From Bloch to Néel: Anisotropy-dependent Domain-Wall Character in FePd Thin Films

We report an experimental investigation of the depth-dependent domain wall formation in L1$_0$-FePd thin films with high perpendicular magnetic anisotropy. Using circular dichroism X-ray resonant magnetic scattering (CD-XRMS) as a function of the incident X-ray angle, we explore the depth evolution of chiral spin textures in two samples with different strengths of magnetocrystalline anisotropy. Combined with CD-STXM, CD-ptychography, and macroscopic characterization of the structural order, magnetic properties, and surface morphology, we relate these observations to differences in the long-range order of the L1$_0$ phase of FePd. One FePd thin film with very high magnetocrystalline anisotropy, characterized by $Q_{PMA}=1.8$, exhibits an unexpectedly large N\'eel contribution. Angular-dependent CD-XRMS directly reveals a smooth transition from a hybrid Bloch-N\'eel chirality within the upper FePd layer towards a purely N\'eel-type structure at the lower FePd interface. In the second FePd sample, despite a still relatively large $Q_{PMA}=1.45$, the domain walls were found to be purely N\'eel type. These results indicate a crucial role of the long-range structural order in determining the formation of the magnetic structure.