the magnetocaloric effect (mce) is one of the more relevant topics in the scientific research owing to its very important technological applications, which derive from the attempts made to replace the gas refrigerating technology involving, among other transcendental aspects, a low impact in the environment and an expected higher energetic efficiency. Furthermore, new and improved magnetocaloric materials are one of the cornerstones in the development of room temperature magnetic refrigeration. Therefore, this ph.D work has mainly been concerned to develop a magnetic refrigerant with large magnetocaloric properties near room temperature, whereas in the other part, the aim is to develop a deep and systematic study of structural, magnetic and magnetocaloric properties of heusler alloy family of materials with compositions susceptible of exhibiting mce. Furthermore, the goal is to explore the concept of spin polarization of some selected alloys. For this purpose the present research work reports experimental studies on the phase transformation, microstructural, magnetocaloric properties and spin polarization of a novel series of off-stoichiometric heusler alloys co50mn30inxsn20-x( x ¿ 20), ni50mn50-xsnx (x ¿ 14), ni49mn51-xsnx ( x = 12 and 13) and stoichiometric alloys of x2mnsn (x = co, ni) and x2crsn( x= co, ni) in forms of bulk ingots and ribbons.The results obtained on these four alloy series under different selective experimental conditions in terms of structural, magnetic, magnetocaloric properties and spin polarization are presented in five different chapters from 3-7. Chapter one gives the general introduction about mce with their thermodynamical aspects, magnetic refrigeration and their materials, heusler alloys, review of the literature, spin polarization along with the aim and objective behind selecting this specific class of the work. Chapter 2 described details of experimental techniques of the alloy formation and sample preparations. Chapter 3 discussed about the structural and magnetic properties at high temperature co rich of co50mn30inxsn20-x (x ¿ 20) ribbon alloys. A very complex magnetostructural transformation occurs in the 800-900 k temperature regions and substitution of in ¿ sn in co rich co50mn30inxsn20-x (x ¿ 20) alloy series established l21 structural phase at room temperature. The structural phase transformation and magnetocaloric properties of ni rich ni50mn50-xsnx (x ¿ 14) andni49mn51-xsnx (x = 12 and 13) alloys are described in chapters 4 and 5 in forms ofbulk ingots and ribbons, respectively. A partial substitution of mn ¿ sn in ni richbulk and ribbon heusler alloys series causes martensitic phase transformation nearroom temperature and the effect of varying either ni/mn or mn/sn on the structuraltransition temperature was found to be the same. As large magnetic entropy change as(-) 9.70 j/kg k has been obtained in ribbon (x = 12.5) alloys at 282 k and (-) 6.23j/kg k at 310 k in bulk (x = 12.5) alloy. It has been seen that the ribbon alloys aremore significant than the bulk alloys. Chapter 6 described the effect of annealing onstructural, magnetic and magnetocaloric properties of ni50mn37.5sn12.5 andni49mn38sn13 alloys. It is found that the tm and ac t value slightly increased afterannealing. This increasing transformation temperatures ™ and ( ac t ) in contrast withthe melt spun ones, which would be ascribed to the change of mn-mn distanceinduced by the stress and structural relaxations. However, the maximum ¿sm valuedecrease after annealing. Influence of cr on the structural and magnetic properties andspin polarization of ni-mn-sn and co-mn-sn bulk rod and ribbon heusler alloys forhigh spin polarization are described in chapter 7. It is found that the substitution of crfor mn of these heusler alloys, drastically decrease the saturation magnetization andcurie temperature whereas it has conformed 100% spin polarization in co2crsn alloywhich could be numerous application for these materials in the field of spintronicsdevices. A summary of the work with important implications achieved in this work isreproduced in the last chapter 8 along with the future scope of the work in this series.
Datos académicos de la tesis doctoral «Structural, magnetic, magnetocaloric and spin transfer properties in new co- and ni-based bulk and ribbon heusler alloys«
- Título de la tesis: Structural, magnetic, magnetocaloric and spin transfer properties in new co- and ni-based bulk and ribbon heusler alloys
- Autor: Nazmunnahar Mst —
- Universidad: País vasco/euskal herriko unibertsitatea
- Fecha de lectura de la tesis: 15/07/2013
Dirección y tribunal
- Director de la tesis
- Juan Jose Del Val Altuna
- Tribunal
- Presidente del tribunal: blanca Hernando grande
- roland Grossinger — (vocal)
- Francisco Castro — (vocal)
- Juan José Suñol martínez (vocal)