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Exchange bias in metamagnetic heterostructures
Zadorozhnii, Oleksii ; Schneeweiss, Oldřich (oponent) ; Uhlíř, Vojtěch (vedoucí práce)
Exchange bias is an intriguing physical phenomenon occuring at the interface of antiferromagnet (AF) and ferromagnet (FM) materials, which has already been widely applied in electronics and magnetic recording industry. Despite being intensely studied for a long time, the exact mechanism behind it remains an unsettled matter. This work presents an overview of the relevant studies documenting exchange bias in thin film bilayer systems, including both experimental evidence and theoretical models developed. The experimental tasks of this diploma thesis covered both manufacturing and measurement of different exchange bias model systems. An Fe/FeRh bilayer (here the FeRh layer features a phase transition from AF to FM at 360K), provides convenient tunability of the exchange bias. Next, the exchange bias and shape anisotropy effects were investigated in Fe/FeRh microstructures. Lastly, the presence of exchange bias was investigated between the coexisting FM and AF phases in submicron FeRh nanowires. The samples were fabricated using magnetron sputtering and E-beam lithography. All the presented systems were analyzed using Magneto-Optical Kerr Effect microscopy. Exchange bias was successfully found in the Fe/FeRh system nearly identical in magnitude and orientation to the results in literature, having an inferior FM-AF interface quality. Training effect as well as rotational asymmetry were also proven to exist within this system, solidifying the presence of exchange bias. In nanowires, significant exchange bias was measured between the coexisting FM and AF phases during cooling from the FM phase to the AF phase.
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Atomární struktura povrchu FeRh(001)
Ondračka, Václav ; Bábor, Petr (oponent) ; Čechal, Jan (vedoucí práce)
Slitina železo-rhodia, Fe50Rh50, vykazuje nízkoteplotní metamagnetickou přeměnu z antiferomagnetické na feromagnetickou fázi. Detailní popis této přeměny pomocí nástrojů povrchové analýzy nebyl dosud proveden. Tato bakalářská práce se věnuje přípravě a charakterizaci čistého povrchu slitiny pomocí nástrojů rentgenové fotoelektronové spektroskopie a nízkoenergiové elektronové difrakce. Je provedena korekce obrazu a kvantifikace měření difrakce pomocí kalibrace vzorkem známé krystalové mřížky.
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Effect of ion beam irradiation and annealing on magnetic properties of FeRh nanostructures
Zadorozhnii, Oleksii ; Turčan, Igor (oponent) ; Staňo, Michal (vedoucí práce)
The first order phase transition from antiferromagnetic to ferromagnetic state in Fe50Rh50 at 370 K make it a suitable material for next generation spin electronic devices with a low power consumption. This work deals with the ways how the phase transition temperature of iron-rhodium (FeRh) can be tuned locally in thin films, using focused ion beam (FIB) and thermal annealing. FIB irradiation approach was chosen due to the fact that FeRh displays magnetic sensitivity to the degree of its chemical ordering, which is characteristic to all alloys of ferromagnetic and non-ferromagnetic metals. Thermal annealing enables the relaxation of the structure and restoration of its crystallinity. The magnetic patterns were manufactured using gallium-based FIB and annealed under ultra high vacuum. The topography as well as magnetic behaviour of these ion irradiated patterns were investigated using atomic and magnetic force microscopies at different temperatures, showing a clear dependence between ion irradiation dose and the magnetic response in pre- and post-annealed states.
