Spin-Reorientierung in epitaktischen NdCo5-Schichten

Die vorliegende Arbeit präsentiert die ersten detaillierten Untersuchungen des Spin-Reorientierungs-Übergangs in epitaktischen NdCo5-Schichten. Die Proben, die mit gepulster Laserdeposition hergestellt wurden, konnten sowohl als in-plane- als auch als out-of-plane-texturierte Schichten präpariert werden. Für beide Wachstumsvarianten ergaben Röntgendiffraktometrie- und Texturmessungen eine sehr gute Texturierung mit einer nahezu einheitlichen Orientierung der c-Achse, die eine Untersuchung der magnetischen Eigenschaften entlang ausgewählter kristallografischer Richtungen ermöglichte. Die globalen Magnetisierungsmessungen der In-plane-Proben zeigten einen Spin-Reorientierungs-Übergang von einer magnetisch leichten c-Achse für Temperaturen oberhalb von 310 K über einen magnetisch leichten Kegel hin zu einer magnetisch leichten Ebene (a-Achse) unterhalb von 255 K. Die Übergangstemperaturen liegen damit geringfügig über den bisher an Massivproben gemessenen Werten. Aus den magnetischen Hysteresemessungen wurden die magnetokristallinen Anisotropiekonstanten erster und zweiter Ordnung für den Temperaturbereich der magnetisch leichten c-Achse und der magnetisch leichten Ebene ermittelt. Die Untersuchungen der Out-of-plane-Proben wiesen die Existenz einer magnetokristallinen Anisotropie höherer als zweiter Ordnung nach. Sie bewirkt ein unterschiedliches Schaltverhalten der parallel zur a- bzw. b-Achse gemessenen magnetischen Hysteresekurven im Temperaturregime der magnetisch leichten Ebene. Für die in-plane-texturierten Schichten wurde das Domänenmuster und dessen Änderung mit der Temperatur im gesamten Spin-Reorientierungs-Bereich analysiert. Diese Untersuchungen basieren auf in Kooperation mit der Universität Hamburg durchgeführten SEMPA-Messungen. Oberhalb von 318 K liegt eine Zweidomänenkonfiguration mit einer Ausrichtung der Magnetisierung parallel zur c-Achse vor, die beim Abkühlen in das Regime des magnetisch leichten Kegels in einen Vierdomänenzustand übergeht. Unterhalb von 252 K bildet sich eine Zweidomänenkonfiguration mit parallel zur a-Achse orientierter Magnetisierung. Diese lokalen Messungen bestätigten den Spin-Reorientierungs-Übergang mit zu den globalen Magnetisierungsmessungen vergleichbaren Übergangstemperaturen. Für charakteristisch orientierte Domänenwände erfolgten genauere Analysen der Magnetisierungsprozesse in den angrenzenden Domänen. Um ein erweitertes Verständnis der Domänenkonfiguration, deren Temperaturabhängigkeit und der vorhandenen Domänenwände zu erarbeiten, erfolgten mikromagnetische Simulationsrechnungen für ausgewählte Temperaturen. Die Berechnungen wurden sowohl für homogene Systeme als auch für Geometrien mit verschiedenen Pinningzentren durchgeführt. Die Analyse der Domänenwände ergab, dass ihr Bloch- oder Néel-Charakter und die Domänenwandweite von der Temperatur sowie ihrer Ausrichtung parallel zur c- oder a-Achse abhängt.This thesis presents the first detailed investigation of the spin-reorientation-transition in epitaxial NdCo5 thin films. The samples were prepared by pulsed laser deposition as in-plane or out-of-plane textured films. For both kinds of samples X-ray diffraction and texture measurements revealed a high degree of texture with one common orientation of the c-axis within the film, which allowed an investigation of the magnetic properties along distinct crystallographic directions. Global magnetization measurements of the in-plane textured films showed a spin-reorientation from a magnetic easy axis (c-axis) at temperatures above 310 K via a magnetic easy cone to a magnetic easy plane (a-axis) at temperatures below 255 K. The transition temperatures are slightly higher than values reported for bulk samples. The magnetocrystalline anisotropy constants of first and second order were determined for the regime of the magnetic easy axis and plane. Measurements of the out-of-plane textured films verified the existence of a magnetocrystalline anisotropy of order larger than two, which becomes obvious from a different magnetic switching behavior along the a- and b-axis in the temperature regime of the magnetic easy plane. The domain structure and its changes with temperature were investigated for the in-plane textured films. There exists a two domain state at temperatures above 318 K with an orientation of the magnetization parallel to the c-axis from which a four domain state evolves when cooling down the sample to the easy cone state. Finally, a two domain state exists in the regime of the magnetic easy plane (easy a-axis) with an orientation of the magnetization parallel to the a-axis at temperatures below 252 K. The local measurements confirm the spin-reorientation-transition with transition temperatures comparable to those derived from global magnetization measurements. In addition, a detailed analysis of the magnetization processes for some characteristically oriented domain walls was performed. Micromagnetic simulations were carried out for selected temperatures to achieve a deeper understanding of the temperature dependence of the domain configuration and of the domain walls. The simulations considered homogeneous systems as well as systems with pinning centers. An analysis of the domain walls showed that their character and width depend on temperature and the orientation parallel to the a- or c-axis

