Synchrotron X-ray study of the epitaxial Co/Pt multilayers on Al₂O₃$\text{(1}\text{1}\text{2}\text{̄}\text{0)}$ substrates with Pt/Mo buffer layers

Abstract

In this study, epitaxial [Co(t nm)/Pt(1 nm)]₃₀ multilayer samples (t=1, 0.5, 0.4, 0.3, 0.25 and 0.2 nm) were studied using X-ray measurements. The samples were prepared on Pt(1 1 1)/Mo(1 1 0) buffered Al₂O₃(1 1 2̄ 0) substrates in the MBE chamber. It was found that the Pt layer in the multilayer has a compressible strain of about 2–3.5% along the in-plane direction and the saturation magnetization is roughly proportional to this strain. Polarized X-ray absorption spectroscopy study showed that the Co layer is more like an FCC pseudomorphic structure for Co thickness less than 0.5 nm.

Introduction

The existence of large perpendicular magnetic anisotropy (PMA) [1], [2], [3], [4], [5] is of great interest for a high-density magnetic optical device. The PMA can exist in the ultra thin multilayer as a consequence of symmetric breaking at the ferromagnetic and non-ferromagnetic interface. Therefore, to understand the interface structure, it is of great importance to understand the PMA effect. For instance, it was predicted that the PMA for the transition metal–noble metal multilayer might be strain-related and interface roughness related [6]. In previous works involving the study of MCD effect of Co/Pt thin film using soft X-ray absorption spectroscopy [7], [8], an FCC pseudomorphic layer of 5 ML was reported and the PMA effect was found for Co less than 5 ML. Sato [9] studied this thin film by changing the layer thickness between Co and Pt, where an FCC-like structure can be found if the Co layer is smaller than that of Pt. Galeotti et al. [10] used X-ray photoelectron diffraction to examine the growth of thin Co on Pt(1 1 1) surface and found that Co formed an FCC pseudomorphic structure. H. Bulou et al. [11], [12] later used grazing incidence X-ray scattering and X-ray photoelectron diffraction to examine the growth of thin Pt on Co(0 0 0 1) surface and found the Pt that has formed in a twinned FCC structure. We also reported that in the [Co(0.3 nm)/Pt(1 nm)]₃₀ multilayer on Al₂O₃(1 1 2̄ 0), the Pt(1 1 3) peak was shifted to a position with a strain of 2.7% [13]. The samples have also been studied under Kerr effect and vibrating-sample magnetometer. The magnetic moment was found to be perpendicular to the surface when the thickness of Co layer is less than 0.5 nm. For the Co layer thicker than 0.5 nm, the magnetic moment is parallel to the interface plane. It is interesting to see the structural differences among different Co thicknesses in order to understand this PMA effect in detail. It is also interesting to see how the 10% lattice mismatch between the Co and Pt layer can be accommodated in these epitaxial multilayer thin films.