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This thesis describes the growth and characterization of epitaxial MnSi thin films on Si substrates. The interest in this material system stems from the rich magnetic phase diagram resulting from the noncentrosymmetric B20 crystal structure. Here neighboring spins prefer a tilted relative arrangement in contrast to ferro- and antiferromagnets, which leads to a helical ground state where crystal and spin helix chirality are linked [IEM+85]. This link makes the characterization and control of the crystal chirality the main goal of this thesis.
After a brief description of the material properties and applied methods, the thesis itself is divided into four main parts. In the first part the advancement of the MBE growth process of MnSi on Si\((111)\) substrate as well as the fundamental structural characterization are described. Here the improvement of the substrate interface by an adjusted substrate preparation process is demonstrated, which is the basis for well ordered flat MnSi layers. On this foundation the influence of Mn/Si flux ratio and substrate temperature on the MnSi layer growth is investigated via XRD and clear boundaries to identify the optimal growth conditions are determined. The nonstoichiometric phases outside of this optimal growth window are identified as HMS and Mn\(_5\)Si\(_3\).
Additionally, a regime at high substrate temperatures and low Mn flux is discovered, where MnSi islands are growing incorporated in a Si layer, which could be interesting for further investigations as a size confinement can change the magnetic phase diagram [DBS+18]. XRD measurements demonstrate the homogeneity of the grown MnSi layers over most of the 3 inch wafer diameter and a small \(\omega\)-FWHM of about 0.02° demonstrates the high quality of the layers. XRD and TEM measurements also show that relaxation of the layers happens via misfit dislocations at the interface to the substrate.
The second part of the thesis is concerned with the crystal chirality. Here azimuthal \(\phi\)-scans of asymmetric XRD reflections reveal twin domains with a \(\pm\)30° rotation to the substrate. These twin domains seem to consist of left and right-handed MnSi, which are connected by a mirror operation at the \((\bar{1}10)\) plane. For some of the asymmetric XRD reflections this results in different intensities for the different twin domains, which reveals that one of the domains is rotated +30° and the other is rotated -30°. From XRD and TEM measurements an equal volume fraction of both domains is deduced. Different mechanisms to suppress these twin domains are investigated and successfully achieved with the growth on chiral Si surfaces, namely Si\((321)\) and Si\((531)\). Azimuthal \(\phi\)-scans of asymmetric XRD reflections demonstrate a suppression of up to 92%. The successful twin suppression is an important step in the use of MnSi for the proposed spintronics applications with skyrmions as information carriers, as discussed in the introduction.
Because of this achievement, the third part of the thesis on the magnetic properties of the MnSi thin films is not only concerned with the principal behavior, but also with the difference between twinned and twin suppressed layers. Magnetometry measurements are used to demonstrate, that the MnSi layers behave principally as expected from the literature. The analysis of saturation and residual magnetization hints to the twin suppression on Si\((321)\) and Si\((531)\) substrates and further investigations with more samples can complete this picture. For comparable layers on Si\((111)\), Si\((321)\) and Si\((531)\) the Curie-Weiss temperature is identical within 1 K and the critical field within 0.1 T.
Temperature dependent magnetoresistivity measurements also demonstrate the expected \(T^2\) behavior not only on Si\((111)\) but also on Si\((321)\) substrates. This demonstrates the successful growth of MnSi on Si\((321)\) and Si\((531)\) substrates. The latter measurements also reveal a residual resistivity of less then half for MnSi on Si\((321)\) in comparison to Si\((111)\). This can be explained with the reduced number of domain boundaries demonstrating the successful suppression of one of the twin domains. The homogeneity of the residual resistivity as well as the charge carrier density over a wide area of the Si\((111)\) wafer is also demonstrated with these measurements as well as Hall effect measurements.
