Annealing Temperature and Thickness Dependencies of Perpendicular Magnetic Anisotropy and Dzyaloshinskii– Moriya Interaction of Pt/Co/MgO Thin Films

Effects of the ferromagnetic layer thickness and annealing temperature were studied in Pt-/Co-/MgO-based systems grown on thermally oxidized Si substrates using magnetron sputtering system. The Co thicknesses were varied in the range of 1–6 nm and the samples were annealed ex situ at 200 °C, 300 °C, and 400 °C. Vibrating sample magnetometry was used to determine the magnetization at saturation and the magnetic dead layer thickness that were found to be insignificantly affected by the annealing. Brillouin light scattering, in Damon–Eshbach configuration, was used to measure the thermally excited spin wave (SW) frequencies versus the in-plane applied magnetic field and the SW vector which were used to investigate the perpendicular magnetic anisotropy (PMA) and interfacial Dzyaloshinskii–Moriya interaction (iDMI), respectively. The Co effective thickness dependence of the effective magnetization ( $M_{\mathrm {eff}}$ ), which provides information on the PMA constants, shows the existence of two linear regimes with different slopes due to misfit strain-induced magnetoelastic anisotropy contributions to volume and surface terms. The volume magnetocrystalline anisotropy constant increases monotonously with annealing temperature, whereas the pure surface (Néel type) constant increases with annealing up to 300 °C and degrades at 400 °C. The surface iDMI constant decreases with increasing annealing temperature and no obvious correlation with interface PMA constant has been found, suggesting different interfaces contribution to PMA and iDMI.

I. Introduction

Interfacial Dzyaloshinskii–Moriya interaction (iDMI) [1], which is an antisymmetric exchange interaction between neighboring spins, is an emerging magnetic interface-related phenomenon that affects both static and dynamic magnetic properties of ultrathin film-based systems. Indeed, in such structures, if the layers on bottom and top of the ferromagnetic (FM) ultrathin film are not similar, then the symmetry is broken in the direction perpendicular to the sample plane. Furthermore, if (at least) one of the bottom or the top layers with FM has strong spin–orbit coupling, iDMI arises and manifests itself by an internal magnetic field that would act like the external in-plane field favoring a chiral arrangement of the magnetization. For instance, if iDMI is large enough, it changes the domain wall ground state from Bloch wall to homochiral Néel domain wall in perpendicular magnetic anisotropy (PMA) systems [2]. Moreover, skyrmions [3] are stabilized by interplay between iDMI and other interactions like symmetric exchange, dipolar, and magnetic anisotropy. Therefore, a sizeable iDMI is an essential component in generating magnetic skyrmions, which can be used in high-density digital technologies for racetrack memory [4] and logic devices [5].