Enhancement of Skyrmion Density Achieved via Interface Engineering

Abstract

Skyrmions are chiral spin textures, which have gained considerable interest due to their promising applications in memory and synaptic devices [1]. These textures can exist in a low-density state or in a high-density state [2]. A high density skyrmion state is more favourable for high-density memory devices. However, formation of the high density state requires a subtle balance of magnetic properties . One of the key requirements needed for the high density state is a relatively high value of Dzyaloshinskii Moriya interaction (DMI) as compared to the effective magnetic anisotropy (Keff) and exchange coupling energy (Aex) [3]. DMI is an anti-symmetric exchange interaction induced by spin-orbit coupling in the presence of broken inversion symmetry [4]. In a specific thin material system, DMI can be tuned by either changing the thickness of ferromagnet or by varying its stoichiometry. Former has been reported for materials like Pt/Co/AlOx, Pt/CoFeB/AlOx, and Ta/FeCoB/TaOx [5, 6]. In these studies, decreasing the thickness of ferromagnet results in a higher DMI. However, a decrease in thickness also results in an enhanced value of Keff, which limits the skyrmion density to a certain number. In the latter report, which was carried out on Ir/Fe/Co/Pt structures, the thickness and composition of the ferromagnetic layers was varied to modulate skyrmion size and density [7]. However the used material system, because of polycrystalline nature of FM resulting in grains, is inadequate for an unhindered skyrmion motion.Here, we report a reduction of effective anisotropy energy density Keff ,by using post growth 20 keV He+ ions induced interface intermixing, without compromising DMI. We have used Pt/CoFeB/MgO stacks to study the effect of varying ion irradiation dose and carried out Monte Carlo simulations to get the probability of the ions’ distributions and the intermixing in the sample [8]. The displacement of ferromagnetic layer atoms increases linearly with increasing dose (inset of Fig. 1a). To study the effect of such intermixing on magnetic properties, we have used a vibrating sample magnetometer. We have found that Keff is reduced as atomic intermixing at interfaces between the ferromagnetic and non magnetic layers increase, which corroborates the behaviour of Keff from previous reports [9]. Interestingly, even at high dose, Keff still remained positive, indicating an out-of-plane magnetization. (Fig. 1a). Moreover, we have observed an increase in the saturation magnetisation (Ms) (fig 2a), due to increased polarization of the ferromagnetic component . The increase in polarization is caused by formation of CoPt alloy at the Co-Pt interface, which results in the increase of the spin moment at interface up to ~17% [10]. Furthermore, we speculate the increase in magnetisation also includes contribution from the migration of B atoms at interstitial sites between Co and Fe atoms. Besides, we have measured the interfacial Dzyaloshinskii-Moriya interaction (DMI) by the Brillouin light scattering spectroscopy . We have noticed an increase in the DMI values of the irradiated samples. Considering a possibility of change in the effective thickness of FM, we have calculated surface Ms and hence, interface DMI (Fig. 2a inset and 1c). We find that the interface DMI increases under intermixing. Moreover, we have used micromagnetic simulations to estimate the exchange coupling energy (Aex) by assessing the periodicity of equilibrium state achieved at zero field (Fig. 1d). Noticeably, both DMI (interface DMI) and Aex follows a similar trend as Ms.To probe the change in magnetization dynamics due to the modification of the magnetic properties, we have carried out magnetic force microscopy (MFM) as shown in Fig. 2a and 2b. We notice the zero-field periodicity of labyrinthine textures is reduced by ~50 nm, depicting a higher density of striped domains after processing the sample (Fig. 2c). The skyrmion density is increased up to six folds as compared to the pristine sample (Fig. 2d). We speculate that the increase in skyrmion density and reduction in periodicity is due to the reduction of domain wall energy. Furthermore, we have also measured the skyrmion size and we observe a reduction by ~50 nm, down to 75 nm for large intermixing (Fig. 2d). Besides, we have carried out a comparatively low energy treatment (annealing) of deposited stack to explain the physical phenomena of skyrmion density enhancement. Our study demonstrates that post growth He+ irradiation can achieve a high density skyrmion state, which is promising for memory, logic and neuromorphic applications. **