The fourth part shows the AMR and PHE of MnSi depending on the angle between in plane current and magnetic field direction with respect to the crystal direction. This was proposed as a tool to identify skyrmions [YKT+15]. The influence of the higher C\(_{3\mathrm{v}}\) symmetry of the twinned system instead of the C\(_3\) symmetry of a B20 single crystal is demonstrated. The difference could serve as a useful additional tool to prove the twin suppression on the chiral substrates. But this is only possible for rotations with specific symmetry surfaces and not for the studied unsymmetrical Si\((321)\) surface. Measurements for MnSi layers on Si\((111)\) above the critical magnetic field demonstrate the attenuation of AMR and PHE parameters for increasing resistivity, as expected from literature [WC67]. Even if a direct comparison to the parameters on Si\((321)\) is not possible, the higher values of the parameters on Si\((321)\) can be explained considering the reduced charge carrier scattering from domain boundaries. Below the critical magnetic field, which would be the region where a skyrmion lattice could be expected, magnetic hysteresis complicates the analysis. Only one phase transition at the critical magnetic field can be clearly observed, which leaves the existence of a skyrmion lattice in thin epitaxial MnSi layers open.
The best method to solve this question seems to be a more direct approach in the form of Lorentz-TEM, which was also successfully used to visualize the skyrmion lattice for thin plates of bulk MnSi [TYY+12]. For the detection of in plane skyrmions, lamellas would have to be prepared for a side view, which seems in principle possible.
The demonstrated successful twin suppression for MnSi on Si\((321)\) and Si\((531)\) substrates may also be applied to other material systems.
Suppressing the twinning in FeGe on Si\((111)\) would lead to a single chirality skyrmion lattice near room temperature [HC12]. This could bring the application of skyrmions as information carriers in spintronics within reach.
Glossary:
MBE Molecular Beam Epitaxy
XRD X-Ray Diffraction
HMS Higher Manganese Silicide
FWHM Full Width Half Maximum
TEM Tunneling Electron Microscopy
AMR Anisotropic MagnetoResistance
PHE Planar Hall Effect
Bibliography:
[IEM+85] M. Ishida, Y. Endoh, S. Mitsuda, Y. Ishikawa, and M. Tanaka. Crystal Chirality and Helicity of the Helical Spin Density Wave in MnSi. II. Polarized Neutron Diffraction. Journal of the Physical Society of Japan, 54(8):2975, 1985.
[DBS+18] B. Das, B. Balasubramanian, R. Skomski, P. Mukherjee, S. R. Valloppilly, G. C. Hadjipanayis, and D. J. Sellmyer. Effect of size confinement on skyrmionic properties of MnSi nanomagnets. Nanoscale, 10(20):9504, 2018.
[YKT+15] T. Yokouchi, N. Kanazawa, A. Tsukazaki, Y. Kozuka, A. Kikkawa, Y. Taguchi, M. Kawasaki, M. Ichikawa, F. Kagawa, and Y. Tokura. Formation of In-plane Skyrmions in Epitaxial MnSi Thin Films as Revealed by Planar Hall Effect. Journal of the Physical Society of Japan, 84(10):104708, 2015.
[WC67] R. H. Walden and R. F. Cotellessa. Magnetoresistance of Nickel-Copper Single-Crystal Thin Films. Journal of Applied Physics, 38(3):1335, 1967.
[TYY+12] A. Tonomura, X. Yu, K. Yanagisawa, T. Matsuda, Y. Onose, N. Kanazawa, H. S. Park, and Y. Tokura. Real-Space Observation of Skyrmion Lattice in Helimagnet MnSi Thin Samples. Nano Letters, 12(3):1673, 2012.
[HC12] S. X. Huang and C. L. Chien. Extended Skyrmion Phase in Epitaxial FeGe(111) Thin Films. Physical Review Letters, 108(26):267201, 2012.
