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Topological insulators and magnetic order
Šebesta, Jakub ; Carva, Karel (advisor) ; Hamrle, Jaroslav (referee) ; Železný, Jakub (referee)
The occurrence of various defects can strongly influence the physical properties of real materials. Depending on their type, new outstanding features can arise, but also charac- teristic behavior might be lost. This thesis is dedicated to the impact of diverse disorder's presence on the physical behavior of three-dimensional topological insulators, namely, the well-known Bi2Se3 compound. Despite possessing a band gap in the bulk, these materials host unique metallic surface states protected by the time-reversal symmetry. Thus, the surface states are robust against perturbation, which respects the protecting symmetry. Breaking the symmetry, e.g. by the magnetic field, can open the surface gaps. In this work, the influence of the magnetic as well as native chemical defects, occurring in the real samples, on the electronic structure is described. Notably, the dispersion of the sur- face metallic states is followed up. Since the surface gap can be opened by the magnetic field, magnetic exchange interactions and related Curie temperatures of magnetic defects are thoroughly studied. Particularly, the influence of disorder on the character of the exchange interactions and related Curie temperatures is described. Dealing with layered systems, the behavior of planar defects might be inspected. Therefore, the...
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New magnetic state of intermetallics - parimagnetism - in RCo2-type compound
Šebesta, Jakub ; Prchal, Jiří (advisor) ; Prokleška, Jan (referee)
A sample of TmCo2 compound was prepared and studied by means of temperature dependence of an electric resistivity, specific heat, magnetisation and AC magnetic susceptibility, all in ambient pressure. We have observed anomalies in the temperature dependence of specific heat at the temperatures TC Tm = (3,60 ± 0,05) K and TC Co = (4,6 ± 0,1) K, which are in the agreement with anomalies in the data of other physical variables. By comparison with HoCo2 and ErCo2 compounds, there was made a suggestion, that the characteristic temperatures correspond to the magnetic order of the Tm and Co sub-lattices. After that, temperature dependencies of AC magnetic susceptibility under external hydrostatic pressures up to 3 GPa were measured. We observed anomalies mentioned above in the measured data and moreover one more anomaly around 35 K. This anomaly corresponds most probably to the flipping temperature Tf of the magnetisation of Co clusters through the plain perpendicular to the magnetic moment of the nearest rare-earth atom. From the set of the realized pressure points we estimated pressure coefficients for the characteristic temperatures ∂TC Tm /∂p = (0,40±0,02) K·GPa-1 and ∂Tf/∂p ~ 0 K·GPa-1, . Roughly zero values of coefficients are significant, because of comparison with other RCo2-type compound.
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Magnetism in the RCo2-type compounds
Šebesta, Jakub ; Prchal, Jiří (advisor) ; Prokleška, Jan (referee)
Magnetic properties of RCo2 compounds have been studied since the sec- ond half of the last century. However, there was recently observed new mag- netic state, so-called parimagnetism. The aim of this thesis was to find the real behavior of TmCo2, which exhibits many discrepancies in the published results, by means of preparation of high-quality single-crystals, its characteri- zation around the ordering temperature and to study parimagnetic behavior. Obtained results showed differences between the samples, which should be caused by the instability of Co and Tm magnetism. It might be the reason of discrepancies in the literature. The results however show the same struc- ture of measured curves with two present phase transition. We suggest two possible interpretations: a) magnetic reorientation of Tm magnetic moments below the ordering temperature of the Tm sub-lattice or b) consequent or- dering of the rare-earth sub-lattice followed by ordering of the Co sub-lattice at lower temperature, which seems to be more realistic. We observed pari- magnetic anomalies at 38 K, 80 K, 168 K only in the case of single-crystals containing small amount of impurity phase, which probably affects the for- mation of magnetic clusters. 1
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New magnetic state of intermetallics - parimagnetism - in RCo2-type compound
Šebesta, Jakub ; Prchal, Jiří (advisor) ; Prokleška, Jan (referee)
A sample of TmCo2 compound was prepared and studied by means of temperature dependence of an electric resistivity, specific heat, magnetisation and AC magnetic susceptibility, all in ambient pressure. We have observed anomalies in the temperature dependence of specific heat at the temperatures TC Tm = (3,60 ± 0,05) K and TC Co = (4,6 ± 0,1) K, which are in the agreement with anomalies in the data of other physical variables. By comparison with HoCo2 and ErCo2 compounds, there was made a suggestion, that the characteristic temperatures correspond to the magnetic order of the Tm and Co sub-lattices. After that, temperature dependencies of AC magnetic susceptibility under external hydrostatic pressures up to 3 GPa were measured. We observed anomalies mentioned above in the measured data and moreover one more anomaly around 35 K. This anomaly corresponds most probably to the flipping temperature Tf of the magnetisation of Co clusters through the plain perpendicular to the magnetic moment of the nearest rare-earth atom. From the set of the realized pressure points we estimated pressure coefficients for the characteristic temperatures ∂TC Tm /∂p = (0,40±0,02) K·GPa-1 and ∂Tf/∂p ~ 0 K·GPa-1, . Roughly zero values of coefficients are significant, because of comparison with other RCo2-type compound.
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