Magnetic force microscopy (MFM) is a widely-used technique for measuring the local magnetic properties of a variety of materials. This method covers a large fi eld of applications ranging from fundamental research of micro-magnetic phenomena to industrial applications in the development of magnetic recording components. The image formation in MFM measurements is based on the magneto-static interaction of a sharp magnetic tip with the probed sample. Despite the fact that MFM is quite easy to perform, image interpretation remains challenging. This is due to the accurate characterization of the probing tip that is needed for a quantitative interpretation of the MFM data in terms of the local magnetic properties of the sample. This thesis examines the fabrication and utilization of special MFM probes based on single ferromagnetic nanoparticles as the magnetically interacting element. A magnetic probe that consists of a very small magnetic single-domain particle can be accurately described by a magnetic point dipole. Such a probe potentially yields an improved lateral resolution and a simplifi ed quantitative interpretation of MFM images compared to a standard thin-fi lm coated MFM tip. First of all, one part of this thesis examines the fabrication of suitable single-domain particles. In particular, this part is focussed on experiments concerning the protection of these particles from oxidation in ambient conditions. To this end, these ferromagnetic particles were coated with gold using the light-induced deposition of gold in a photoactive metal-salt solution. The chemical surface passivation of the particles by the gold coating was verifi ed using diff erent techniques (SQUID, MFM). In the next step, these particles were mechanically affi xed to a standard silicon tip of atomic force microscopy (AFM). The controlled attachment of a single particle, as well as the attachment of multiple particles to such a Si-AFM tip was demonstrated. Another part of the thesis examines the magnetic imaging with particle based probes in MFM experiments. A minimum of four cobalt particles affi xed to the tip was determined as the threshold for obtaining a reasonable MFM signal. It was possible to image the magnetic domain structure of a hard disk drive sample with these particle probes. Using a simulation of the MFM data, the orientation and the position of the e ffective tip dipole were determined. The e ffective dipole moment of the tip is found by a calibration experiment assuming a magnetic dipole-dipole interaction between the tip and another magnetic particle.

III-Mn-V dilute ferromagnetic semiconductors (DFSs) have been treated as a candidate material for semiconductor spintronics due to their intrinsic ferromagnetism mediated by holes. In this thesis, three different Mn doped III-V DFSs, (In,Mn)As, (Ga,Mn)As, and (Ga,Mn)P, have been produced by ion implantation and pulsed laser melting. The comparison of magnetic anisotropy, magnetization, Curie temperature, as well as the electrical property is performed between three different materials to understand the nature of hole-mediated ferromagnetism in DFSs. An in-plane magnetic easy axis is observed in (Ga,Mn)As and (Ga,Mn)P, while an out-of-plane magnetic easy axis is found in (In,Mn)As due to the contribution of different inner strain resulting from the lattice mismatch between the DFS layer and the corresponding substrate. Most importantly, the direct proof of interplay between localization and magnetism is provided by a systematic comparison between (Ga,Mn)As and (In,Mn)As. When the Mn concentration is increased in the regime of the insulator-metal transition, the long-range ferromagnetic coupling is gradually built up accompanied with the appearance of metallic features. The generation of long-range global ferromagnetism is strongly influenced by the p-d coupling between hole and Mn local spins: The global ferromagnetism (metallic feature) happens at lower Mn concentration in (In,Mn)As than in (Ga,Mn)As due to the stronger p-d coupling from the smaller lattice parameter of GaAs. Moreover, for the case of (Ga,Mn)P with the strongest p-d coupling in comparison with (In,Mn)As and in (Ga,Mn)As, the super-exchange model has to be considered, since metallic features does not appear at the highest obtainable Mn concentration. Through the comparison between three different DFS materials, our findings strongly advocate for the heterogeneous model of electronic states at the localization boundary and point to the crucial role of weakly localized holes in mediating efficient spin-spin interactions even on the insulator side of the insulator-metal transition at least for (In,Mn)As and in (Ga,Mn)As.

