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It has been suggested that thin FePt (or CoPt) layer can be formed by intermixing in general Pt/FM structures such as Pt/Fe(Co) or Pt/CoFeB structures. The addition of the inter-mixed layer, such as a dusting layer, is expected to affect various interfacial properties such as interfacial perpendicular magnetic anisotropy and spin Hall angle, and so on. In the case of the Dzyaloshinskii-Moriya interaction (DMI), which is a representative interfacial magnetic property, it has been predicted that such inter-mixing has notable influence [1], but related experimental results have not been reported yet.
Here, we present the trend of the surface DMI (Ds = D tFe) according to the presence or absence of an FePt insertion layer (1-nm thick) and the composition ratio of FePt in the Pt/(FePt)/Fe/MgO structure. As a consequence, it was confirmed that the Ds notably decrease (1.3 to 0.7 pJ/m) when the FePt layer is inserted, and mere change (0.72 ± 0.01 pJ/m) according to the composition ratio of FePt was confirmed. The composition of the FePt layer was controlled by the deposition watt ratio of the two targets during co-sputtering as 40:20, 30:20, 20:20 watt ratio of Fe:Pt (the exact composition will be determined later through x-ray photoelectron spectroscopy measurement). This trend of DS can be interpreted through the correlation between the work function difference (ΔW) and DMI suggested by Park et al. [2] They presented experimental results associating the scattering potential and the ΔW between nonmagnetic and ferromagnetic metal, based on the theory stating that the strength of DMI is determined by the extent of the spin-orbit scattering of conduction electron caused by nonmagnetic impurities. This association is also consistent with the results of Kim et al., which confirmed the trend of D according to the thickness of the Cu insertion layer above and below the Co layer in the Pt/Co/AlOx structure. [3] In this background, the notable decrease in DS by FePt insertion could be attributed to the lower ΔW between FePt-Fe than Pt-Fe, since FePt layer is known to have relatively small work function value compared to Pt by Fe addition. [2,4] Note that the 1-nm-thick FePt layer in this experiment was confirmed to have almost negligible magnetic moment at room temperature (up to 20 kA/m). In summary, here we confirmed that the magnitude of Ds notably decreased (1.3 to 0.7 pJ/m) by the addition of 1-nm thick FePt insertion layer. And this reduction could be ascribed to work function of FePt, which is known to be relatively lower than Pt layer.
[1] Zimmermann, B. et al. Appl. Phys. Lett. 113, 232403 (2018)
[2] Park Y.-K. et al. NPG Asia Materials 10, 995–1001 (2018)
[3] Kim N.-H. et al. AIP Advances 7, 035213 (2017)
[4] Yin C.-K. et al. Jpn. J. Appl. Phys. 46, 2167 (2007)
