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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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Magneto-optical gradient effect imaging of magnetic textures
Molnár, Tomáš ; Hamrle, Jaroslav (referee) ; Arregi Uribeetxebarria, Jon Ander (advisor)
Základy magnetooptiky (MO), oboru zkoumajícího vliv magnetických polí a magnetizace na šíření světla v hmotě, byly položeny před více než půldruhým stoletím. Od té doby se MO stala jednou z nejpoužívanějších metod magnetického zobrazování v mikroměřítku. Kromě MO Faradayova a Kerrova jevu, které jsou lineární v závislosti na magnetizaci, byly později objeveny efekty kvadratické v závislosti na magnetizaci (Voigtův nebo Cottonův-Moutonův jev) nebo závislé na gradientech magnetizace (gradientní jev). Zejména MO gradientní jev byl objeven jako poslední, ale protože zdobí pouze hranice magnetických domén, nepřitahoval zatím takový zájem jako Kerrův jev pro magnetické zobrazování. Tato práce zkoumá užitečnost magnetooptického gradientního jevu pro charakterizaci spinových konfigurací v nanorozměrech, jako jsou doménové stěny v kolmo zmagnetovaných materiálech. Představujeme nový experimentální přístup k odhalení vlastností MO gradientního jevu využitím jeho symetrie vzhledem k polarizaci světla. Dále analyticky vyjadřujeme odpovídající signál pomocí dostupného teoretického modelu. Porovnáním simulovaných signálů MO gradientního jevu získaných metodou přenosové matice a experimentálně získaných signálů zkoumáme možnost charakterizovat vnitřní spinovou strukturu magnetických doménových stěn v nanorozměrech.
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Magneto-optical Kerr magnetometer with variable light wavelength
Plaskurová, Beáta ; Zázvorka, Jakub (advisor) ; Hamrle, Jaroslav (referee)
This bachelor thesis deals with methods for measuring hysteresis loops. We have successfully constructed three different magneto-optical Kerr magnetometers using two light sources of different wavelengths. For each method, we measured the Kerr effect, specifically Kerr rotation, at different settings of several parameters. We found that these settings indeed affect the accuracy of the measurements. We determined the measurement error for each setting of a given method. Then we were able to decide which methods of measuring Kerr rotation are appropriate, and which are not.
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Laser spectroscopy of magnetically-ordered materials
Wohlrath, Vladislav ; Němec, Petr (advisor) ; Hamrle, Jaroslav (referee)
For a relatively long time, a new experimental technique for the study of magnetic materials by magneto-optical effects quadratic in sample magnetization, like Voigt effect, is being developed at Laboratory of OptoSpintronics. Thanks to the used experimental geometry, in our setup, unlike in different methods, it is possible to use a cryostat. Thanks to this the investigated samples can be studied both at low and high temperatures. In this master thesis we identified and eventually solved several problems that prevented the practical utilization of this technique in the past. We successfully demonstrated the appli- cation of this method both in transmission and reflection geometries using ferromagnetic samples of CoFe and FeRh. Our measurements revealed that the coefficient describing the quadratic magneto-optical response can be strongly anisotropic with a wavelength- -dependent magnitude and sign. This observation has strong consequences for the design and/or interpretation of experiments based on quadratic magneto-optics, which are gai- ning an increasing attention nowadays thanks to the growing interest in antiferromagnetic spintronics. 1
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Magneto-optical magnetometry of ferromagnetic thin films with flexible substrate
Střihavková, Julie ; Schmoranzerová, Eva (advisor) ; Hamrle, Jaroslav (referee)
This work is focused on measurements of magnetooptical effects in thin magnetic multilayers prepared on a flexible substrate. A main goal of this thesis was to deter- mine ideal conditions of the experiment, suitable for studying this type of sample with a magnetooptical method. In the measurements were determined artifacts caused both by the experimental setup and by effect of the flexible substrate on the polarization state of transmitted light. An approximate value of the magnetic anisotropy of the ferromagnetic layer (Co), connected with apresence of strain in the system, was determined using a ROT-MOKE method. The calculated values of the anisotropy field were burdened with a significant measurement error. We can thus conclude that the magnetooptical method can be used for the study of the changes of magnetic properties (i.e. thermally induced strain). However, regarding determination of the exact values of the magnetic anisotropy, the method has its limitations. 1
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Ultrafast laser spectroscopy of antiferromagnets
Saidl, Vít ; Němec, Petr (advisor) ; Kužel, Petr (referee) ; Hamrle, Jaroslav (referee)
This work is dedicated to the study of two antiferromagnetic materials that are suitable for use in spintronic devices. In series of FeRh samples we studied the transition temperature between the antiferromagnetic and ferromagnetic phases. We developed a method based on material optical response for a quick determination of this temperature, which enabled us to study with a spatial resolution of 1 μm a magnetic inhomogeneity of prepared samples.We also developed a method for a determination of the Néel temperature and the magnetization easy axis position in thin films prepared from compensated antiferromagnetic metal. We successfully applied this method on an uniaxial sample of CuMnAs and we discussed its applicability for a research of samples with a biaxial magnetic anisotropy.
