Chiral nanomagnetic systems and logic

Magnetically coupled nanomagnets have many possible applications including non-volatile memories, logic gates, and sensors. So far, the most effective couplings have been found to occur between the magnetic layers in vertically layered stacks. We have exploited the Dzyaloshinskii-Moriya interaction to control the lateral coupling between out-of-plane and in-plane non-homogeneous magnetic structures, and demonstrate unique phenomena associated with such chirally coupled nanomagnets [1, 2]. Based on this chiral coupling, we have further develop a method of performing all-electric logic operations and cascading with domain-wall racetracks, providing a viable platform for novel logic-in-memory architectures [3, 4].

It is possible to design complex coupled nanomagnetic systems with lateral magnetic coupling such as those used for nanomagnet logic or artificial spin ice. Currently, most laterally coupled nanomagnet systems make use of the dipolar interaction, for which the coupling strength is proportional to the volume of nanomagnet. However, in order to electrically control these systems via spin-orbit torques, the nanomagnets should be extremely thin, which means that the dipolar coupling is very weak. Here, we couple the nanomagnets via the interfacial Dzyaloshinskii-Moriya interaction (iDMI). iDMI can be expressed by an energy term H_DM = - D_ij·(m_i × m_j), which represents the antisymmetric exchange coupling between two neighbouring magnetic moments m_i  and m_j  induced by the spin-orbit interaction in a structurally asymmetric environment (Fig. 1a). Since it is an interfacial effect, iDMI-induced coupling is only effective in thin magnetic films. In a homogeneous film, the presence of iDMI results in chiral domain walls and topological non-trivial magnetic textures such as cycloids or skyrmions.

We have patterned regions with in-plane (IP) and out-of-plane (OOP) magnetic anisotropy in a magnetic element using selective oxidation of Pt/Co/Al films (Fig. 1b). In addition to classical non-chiral interactions, the magnetization in the OOP and IP parts of the islands are coupled via iDMI arising from the Pt underlayer. As shown in Fig. 1b, this results in chirally coupled magnetization so that, when the IP region is set to point to the right (→) with an in-plane magnetic field, the OOP regions on the left and right of the IP region point down (⊗) and up (⊙) accordingly, giving either ⊗→ or →⊙ configurations. This agrees with the left-handed chirality generally expected from the negative sign of the iDMI in Pt/Co/AlOx and shown in the schematics in Fig. 1a.