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Ultrapure metallic materials
Janatová, Marcela ; Kalbáčová Vejpravová, Jana (advisor) ; Prchal, Jiří (referee)
This thesis deals with crystal structure and magnetic properties of RCr2Si2 (R = Pr, Tb, Er) and RCr2Si2C (R = La, Ce, Pr, Nd) compounds. All prepared samples were characterized by powder X-ray diraction and investigated by specic-heat, magnetization and AC-susceptibility measurements in the temperature range 2-900 K and in magnetic elds up to 9 T. The results suggest non-trivial magnetic ordering of the Cr sublattice in all studied compounds, accompanied with ferromagnetic ordering of the Pr and Nd magnetic moments below the Curie temperature of 30 and 21 K in PrCr2Si2C and NdCr2Si2C, respectively, and possible metamagnetism in CeCr2Si2C. For RCr2Si2C compounds, the experimental evidence of Cr magnetism is corroborated by results of extended rst-principles calculations based on the density functional theory.
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Influence of the external pressure on the magnetic behavior of the material
Valenta, Jaroslav ; Prchal, Jiří (advisor) ; Prokleška, Jan (referee)
This thesis deals with magnetic properties of the HoCo2 compound, which is composed of rare earth and transition metal elements. The study is focused on properties at ambient pressure and changes of the magnetic behavior of the material under hydrostatic pressure. At the ambient pressure, there was performed analysis of the sample quality with a finding of presence of three magnetic transitions: flipping temperature magnetization of Co clusters Tf = (125 ± 1) K, Curie temperature TC = (79,5 ± 0,1) K and spin-reorientation temperature TR = (16,4 ± 0,5) K. When applying the hydrostatic pressure up to 3 GPa, there was observed a shift of the temperatures of the magnetic transitions and in the case Tf and TR their consequent vanishing. From the shifts of the TC and Tf towards lower temperatures and from the vanishing of the Tf, it was concluded that Co magnetism is being suppressed by the influence of the hydrostatic pressure.
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Structure discontinuity in the RTX hexagonal materials
Rusňák, Martin ; Prchal, Jiří (advisor) ; Javorský, Pavel (referee)
As it was discovered earlier the crystal lattice of ternary hexagonal RTX compounds exhibits a criticality for specific values of the ratio of the lattice parameters c/a around the interval of 0.565 - 0.575. To explore this effect in more detail, we have prepared polycrystalline samples of Ce1-xYxPdAl and SmPdAl and monocrystalline SmPdAl, SmNiAl. In case of Ce1-xYxPdAl series the forbidden range of c/a values was found to evolve with composition and temperature. Structural transformation is temperature independent in these compounds. SmPdAl compound is single-phased in the whole temperature range even though the c/a ratio crosses the gap of forbidden values common for other compounds. However a criticality in evolution of c/a is present at temperature 500 K. Anomaly visible on magnetic and transport properties, which is connected with change of structure, was found at approximately the same temperature. Explanation based on enhanced thermal movement of the atoms is proposed. Besides, also magnetic measurements at low temperatures were performed. The magnetic ordering temperature Tc = 15.3 K and dominant antiferromagnetic order was found. Monocrystalline SmNiAl exhibits very similar behaviour to the SmPdAl.
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CeT2Al2 compounds with enhanced electron-phonon interaction - preparation and characterisation of single crystalline samples
Mičková, Zuzana ; Doležal, Petr (advisor) ; Prchal, Jiří (referee)
This thesis is focused on preparation and characterisation of CePd2Al2, which is an intermetallic, incongruently melting compound. Due to its incongruent nature a number of polycrystalline samples was made to find an ideal nominal stoichiometry for a precursor for Czochralski growth of a single crystalline sample. Seven polycrystalline samples of different nominal stoichiometries were prepared in mono- arc furnace and studied via electron microscopy, differential scanning calorimetry and four of the samples also by powder diffraction. The result of analyses showed a presence of a hexagonal phase of stoichiometry 16.6:33.3:50 in multiple of the samples. This phase crystallizes at temperature higher than the rest of the present phases. The hexagonal phase was found to not occur in four of polycrystalline as-cast samples, namely of stoichiometries 22:40.7:37.3, 22:41.1:36.9, 22:41.5:36.5 and 22:42.5:35.5 atomic percent. The most suitable stoichiometry of the precursor for Czochralski growth of single crystalline sample in tri-arc furnace was found to be stoichiometry 22:41.1:36.9.