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Korelovaná sondová a elektronová mikroskopie pro studium moderních magnetických nanomateriálů
Novotný, Ondřej ; Flajšman, Lukáš (oponent) ; Pavera, Michal (vedoucí práce)
Silný tlak na vývoj nových magnetických materiálů a jejich miniaturizaci, klade také důraz na vývoj nových analytických technik. Tato diplomová práce se zabývá vývojem a demonstrací korelované mikroskopie magnetických sil a elektronové mikroskopie, která je slibným nástrojem pro analýzu magnetických nanomateriálů. První část práce je věnována popisu fyzikálních základů mikromagnetismu se zaměřením na cylindrická nanovlákna. Na následujících stranách jsou popsány světelné, sondové, elektronové a synchrotronové metody pro mapování magnetických vlastností materiálů. V další části je popsán výzkum pohybu magnetických doménových stěn v cylindrických nanovláknech, který byl prováděný jako součást rozsáhlejší materiálové studie. Poslední část práce je věnována popisu vývoje korelované mikroskopie magnetických sil a elektronové mikroskopie na zařízení LiteScope. V rámci vývoje byl navržen a úspěšně otestován postup výroby magnetických sond. K výrobě sond bylo využito depozice indukované fokusovaným elektronovým svazkem a prekurzoru Co2(CO)8. V závěru práce je demonstrována vyvinutá technika na vícevrstevnatém Pt/Co vzorku, CoNi cylindrických nanovláknech, NiFe mikrostrukturách a FeRh metamagnetických nanoostrůvcích.
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Imaging of metamagnetic thin films using TEM
Hajduček, Jan ; Buršík,, Jiří (oponent) ; Uhlíř, Vojtěch (vedoucí práce)
Complex magnetic materials at the nanoscale are essential in many areas of modern devices, such as digital memories or sensors. Novel technological approaches require the control and understanding of modern magnetic materials down to the atomic scale. One possibility is to exploit high-resolution transmission electron microscopy (TEM), characteristic for its outstanding subatomic resolution. This thesis investigates the options of TEM imaging of metamagnetic materials. These materials are characteristic by displaying coexistence of magnetic phases upon external control. Thin films of metamagnetic FeRh are used as an experimental platform to investigate the various aspects of TEM imaging. FeRh undergoes the metamagnetic phase transition from antiferromagnetic to ferromagnetic phase upon heating. We start with evaluating the sample fabrication processes suitable for our system, which is essential for successful TEM analysis. The differential phase contrast (DPC) technique in TEM is used for the magnetic analysis due to its direct access to the sample magnetic field configuration. An in-depth discussion of DPC signal formation is presented, which is crucial for understanding and analysis of resulting images. Furthermore, we perform structural, chemical, and particularly magnetic imaging of both magnetic phases present in FeRh. Finally, the process of in-situ heating of metamagnetic FeRh lamellae is presented.
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Magnetic properties of self-assembled FeRh nanomagnets
Motyčková, Lucie ; Fruchart, Olivier (oponent) ; Arregi Uribeetxebarria, Jon Ander (vedoucí práce)
Magnetic nanoparticles and nanostructured materials are of great promise in many domains, including biomedicine, environmental remediation, or energy harvesting. Therefore, there is an ever-growing interest in their unique nanoscale functionalities as well as in the development of viable fabrication routes. This thesis investigates a self-assembly route, specifically solid-state dewetting of thin films, to produce epitaxial nanoisland arrays of the FeRh alloy on different single-crystal substrates. It is shown that using this fabrication route, the metamagnetic phase transition is preserved in nanoscale confined geometries. The morphology and magnetic properties of the self-assembled FeRh nanomagnets are characterized by a combination of experimental techniques and modeling, finding that their equilibrium shapes and magnetic order are closely interconnected. Furthermore, a route for obtaining free-standing nanoparticles is devised, which could potentially allow using metamagnetic nano-objects in cell cultures and biomedicine in general. To do so, the supported FeRh nanomagnets are released from the substrate via chemical wet etching. The behavior of the nanoparticles and their response to temperature cycling and magnetic field is subsequently studied in a liquid environment. The metamagnetic properties of separated nanoparticles are characterized using vibrating sample magnetometry.
