Bulk-like Magnetic Moments in Epitaxial Two-dimensional Superlattices

Abstract

The family of two dimensional (2D) magnets has been expanded with tremendous speed since the two pioneering 2D magnetic insulators, Cr2Ge2Te6 [1] and CrI3 [2], were isolated in 2017, and has exhibited great potential in developing spintronic devices for next generation data storage [3]. Over the past four years, the magnetism of 2D magnets has been extensively studied by the full arsenal of probing techniques. Due to their sensitivity to external stimuli, various modulations, such as electron gating [4]–[6], have been taken to manipulate the magnetic properties of van der Waals (vdWs) systems. Such 2D magnets can be incorporated to form heterostructures with clean and sharp interfaces, which gives rise to exotic phenomena as a result of the interfacial proximity effect. Very recently, we have demonstrated the first inch-scale epitaxial two-dimensional ferromagnetic superlattices (Fe3GeTe2/CrSb)n, in which Fe shows a significantly reduced moment (1.05 µB/atom) [7] comparing to bulk Fe3GeTe2 [8]. Here we report a detailed x-ray magnetic circular dichroism (XMCD) study of the spin and orbital moments of this system. Fig. 1a shows the schematic diagram of the (4-layer Fe3GeTe2/CrSb)6 superlattices. These were grown in a Perkin Elmer 430 MBE on sapphire(0001) with a base vacuum of 10-9 Torr. The Fe3GeTe2 layers were grown at the substrate temperature of ∼310 °C, with the source temperatures of Fe (99.99%), Ge (99.999%) and Te (99.999%) at 1165 °C, 1020 °C and 285 °C, respectively, co-evaporated from Knudsen cells. The substrate temperature was changed to 280 °C when growing CrSb films with Cr (99.99%) and Sb (99.999%) cell temperatures of 1180 °C and 400 °C, respectively. The synchrotron-based x-ray magnetic circular dichroism (XMCD) technique was performed to unambiguously determine the spin and orbital moments of the superlattices. Oppositely circular polarized x-rays with 100% degree of polarization were used successively to resolve XMCD signals from each of the magnetic elements. The light-helicity was switched in a fixed magnetic field (5 T), which was applied in normal incidence with respect to the film plane and in parallel with the beam, as shown in Fig. 1b. The XMCD was obtained by subtracting the two x-ray absorption spectra (XAS), (σ- - σ+), with different helicity. Fig. 1c and 1d show typical pairs of XAS and XMCD spectra of the (Fe3GeTe2/CrSb)6 superlattices obtained at 3 K in total electron yield (TEY). The XAS of Fe L2,3 edges well resembles that of bulk Fe3GeTe2 [8], whilst that of Cr show multiple structures for both spin-orbit split core levels. The strongly dichroic spectra of Fe and Cr clearly indicate the ferromagnetic coupling between the Fe3GeTe2 and the CrSb layers in the superlattices. By applying sum rules, we obtained ms = 1.58 ± 0.1 µB/atom and ml = 0.22 ± 0.02 µB/atom for the Fe and ms = 0.94 ± 0.09 µB/atom and ml = 0.29 ± 0.03 µB/atom for the Cr, respectively. Unlike the reduced moment of the Fe3GeTe2 in (Fe3GeTe2/CrSb)3 [7], that in (Fe3GeTe2/CrSb)6 shows a bulk-like moment [8]. Future work to explore the tuning of the spin polarized band structure of both the 2D ferromagnetic superlattices via the interface engineering will be of great interest and have strong implications for both fundamental physics and the emerging spintronics technology. **