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Experimental study of the electronic properties of the RT1-xT'xAl system
Prchal, Jiří
The RNiAl and RCuAl compounds belong to a large group of RTX compounds (R ~ rare-earth, T ~ transition metal, X ~ p-metal) crystallizing in the ZrNiAl-type hexagonal structure, space group P-62m (group No.189). Very interesting behaviour has been observed for TbNi1-xCuxAl system, what has been attributed to a change of mechanisms responsible for the magnetic ordering. In comparison to it, ErNi1-xCuxAl series represent a more complex system. Change of magnetocrystalline anizotropy between the boundary stoichiometric concentrations is involved additionally. ErNiAl is an antiferromagnet (AF) with magnetic moments aligned within the basal plane below 6.2 K, ErCuAl is a simple ferromagnet (F) with moments aligned along the c-axis below 6.8 K. While increasing parameter x, a dramatic 'jump' of lattice constants occurs between x = 0.5 and 0.6 in temperatures down to 5 K. The magnetization and heat capacity data indicate that the change from AF to F ordering occurs between x = 0.2 and x = 0.4. In the range 0.05 x 0.20 there is possible the coexistence of F and AF alignment, the samples in this region embody two phase transitions. This was for x = 0.05 confirmed by heat capacity measurements. The samples with x 0.4 exhibit ferromagnetic behaviour. The neutron diffraction measurements are desired for the final...
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Structural changes in intermetallic compounds
Doležal, Petr ; Prchal, Jiří (advisor)
This diploma thesis is focused on the study of the structural phase transitions in intermetallic compounds LaPd2(Al,Ga)2 by low-temperature x-ray powder diffraction and macroscopic measurements on single crystals. The results obtained from powder diffraction at room temperature confirm that the compounds crystallize in tetragonal CaBe2Ge2 structure type (space group P4/nmm). At low temperature all the compounds undergo the same structural phase transition by lowering the symmetry to the orthorhombic space group Cmma. Although the structural transformation is of first order, the transition is not-manifested in step-like change of the phase at a certain temperature, but occurs in a broader temperature interval. In this interval the different phases coexist and transform gradually from high- to low- temperature phase. After application of hydrostatic pressure the structural transition is shifted to higher temperatures. Despite that the low temperature structures are of the same type, there is a difference between compounds containing Ga and Al in the discontinuity of the lattice parameters resulting in a distinct anomaly in electrical resistivity. In pressures >0.6 GPa this characteristic anomaly on single crystalline CePd2Al2 is changed, indicating a pressure- induced change of the low-temperature...
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Influence of Fe substitutions on tranformation temperatures in Ni2MnGa alloys
Drastichová, Kristýna ; Klicpera, Milan (advisor) ; Prchal, Jiří (referee)
In this bachelor thesis we are interested in studying the properties of Ni2MnGa alloy and their development with the other element substitution, for example with iron. Overally, we were studying five samples with four different compositions described by the formula Ni50Mn 25Ga25-xFex where the concentration of iron was in the interval . Consequently we performed on the prepared single crystals an extensive characterization by the methods of x-ray and electron diffraction, differential scanning calorimetry and dilatometry. Special emphasis was devoted to measuring the magnetization with vibrational magnetometer. The obtained data enabled us to confirm the quality of the prepared single crystals and to observe a development of Ni2MnGa properties, especially the temperatures of the phase transformations.
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