Enhancement of spin-orbit coupling at manganite surfaces

Spin-orbit coupling in magnetic systems lacking inversion symmetry can give rise to nontrivial spin textures. Magnetic thin films and heterostructures are potential candidates for the formation of skyrmions and other noncollinear spin configurations as inversion symmetry is inherently lost at their surfaces and interfaces. However, manganites, in spite of their extraordinarily rich magnetic phase diagram, have not yet been considered of interest within this context as their spin-orbit coupling is assumed to be negligible. We demonstrate here, by means of angular dependent x-ray linear dichroism experiments and theoretical calculations, the existence of a noncollinear antiferromagnetic ordering at the surface of ferromagnetic ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ thin films whose properties can only be explained by an unexpectedly large enhancement of the spin-orbit interaction. Our results reveal that spin-orbit coupling, usually assumed to be very small in manganites, can be significantly enhanced at surfaces and interfaces adding a new twist to the possible magnetic orders that can arise in electronically reconstructed systems.

Figure 1

(a) Sketch of the experimental setup and geometry used for the XLD measurements. (b) Comparison of the temperature dependence of the normalized XMCD and XLD measured at the Mn $L_3$ and $L_2$ edges, respectively (see Appendix pp1). The XLD data is square rooted to account for its square dependence on magnetization as opposed to the XMCD linear dependence. Both curves show the same $T$ dependence confirming the AFM origin of the XLD signal (see text for discussion).

Figure 2

Experimentally measured angular dependence of the XLD (red dots) at the Mn $L_2$ edge for a thin (9.4 nm) LSMO thin film. The dotted line is a guide to the eye. For comparison, we show the expected XLD angular dependent behavior for the case of AFM order with magnetic axis oriented parallel (dashed black) or orthogonal (dashed-dotted blue) to the surface plane of the sample.

Figure 3

(a) Predicted misalignment between the FM and AFM axis $|\theta -{\theta }_s|$ vs $\theta$ for different values of the interfacial SOC $g$. (Inset) Sketch of the spins showing the angle between the bulk FM ($\theta$) and surface AFM (${\theta }_s,{\theta }_s^{\prime }$) axis with respect to the sample's surface. The ground state for the surface gives ${\theta }_s^{\prime }={\theta }_s+\pi$. (b) Comparison of the experimentally measured (red dots) and the predicted (solid line) XLD angular dependence for $g=0.05t$.

Figure 4

(a) Mn $L_3$-edge XAS vs LSMO thin film thickness. The 45-nm sample shows bulk like spectral shape. Decreasing the film thickness leads to an enhancement of the spectral weight at the low-energy side of the $L_3$ spectral feature related to the increase of a ${\mathrm{Mn}}^{3+}$ component. The 9.4-nm sample shows a clear peak at approximately 641 eV related to the presence of ${\mathrm{Mn}}^{2+}$. (b) Sketch of the experimental setup used for the XMCD measurements. (c) (Main panel) Normalized spin contribution to the magnetic moment “(Ms)” of Mn vs thickness as deduced from the application of the sum rules to the XMCD spectra obtained at normal incidence at 5 K and $H=3T$. Data have been normalized to the magnetization obtained for the thicker bulklike film. (Inset) Magnetic hysteresis loops obtained at $T=10$ K for magnetic fields applied parallel and orthogonal to the sample plane. (d) Sketch of the experimental setup used for the XLD measurements. (e) Orbital contribution to the XLD obtained at 350 K. At low thicknesses, the sign and shape of the XLD at the Mn $L_2$-edge indicates a preferential $3z^2-r^2$ orbital occupancy. Increasing the thickness leads to the observation of an XLD compatible with a preferential $x^2-y^2$ orbital occupation.

Figure 5

XLD spectra obtained at 4 K with a magnetic field applied perpendicularly to the propagation direction of the incoming beam for samples with thickness 8.9, 9.4, and 45 nm (continuous lines). For comparison, we show the orbital contribution for the 9.4-nm sample (dotted line). The low-temperature XLD is similar in all cases and does not depend on the presence of other Mn oxidation states besides that of the mixed ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ valence one.