Strong Dzyaloshinskii-Moriya interaction in symmetric crystalline [Co/Pd(111)]n superlattices

Abstract

When creating asymmetric superlattices [heavy metal 1 / ferromagnetic / heavy metal 2] n , the energies of perpendicular anisotropy and the Dzyaloshinskii-Moriya interaction (DMI), as a rule, depend weakly on the period of superlattices $n$ and remain approximately equal to the energies observed in the structure consisting of one period [1]. This is expected, since both the perpendicular anisotropy and the DMI in such structures are believed to be of an interface origin. The increase in the number of interfaces is compensated by an increase in the volume of the ferromagnetic layers. However, more recently in symmetric polycrystalline superlattices [Co/Pd] n , an increase in the effective DMI was observed with an increase in the number of repetitions of bilayers $n$ [2]. Similar results, but in the single-crystal system [Co/Pd(111)] n with higher values of the perpendicular magnetic anisotropy and DMI, were found by our group. Crystalline [Co(0.8 nm)/Pd(2 nm)] n superlattices were grown by molecular beam epitaxy on Si(111) substrates with a Cu buffer layer. Period of superlattices $n$ was varied from 1 to 20. Structural properties of superlattices were analyzed by reflection high energy electron diffraction. The structure and epitaxial orientation of all the layers remained the same as in the bottom layers. Topography and growth processes were investigated by scanning tunneling microscopy. With an increase in the overall thickness of the structures, the amplitude and period of roughness increased. Magnetic properties of the superlattices were investigated by vibrating sample magnetometer. An energy of perpendicular magnetic anisotropy slightly increased from 0.66 to 0.87 MJ/m3 with an increasing of the period of superlattices from 1 to 20. Measurements of DMI were performed by Brillouin light scattering spectroscopy based on DMI-driven asymmetric dispersion shift of long-wavelength thermal spin waves in the Damon-Eshbach surface mode. We found an increase of effective DMI constant $D_{eff}$ from 2 to 4.5 mJ/m2 with an increasing of the period of superlattices from 1 to 20. In the Fig. 1 Brillouin light scattering spectrum of the [Co(0.8 nm)/Pd(2 nm)] 15 superlattice is outlined. The increase in the DMI constant more than twice led to significant changes in the magnetic structure of the samples during magnetization reversal. In the single magnetic layered structures with $n =1$ DMI was relatively weak to influence on magnetic structure of domains, but it stabilized chiral Neel domain walls. In these samples by means of Kerr-microscopy, we observed an asymmetric growth of domains in the combination of longitudinal and perpendicular magnetic fields. Magnetization reversal in the single magnetic layered structures occurred by nucleation of oppositely magnetized domains and domain walls propagation. Increasing of the period of superlattices $n$ to 3–5 led to increasing of the DMI constant. In the magnetic structure during magnetization reversal, it resulted in increasing of the density of nucleating domains. The magnetic moments in all magnetic layers of a chosen domain were oriented in the same direction due to ferromagnetic coupling between Co layers. In sufficiently high magnetic fields, chiral Neel domain walls repelled from each other forming net of topologically protected 360° domain walls 360° domain walls transformed to isolated skyrmions in magnetic fields just below the saturation magnitudes. We stabilized skyrmions in such samples in zero magnetic field and detected them by magnetic-force microscopy. Micromagnetic simulations indicate that at such high values of DMI constant chiral Neel domain walls and skyrmions may be formed in [Co/Pd] 3-5 superlattices. Further increasing of the period of superlattices $n$ to 10–20 led to extremely strong DMI. Strong DMI overcame perpendicular magnetic anisotropy and demagnetized superlattices in the zero magnetic fields. In the remanent state, the magnetic structure of the superlattices corresponded to demagnetized stripe domain structure with chiral Neel domain walls and stable isolated skyrmions (Fig. 2). The period of the stripe domain structure decreased with increasing of the DMI constant in [Co/Pd] 10-20 superlattices. Strong dependence of the DMI constant on the period of [Co/Pd(111)] n superlattices indicates that DMI in this system is not simply additive. Despite the interface nature, DMI from the interfacial Co atoms of the selected magnetic layer extends not only to the volume of a given layer, but also to other neighboring layers. Probably this behavior of DMI is general for magnetic superlattices, but in other systems it may be hindered by increasing amount of structure imperfections during growth of thick superlattices. In conclusion, we demonstrate the symmetric crystalline [Co/Pd(111)] n system, in which the DMI constant may be varied over a wide range by choosing the appropriate period of superlattice, making possible skyrmions and chiral stripe domain structure formation in the zero magnetic fields. The reported study was funded by RFBR (research project 18-02-00205) and the Grant program of the Russian President (MK-5021.2018.2).