Dr. Hans nembach of JILA, University of Boulder

The Dzyaloshinskii-Moriya interaction (DMI) is of large interest. It can give rise to chiral spinchains, chiral domain walls and skyrmions. Interfacial DMI can be described by a three-site exchange mechanism, where the coupling between two spins in the ferromagnet is mediated by an atom in the material with large spin-orbit coupling. The DMI causes a non-reciprocal frequency-shift for Damon-Eshbach spin-waves. The sign of the frequency-shift depends on the polarity of the magnetization and the propagation direction of the spin-waves. We use Brillouin Light scattering spectroscopy to determine the frequency-shift for both fieldpolarities. We use M and g, which are determined by SQUID and ferromagnetic resonance spectroscopy (FMR), to then extract the DMI. In order to gain deeper insight into the underlying physics of the interfacial DMI, we prepared different multilayer systems with DMI. We found that for a Ni₈₀Fe₂₀ x)/Pt sample series with x ranging from 1 nm to 13 nm the symmetric Heisenberg exchange and the DMI both show the almost identical thickness dependence . This was originally predicted for magnetic oxides and for metallic spin-glasses. With another sample series we studied the role of the direct exchange coupling between CoFeB and Pt, where we inserted a Cu spacer between the two materials with a thickness x ranging from 0.2 nm to 2.0 nm. We find that the DMI and the proximity magnetization in the Pt both show a similar exponential decrease with Cu thickness. This underlines the importance of the direct exchange for these two phenomena. Moreover, we find that the Gilbert damping is highly correlated with the magnitude of the proximity magnetization. This suggest that spin-memory loss due to the proximity magnetization strongly contributes to the total damping. Bulk magnetic oxides like for example Fe₂O₃ are well known for the presence of DMI. We in-situ oxidized a Pt/Co₉₀Fe₁₀ and a Cu/Co₉₀Fe₁₀ sample series for different times and subsequently capped these samples to prevent further oxidation. Both sample series showed an increase in DMI with oxidation, which demonstrates unambiguously that interfacial oxide gives rise to DMI. The spectroscopic splitting factor, which we determined by perpendicular FMR, changes with the oxidation and is correlated with the increase in the DMI. The change in indicates changes of the hybridization and the associated charge transfer at the oxide interface. This was also predicted by recent density functional calculations (DFT). Finally, we addressed the importance of the in-plane symmetry of the crystal lattice. So far, most of the experimental work has been done on (111) textured films exhibiting C3v symmetry, for which the DMI is isotropic. We prepared by molecular beam epitaxy the Pt(110)/Fe system, which has C2V symmetry. Our measurements show a two-fold symmetry for the DMI. The ratio for the DMI along the [-110] and the [001] agrees with DFT calculations.

References

[1] H.T. Nembach et al., Nat. Phys. 11, 825 (2015).

[2] A. Belabbes et al. Sci. Rep., 6, 24634 (2016).

Email:

Address:University of Colorado, JILA, Boulder, Colorado, United States, 80305