Magnetic X-ray Reflectivity

The scope of the thesis is to demonstrate the feasibility to examine magnetization profiles of thin films and multilayer systems via magnetic soft and hard x-ray reflectivity. The focus here is on 3d transition metals, which are used mainly for development of numerous noval magnetic devices, that are both technologically and scientifically interesting. Complementary to Neutron diffraction, which is the standard tool for the examination of magnetic structures in matter, magnetic x-ray diffraction permits to study small samples and exhibits better Qz-resolution due its small and only slightly divergent beam. The biggest advantage is its element specificity, which enables one to probe different magnetic sites separately. The method of magnetic x-ray reflectivity combines the strong magnetic circular dichroism (MCD) effect, significantly enhancing the magnetic sensitivity of x-rays, with the technique of conventional specular reflectivity, a well established tool for the structural studies of the chemical makeup of thin films and artificial multilayer systems. The theory of resonant magnetic scattering within dipole approximation combined with the specular reflectivity condition suggests that the strongest effects are in the lower incident angle regime using circularly polarized x-rays. By using soft and hard x-rays structures on a scale of a few to several hundreds of Å are probed, which is the dimensions of the thicknesses of the layers of most thin film and multilayers systems. In order to retrieve quantitative information from the measured magnetic reflectivity curves, an approach for visible light magneto-optical effects based on known dielectric tensors of the sample has been adopted and applied for soft and hard x-ray resonant scattering. Sample absorption and polarization changes in the sample are accounted for. Besides the structural composition, the thickness of the individual layers and the index of refraction, also the magnetic spin configuration can be chosen with arbitrary moment direction and magnitude by modifying the off-diagonal terms in the dielectric tensor. The magnetic optical constants, which determine the magnitude of the magnetic moments, are experimentally determined via MCD absorption measurements and then retrieving the real part through the Kramers-Kronig transformation of the measured imaginary part. This is shown in this work for several 3d transition metals and edges. The simulations are sensitive to a variety of different spin configurations: spiral spin structures, magnetic dead layers and of collinear alignment. Experimentally the magnetic reflectivity of 3d transition metals has to distinguish between the two available possible absorbtion edges, L and K, lying in different x-ray regions. The L-edges are situated in the soft x-ray region and exhibit large enhancements of the magnetic cross section, while the K-edges lie in the hard x-ray regime and show much smaller effects. In spite of this handicap, the latter can be important due to the much larger penetration depth and better Qz-resolution. The X13 beamline at the NSLS at Brookhaven National Laboratory consisting of two branches for soft and hard-x ray operations, respectively, uses an elliptical polarized wiggler (EPW), which produces circularly polarized x-rays in the orbit plane and allows fast switching between left and right circular polarization. Lock-in detection is used to improve the signal-to-noise ratio at the soft x-ray branch and single photon detection at the hard x-ray branch to measure the magnetic signal. The EPW and the experimental setup was commissioned to demonstrate the feasibility of magnetic x-ray experiments. Especially at the hard x-ray beamline branch the small magnetic effects, less than 0.1% of the charge scattering, were possible to detect. In order to satisfy the need for high flux the CMC-CAT beamline at the APS in Argonne was used for magnetic hard x-ray reflectivity, providing an undulator beamline where the high flux of linear polarized photons was converted into circular polarization via a diamond phase plate, delivering much higher flux and better circular polarization. The sample used to demonstrate the feasibility of the method of magnetic reflectivity consists of two multilayer structures of Fe/Cr on top of each other, where the iron spins of the upper are ferromagnetically and of the lower antiferromagnetically coupled, representing an exchange bias system. The sample was characterized with conventional x-ray reflectivity and MOKE measurements in order to accurately determine the structural composition and magnetic configuration (hysteresis loops), respectively. Magnetic reflectivity experiments on the L-edges at the X13A beamline showed strong magnetic effects, which could be clearly identified as ferromagnetic and antiferromagnetic Bragg peak contributions and simulation confirmed the collinear alignment and full magnetization of the iron spins throughout the iron layers. Energyand magnetic field dependent measurements complete the picture. By tuning the x-ray energy to the chromium L-edge, a signal 20 times weaker compared with iron, demonstrates that the weak magnetic moment in the chromium layers could be detected. Especially the AFM contribution shows strong effects which could be qualitatively and quantitatively evaluated. Simulation show clearly that the magnetic moment is concentrated at the interfaces and could be approximated to a magnetic layer with an effective thickness of about 0.5 Å assuming a step function in the magnetization profile. Soft x-ray data usually suffer from strong absorption and the limited Qz-range and resolution and therefore the use of hard x-rays seems desirable to probe the whole sample. Magnetic hard x-ray reflectivity measurements on the Fe/Cr double multilayer carried out at the CMC beamline by switching the magnetic field on the sample show clear magnetic Bragg reflection at the ferromagnetic structural peaks. They are very well reproduced by simulations and thus confirm the collinear alignment of the iron spins. In order to probe the AFM spin configuration the helicity of the photon beam has to be switched with constant magnetic field. In spite of complications in the reflectivity spectra it was possible to extract the relative orientation of the AFM to FM spin configuration in the two multilayers. In summary the work showed for the example of an Fe/Cr double multilayer that magnetic soft and hard x-ray reflectivity can be applied to retrieve information about the magnetization profile of thin magnetic films and multilayer, and can compliment polarized neutron scattering