In this work the epitaxial growth and characterization of the half-metallic ferromagnet NiMnSb was presented. NiMnSb crystallizes in the C1b structure which is similar to the zinc blende structure from widely used III-V semiconductors. One special property of NiMnSb is the theoretical 100% spin-polarization at the Fermi edge. This makes it a perfect candidate for spintronic experiments and the material of choice for building novel spintronic devices. Another important topic in this work were the magnetic properties of NiMnSb, especially the low magnetic damping of the grown thin films. All grown layers were fabricated with the technique of MBE. The layer stacks for all different experiments and devices were grown on InP substrate in (001) or (111)B orientation. Before the NiMnSb layer a buffer layer of undoped (In,Ga)As was grown. Additional for some samples on InP(111)B, a Si doped (In,Ga)As layer was grown on top of the undoped (In,Ga)As layer. The dopant concentration of this n-doped layer was determined by ETCH-CV. All layers were investigated by structural and the NiMnSb layer additional by magnetic properties. For the structural investigation the in-situ technique RHEED and ex-situ tool HRXRD were used. RHEED observations showed a good quality of the grown buffer and half-metallic ferromagnet layers on both orientations. These results were strengthened by the HRXRD measurement. The vertical lattice constant could be determined. The received value of a(NiMnSb_vertical) = 5.925 Å for NiMnSb on InP(001) is in good agreement to the value a(NiMnSb_Lit) = 5.903 Å found in literature [Cas55]. For NiMnSb on InP(111)B a vertical lattice constant of a(NiMnSb_vertikal) = 6.017 Å could be determined. The horizontal lattice constant of the buffer and the half-metallic ferromagnet layer could be determined as the same of the substrate. For NiMnSb this conclusion is only valid up to a thickness of ≈40nm. To increase this maximum thickness, NiMnSb samples were grown on InP(001) substrates and capped with Ti/Au layers. Afterwards a reciprocal space map of the (533) reflex was drawn with GIXRD at the synchrotron beamline BW2 of HASYLAB [Kum07]. It has been shown that the critical thickness is more than doubled by depositing a Ti/Au capping directly after growth of NiMnSb without breaking the ultrahigh vacuum (UHV). The magnetic properties were determined with FMR experiments and SQUID measurements. The received magnetic damping parameter α from a 40nm thick NiMnSb layer on InP(001) could be determined to 3.19e−3 along [1-10]. The resulting line width of our NiMnSb layers on InP(001) is more than 4.88 times smaller than measured before [Hei04]. Another result is the direction dependence of the damping. It has been measured that the difference of the damping is changed by more than 42% when rotating the applied field by 45° from [1-10] to [100].With SQUID we measured a saturation magnetization of a 40nm thick NiMnSb layer as 4µB. NiMnSb layers on InP(111)B substrate where also measured with FMR with a surprising result. These layers not only showed a decreasing in the anisotropy field with increasing thickness but also an uniaxial anisotropy. This behaviour can be explained with defects on these samples. With an AFM triangle-like defects were measured. These defects originated from the buffer layer and influenced the magnetic properties. Another part of this work is dedicated to the behaviour of NiMnSb at temperatures around 80K. With our samples, no phase transition can be observed in the data of the Hall, anomalous Hall term and resistivity. The last part of this work discusses different spintronic devices build with our NiMnSb layers. In a first device the magnetization acts on the current. This Giant Magneto Resistance (GMR) device consisted of InP:S(001) - 180nm undoped (In,Ga)As - 40nm NiMnSb - 10nm Cu - 6nm NiFe - 10nm Ru in current perpendicular to plane (CPP) geometry. We received a Magneto-Resistance-Ratio of 3.4%. In a second device the current acts on the magnetization and makes use of the spin torque phenomena. This so called Spin Torque Oscillator (STO) emitted frequencies in the GHz range (13.94GHz - 14.1GHz). The last fabricated device is based on the magnetic vortex phenomena. For switching the core polarity the gyrotropic frequencies f + = 254MHz f − = 217MHz and a total static magnetic field of only mµ0H = 65mT were necessary. The reversal efficiency has been determined as better than 99% [Lou09].
