Investigation of spin-caloric and spin-orbit torque materials by vectorial spin Seebeck measurements and an advanced x-ray magnetic reflectivity analysis

Krieft J. Investigation of spin-caloric and spin-orbit torque materials by vectorial spin Seebeck measurements and an advanced x-ray magnetic reflectivity analysis. Bielefeld: Universität Bielefeld; 2021.Over the past decade there has been considerable interest in magnetic Heusler compounds in the field of spintronics based on their wide range of magnetic properties and susceptibility to manipulation by elemental substitution. In particular half-Heusler alloys are currently attracting growing attention due to their non-centrosymmetric C1b magnetic structure with broken inversion symmetry. This characteristic, in combination with a pronounced spin-orbit coupling, is of particular interest in current spintronic research. Most fundamentally, the inverse spin galvanic effect (ISGE) can directly generate a non-equilibrium spin polarization which subsequently induces a spin-orbit torque (SOT) on the magnetization. Phenomena like the ISGE take advantage of the relativistic momentum transfer between electron and spin just as another key mechanism of spin-orbitronics in form of the so called spin Hall effect (SHE). Here, an electric current transforms into a spin current due to a spin-orbit interaction in combination with either intrinsic material asymmetries or spin-dependent scattering which results in an interface spin accumulation. In this framework, the half-Heusler PtMnSb is an outstanding candidate for future SOT applications due to its unusual magneto-optical properties and electronic structure. This alloy exhibits a half-metallic band structure and a considerable magneto-optic Kerr effect (MOKE). In addition, strong intrinsic spin-orbit torque effects have been observed in this system. When these are combined with the half-metallic band structure of PtMnSb, a pronounced spin polarization can be generated which is very useful in the optimization of magnetoresistive effects. Few years ago, a novel spin Hall magnetoresistance (SMR) was discovered which is best described as an inverted GMR effect. This unidirectional SMR (USMR) is observed in a bilayer consisting of a ferromagnet and a heavy metal where the inversion symmetry is broken by either the interface or an intrinsic lack of inversion symmetry in the crystal structure or local environment of the magnetic material. In this inversion-asymmetric system, the resistivity of the heavy metal layer depends on the relative orientation of the magnetization of the ferromagnet to the SHE induced spin polarization at the interface. The next logical step is to combine both fundamental ways of symmetry breaking which is realized in PtMnSb/Pt bilayers due to their lack of inversion symmetry by both the interface and the local half-Heusler environment. This bilayer system is investigated as a central focus of this work, foremost to promote the understanding of spin-orbit torques in bulk non-centrosymmetric crystals. Therefore, the first experimental chapter of this work sets a focus on the preparation and optimization of half-Heusler PtMnSb alloy thin films and corresponding Pt capped bilayer systems for the application in spintronic devices. The structural and magnetic properties are studied in-depth to optimize the fabrication conditions and gain high quality thin films with epitaxial growth. However, the nonmagnetic Pt in these bilayers is close to the Stoner criterion and has the ability to become magnetic in the proximity to the ferromagnetic PtMnSb. This magnetic proximity effect (MPE) is another key aspect in spintronics and of particular importance since it can prevent the generation of pure spin currents or induce secondary magnetoresistive effects. In order to distinguish these SOT phenomena from magnetic-proximity induced effects, which spin polarize both the Pt in the Pt film and also within the half-Heusler compound, an in-depth magnetooptical characterization based on x-ray resonant magnetic reflectivity (XRMR) is performed. The XRMR technique allows to study the spin polarizations of buried magnetic layers which are not accessible by standard x-ray magnetic circular dichroism (XMCD) measurements. However, the in-depth analysis of the structural and magnetic depth profiles of the complexer half-Heusler bilayers turns out to be a challenge since there is no standard procedure to analyze XRMR asymmetry ratios. Another central aspect of this work is therefore the development of a step-by-step guide to take advantage of the excellent magnetic sensitivity and depth resolution of XRMR which should be universally applicable to allow for the precise evaluation of even minimal spin polarization depth profiles. By means of three specific systems, where Pt is integrated in the sample in an increasingly complex manner from simple bilayer over multi-layered stacks to the PtMnSb half-Heusler compound, one chapter of this work discusses a recipe procedure for the determination of an optimal XRMR asymmetry ratio simulation. This includes an iterative optimization approach based on various fitting algorithms as well as a detailed analysis of the asymmetry ratio features and the multidimensional error landscape. Following this process, this thesis addresses the XRMR investigation of the half-Heusler structure and interface quality as well as the element specific spin polarization depth profiles based on the previously established procedures. The obtained XRMR results are tested for reliability and compared to additional qualitative and quantitative studies of the magnetic PtMnSb characteristics to confirm the results of the XRMR simulation procedure. This precise determination of the magnetooptical depth profiles opens the route for further exploration and a potential application of spin-orbit torques in devices based on non-centrosymmetric Heusler alloys. The rise of the field of spintronics interconnects the conventional charge based electronics with the intrinsic spin of the electron. In the broader sense, as most prominently shown by the rapidly increasing energy consumption for heat transport in modern electronic data processing centers, this field of research is also intrinsically tied to the field of thermoelectricity. A decade ago, the discovery of the spin Seebeck effect (SSE), which specifies the generation of a spin voltage as a result of a temperature gradient, introduced the new fields of spin caloric transport. Here, the longitudinal spin Seebeck effect (LSSE) is the most prominent caloritronic effect, enabling an easy and versatile generation of spin currents based on heat. In an insulating material without any mobile charge carriers, the spin is transported by magnonic spin waves which propagate nearly free off dissipation and consequently are a candidate to reduce the steadily growing energy consumption of reading and writing information. The LSSE is commonly utilized to generate a pure spin current in a ferri- or ferromagnetic insulator (FMI) such as Yittrium Iron Garnet (YIG) or Nickelferrite (NFO). These FMIs represent a promising class of material for the implementation in spin caloric devices since the insulating character suppresses inadvertent charge currents which possibly prevent the detection of spin currents due to the induction of additional magnetoresistive effects. Such as the generation of pure spin currents, the detection is equally more complex compared to their electric counterparts. One particular robust method is the conversion by the inverse spin Hall effect (ISHE). Here, a material with a significant spin Hall angle, usually a heavy 4d and 5d transition metals such as Pd or Pt, is subjected to the spin current. Once again, a MPE has the potential to generate unintended charge related parasitic effects, underlining the importance of a reliable technique to study the element specific interfacial magnetic properties. In order to design spin caloritronic devices, it is of particular interest to examine the magnetization reversal process of these thin films. This is also of a more general interest, since the current day mass storage capabilities are realized based on the switching of magnetic memory, predominantly in hard drives. However, the study is usually realized by magnetooptical means via combinations of magneto-optic Kerr effects which are not applicable to key spintronic FMIs due to their vanishingly small Kerr rotation in the visible light range. Therefore, a magnetometry technique based on the vectorial observation of the spin Seebeck effect has been developed as part of this thesis. It utilizes the LSSE and ISHE voltage detection as an alternative to magnetooptical instruments to visualize the complete reversal process of the magnetization, demonstrating a spin caloric technique with broad applicability. This method is demonstrated in the study of NFO on lattice-matched substrates such as MgGa2O4. These substrates show significantly improved magnetic properties as well as enhanced SSE and enhanced spin transport characteristics with potential applications in high-frequency microwave and spintronics devices