Thermal analysis of high-bandwidth and energy-efficient 980 nm VCSELs with optimized quantum well gain peak-to-cavity resonance wavelength offset

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 111, 243508 (2017) and may be found at https://doi.org/10.1063/1.5003288.The static and dynamic performance of vertical-cavity surface-emitting lasers (VCSELs) used as light-sources for optical interconnects is highly influenced by temperature. We study the effect of temperature on the performance of high-speed energy-efficient 980 nm VCSELs with a peak wavelength of the quantum well offset to the wavelength of the fundamental longitudinal device cavity mode so that they are aligned at around 60 °C. A simple method to obtain the thermal resistance of the VCSELs as a function of ambient temperature is described, allowing us to extract the active region temperature and the temperature dependence of the dynamic and static parameters. At low bias currents, we can see an increase of the −3 dB modulation bandwidth f−3dB with increasing active region temperature, which is different from the classically known situation. From the detailed analysis of f−3dB versus the active region temperature, we obtain a better understanding of the thermal limitations of VCSELs, giving a basis for next generation device designs with improved temperature stability

81 fJ/bit energy-to-data ratio of 850 nm vertical-cavity surface-emitting lasers for optical interconnects

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 98, 231106 (2011) and may be found at https://doi.org/10.1063/1.3597799.Extremely energy-efficient oxide-confined high-speed 850 nm vertical-cavity surface-emitting lasers for optical interconnects are presented. Error-free performance at 17 and 25 Gb/s via a 100 m multimode fiber link is demonstrated at record high dissipation-power-efficiencies of up to 69 fJ/bit (<0.1mW/Gbps) and 99 fJ/bit, respectively. These are the most power efficient high-speed directly modulated light sources reported to date. The total energy-to-data ratio is 83 fJ/bit at 25°C and reduces to 81 fJ/bit at 55°C. These results were obtained without adjustment of driving conditions. A high -factor of 12.0GHz/(mA)0.5 and a -factor of 0.41 ns are measured.EC/FP7/224211/EU/VISIT - Vertically Integrated Systems for Information Transfer/VISITDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Frequency response of large aperture oxide-confined 850 nm vertical cavity surface emitting lasers

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 95, 131101 (2009) and may be found at https://doi.org/10.1063/1.3231446.Small and large signal modulation measurements are carried out for 850 nm vertical cavity surface emitting lasers (VCSELs). The resonance frequency, damping factor, parasitic frequency, and -factor are extracted. Small signal modulation bandwidths larger than 20 GHz are measured. At larger currents the frequency response becomes partially limited by the parasitics and damping. Our results indicate that by increasing the parasitic frequency, the optical 3 dB bandwidth may be extended to ∼25GHz. A decrease in the damping should enable VCSEL bandwidths of 30 GHz for current densities not exceeding ∼10kA/cm2 and ultimately error-free optical links at up to 40 Gbit/s.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeEC/FP7/224211/EU/VISIT - Vertically Integrated Systems for Information Transfer/VISI

Highly temperature-stable modulation characteristics of multioxide-aperture high-speed 980 nm vertical cavity surface emitting lasers

