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Magnetokrystalová anizotropie ve sloučeninách TbTX
Klicpera, Milan ; Javorský, Pavel (advisor) ; Prokleška, Jan (referee)
Title: The magnetocrystalline anizotropy in the TbTX compounds Author: Milan Klicpera Department: Department of Condensed Matter Physics Supervisor: doc. Mgr. Pavel Javorský, Dr. Supervisor's e-mail address: javor@mag.mff.cuni.cz Abstract: The subject of this work is the study of structural, magnetic and trans- port properties of the TbNi(Al,In) series to finding connections between magne- tocrystalline anisotropy and lattice parameters of the structure of the TbTX com- pounds. Polycrystalline TbNiAl1−xInx samples were prepared by melting. The phase and crystal structure analysis were provided on samples. We performed the measurements of the magnetization, susceptibility, specific heat, resistivity, low temperature X-ray diffraction and powder neutron diffraction. From the neutron diffraction data we refined lattice parameters and propagation vectors of the series. The main propagation is (000) and second weaker component has the propagation vector (1 2 0 1 2 ). The change of the magnetocrystalline anisotropy from uniaxial to planar type occurs for compounds with x between 0.4 and 0.5. Keywords: magnetization, X-ray and neutron diffraction, electrical resistivity.
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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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Pressure influence on the magnetic and transport properties of the f-electron compounds
Míšek, Martin ; Javorský, Pavel (advisor) ; Kamarád, Jiří (referee)
In this thesis, we describe the work with a new pressure cell, its implementation in Joint Laboratory for Magnetic Studies, together with testing measurements necessary for ensuring good environmental conditions control and trouble-free operation. We have chosen a single crystal of the CePtSn compound to be the first representative measurement. We measured electrical resistivity along crystallographic b-axis in conditions: T = 2-300 K, p = 0-2.5 GPa and magnetic field B = 0-14 T applied along b-axis. We observed only minimal pressure influence on temperatures of magnetic phase transitions, however interesting and rather unusual behavior in magnetic fields was revealed. Previous works reported two field-induced transitions at low temperatures and ambient pressure, with values of the critical fields BcLF ~ 4 T and BcHF ~ 11.5 T, accompanied with large magnetoresistence step of -30% and +10%, respectively. With application of pressure, we observed shift of the critical field of low-field transition BcLF to the higher fields, whereas the value of critical field of high-field transition BcHF was unaffected with applied pressure. At a p ~ 1.5 GPa the two transitions merged and only one step was observed. With pressure further increasing above 1.5 GPa, situation with two transitions, similar to the lower pressure...
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The development of magnetic behavior in the PrNi1-xCuxAl compounds
Fikáček, Jan ; Prchal, Jiří (referee) ; Javorský, Pavel (advisor)
In the present work we studied magnetic properties of the substituted PrNi1-xCuxAl series. A very interesting behavior had been reported in the previously studied systems with the same substitution, but with different rare earths contained, e.g. loss of long range order for certain concentrations of copper. We prepared polycrystalline samples of the PrNi1-xCuxAl (x = 0.1 - 0.9, 9 different stoichiometries) and measured them using X-ray diffraction, specific heat, magnetization and AC susceptibility. Additionally, powder neutron diffraction measurement on PrNi0.2Cu0.8Al sample was performed in ILL (Institut Laue Langevin) in Grenoble. Results of measurements on these compounds show on the presence of a long order in case of compounds with x = 0.1 - 0.4 below the ordering temperatures in the range of 3.4 - 5 K. Measurements of AC susceptibility and neutron diffraction revealed the loss of long range order in the rest of the series. In these compounds, the development of a spin glass state occurs below the temperatures between 3.4 and 4.5 K.
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Magnetic properties of Ce compounds studied by specific heat
Čermák, Petr ; Javorský, Pavel (advisor) ; Havela, Ladislav (referee)
Materials containing the 4f (rare earth) or 5f (actinides) exhibit a large variety of interesting physical properties. The Ce-based compounds have a special place among the rare-earth compounds. The Ce atom contains only a single f-electron that is responsible for the magnetic behavior. The 4f states in compounds with the heavy rare earths have a well localized character, whereas many Ce-based compounds are on the borderline between the localized and itinerant behavior. These compounds show large variety of the magnetic ground states what is a result of the competition between the long-range order of the RKKY type and the screening of the localized moments by conduction electrons. We observe nonmagnetic states with a mixed valence (between Ce3+ and Ce4+), metallic systems with a long-range order of the Ce moments (ferromagnetic, antiferromagnetic or more complex structures). To analyze the electronic properties, the heat capacity data, and namely their low-temperature part, play an indispensable role. This diploma work comprise the sample preparation of selected cerium compounds, their phase characteristics and the heat capacity measurements at low temperatures (0.4 - 300 K). The main part is focused on the data analysis and comparison with theoretical models.
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Generation of computer program for specific-heat analysis
Majer, Peter ; Ježek, Lukáš (referee) ; Javorský, Pavel (advisor)
This work is about to design and implement the speci c-heat analysis application. The application consists of two parts: the rst one provides for data manipulation, the second one plots a graph. The data manipulation part resembles a spreadsheet program - each column in a dataset stands for one measurement. Columns containing a temperature data could be used for a speci c-heat calculation. This calculation is based on this model functions: conduction electrons consideration, phonon contribution and Schottky contribution. It is also possible to arrange your own formulas by using basic mathematical operations (namely summation, substraction, multiplication and division). Fitting data to the selected model is also possible. Levenberg-Marquardt and simplex method are being used for it. Hamiltonian matrix used for energy levels calculations of magnetic ions in crystalline electric elds could also be calculated. The application is written in C# programming language.
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