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Magneto-optical imaging and analysis of magnetic domain microstructures
Molnár, Tomáš ; Wojewoda, Ondřej (oponent) ; Arregi Uribeetxebarria, Jon Ander (vedoucí práce)
Magneto-optical effects are associated with the interaction of polarized light with magnetized media. Although discovered nearly two centuries ago, magneto-optics is a widely utilized tool for magnetic characterization due to its high sensitivity and versatility. For example, it enables visualization of magnetic domain structures in microscale magnetic systems via wide-field magneto-optical microscopy. The majority of research on this topic has employed linear effects in magnetization to image domains in ferromagnetic materials, where one measures an optical contrast for regions with opposite magnetization orientations. It has been recently shown that even antiferromagnetic materials can be studied using quadratic magneto-optical effects (Voigt effect), making it possible to visualize regions with different spin axis orientations. In this work, we perform a fundamental magneto-optical microscopy study of magnetic thin film and microstructure systems using linear and quadratic effects. Consequently, the microscale magnetization configuration of continuous and patterned magnetic thin films is interpreted via the image processing strategy developed in this thesis, leading to quantitative vector magnetometry. Moreover, the coexistence of the ferro- and antiferromagnetic phase domains in FeRh films was investigated using optical and magneto-optical microscopy.
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Magneto-optical investigation of spintronic materials
Vojáček, Libor ; Flajšman, Lukáš (oponent) ; Arregi Uribeetxebarria, Jon Ander (vedoucí práce)
The magneto-optical Kerr effect is a useful tool for accessing the magnetic properties of metallic surfaces. The magnetization reversal process controlled by an external applied field and magneto-optically measured can give us information about the anisotropic properties of magnetic systems. In the presented work, we review the theoretical aspects of magnetization reversal and the changes of light polarization upon reflection from a magnetized medium. Description of a functional, high-sensitivity magneto-optical setup based on the polarization modulation technique is given. The apparatus was assembled by modifying an already existing similar setup. Measurements on thin-film magnetic layers prove its functionality and reveal the properties of metamagnetic thin films from FeRh, which is a material with potential applications in spintronics.
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Epitaxial growth and characterization of metamagnetic nanoparticles for biomedical applications
Motyčková, Lucie ; Gröger,, Roman (oponent) ; Arregi Uribeetxebarria, Jon Ander (vedoucí práce)
Magnetic nanoparticles represent a promising platform for a vast number of biomedical applications in continuously developing domains of diagnostics and therapeutics. Novel magnetic nanoscale technologies based on metamagnetic materials may provide significant benefits, for instance, in terms of easily controllable actions on biological species in the human body. This thesis investigates the growth mechanisms and magnetic properties of supported nanoparticles made of the iron-rhodium (FeRh) alloy on the MgO substrate. The FeRh compound was chosen for its specific transition from the antiferromagnetic to ferromagnetic phase occurring slightly above room temperature, thus allowing the control of magnetic properties of nanoparticles in the temperature range close to the human body. The presented nanostructures have been fabricated via magnetron sputtering using the bottom up preparation approach. The morphology and magnetic behavior of such deposited nanostructures have been investigated via atomic and magnetic force microscopy, which provide spatially resolved antiferromagnetic and ferromagnetic domain structure in the individual nanoislands.
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Ultrarychlá laserová spektroskopie antiferomagnetů
Saidl, Vít ; Němec, Petr (vedoucí práce) ; Kužel, Petr (oponent) ; Hamrle, Jaroslav (oponent)
Tato práce se zabývá studiem dvou antiferomagnetických materiálů, které jsou principielně použitelné pro spintronické aplikace. V sérii vzorků FeRh jsme studovali teplotu přechodu mezi antiferomagnetickou a feromagnetickou fází. Vyvinuli jsme na optické odezvě založenou metodu na rychlé určení této teploty, která nám následně umožnila studovat s prostorovým rozlišením 1 μm magnetickou nehomogenity připravených vzorků. Dále jsme vyvinuli metodu pro nalezení Néelovy teploty a snadné osy magnetizace v tenkých filmech připravených z kovového kompenzovaného antiferomagnetu. Tuto metodu jsme úspěšně aplikovali na uniaxiální vzorek CuMnAs a diskutovali jsme její použitelnost i pro vzorky s biaxiální magnetickou anizotropií.
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