Dzyaloshinskii\u2013Moriya interaction in noncentrosymmetric superlattices

Woo Seung Ham,Abdul-Muizz Pradipto,Teruo Ono,Sanghoon Kim,Yoichi Shiota,Kwangsu Kim,Kohji Nakamura,Kay Yakushiji,Sonny H Rhim

doi:10.1038/s41524-021-00592-8

Abstract

Dzyaloshinskii–Moriya interaction (DMI) is considered as one of the most important energies for specific chiral textures such as magnetic skyrmions. The keys of generating DMI are the absence of structural inversion symmetry and exchange energy with spin–orbit coupling. Therefore, a vast majority of research activities about DMI are mainly limited to heavy metal/ferromagnet bilayer systems, only focusing on their interfaces. Here, we report an asymmetric band formation in a superlattices (SL) which arises from inversion symmetry breaking in stacking order of atomic layers, implying the role of bulk-like contribution. Such bulk DMI is more than 300% larger than simple sum of interfacial contribution. Moreover, the asymmetric band is largely affected by strong spin–orbit coupling, showing crucial role of a heavy metal even in the non-interfacial origin of DMI. Our work provides more degrees of freedom to design chiral magnets for spintronics applications.

Highlights

The lack of inversion symmetry at the interface between a heavy metal (HM) and a ferromagnet (FM) induces the antisymmetric exchange interaction so-called Dzyaloshinskii–Moriya interaction (DMI)[1,2,3,4]
DMI has been intensively studied in the material combinations possessing perpendicular magnetic anisotropy (PMA) due to their necessities in creating magnetic chiral textures, such as magnetic skyrmions for the new type of racetrack memory device[5,6,7,8]
In order to stabilize skyrmions at room temperature, multilayer structures with repetitive stacking of FM/HM bilayer are utilized because multi-stacking of the bilayer unit provide the PMA and the sizable DMI at the same time, both of them arising from the same physical origin, i.e., interfacial SOC9–13

Summary

INTRODUCTION

The lack of inversion symmetry at the interface between a heavy metal (HM) and a ferromagnet (FM) induces the antisymmetric exchange interaction so-called Dzyaloshinskii–Moriya interaction (DMI)[1,2,3,4]. In order to stabilize skyrmions at room temperature, multilayer structures with repetitive stacking of FM/HM bilayer are utilized because multi-stacking of the bilayer unit provide the PMA and the sizable DMI at the same time, both of them arising from the same physical origin, i.e., interfacial SOC9–13. Note that not interfaces but asymmetry of bulk-type band formation in the [Co/ Pd/Pt]-SL as illustrated in Fig. 1b is essential to give rise to such a chiral phenomenon, resulting in strong PMA Such inversion symmetry breaking (ISB) in the SL with ABC-type stacking order would traditionally be accounted for by involving the Rashba model Hamiltonian, HR 1⁄4 αRðk ^zÞ Á σ, which was initially proposed for a surface, where ^z is the direction of inversion-symmetry-breaking-induced potential gradient[17]. The observed behavior of DMI upon increasing the repetitions of the ABC-layer unit in the SL suggests that while the interfacial and bulk DMI co-exist with small N, the enhancement of DMI with larger N can be attributed to the bulk-type asymmetric band formation around the Fermi level

RESULTS AND DISCUSSION

METHODS

CODE AVAILABILITY