XPS chemical state analysis of sputter depth profiling measurements for annealed TiAl-SiO2 and TiAl-W layer stacks

For the application of surface acoustic wave sensors at high temperatures, both a high-temperature stable piezoelectric substrate and a suitable metallization for the electrodes are needed. Our current attempt is to use TiAl thin films as metallization because this material is also known to be high temperature stable. In this study, Ti/Al multilayers and Ti-Al alloy layers were prepared in combination with an SiO2 cover layer or a W barrier layer at the interface to the substrate (thermally oxidized Si or Ca3TaGa3Si2O14) as an oxidation protection. To form the high-temperature stable γ-TiAl phase and to test the thermal stability of the layer systems, thermal treatments were done in vacuum at several temperatures. We used X-ray photoelectron spectroscopy (XPS) sputter depth-profiling to investigate the film composition and oxidation behavior. In this paper, we demonstrate how the semiautomatic peak fitting can help to extract beside the elemental information also the chemical information from the measured depth profiles. © 2020 The Authors. Surface and Interface Analysis published by John Wiley & Sons Lt

Long-term high-temperature behavior of Ti–Al based electrodes for surface acoustic wave devices

The long-term high-temperature behavior of Ti–Al based electrodes for the application in surface acoustic wave (SAW) sensor devices was analyzed. The electrodes were obtained by e-beam evaporation of Ti/Al multilayers on the high-temperature stable piezoelectric Ca3TaGa3Si2O14 (CTGS) substrates and structuring via the lift-off process. AlNO (25 at.% Al; 60 at.% N and 15 at.% O) cover and barrier layers were applied as protection against oxidation from the surrounding atmosphere and to prohibit a chemical reaction with the substrate. The samples were annealed at temperatures up to 600 °C in air for a duration of up to 192 h. Scanning and transmission electron microscopy were used to evaluate the morphology and degradation of the electrodes as well as of the extended contact pads. The results revealed that the Ti–Al based electrodes remained unoxidized after annealing for 192 h at 400 and 500 °C and for 24 h at 600 °C. After the heat treatment for 192 h at 600 °C, a strong oxidation of the structured electrodes occurred, which was less pronounced within the pads. In summary, the investigation showed that Ti–Al based SAW devices are a cost efficient alternative for long-term applications up to at least 500 °C and short- and medium-term applications up to 600 °C

Aluminum based high temperature thin film electrode system for wireless sensors

Self-sustained, wireless high-temperature stable sensors are developed, which are based on an aluminum alloy as the electrode metallization. Due to its cost-effectiveness accompanied by a high-temperature stability, this alloy substitutes and outperforms the commonly applied expensive Pt- and Ir-based metals. For the first time, a comprehensive structural, electrical and high-frequency characterization of these surface acoustic wave (SAW) sensors is shown. They are based on Catangasite (Ca3TaGa3Si2O14, CTGS) in combination with properly structured cover and barrier layers for the metallization. The frequency characteristics is determined up to 700 °C by ex situ and in situ methods. In addition, the morphology of the AlRu electrodes is analyzed after the thermal loadings and the temperature dependent sheet resistance is measured. The results reveal a reproducible and linear correlation between the applied temperature and the sheet resistance as well as the resonant frequency. In addition, hardly any degradation of the electrodes is detected after the thermal loadings. The observed high-temperature stability of the devices up to at least 700 °C demonstrates the large potential of the AlRu based SAW sensors as a cost-efficient alternative to expensive noble metal based sensors in industrial applications for the support of energy efficient operation

Durability of TiAl based surface acoustic wave devices for sensing at intermediate high temperatures

TiAl based surface acoustic wave (SAW) devices, which offer a promising cheap and easy to handle wireless sensor solution for intermediate high temperatures up to 600 °C, were prepared and investigated with respect to their durability. To obtain the devices, Ti/Al multilayers were deposited on high-temperature stable piezoelectric catangasite (CTGS) substrates and structured as electrodes via the lift-off technique. AlNO cover layers and barrier layers at the substrate site served as an oxidation protection. The devices were characterized regarding their electrical behavior by ex-situ measurements of their frequency characteristics after heat treatments up to 600 °C in air. In addition, long-term in situ measurements up to 570 °C were performed to analyze a possible drift of the resonant frequency in dependence on the temperature and time. Scanning electron microscopy of the surfaces of the devices and scanning transmission electron microscopy of cross sections of TiAl interdigital transducer electrode fingers and the contact pads were conducted to check the morphology of the electrode metallization and to reveal if degradation or oxidation processes occurred during the heat treatments. The results demonstrated a sufficient high-temperature stability of the TiAl based devices after a first conditioning of system. A linear dependence of the resonant frequency on the temperature of about −37 ppm/K was observed. In summary, the suitability of TiAl based SAW sensors for long-term application at intermediate temperatures was proven

