Interface roughness driven magnetic anisotropy and Dzyaloshinskii- Moriya interaction in thin films with broken structural inversion symmetry

Abstract

An interface between ${3d}$ transition metal and ${5d(4d)}$ heavy metal (HM) is a host of intriguing spin-related effects desirable for spin-orbitronic applications [1]. Chiral interaction between two ${3d}$ spins due to a strong spin-orbit coupling (SOC) in an underlying atomic layer of heavy metal is associated with the Dzyaloshinskii-Moriya interaction (DMI) [2]. The and interfacial DMI (iDMI) value is extremely sensitive to the ferromagnetic layer thickness $( \mathrm {t}_{FM})$, where the non-linear dependence of iDMI on $\mathrm {t}_{FM}$ implies the degradation of interfaces at small $\mathrm {t}_{FM}$ [3]. Recently, Wells et al. [4] has demonstrated the effect of interface quality, reflecting both roughness and intermixing of interfaces, on iDMI in symmetrical Pt/Co/Pt trilayers. There is a radically different situation in the inversion symmetry broken systems, where the ideal (which is difficult to achieve in polycrystalline films) or having identical quality of top and bottom interfaces of a ferromagnetic layer are the main source of the enhanced iDMI [5], [6]. One can suppose that if in such systems the quality parameter, defined as the difference between quality of top and bottom interfaces of the magnetic layer, tends to zero, iDMI will be strong. The case of the coherent roughnesses of top and bottom interfaces corresponds to this assumption, which arises from the sensitivity of iDMI to the magnetic layer thickness variation. We report on results of the investigation of the magnetic anisotropy and interfacial DMI in the magnetic layer sandwiched between two ${5d}$ HM (Pt and Ta) (Series 1 - Pt(2)/CoFeSiB(1.5)/Ta(5)) or one ${5d}$ HM (Pt) from the bottom and ${4d}$ HM (Ru) from the top (Series - 2 Pt(2)/CoFeSiB(1.5)/Ru(3)/Ta(5)), where the layer's thickness is indicated in nm. The two series of samples were deposited by magnetron sputtering on the top of the Pd surface with artificially introduced interface roughness: The substrates with Pd seed layer of different thickness ranging from 0 to 56 ML (or from 0 to 12.6 nm were epitaxially grown on the atomically smooth Si(111)/Cu surface. Roughness of the Pd surface in dependence on the thickness was measured by and AFM. The surface is represented by well-ordered atomic terraces of Si, covered by Cu seed layer with the Pd islands grown on top of it. The island growth leads to the significant increase of the mean-square roughness (R q ) as well as the amplitude of roughnesses of the Pd surface. But the period of roughnesses remains constant within the value distribution: $\mathrm {p}= 70 \pm 10$ nm. In order to analyze the interface quality (roughness, intermixing, thickness variation) we used the X-ray reflectivity (XRR) measurement technique. The study was performed on a SmartLab (RIGAKU) X-ray diffractometer at CuK $\alpha $ radiation wavelength (1.54A). The simulations of XRR spectra were performed with GlobalFit software. The main trend is the increase of the interface roughness for all layers with the rising $\mathrm {t}_{Pd}$. Noteworthy, the initial R q for CoFeSiB is higher than for rest layers. It can relate to the amorphous nature of the magnetic layer. With increase of $\mathrm {t}_{Pd}$ up to 10.2 nm roughness of CoFeSiB layer approaches to R q of the underneath Pd and Pt layers, which corresponds to the roughness coherency. The experimental data revealed that Ta capping layer smooths the surface roughness. We observed the correlation between the interface quality factor $(\Delta \sigma / \sigma )$ and iDMI for both series. The direct measurement of DMI was performed by Brillouin light scattering (BLS) spectroscopy based on DMI-driven asymmetric dispersion shift of long-wavelength thermal spin waves in the Damon-Eshbach surface mode. At the small Pd roughness the magnitude of $\Delta \sigma / \sigma $ has maximum that means the highest incoherency between top and bottom interfaces of CoFeSiB layer leading to the relatively small iDMI. The flowing tendency of $\Delta \sigma / \sigma $ with rising $\mathrm {t}_{Pd}$ reflects the growing coherence of interface roughnesses causing in significant increase of iDMI about twice for series 1 and 2. We demonstrate the direct dependence between the quality of CoFeSiB interfaces, the iDMI value and magnetic anisotropy. The coherent roughnesses of top and bottom interfaces can enhance the iDMI value up to 2.5 times with the maximum obtained value $\mathrm {D}_{eff} \quad = -0.9$ mJ$/ \mathrm {m}^{2}($ or surface DMI is Ds $= -1.05$ pJ/m), which is largest known iDMI for CoFeB-based systems. For samples with high iDMI we observed the isolated skyrmions (or skyrmion bubbles) formation with the size about 200–300 nm. This work was supported by the Russian Foundation for Basic Research (grant 17–52-50060), by the Russian Ministry of Education and Science under the state task (3.5178.2017/8.9 and 3.4956.20 17/VU) and Far Eatern Federal University.