In dieser Dissertation wird die gute Einstellbarkeit von synthetischen Antiferromagneten mit dem kollektiven Surface Spin-Flop-Verhalten kombiniert. Es wird der Einfluss der Gesamtschichtdicke untersucht, welche mit dem Abstand der magnetische Oberflächen korreliert. Zudem werden die Dicken der ferromagnetischen Untereinheiten an den Außenseiten verändert, womit die Beiträge der Oberflächen unterdrückt oder verstärkt werden können. Darauffolgend wird die Kontrolle der Oberflächenbeiträge angewendet, um Exchange Bias-Strukturen auf Basis synthetischer Antiferromagnete zu erzeugen. Da diese nicht aus Heterostrukturen intrinsischer Antiferromagnete und Ferromagnete bestehen, wird nicht nur eine gute Abstimmbarkeit erreicht, sondern auch die Materialwahl wird potentiell vereinfacht. Zudem kann der Exchange Bias in synthetischen Antiferromagneten vollständig bei Raumtemperatur beobachtet und gesteuert werden. Im Weiteren wird ein zuvor untersuchtes Konzept zur Stabilisierung der vom Surface Spin-Flop erzeugten vertikalen antiferromagnetischen Domänenwände erweitert. Es wird demonstriert, wie ein Paar koexistierender antiferromagnetischer Domänenwände in Abwesenheit äußerer Magnetfelder und bei tiefen Temperaturen stabil gehalten werden kann. Damit können in Erweiterung der ursprünglichen Konzeptes nun acht anstatt sechs remanenter Zustände durch geeignete Magnetfeldroutinen eingestellt werden.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste / In this thesis, the high degree of tunability in the SAFs is combined with the collective surface spin-flop reversal. The influence of the total thickness and thus the distance of the magnetic surfaces is explored. Furthermore, the thickness of the ferromagnetic surface subunits is altered to selectively suppress or enhance the surface contribution. The control of the surface contribution is subsequently employed to create magnetic exchange bias structures based on the synthetic antiferromagnets. If compared to conventional exchange bias systems in heterostructures of intrinsic antiferromagnetic and ferromagnetic materials, an exchange bias with full room temperature operation, high tunability and a potential potential much more flexible choice of materials becomes available. Additionally, a previously established concept to stabilize the vertical antiferromagnetic domain walls that originate from the surface spin-flop at remanence is extended to a coexisting pair of antiferromagnetic domain walls. At low temperatures, the coexisting vertical antiferromagnetic domain walls can be stabilized at remanence, too. Furthermore, the total number of different remanent states, which are accessible through different field routines, can be increased from six in the original concept to eight in the more sophisticated concept presented here.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste

Heavy fermion metals often exhibit novel electronic states at low temperatures, due to competing interactions and energy scales. In order to characterize these states, precise determination of material electronic properties, such as the Fermi surface topology, is necessary. Magnetic field is a particularly powerful tool, since it can be used as both a tuning parameter and probe of the fundamental physics of heavy fermion compounds. In CePb3, I measured magnetoresistance and torque for 23 mK ≤ T ≤ 400 mK, 0 T ≤ H ≤ 18 T, and magnetic field rotated between the (100), (110), and (111) directions. For H||(111), my magnetoresistance results show a decreasing Fermi liquid temperature range near Hc, and a T^2 coefficient that diverges as A(H) ∝ |H −Hc|^−α, with Hc ~ 6 T and α ~ 1. The torque exhibits a complicated dependence on magnetic field strength and angle. By comparison to numerical spin models, I find that the “spin-flop” scenario previously thought to describe the physics of CePb3 does not provide a good explanation of the experimental results. Using novel data acquisition software that exceeds the capabilities of a traditional measurement set-up, I measured de Haas–van Alphen oscillations in YbRh2Si2 for 30 mK ≤ T ≤ 600 mK, 8 T ≤ H ≤ 16 T, and magnetic field rotated between the (100), (110), and (001) directions. The measured frequencies smoothly increase as the field is decreased through H0 ≈ 10 T. I compared my measurements to 4f-itinerant and 4f-localized electronic structure calculations, using a new algorithm for extracting quantum oscillation information from calculated band energies, and conclude that the Yb 4f quasi-hole remains itinerant over the entire measured field range, with the behaviour at H0 caused by a Fermi surface Lifshitz transition. My measurements are the first to directly track the Fermi surface of YbRh2Si2 across this field range, and rule out the 4f localization transition/crossover that was previously proposed to occur at H0.

Heavy fermion metals often exhibit novel electronic states at low temperatures, due to competing interactions and energy scales. In order to characterize these states, precise determination of material electronic properties, such as the Fermi surface topology, is necessary. Magnetic field is a particularly powerful tool, since it can be used as both a tuning parameter and probe of the fundamental physics of heavy fermion compounds. In CePb3, I measured magnetoresistance and torque for 23 mK ≤ T ≤ 400 mK, 0 T ≤ H ≤ 18 T, and magnetic field rotated between the (100), (110), and (111) directions. For H||(111), my magnetoresistance results show a decreasing Fermi liquid temperature range near Hc, and a T^2 coefficient that diverges as A(H) ∝ |H −Hc|^−α, with Hc ~ 6 T and α ~ 1. The torque exhibits a complicated dependence on magnetic field strength and angle. By comparison to numerical spin models, I find that the “spin-flop” scenario previously thought to describe the physics of CePb3 does not provide a good explanation of the experimental results. Using novel data acquisition software that exceeds the capabilities of a traditional measurement set-up, I measured de Haas–van Alphen oscillations in YbRh2Si2 for 30 mK ≤ T ≤ 600 mK, 8 T ≤ H ≤ 16 T, and magnetic field rotated between the (100), (110), and (001) directions. The measured frequencies smoothly increase as the field is decreased through H0 ≈ 10 T. I compared my measurements to 4f-itinerant and 4f-localized electronic structure calculations, using a new algorithm for extracting quantum oscillation information from calculated band energies, and conclude that the Yb 4f quasi-hole remains itinerant over the entire measured field range, with the behaviour at H0 caused by a Fermi surface Lifshitz transition. My measurements are the first to directly track the Fermi surface of YbRh2Si2 across this field range, and rule out the 4f localization transition/crossover that was previously proposed to occur at H0.