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Investigation of spin structure and dynamics in magnetically ordered thin films
Janda, Tomáš ; Němec, Petr (advisor) ; Hamrle, Jaroslav (referee) ; Reiss, Günter (referee)
Title: Investigation of spin structure and dynamics in magnetically ordered thin films Author: Tomáš Janda Abstract: This thesis is devoted to the development of methods for imaging and ultrafast manipulation of magnetic textures, such as magnetic domains and domain walls (DWs), and to the investigation of the corresponding magnetiza- tion dynamics. We focused on techniques that are, besides ferromagnets (FMs), applicable also to antiferromagnets (AFs), in particular, AF thin films. The employed excitation techniques were mostly based on direct or indirect effects of ultrashort laser pulses. We studied the DW motion induced by a transfer of angular momentum from circularly polarized femtosecond laser pulses in a FM GaMnAsP and we found that the observed macroscopic DW displacement is only possible due to its inertia. In a distinct experiment, picosecond current pulses were generated by an absorption of the ultrashort laser pulses in a verti- cal PIN diode-based photoconductive switch and used to excite a magnetic film deposited directly on top of the diode. For imaging of AF domain patterns, we developed a simple table-top laser-scanning technique, which is based on a magneto-thermoelectric response of the AF textures in the presence of a local laser-induced heating. We first used the method to image the...
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Investigation of spin dynamics in hybrid structures based on ferromagnetic semi-conductor (Ga,Mn)As
Butkovičová, Dagmar ; Němec, Petr (advisor) ; Hamrle, Jaroslav (referee) ; Žídek, Karel (referee)
Investigation of spin dynamics in hybrid structures based on ferromagnetic semi- conductor (Ga,Mn)As Abstract: This dissertation deals with the study of hybrid ferromagnet/semiconductor structures, which are of particular for spintronics. We focused on heterostructures that contain ferromagnetic semiconductor (Ga,Mn)As, which is the most studied model ma- terial from the group of diluted magnetic semiconductors. The main goal of this work was a detailed study of the Optical Spin Transfer Torque (OSTT) phenomenon, which is an optical equivalent of the STT effect, which is used in ferromagnetic metal layers for non-thermal switching of the direction of magnetization. In the first part of the work, we describe experiments aimed at achieving non-thermal control of the direction of mag- netization in (Ga,Mn)As with the contribution of control of magnetic anisotropy using mechanical strain induced by a piezo-transducer (PZT) in hybrid structure (Ga,Mn)As/GaAs/PZT. For this purpose, the preparation of the structure was first opti- mized, which was tested in detail by means of X-ray diffraction and magneto-optical methods. However, we were unable to achieve magnetization switching due to the OSTT phenomenon. In addition, we found that the results measured at low temperature are very poorly reproducible, despite...
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Investigation of spin structure and dynamics in magnetically ordered thin films
Janda, Tomáš ; Němec, Petr (advisor) ; Hamrle, Jaroslav (referee) ; Reiss, Günter (referee)
Title: Investigation of spin structure and dynamics in magnetically ordered thin films Author: Tomáš Janda Abstract: This thesis is devoted to the development of methods for imaging and ultrafast manipulation of magnetic textures, such as magnetic domains and domain walls (DWs), and to the investigation of the corresponding magnetiza- tion dynamics. We focused on techniques that are, besides ferromagnets (FMs), applicable also to antiferromagnets (AFs), in particular, AF thin films. The employed excitation techniques were mostly based on direct or indirect effects of ultrashort laser pulses. We studied the DW motion induced by a transfer of angular momentum from circularly polarized femtosecond laser pulses in a FM GaMnAsP and we found that the observed macroscopic DW displacement is only possible due to its inertia. In a distinct experiment, picosecond current pulses were generated by an absorption of the ultrashort laser pulses in a verti- cal PIN diode-based photoconductive switch and used to excite a magnetic film deposited directly on top of the diode. For imaging of AF domain patterns, we developed a simple table-top laser-scanning technique, which is based on a magneto-thermoelectric response of the AF textures in the presence of a local laser-induced heating. We first used the method to image the...
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Optical and magneto-optical properties of thin films of topological insulator Bi2Se3
Jambrich, Jakub ; Veis, Martin (advisor) ; Hamrle, Jaroslav (referee)
Topological insulators have attracted intensive research interest in the field of material research in the last two decades. Their main advantage is that metallic conductive states are formed on their surface, in which the electrons can move as if their effective mass is nearly zero. This feature has huge potential for use in the new generation of high-speed electronics. In this work, we investigate the optical and magneto-optical properties of the topological insulator Bi2Se3, which are key to its further potential use. By measuring the absorption at Landau levels in a high magnetic field, we determine the shape of the dispersion of the electronic structure as well as the bandgap. Subsequently, by spectroscopic ellipsometry in the range of the visible and near-ultraviolet spectrum, we measure the energy dependence of complex relative permittivity, which is the key for the description of optical properties.
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