Spin Vortices in Magnetic Multilayers


Spin Vortices in Magnetic Multilayers

Wintz, S.

The focus of this thesis are spin vortices in magnetic multilayers. A spin vortex is a topological spin texture, characterized by a planar magnetization curl that tilts out- of-plane in the nanoscopic core region at the center. There are two different states possible for both the rotation sense of the curl (circulation) and the orientation of the core (polarity), leading to a binary vortex chirality. The spin vortex is typically the ground state of micron sized ferromagnetic thin film disks or square-shaped elements. During the past decades, spin vortices were studied intensively with respect to their magnetostatic and—in particular—their intriguing magneto-dynamic properties. Along with these fundamental studies, proposals were made for the application of vortices for example in data storage and radio-frequency oscillator technology. With respect to density and synchronization in both these application concepts—but also from a fundamental point of view—the interaction between vortices is a crucial issue to address. This holds true especially for the vertical geometry of trilayer vortices (ferromagnet/non-ferromagnet/ferromagnet) in which giant- or tunnel magneto resistance effects as well as spin-transfer torques are exploitable. However, the knowledge on such coupled vortices has been fairly limited. Therefore it was the aim of this work to investigate the magnetic properties of vertically coupled vortex pairs. Investigations were mainly made by means of synchrotron-based transmission x-ray microscopy. This method uniquely allows for a direct and element selective imaging of the magnetic orientation in spin textures with the lateral resolution required. The vortex pair samples were fabricated by electron beam lithography and thin film deposition techniques. Additional magnetometry measurements were employed to address the basic material and coupling properties. Also micromagnetic simulations were carried out in order to complement the experimental results. By this approach, different vortex pair configurations concerning the relative orientation of circulations and polarities, are identified. It is shown that interlayer exchange coupling can be exploited to deterministically set the relative circulation state, and that ion irradiation provides a method to finely tune this coupling after the multilayer has been deposited. In contrast, the polarity states are found not to be controllable by any of the interlayer coupling strengths accessible. Both states are stable in fact, with the parallel configuration representing the ground state. The field response of a vortex pair is evaluated to range from a quasi-independent behavior to a strict coupling between both layers, where the latter can be described by a single layer vortex response with effective magnetic properties. When a relevant non-collinear interlayer coupling is introduced to the vortex pair state, the system is found to exhibit radial magnetization components with opposing sign between both ferromagnetic layers, which in turn results in the formation of a three-dimensional torus vortex accompanied by a symmetry break of the polarity states. The findings achieved in this work mark a significant advance of the state of the art in the field of spin vortices and coupled spin textures, and they may also be of value for future vortex-based applications in technology.

Keywords: magnetism; vortex; multilayer; coupling; x-ray microscopy

Related publications

  • Doctoral thesis
    TU Dresden, 2014
    Mentor: Prof. Dr. Jürgen Faßbender
    153 Seiten

Permalink: https://www.hzdr.de/publications/Publ-20230
Publ.-Id: 20230