The present work gives an overview of the authors work in the field of electronic structure calculations. The main objective is to show how electronic structure methods in particular density functional theory (DFT) can be used for the description and interpretation of experimental results in order to enhance our understanding of physical and chemical properties of materials. The recently found superconductor MgB2 is an example where the electronic structure was the key to our understanding of the surprising properties of this material. The experimental confirmation of the predicted electronic structure from first principles calculations was very important for the acceptance of earlier theoretical suggestions. Molecular crystals build from magnetic clusters containing a few transition metal ions and organic ligands show fascinating magnetic properties at the nanoscale. DFT allows for the investigation of magnetic ordering and magnetic anisotropy energies. The magnetic anisotropy which results mainly from the spin-orbit coupling determines many of the properties which make the single molecule magnets interesting.

In this thesis, specific heat, magnetic susceptibility and resistivity studies on the iron-pnictide superconductors LiFeAs, NaFe1-xCoxAs, AFe2As2 (A = K, Ca, Ba), M1-xNaxFe2As2 (M = Ca, Ba), and Ca(Fe1-xCox)2Fe2As2 are presented, from which different intrinsic physical properties are resolved. The combined first-order spindensity wave/structural transition which occurs in the parent compounds of the 122 pnictide systems is shown to gradually shift to lower temperature for low doping levels. Upon higher doping, this transition is completely suppressed and simultaneously, superconductivity appears at lower temperature. In contrast, the phase diagram in Ca(Fe1-xCox)2Fe2As2 is shown to exhibit a pronounced region of coexistence of magnetism and superconductivity. Further important results reported in this work concern the electronic properties and superconducting-gap characteristics. In LiFeAs, the zero-field temperature dependence of the electronic specific heat can be well described by two s-wave gaps, whose magnitudes are in agreement with ARPES results. Our gap analysis in KFe2As2, Ca0.32Na0.68Fe2As2, and Ba0.65Na0.35Fe2As2 single crystals also implies the presence of two s-wave-like gaps. The magnetic phase diagram of LiFeAs and KFe2As2 for magnetic fields along both principal orientations has been constructed and an anisotropy of Hc2(T) of 3 and 5, respectively, has been obtained.

In my work, magnetic and structural transitions of three categories of compounds are investigated by density functional calculations under local spin density approximation (LSDA). The first compound is Rh2MnGe with full Heusler structure at ambient condition. However, the structure is unstable at T=0 according to our calculations. A more stable structure we found is tetragonal one with either extension or compression along c-axis. The electronic reason of this distortion is the band Jahn-Teller effect where the Jahn-Teller active states are 4d states of Rh which is accidently put at the Fermi level by spin splitting. Then, magnetic moment behavior under pressure in itinerant compounds is investigated in four cubic Laves phase compounds (YFe2, ZrFe2, HfFe2, and LuFe2). The magnetic spin moment is decreased under pressure. A magnetic collapse where the spin moment vanishes is predicted under pressure around 20 GPa for Zr and Hf compounds, 40 GPa for Y and Lu compounds. The behavior of the magnetic moment is the result of competition between magnetic exchange interactions and kinetic energy during the compression of the volume, as described by the Stoner model. The last material investigated is CoO using LDSA+U in order to describe the strong Coulombic interaction of the transition metal ion. The pressure induced a magnetic transition, which was discovered in experiments, is explained by the competition between ligand field splitting and exchange energy. The ligand field splitting is increased under pressure, and suppresses the intraatomic exchange. As a result, the spin state changes from high spin to low spin, and at last to nonmagnetic state.

