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Study of conditions of crystallization and of thermodynamic properties of selected intermetallic compounds of lanthanides
Štěpánová, Karla ; Svoboda, Pavel (advisor) ; Javorský, Pavel (referee)
In this diploma work, I present the results of structural and thermodynamic studies of selected RNi4Si (R - La, Pr, Nd, Sm, Gd, Er, Ho, Dy) compounds. Contrary to its parent compound RNi5, these compounds do not crystallize in the hexagonal structure (CaCu5-type, space group P6/mmm). Instead of this, the orthorhombic structure with the space group Cmmm was found. The phase transition temperatures were determined from the zero-field specific heat measurement. Apart of PrNi4Si all compounds order ferromagnetically below the transition temperature. The phase transition temperature dependence of these compounds is in very good agreement with the de Gennes factor. Specific heat was analysed using a sum of the electronic, phonon and magnetic contribution. The significant magnetic contribution was fully described by the Schottky formula for n-level system. The magnetic entropy was calculated and is approaching the expected free ion entropy. Specific heat measurement of NdNi4Si compound showed structure transition at higher temperatures. Anisotropic properties cannot be studied on a polycrystalline sample and further studies must be carried out on the single-crystal. Trials to prepare single-crystals confirmed an incongruency of these samples. The mozaicity of measured samples is very high having the hexagonal...
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The influence of substitutions on the magnetocalloric effect in selected rare-earth compounds
Kaštil, Jiří ; Javorský, Pavel (advisor) ; Svoboda, Pavel (referee)
The anisotropy of the magnetocaloric effect was studied on DyNiAl single crystal. Our data reveal a strong anisotropy of the magnetocaloric effect. The large effect occurs for field applied along the c-axis whereas the entropy change is small for the perpendicular field direction. The influence of substitution on the magnetocaloric effect was measured in the Gd(Co1-xRhx)2 compounds (where x was from 0.05 to 0.30) and Dy(Co1-xFex)2 compounds (where x was from 0.00 to 0.03). The substitution of Co by Rh leads to a decrease of the transition temperature, Tc, and increase of the maximum entropy change. The Tc increases but the maximum of entropy change quickly decreases in case of Fe substitution. The last studied systems were the Gd1-xTbx alloys with x in range from 0.00 to 0.40. The influence of the sample shape and orientation was studied. The thin ribbon oriented along the magnetic filed gave the highest value of the entropy change. The three measurement techniques were used: the heat capacity measurement, the magnetization measurement and the direct measurement of the temperature change.
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