Interfacial Dzyaloshinskii-Moriya Interaction in Pt/CoFeB Films: Effect of the Heavy-Metal Thickness

Recently, the Dzyaloshinskii-Moriya interaction (DMI) [1], i.e., the antisymmetric exchange interaction, has attracted a great deal of attention due to its crucial role in the stabilization of chiral spin textures such as spin spirals and magnetic skyrmion lattices. A considerable interfacial DMI arises from the inversion symmetry breaking at interface between a ferromagnetic layer and a nonmagnetic one having a large spin-orbit coupling. Due to the enormous potential of chiral magnetic domain walls and skyrmions for the development of a new generation of spintronic devices, it’s now crucial to obtain a detailed knowledge of the interfacial DMI in ultrathin films. In this context Brillouin light scattering (BLS) revealed to be a powerful tool to investigate interfacial DMI. In a BLS experiment, the presence of an interfacial DMI manifests itself in an asymmetric dispersion relations of the counterpropagating Damon-Eshbach spin wave modes. In this work, BLS has been exploited to study the interfacial DMI in ultrathin CoFeB films in contact with a Pt layer with variable thickness.[2] We found that the strength of the interfacial DMI increases with Pt thickness (d_Pt), up to reach a saturation value for d_Pt larger than a few nanometers. The experimental results are quantitatively explained by a theory based on an extension of the 3-site indirect exchange mechanism [3], where the asymmetric exchange interaction between two neighboring ferromagnetic atoms is mediated by a third nonmagnetic atom having a large spin-orbit coupling. We found that the increase of the interface DMI as a function of the Pt thickness is due to the cumulative electron hopping between the atomic spins at the interface and the non-magnetic atoms in the heavy metal.

[1] I. Dzyaloshinskii, Sov. Phys. JETP 5 , 1259 (1957); T. Moriya, Phys. Rev. Lett. 4 , 228 (1960).
[2] S. Tacchi et al., Phys. Rev. Lett. 118 , 147201 (2017).
[3] P. M. Levy and A. Fert, Phys. Rev. B 23 , 4667 (1981).