2 Title: Interactive GeoGebra applets for Collection of Solved Problems in Physics Author: Bc. Ondřej Broža Department: Department of Physics Education Supervisor: RNDr. Zdeňka Koupilová, Ph.D., Department of Physics Education Abstract: The aim of the diploma thesis was to focused on designing of interactive applets and desiging of solution to physical problems in the field of electricity and magnetism for Electronic Collection of Solved Problems. The thesis is also focused on the useability of the interactive applets during the physics education. Within this thesis was totaly created 22 interactive applets, a description and questions for working with them and 13 solutions to physical problems. Another parts of the thesis are: manual how to design interactive applets in the GeoGebra system, manual how to integrate these interactive applets into Collection of Solved Problems and library of partial construction in GeoGebra. The last part of the thesis is about verification of usability of these interactive applets in physics education, which was performed via observation of the students during the work with the interactive applets and via feedback questionnaire. Keywords: physics, solved problems, GeoGebra, applets, electricity and magnetism

In this thesis the superconducting and magnetic phases of LiOH(Fe,Co)(Se,S), CuFeAs/CuFeSb, and LaFeP_1-xAs_xO - belonging to the 11, 111 and 1111 structural classes of iron-based arsenides and chalcogenides - are investigated by means of 57Fe Mössbauer spectroscopy and muon spin rotation/relaxation (μSR). Of major importance in this study is the application of high magnetic fields in Mössbauer spectroscopy to distinguish and characterize ferro- (FM) and antiferromagnetic (AFM) order. A user-friendly Mössbauer data analysis program was developed to provide suitable model functions not only for high field spectra, but relaxation spectra or parameter distributions in general. In LaFeP_1-xAs_xO the reconstruction of the Fermi surface is described by the vanishing of the Γ hole pocket with decreasing x. The continuous change of the orbital character and the covalency of the d-electrons is shown by Mössbauer spectroscopy. A novel antiferromagnetic phase with small magnetic moments of ~ 0.1 μ_B state is characterized. The superconducting order parameter is proven to continuously change from a nodal to a fully gapped s-wave like Fermi surface in the superconducting regime as a function of x, partially investigated on (O,F) substituted samples. LiOHFeSe is one of the novel intercalated FeSe compounds, showing strongly increased T_C = 43 K mainly due to increased interlayer spacing and resulting two-dimensionality of the Fermi surface. The primary interest of the samples of this thesis is the simultaneously observed ferromagnetism and superconductivity. The local probe techniques prove that superconducting sample volume gets replaced by ferromagnetic volume. Ferromagnetism arises from magnetic order with T_C = 10 K of secondary iron in the interlayer. The tendency of this system to show (Li,Fe) disorder is preserved upon (Se,S) substitution. However, superconductivity gets suppressed. The results of Mössbauer spectroscopy indicate that the systems tends to a secondary structural phase, where the local iron environment observed in pure FeS is absent. Moreover, two interlayer positions of the iron are identified. The absence of enhanced superconducting T_C in LiOHFeS thus is related to a structural instability. Also, in CuFeAs the role of secondary iron at the Cu position turns out to be decisive for the observed magnetic behaviour. As in LiOHFeSe, it orders ferromagnetically at T_C ~ 11 K and superimposes with the magnetic instability of the main iron site. It is shown that a small charge doping of 0.1e/Fe, which is expected from (Cu,Fe) disorder, is sufficient to switch the system between a paramagnetic and an AFM ground state. Both magnetic orders are indistinguishable, because the magnetic order parameters are strongly coupled. This coupling was observed in the structurally identical CuFeSb, where the magnetic order parameters of both iron sites scale perfectly. The magnetically unstable CuFeAs and the ferromagnetic CuFeSb can be classified according to the theory of As height driven magnetism, predicting a change from paramagnetism to AFM and finally FM with increasing As height.

No description available.

Die vorliegende Promotionsarbeit “Density functional study of the electronic and magnetic properties of selected transition metal complexes” beschäftigt sich mit dem Zusammenhang zwischen strukturellen Merkmalen sowie elektronischen und magnetischen Eigenschaften von Einzelmolekül-Magneten. Im Wesentlichen konnte dabei gezeigt werden, dass die magnetischen Eigenschaften sowohl von strukturellen Merkmalen als auch von den elektronischen Eigenschaften bestimmt werden. Des Weiteren ergab sich, dass verschiedene Kenngrößen der magnetischen Eigenschaften (im speziellen der magnetische Grundzustand S sowie die magnetische Anisotropie D) miteinander korreliert sind. Dies ist im Besonderen für eine mögliche Anwendung von Einzelmolekül-Magneten im Bereich der Datenspeicherung von Bedeutung.

