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Simulations of the magnetic domains dynamics in antiferromagnetic CuMnAs
Pařízek, Vojtěch ; Němec, Petr (advisor) ; Pokorný, Vladislav (referee)
Antiferromagnetic materials are promising materials for implementation in spintronic memory devices. In contrast to the more well-known ferromagnetic materials, which are already used in magnetoresistive random access memory (MRAM) devices, they possess multiple advantages, such as no net magnetization and ultrafast dynamics. Antiferro- magnetic memories store information through the orientation of the antiferromagnetic ordering. The magnetoresistance of the materials could be used for the electrical readout of the antiferromagnetic structure. In recent experiments in an antiferromagnet CuMnAs after applying a series of electrical or optical pulses, a change in resistivity associated with a significant decrease in the size of antiferromagnetic domains was observed. This means that one is able to perform electrical or optical writing in antiferromagnets. The state persists for timescales that exceed the magnetic dynamics timescales by many orders of magnitude. Here, we present the findings of the antiferromagnetic domain dynamics simulations in CuMnAs, specifically focusing on the process of small domain relaxation leading to the formation of larger domains. The simulations were based on atomistic spin dynamics. For all temperatures, a growth of magnetic domains was detected. However, the size of the...
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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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The Analysis of impedance properties of the bridge in string instruments
Němec, Petr ; Nop, Patrik (referee) ; Jirásek, Ondřej (advisor)
It is a known fact that stringed instruments bridge makes an important part of "acoustic circuit" of their bodies. Bridge acts as an acoustic filter which has the biggest impact on 2 – 6 kHz band and high frequency bands. To find the connection between the impact of a specific acoustic and mechanical properties of a bridge and acoustical behavior of the entire instrument remains as a matter of further exploration. Aim of this thesis is to find the mentioned relations of a chosen materials in an accordance to their mechanical properties within a full range of materials with common bridge properties. The experiments are designed to determine mechanical and acoustic properties of chosen materials which are then used to fabricate bridges. Subsequently the acoustical behavior of instruments with chosen bridges is determined. Based upon these findings we propose specific solutions and suggest further research.
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Investigation of Fresnel rhomb
Jesenská, Eva ; Němec, Petr (advisor) ; Jakubisová, Eva (referee)
This bachelor thesis is devoted to the investigation of the polarization propertis of the Fresnel rhomb. First of all, we was interested in the quality of the polarized light which can be created using the Fresnel rhomb. In particular, we were checking the accuracy of the circular and linear polarization which is produced by Fresnel rhomb. Secondly, we tested the wavelength dependence of the quality of polarized light. We also investigated the dependence of the polarization state on the light angle of incidence. Finally, we compared the obtained results with those for the wave plates.
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Spatially- and time-resolved laser spectroscopy
Kuchařík, Jiří ; Němec, Petr (advisor) ; Preclíková, Jana (referee)
Speed is one of the most important characteristics of the majority of electro- nic and optoelectronic components. Ultrafast laser spectroscopy is therefore a very effective tool for studying the properties of materials suitable for their pro- duction. However, for more detailed information it is also necessary to achieve a high spatial resolution. In this bachelor thesis we deal with several experimental approaches which allow us to achieve a high spatial resolution in time-resolved experiments. In particular, we describe experimental setups using microscope ob- jectives and we also mention the use of transient grating technique. Furthermore, we present the results of our measurement of a time length of ultrashort laser pulses which are generated by the pulse laser Mai Tai HP and their expansion due to propagation through microscope objectives. 1
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Investigation of spin polarization in semiconductors by laser spectroscopy
Butkovičová, Dagmar ; Němec, Petr (advisor) ; Kadlec, Filip (referee)
This work is devoted to the investigation of a spin polarization in ferromagnetic semiconductor Ga1-xMnxAs with a broad nominal concentration of manganese ions using one method of the ultrafast laser spectroscopy - the time-resolved Kerr rotation. At first, the experimental setup was optimized for the investigation of the dynamics of spin polarized charge carriers in semiconductors which were photo-generated by circularly polarized laser pulses. It was observed that the measured signal is induced by spin-polarized electrons. Due to a small thickness of the investigated ferromagnetic films the measured signal probably monitored the dynamics of fotogenerated electrons in GaAs substrate. Nevertheless, the measured data show that the electron spin dynamics in the substrate is significantly influenced by proximity effect due to the deposited ferromagnetic layer.
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Investigation of transport phenomena in GaAs/AlGaAs based materials for measurements of the spin Hall effect
Darebník, Peter ; Němec, Petr (advisor) ; Reichlová, Helena (referee)
Title: Investigation of transport phenomena in GaAs/AlGaAs based materials for measurements of the spin Hall effect Author: Peter Darebník Department: Department of Chemical Physics and Optics Supervisor: doc. RNDr. Petr Němec, Ph.D. Abstract: The content of this thesis is the study of transport phenomena in semiconducting structures based on GaAs/AlGaAs. These heterostructures are nowadays widely used for the study of the spin Hall effect, which can be partly considered a transport phenomenon, and it is therefore important to know the transport characteristics of these structures. We will mainly be focusing on electron/hole mobility since this parameter characterizes the scattering rate in the studied material. Our goal is to determine how the mobility will change at different temperatures in a system, which dimensions are comparable to the mean free path of electrons. Part of this thesis is dedicated to theoretical description of the studied phenomena and to the description of manufacturing technology of aforementioned heterostructures. Keywords: electron, spin, Drude model, 2DEG, GaAs/AlGaAs heterostructures
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Constructions of Commutative Semirings and Radical Rings
Korbelář, Miroslav ; Kepka, Tomáš (advisor) ; Němec, Petr (referee) ; Příhoda, Pavel (referee)
In this dissertation we deal with constructive methods applied to the commutative semirings and commutative radical rings. In Chapter 2 we study the class S of the commutative subdirectly irreducible radical rings. We present a few constructive methods for them and using the reflection of the category of the commutative rings into the category of the commutative radical rings we derive a lot of examples of rings in S with various properties. We prove that a ring S 2 S is noetherian if and only if it is finite. We show partial results in the classification of factors of S modulo monoliths. In Chapter 3 we introduce, using the p-prime valuation for all primes p, a set of characteristic sequences that can be assign to every subsemiring of Q+. We find and classify all maximal subsemirings of positive rational numbers and show that every proper subsemiring of Q+ is contained in at least one of them. This results was published in [16]. In Chapter 4 we construct, using the approach from the Chapter 4, a new large subclass of the class CongSimp of all proper congruence-simple subsemirings of Q+, classify all the maximal elements of CongSimp and show that every element of CongSimp is contained in at least one of them. In Chapter 5 we find an equivalent condition under which is the semiring Q+[ ] C, 2 C, contained in...
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