Capability study of Ti, Cr, W, Ta and Pt as seed layers for electrodeposited platinum films on γ-Al2O3 for high temperature and harsh environment applications

High temperature surface acoustic wave sensors based on radio frequency identification technology require adequate antennas of high efficiency and thermal stability for the signal transmission. Platinum is well known and frequently used as a material of choice for high temperature and harsh environment applications because of the high melting point and its chemical stability. Therefore, one way to realize high temperature stable antennas is the combination of a Pt metallization on an Al 2 O 3 substrate. As a cost-effective technique, the Pt film is deposited via electrochemical deposition. For this growth procedure, a pre-deposited metallization on the Al 2 O 3 layer is required. This paper analyzes the influence of various seed layers (Ta, Ti, W, Cr, Pt) on the morphology, stability and electrical properties of the electrochemically-grown Pt thick film after heat treatments up to 1000 ∘ C in air. We find an oxidation of all adhesion layers except for Pt, for which the best electrical properties were measured. Although significant areas of the films delaminate from the substrate, individual anchor structures retain a stable connection between the Pt layer and the rough Al 2 O 3 substrate

The Influence of the Composition of Ru100−xAlx (x = 50, 55, 60, 67) Thin Films on Their Thermal Stability

RuAl thin films possess a high potential as a high temperature stable metallization for surface acoustic wave devices. During the annealing process of the Ru-Al films, Al2O3 is formed at the surface of the films even under high vacuum conditions, so that the composition of a deposited Ru50Al50 film is shifted to a Ru-rich alloy. To compensate for this effect, the Al content is systematically increased during the deposition of the Ru-Al films. Three Al-rich alloys—Ru45Al55, Ru40Al60 and Ru33Al67—were analyzed concerning their behavior after high temperature treatment under high vacuum and air conditions in comparison to the initial Ru50Al50 sample. Although the films’ cross sections show a more homogeneous structure in the case of the Al-rich films, the RuAl phase formation is reduced with increasing Al content

Stress and Microstructure Evolution in Mo Thin Films without or with Cover Layers during Thermal-Cycling

The intrinsic stress behavior and microstructure evolution of Molybdenum thin films were investigated to evaluate their applicability as a metallization in high temperature microelectronic devices. For this purpose, 100 nm thick Mo films were sputter-deposited without or with an AlN or SiO2 cover layer on thermally oxidized Si substrates. The samples were subjected to thermal cycling up to 900 °C in ultrahigh vacuum; meanwhile, the in-situ stress behavior was monitored by a laser based Multi-beam Optical Sensor (MOS) system. After preannealing at 900 °C for 24 h, the uncovered films showed a high residual stress at room temperature and a plastic behavior at high temperatures, while the covered Mo films showed an almost entirely elastic deformation during the thermal cycling between room temperature and 900 °C with hardly any plastic deformation, and a constant stress value during isothermal annealing without a notable creep. Furthermore, after thermal cycling, the Mo films without as well as with a cover layer showed low electrical resistivity (≤10 μΩ·cm)

Micromagnetic investigation of domain and domain wall evolution through the spin-reorientation transition of an epitaxial NdCo5 film

The domain pattern and the domain wall microstructure throughout the spin-reorientation transition of an epitaxial NdCo5 thin film are investigated by micromagnetic simulations. The temperature-dependent anisotropy constants K1 and K2, which define the anisotropy energy term in the model, are chosen to reflect the easy axis—easy cone—easy plane spin-reorientation transition observed in epitaxial NdCo5 thin films. Starting at the high-temperature easy c-axis regime, the anisotropy constants are changed systematically corresponding to a lowering of the temperature of the system. The character of the domain walls and their profiles are analysed. The calculated domain configurations are compared to the experimentally observed temperature-dependent domain structure of an in-plane textured NdCo5 thin film