Epitaxial growth is an important technique for the fabrication of film structures with good crystalline quality, e.g., monoatomic overlayers, multilayers, compound materials, and ordered alloys. Such epitaxially grown films are technologically important materials with, e.g., adjustable electronic, magnetic, and optical properties. In case of coherent or pseudomorphic epitaxy, the overlayer adapts the in-plane lattice parameters of the substrate, i.e., the overlayer is strained to match the lattice parameters parallel to the substrate surface (in-plane directions). Simultaneously, a relaxation of the film dimension perpendicular to the substrate-film interface occurs (out-of-plane direction). Thus, coherent epitaxy provides a method to put phases under strain, and it can stabilise a metastable state of the film material, if the substrate lattice matches this metastable structure. Bulk-like properties in thick overlayers, which adopt the body-centred tetragonal (BCT) crystal structure and which grow coherently on a suitable substrate with quadratic surface symmetry, are modelled by the epitaxial Bain path (EBP) in this thesis. The knowledge of the EBP allows to study properties of the overlayer as function of the substrate lattice parameter. In particular, strain effects on the film material, magnetic order in the overlayer, and the existence of possible metastable states are investigated by means of density functional theory (DFT) in the local spin density approximation (LSDA), and in the singular case of uranium, employing the generalised gradient approximation (GGA). Note that a symmetry property of the BCT structure states, that it is identical to the body-centred cubic (BCC) structure or the face-centred cubic (FCC) structure for definite ratios of the tetragonal lattice parameters. Our definition of the EBP has two, previously not considered consequences for EBPs in general: an EBP can be discontinuous, and the high symmetry cubic structures (FCC and BCC) need not be points on the EBP. Both cases occurred for several elements considered in this thesis. If, however, a cubic structure is a point on the EBP, then a symmetry property guarantees that the total energy along the EBP, E(a), is stationary at this cubic structure. We computed the EBPs of all transition metals (TMs), the post TMs Zn, Cd, and Hg, the alkaline earth metals Ca, Sr, and Ba, the lanthanides La and Lu, and the actinide U (35 elements were treated in total). For each element but Zr, Hg, and U, there are exactly two structures whose energies are minima on the EBP, and which exhibit neither in-plane nor out-of-plane stresses; for Zr, Hg, and U there are three minima each. All other states on the EBP exhibit in-plane stresses because they are a strained form of the stress-free structures. The possibility of metastability of these particular, stress-free structures, i.e., stabilisation of these structures without bonding to the substrate, was investigated by stability conditions based on linear elasticity theory (except for U). We predict that ten FCC structures and three BCT structures not known from the respective phase diagrams may be metastable. We studied the properties of ferromagnetic (FM) states on the EBP for the elements Fe, Co, and Ni, and moreover predict, that Mn, Ru, Os, and U order ferromagnetically for certain states of the EBP. The latter three elements are paramagnetic in their ground states. The onset of ferromagnetism in Os and U is not accompanied by a simultaneously fulfilled Stoner criterion. According to our results, antiferromagnetic order (with moment sequences up-down or up-up-down-down on successive (001) planes) is never more stable than FM order on any EBP for any element investigated. On the basis of our comprehensive results for all TMs, we analysed trends across each of the three TM series and similarities among the three series. We demonstrate, that the type of the EBP (a classification of extrema of E(a) by symmetry into types) follows a characteristic trend across each of the three TM series. We discuss exceptions (Mn, Fe, and Zr) to this trend. Another trend, identical for the three series, is found for the BCT­-FCC structural energy difference as function of the d-band filling (evaluated for BCT structures that define extrema of E(a)), which follows a similar trend as the well studied BCC­-FCC structural energy difference. Clear similarities among the three periods of elements are also reflected in the bulk moduli and in the elastic constants of the cubic or tetragonal structures, that define the global and local minima of E(a). The mentioned similarities suggest, that many properties which are associated with the EBPs of TMs, can be attributed to the occupation of the d-band, which is the most dominant feature of the electronic structure of TMs.