Research Area A: Metal-based NANO-SPINTRONIC
The functionality of future metal-based spintronic devices relies critically on the three-dimensional lateral confinement of heterostructures and the magnetic interactions within the device architecture. In order to tailor the magnetic properties it is mandatory to control structural parameters over a wide range of length scales. The investigation of the structural and magnetic properties with highest spatial resolution is one of the prime goals. Likewise, the investigation of the spin dynamics with highest temporal resolution will be a central goal of our studies. The experimental activities will be complemented by a wide range of theoretical investigations, simulation, and modeling activities.
X-ray holography has been established in Hamburg in recent years in a successful cooperation between the University and DESY (Oepen, Grübel). In the framework of the proposed Cluster, the goal is to take images of the domain structure in magnetic nanowires via X-ray holography while measuring magnetoresistance and Anomalous Hall Effect in order to determine the domain wall resistance. In a second step, we will try to push domain walls via current pulses and image the time evolution of the domain structure. Perpendicularly magnetized systems, like Co/Pt, are in the focus of present research as their domain walls are very small. The ballistic transport through narrow walls causes a larger momentum transfer which enables switching at lower current densities.
Thermoelectric effects in magnetic or semi-metallic nanowires under the influence of magnetic fields have recently attracted considerable interest. For instance, the observation of the spin-dependent Seebeck and Peltier effects in nanowires has been reported. This emerging field is called ”Spin Caloritronics”and bridges the gap between spintronics, magnetism, and thermoelectricity. The Nielsch group will study the effect of the Giant Magneto Resistance (GMR) on the thermal conductivity in nanowires. In general the thermal conductivity in metallic systems is proportional to the electrical conductivity (Wiedemann-Franz law). The limits of the Wiedemann-Franz law for nanostructured GMR systems will be investigated. Co/Cu multilayered nanowires will be synthesized via pulsed electrodeposition in self-ordered Al2O3 membranes. Nanowires released from the Al2O3 matrix will be electrically contacted. Thermal conductivity measurements will be performed based on the hot wire approach and the 3-omega technique. In a second step the project will be extended to thermopower and thermo-galvanic measurements of GMR nanostructures and semi-metallic systems, e.g. bismuth, in magnetic fields.
Logic devices based on macro-spins are in the scope of a project that is focused on the fabrication of nanomagnets and the investigation of their interaction in an ordered matrix (Nielsch, Oepen). Imprint lithography and electrochemical oxidation processes will be applied to prepare perfectly ordered Al2O3 membranes consisting of hexagonally arranged pore channels. Individual pore channels will be functionalized via focused ion beam (FIB) and filled selectively with ferromagnetic material, enabling the fabrication of cylindrical nanomagnets with high-aspect ratio, so called macro-spins, which exhibit a bistable behavior caused by the shape anisotropy. The ultimate goal is the development of macro-spin arrays for applications as logic elements. The dipolar interaction will be used to mediate the information along lines of macro-spins. Logic devices will be achieved via branching and crossing of lines of macro-spins and their specific two-dimensional geometrical arrangements. The experimental results will be compared with theoretical studies (Vedmedenko).
Since many spintronic devices require insulating elements as building blocks we also plan to study the properties of magnetic nanostructures in direct contact with insulators. For this purpose we will utilize our recently developed technique of magnetic exchange force microscopy (MExFM), which is sensitive to the force between atomic magnetic moments and hence is not limited to conducting samples (Schwarz). MExFM has a short interaction range of Angstroms which enables the imaging of the magnetic structure with atomic resolution, including antiferromagnetic and complex spin structures. For instance, exchange-bias systems including metallic-magnetic/insulating-magnetic interfaces will be addressed. By recording the energy dissipation in MExFM experiments, we will additionally study spin excitations, i.e. magnons, in laterally confined systems.