Spin-Reorientierung in epitaktischen NdCo5-Schichten

Die vorliegende Arbeit präsentiert die ersten detaillierten Untersuchungen des Spin-Reorientierungs-Übergangs in epitaktischen NdCo5-Schichten. Die Proben, die mit gepulster Laserdeposition hergestellt wurden, konnten sowohl als in-plane- als auch als out-of-plane-texturierte Schichten präpariert werden. Für beide Wachstumsvarianten ergaben Röntgendiffraktometrie- und Texturmessungen eine sehr gute Texturierung mit einer nahezu einheitlichen Orientierung der c-Achse, die eine Untersuchung der magnetischen Eigenschaften entlang ausgewählter kristallografischer Richtungen ermöglichte. Die globalen Magnetisierungsmessungen der In-plane-Proben zeigten einen Spin-Reorientierungs-Übergang von einer magnetisch leichten c-Achse für Temperaturen oberhalb von 310 K über einen magnetisch leichten Kegel hin zu einer magnetisch leichten Ebene (a-Achse) unterhalb von 255 K. Die Übergangstemperaturen liegen damit geringfügig über den bisher an Massivproben gemessenen Werten. Aus den magnetischen Hysteresemessungen wurden die magnetokristallinen Anisotropiekonstanten erster und zweiter Ordnung für den Temperaturbereich der magnetisch leichten c-Achse und der magnetisch leichten Ebene ermittelt. Die Untersuchungen der Out-of-plane-Proben wiesen die Existenz einer magnetokristallinen Anisotropie höherer als zweiter Ordnung nach. Sie bewirkt ein unterschiedliches Schaltverhalten der parallel zur a- bzw. b-Achse gemessenen magnetischen Hysteresekurven im Temperaturregime der magnetisch leichten Ebene. Für die in-plane-texturierten Schichten wurde das Domänenmuster und dessen Änderung mit der Temperatur im gesamten Spin-Reorientierungs-Bereich analysiert. Diese Untersuchungen basieren auf in Kooperation mit der Universität Hamburg durchgeführten SEMPA-Messungen. Oberhalb von 318 K liegt eine Zweidomänenkonfiguration mit einer Ausrichtung der Magnetisierung parallel zur c-Achse vor, die beim Abkühlen in das Regime des magnetisch leichten Kegels in einen Vierdomänenzustand übergeht. Unterhalb von 252 K bildet sich eine Zweidomänenkonfiguration mit parallel zur a-Achse orientierter Magnetisierung. Diese lokalen Messungen bestätigten den Spin-Reorientierungs-Übergang mit zu den globalen Magnetisierungsmessungen vergleichbaren Übergangstemperaturen. Für charakteristisch orientierte Domänenwände erfolgten genauere Analysen der Magnetisierungsprozesse in den angrenzenden Domänen. Um ein erweitertes Verständnis der Domänenkonfiguration, deren Temperaturabhängigkeit und der vorhandenen Domänenwände zu erarbeiten, erfolgten mikromagnetische Simulationsrechnungen für ausgewählte Temperaturen. Die Berechnungen wurden sowohl für homogene Systeme als auch für Geometrien mit verschiedenen Pinningzentren durchgeführt. Die Analyse der Domänenwände ergab, dass ihr Bloch- oder Néel-Charakter und die Domänenwandweite von der Temperatur sowie ihrer Ausrichtung parallel zur c- oder a-Achse abhängt.This thesis presents the first detailed investigation of the spin-reorientation-transition in epitaxial NdCo5 thin films. The samples were prepared by pulsed laser deposition as in-plane or out-of-plane textured films. For both kinds of samples X-ray diffraction and texture measurements revealed a high degree of texture with one common orientation of the c-axis within the film, which allowed an investigation of the magnetic properties along distinct crystallographic directions. Global magnetization measurements of the in-plane textured films showed a spin-reorientation from a magnetic easy axis (c-axis) at temperatures above 310 K via a magnetic easy cone to a magnetic easy plane (a-axis) at temperatures below 255 K. The transition temperatures are slightly higher than values reported for bulk samples. The magnetocrystalline anisotropy constants of first and second order were determined for the regime of the magnetic easy axis and plane. Measurements of the out-of-plane textured films verified the existence of a magnetocrystalline anisotropy of order larger than two, which becomes obvious from a different magnetic switching behavior along the a- and b-axis in the temperature regime of the magnetic easy plane. The domain structure and its changes with temperature were investigated for the in-plane textured films. There exists a two domain state at temperatures above 318 K with an orientation of the magnetization parallel to the c-axis from which a four domain state evolves when cooling down the sample to the easy cone state. Finally, a two domain state exists in the regime of the magnetic easy plane (easy a-axis) with an orientation of the magnetization parallel to the a-axis at temperatures below 252 K. The local measurements confirm the spin-reorientation-transition with transition temperatures comparable to those derived from global magnetization measurements. In addition, a detailed analysis of the magnetization processes for some characteristically oriented domain walls was performed. Micromagnetic simulations were carried out for selected temperatures to achieve a deeper understanding of the temperature dependence of the domain configuration and of the domain walls. The simulations considered homogeneous systems as well as systems with pinning centers. An analysis of the domain walls showed that their character and width depend on temperature and the orientation parallel to the a- or c-axis