Seminar experimentelle Physik der kondensierten Materie

The geometry of magnetic objects at the nanoscale plays a crucial role in their properties. Conventionally, the respective phenomena were considered for a long time as a result of sample boundaries leading, e.g., to the formation of closed-flux magnetization distributions and the interaction of magnetic solitons with notches in racetracks. However, such a simplified picture omits the topological properties and symmetries of the sample. A quantitative approach to predict magnetic responses based on the geometry of magnetic nanoarchitectures is provided by the theory of curvilinear magnetism. In this talk, we will discuss analytical approaches and some experimental validations of theoretical predictions for curvilinear nanomagnets. The local bends and twists of low-dimensional ferromagnets enable chiral and anisotropic responses stemming from the exchange interaction. For the particular sample scale, these responses are complemented by the magnetostatics-induced symmetry breaks and the respective formation of multiple magnetochiral characteristics of the magnetic textures. Antiferromagnetic nanoobjects inherit particular properties of curvilinear ferromagnets and complement them with more complex properties of the Neel order parameter and field-induced spin-reorientation transitions.

Spin Metal-Oxide-Semiconductor Field-Effect transistor (MOSFET) is a promising device [1] that utilizes the fundamental material in electronics: silicon (Si). In 2007, the electrical injection of spin current into Si was achieved, overcoming the challenge of "conductance mismatch" [2]. Over the past decade, extensive research has been devoted to studying the transport of spin current in Si, resulting in the realization of room temperature operation [3]. In our investigations to enhance the magnetoresistance ratio, we have focused on understanding the physics of the metal/Si interface and proposed three different approaches [4-6]. Firstly, we have made progress by improving the crystal alignment between the spin source material and the tunneling barrier through moderate-temperature thermal annealing [4]. Secondly, we have demonstrated the spin-dependent Seebeck effect, a novel spin caloritronic effect, for the first time using semiconducting materials [5]. Lastly, we have achieved a significant reduction in electrical resistance at the interface by implementing ohmic contact on non-degenerate n-type Si with ferrimagnetic material, resulting in a 100-fold improvement [6]. By leveraging these technologies, we anticipate a notable improvement in the magnetoresistance ratio, bringing us closer to practical device applications. Reference [1] S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84, 2307 (2004). [2] I. Appelbaum, B. Huang, and D. J. Monsma, Nature 447, 295-298 (2007). [3] T. Tahara, Y. Ando, M. Shiraishi, et al., Appl. Phys. Express 8, 113004 (2015). [4] N. Yamashita, Y. Ando, M. Shiraishi, et al., AIP Advances 10, 095021 (2020). [5] N. Yamashita, Y. Ando, M. Shiraishi, et al., Phys. Rev. Applied 9, 054002 (2018). [6] N. Yamashita, M. Shiraishi, Y. Ando, et al., Phys. Rev. Materials 6, 104405 (2022).