Thickness dependent rare earth segregation in magnetron deposited NdCo$_{4.6}$ thin films studied by Xray reflectivity and Hard Xray photoemission

TL;DR

This study demonstrates thickness-dependent PMA in amorphous NdCo4.6 thin films and reveals Nd surface segregation as the key structural mechanism. XRR detects a Nd-rich top layer whose thickness grows with film thickness, while HAXPES confirms a near-surface Nd enrichment and a Co-to-Nd concentration below nominal values, indicating Nd diffusion toward the surface. The combined evidence supports a strain-energy minimization scenario: Nd incorporation induces compressive cobalt lattice strain, which Nd atoms alleviate by forming vertically elongated environments, thereby enhancing PMA. These findings connect growth dynamics, interfacial diffusion, and microstructure to magnetic anisotropy in RE-TM amorphous alloys and offer growth-guided routes to tailor PMA in such systems.

Abstract

Magnetic anisotropy in disordered rare-earth-transition metals (RE-TM) compounds arises from RE atoms occupying asymmetric environments within the TM lattice. However, the underlying mechanism that promotes such environments remains not fully understood. In this study, we investigate amorphous NdCo$_{4.6}$ thin films deposited by magnetron sputtering, where the magnetic anisotropy evolves with thickness from in-plane to out-of-plane orientation above 40 nm. X-ray reflectivity measurements revealed the progressive formation of an additional layer between the 3 nm Si capping layer and the NdCo film with increasing film thickness. To probe the composition and distribution of Co and Nd near the surface, Hard X-ray Photoemission Spectroscopy (HAXPES) was performed on films ranging from 5 nm to 65 nm in thickness using incident photon energies of 7, 19, and 13 keV. These correspond to inelastic electron mean free paths of 7.2-12.3 nm in cobalt. The cobalt atomic concentration, deduced from HAXPES at the Nd 3d and Co 2p excitations, was consistently below the nominal value and varied with both thickness and photon energy. This indicates segregation of RE atoms at the film surface, which becomes more pronounced with increasing thickness. Background analysis of the Co 2p and Nd 3d peaks supports this conclusion. The thickness of the Nd segregated layer is estimated in 2-3 nm. These findings demonstrate that incorporating neodymium into the cobalt lattice incurs an energy cost which is associated with strain due to the volume mismatch between the two elements. Reducing this strain energy promotes anisotropic atomic environments for Nd, thereby explaining the emergence of perpendicular anisotropy and its dependence on film thickness in these RE-TM compounds.

Figures (14)

• Figure 1: Hysteresis loops measured by VSM of the in-plane magnetization of (a) pure cobalt of thickness 5, 10, 25 and 50 nm, and NdCo$_{4.6}$ thin films along the direction parallel (blue) and perpendicular (red) to their in-plane EA of thickness (b) 10 nm and 30 nm, (c) 45 nm, (d) 65 nm.
• Figure 2: XRR curves (blue) and their fits (red) of (a) pure cobalt thin films of thickness, from top to bottom, of 50, 30 and 10 nm; (b) NdCo$_{4.6}$ thin films of 65, 30 and 10 nm thickness. (c) XRR curves of 95 nm thick NdCo$_{4.6}$ alloy obtained with the Xray plane of incidence parallel ($//$) and perpendicular ($\bot$) to the plane of incidence of the neodymium and silicon magnetron beams
• Figure 3: Comparison of the HAXPES spectra obtained at 7 keV photon energy of the VB region in the pure cobalt 50 nm thick film (red) and in the 5 nm (green) and 10 nm (blue) NdCo$_{4.6}$ films.
• Figure 4: Fitting components and Shirley type background of the Co 2p HAXPES spectrum of the 65 nm thick NdCo$_{4.6}$ film obtained at 7 keV photon energy.
• Figure 5: Fitting components and Shirley type background of the Nd 3d HAXPES spectrum of the 65 nm thick NdCo$_{4.6}$ film obtained at 7 keV photon energy.