Very small, thioglycerol (TG)-capped CdS nanoparticles were synthesized by a wet chemical technique and investigated in the framework of this thesis. Also glutathione-capped particles were investigated for a comparison of the capping agents. High-resolution photoelectron spectroscopy using high-brilliance synchrotron radiation was applied as the major tool for the characterization of these particles. Additionally, the particles were investigated with UV-VIS absorption spectroscopy, XPS using a laboratory source, valence band photoemission spectroscopy (VBPES), near-edge x-ray absorption spectroscopy (NEXAFS), and micro-Raman spectroscopy to address various aspects of the particles. In the beginning, an overview on size quantization effects is given to create a theoretical background behind the work presented in this thesis. Furthermore, an overview of various conventional techniques for size determination is presented. Exact information about size, shape and size distribution of nanoparticles is not yet achievable because of experimental limitations of the various size determination methods. Nanoparticles, with a range of sizes from 1.1 to 4. 2 nm, were synthesized using non-aqueous preparation and a TG capping. It is demonstrated that the use of the non-aqueous wet chemical synthesis method enables the production of very small particles and prohibits the aging of the particles. Furthermore, TG capping leads to a significant improvement for a narrow size distribution. Moreover, the results are very reproducible with TG capping and non-aqueous synthesis. Monodispersed particles can be produced by a size selective precipitation method, however, the reproducibility is questionable due to the aqueous medium of the synthesis in this case. High-resolution photoemission measurements on the small particles, i.e., 1.1 nm (CdS-A), 1.4 nm (CdS-B), 1.7 nm (CdS-C), and 1.8 nm (CdS-D, glutathione-capped), revealed five components as constituents of the S 2p signal after a careful data evaluation. Furthermore, it was observed that the particles with different sizes and capping show differences in the photoemission spectra and also in the beam damage behaviour. The different components of CdS-B were assigned as S atoms with different Cd neighbors, S atoms from thiol and S atoms in a partially oxidized state, based on the observed intensity changes of these components as a function of photon energy and beam damage, and on previous photoemission work on CdS nanoparticles [23, 45]. Furthermore, it was found that this assignment cannot be directly transferred to other particles. A new approach of structural model-based photoemission intensity calculations in comparison with the experimental data is presented. This enables us to understand subtle features in the photoemission spectra, in particular the intensity changes of the different components as a function of photon energy and beam exposure. This approach is especially applied to CdS-B (as some structural information for this particle is avialable from XRD), using three different structural models. It is found that a structural model with 33 S atoms can explain the experimental intensity changes of CdS-B. Furthermore, it is found that the photoemission spectra can be used to determine the particle size indirectly, as other plausible models show significant deviation from the experimental data. To study the various aspects by calculations, such as the influence of the particle shape and of the value of the mean free path, a program developed with L. Weinhardt and O. Fuchs is used for the intensity calculations. In order to determine a reasonable value of the mean free path for the used photon energies, two different equations from previous reports (Seah et al. and Powell et al.) are applied. As average mean free path values for the two photon energies we chose 5.5 ± 2 Å (254 eV) and 14 ± 2 Å (720 eV). The program calculation confirms the result of simple “manual” calculations of the different models. Moreover, it is tested that the value of , used in the calculations does not produce any significant influence on the calculation results. Another interesting feature is derived from the calculations that a model with a rather round shape produces similar intensity ratios for the different components to those of the data. Thus this new approach of analysis of photoemission spectra offers a way to determine particle sizes and to some extent to give an impression of the approximate particle shape. Furthermore, it is observed that the electronic band gap is larger compared to the optical band gap, which was attributed to an enhanced electron-hole correlation for optical absorption in small particles. The XPS experiments performed in the laboratory using an x-ray tube, show that the thin films produced from a freshly synthesized nanoparticle solution are fairly homogeneous and non-charging. Moreover, annealing experiments indicated that TG-capped particles posses less thermal stability as compared to MPA-capped particles. It was demonstrated that beam-induced effects play a major role. However, the knowledge of the time scale for such effects gives the possibility to record photoemission spectra with fairly good signal quality and to extrapolate to zero radiation damage. Further, particles with different sizes and capping show different beam damage behaviour. The thin film preparation by electrophoresis results in significant changes in the spectrum indicating agglomeration, while the drop-deposition technique points towards spectral changes on the rim of the sample, which can be avoided by focusing the radiation to the centre of the deposited dried drop. Micro-Raman experiments carried out in collaboration with C. Dem, Dr. M. Schmitt and Prof. W. Kiefer exhibited major differences in the spectra of nanoparticles as compared to those of the capping molecule thioglycerol. For instance, the absence of the S-H vibrational modes indicates the consumption or removal of all unreacted capping molecules. There is definitely a need for further detailed investigations concerning various interesting aspects of this work. For instance, it would be of significance to extend the program calculations to more models. Also more information about the band gap opening has to be gathered in order to find out the reason for the larger electronic band gap as compared to the optical band gap. The photoemission analysis approach using a model calculation has to be extended to differently prepared nanoparticles, in particular, to address the differences in the location of the various species in the particle as a function of preparation. The efforts of XRD simulations by C. Kumpf et al. [50] may reveal significant new information about the particle size and the size distribution. It can be expected that the program calculations, if extended to more models, can prove the potential of photoelectron spectroscopy to serve as a tool for size and shape determination of nanoparticles, which is a new contribution to the investigation of nanoparticles.