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 97, 151101 (2010) and may be found at https://doi.org/10.1063/1.3499361.We present multioxide-aperture 980 nm-range vertical cavity surface emitting lasers (VCSELs) with highly temperature stable modulation characteristics operating error-free at 25 Gbit/s at 25 and 85°C. We perform small signal modulation experiments and extract the fundamental physical parameters including relaxation resonance frequency, damping factor, parasitic cut-off frequency, -factor, and -factor, leading to identification of thermal processes and damping as the main factors that presently limit high speed device operation. We obtain very temperature-insensitive bandwidths around 13–15 GHz. Presented results clearly demonstrate the suitability of our VCSELs for practical and reliable optical data transmission systems.EC/FP7/224211/EU/VISIT - Vertically Integrated Systems for Information Transfer/VISITDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Anomalous lasing of high-speed 850 nm InGaAlAs oxide-confined vertical-cavity surface-emitting lasers with a large negative gain-to-cavity wavelength detuning

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 105, 061104 (2014) and may be found at https://doi.org/10.1063/1.4892885.The impact of a large negative quantum well gain-to-cavity etalon wavelength detuning on the static and dynamic characteristics of 850 nm InGaAlAs high-speed oxide-confined vertical-cavity surface-emitting lasers (VCSELs) was investigated. Three distinct lasing regimes were revealed in large square aperture (≥7 μm per side) devices with large detuning including: (1) an anomalous lasing via higher order Hermite–Gaussian modes at low forward bias current; (2) lasing via the lowest order Hermite–Gaussian modes at high bias current; and (3) simultaneous lasing via both types of transverse modes at intermediate bias currents. In contrast to conventional multimode VCSELs a two-resonance modulation response was observed for the case of co-lasing via multiple transverse modes with high spectral separation. The reduction in the oxide aperture area resulted in classical lasing via the lowest order modes with a conventional single-resonance frequency response.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Experiments push the limits of micromagnetic SANS theory

Commentary is provided on recent magnetic SANS experiments on highly inhomogeneous high-pressure-torsion manufactured metals. The ensuing progress in the theoretical description of magnetic SANS using micromagnetic theory is highlighted

Qualitätsorientierte, prozeß-sensitive Softwareentwicklungsumgebungen im MVP-Projekt

Software-Projekte bestehen aus einer Vielzahl von Teilaufgaben, die durch komplexe Wechselbeziehungen miteinander verknüpft sind. Systematische Unterstützung bei der Durchführung von Software-Projekten erfordert deshalb nicht nur die isolierte Unterstützung einzelner Teilaufgaben, sondern insbesondere der Wechselbeziehungen. Außerdem müssen Aktivitäten des Messens und Bewertens durchgeführt werden, um quantitative Aussagen über Produkte und Prozesse ableiten zu können. Ziel des MVP-Projekts (Multi-View Process modeling) ist es, derartige integrierte Unterstützung auf der Basis meßbarer Projektpläne zur Verfügung zu stellen. Projektpläne setzen sich dabei unter anderem aus Prozeß-, Produkt-, Ressourcen- und Qualitätsmodellen zusammen. Meßansätze werden nicht nur zur systematischen Unterstützung von Projekten, sondern auch zur Verbesserung existierender Prozeß-, Produkt-, Ressource- und Qualitätsmodelle aufgrund 'gemessener' Erfahrungswerte verwendet. Die Benutzer des MVP-Entwicklungssystems (MVP-S) werden durch ihre Rollen im Rahmen eines Projekts charakterisiert werden können. Es wird beschrieben, wie Rollen das MVP-System nutzen können. Dies geschieht entweder durch direkte Repräsentation ihrer Aufgaben als Prozesse oder indem die im Projektplan repräsentierte Information ausgewertet und präsentiert wird; entsprechend bezeichnen wir eine Rolle als "zustandsverändernd" oder als "zustandserfragend". Um diese Rollen zu unterstützen, existieren unterschiedliche Möglichkeiten abhängig vom Grad der Automatisierung. Es werden beispielhaft drei Stufen aufgezeigt. Anschließend wird die Realisierung einer prototypischen, qualitätsorientierten, prozeßsensitiven Software-Entwicklungsumgebung diskutiert. Zum Abschluß wird auf gegenwärtige und zukünftige Forschungsfragen im Rahmen des MVP-Projekts eingegangen