Using spin-polarized scanning tunneling microscopy (SP-STM) it is feasible to resolve atomic scale magnetic structures with non-collinear and complex magnetic order (Wiesendanger group). In systems lacking inversion symmetry, like at interfaces and surfaces, spin-orbit coupling gives rise to the Dzyaloshinskii-Moriya interaction (DMI) which induces a canting between magnetic moments of adjacent atoms with specific sense of rotation. DMI can compete with exchange interaction and magnetic anisotropy in metallic nanostructures. The competition of these parameters generates a huge variety of magnetic order such as helical, cycloidal, and skyrmion spin structures. We want to explore this complex magnetic phase space using a combination of density functional theory (Heinze), Monte-Carlo simulations (Vedmedenko), and SP-STM experiments (von Bergmann, Wiesendanger). The first goal is to understand the underlying principles and the microscopic origin of competing magnetic interactions leading to uniaxial spirals or nanoscale two-dimensional magnetic order. In a second step, we will investigate the dynamics of such structures due to excitations by tunneling electrons. Moreover, we will use spin-polarized current pulses from an STM tip to study the stability of non-collinear spin states in nanoscale magnetic systems (Krause). We will also study systematically the thermal stability of such systems by variable-temperature SP-STM (Pietzsch, Krause, Wiesendanger).
Current-induced manipulation of magnetization will facilitate further down-scaling of magnetic random access memories (MRAMs), since magnetization reversal by means of an electric current is much easier to perform than using external magnetic fields. One aim of this initiative is to understand the microscopic processes involved in interactions between spin-polarized currents and nanoscale magneticstructures with complex spin states. The transport parameters will be gained from unified ab-initio calculations (Lichtenstein, Lechermann) and be taken as an input for Monte-Carlo and spin dynamics simulations (Pfannkuche, Vedmedenko, Potthoff, Bolte) as well as an analytic formalism (Gunesch, Vedmedenko, Hinze) bridging the nano- and micro-scales. These multi-scale theoretical approaches will play a decisive role for a fundamental understanding of complex nanodevices in research areas A and C.
When the spin-torque is exploited to control magnetic states in nanostructures, spin excitations are inherently involved. While spin waves in bulk materials are well understood, excitations in nanoscale magnetic structures are in the focus of current research activities worldwide. We plan a joint effort to investigate the spin dynamics in nanostructured materials by inelastic neutron scattering (Schreyer) and inelastic electron tunneling spectroscopy (Kubetzka). Neutron scattering on one hand can access averaged properties of magnetic systems and, with its wave vector resolution, the dispersion of spin waves for the entire Brillouin zone. With inelastic scanning tunneling spectroscopy (ISTS) on the other hand, individual nanoscale elements can be addressed and their dynamic properties can be correlated to their precisely known atomic arrangement. In addition, we will investigate spin waves in superlattices made of monodisperse magnetic nanoparticles separated by polymers (Förster, Weller). These samples can be prepared in a comparatively large volume, providing maximum intensity in neutron scattering experiments. We aim at a detailed understanding of the crossover from quasi-classical spin waves at large length scales to quantum-spin excitations at the atomic limit.
3D-Animation: Current-Induced Magnetization Switching with a Spin-Polarized Scanning Tunneling Microscope
Soft X-ray microscopy with a spatial resolution of 10 nm in combination with time resolution in the picosecond regime is a powerful tool for spatio-temporal investigations. At the Advanced Light Source in Berkeley, the groups of Meier, Bolte, and Merkt will investigate, in collaboration with international partners, soft magnetic, micron- and nanosized thin-film permalloy (Ni80Fe20) structures with magnetic vortices and domain walls. A vortex could serve as bit in future magnetic memory devices. Interesting topics are the influence of the confining potential on the dynamics of the vortex and a possible blue or red shift for larger gyration amplitudes. The planned experiments will be compared to theoretical descriptions by Guslienko, Shibata, and our own analytical models (Pfannkuche) as well as micromagnetic and transport simulations (Bolte, Meier, Möller). Through electric transport measurements and time-resolved X-ray microscopy in collaboration with our partner at the IBM Almaden Research Center, San José, we will advance the understanding of the spin-transfer torque in magnetic nanostructures. We will study the magnetization dynamics of domain walls in nanowires with in-plane and perpendicular magnetic anisotropy.
In conclusion, in the research area A groups that have demonstrated successful cooperation within the SFB 668 and the GrK 1286 will undertake a joint effort to advance metal-based spintronics, particularly in view of the role of lateral confinement and non-collinear spin states. Based on the unique combination of experimental methods, which allow for fabrication and analysis of magnetic nanostructures, including atomically resolved spin mapping and spin state manipulation by SP-STM and MExFM at the Institute of Applied Physics and facilities for investigating the spin dynamics at ultrashort time scales at DESY, we aim at the visionary goal of developing nanoscale spintronic devices based on a detailed knowledge of the atomistic spin-dependent interactions and processes.