Segregation and interdiffusion in (Fe,Co)/Pt superlattices

We report on the chemical structure of (Fe,Co)/Pt superlattices, which recently have shown high uniaxial magnetocrystalline anisotropy combined with high saturation magnetic moments. In particular, the homogeneity of the (Fe,Co) alloy is studied with a combination of x-ray and neutron reflectometry—the latter in a configuration where magnetic scattering is negligible. It is deduced, with support from off-specular x-ray reflectivity patterns and corresponding simulations, that the lower (Fe,Co)-on-Pt interface contains more Co than the upper Pt-on-(Fe,Co) interface. This can occur as Co interdiffuses into Pt more easily than Fe, as shown by density-functional calculations. The effect of this interdiffusion and segregation on the uniaxial anisotropy is discussed, and it is found that the previously observed discrepancy between experimental and theoretical anisotropy values can be quantitatively accounted for.

Figure 1

Unnormalized TOF data for sample FCP1 at fields of 0.5 (upper) and 3 T (lower) plotted with a logarithmic intensity scale. The black lines show the theoretical positions of the Zeeman wings due to spin-flip scattering from components of the magnetization in the plane (Ref. 14). Note that the Zeeman wings have disappeared in the bottom panel. The additional streaks of intensity at 0.5 T are Yoneda wings.

Figure 2

Figure comparing the sensitivity to changes in the (Fe,Co) layer composition of x-ray reflectivity (top) and neutron reflectivity (bottom). The inset in the bottom panel shows the Co concentration per FeCo atom, cf. Fig. 6. Dotted lines represent an inhomogeneous (Fe,Co) layer and full lines represent a homogeneous (Fe,Co) layer both in the concentration plot and in the simulations.

Figure 3

The two different composition profile models used to refine the specular reflectivity data: the linear model (left) and the step model (right). The upper figures show the total (Fe,Co) content and the lower figures show the Co concentration per FeCo atom in the alloy layer. The gray regions show the location of the Pt layers in a perfect sample.

Figure 4

The best fit (line) to the x-ray reflectivity (left) and neutron reflectivity (right) for the two models. The top graphs show the linear model and the lower graphs show the step model. The measured data are shown with circles.

Figure 5

The refined composition profiles for the linear model (upper) and the step model (lower). Dotted lines represent the Pt concentration, dashed lines the Fe concentration, and solid lines the Co concentration. The gray regions show the location of the Pt layers in a perfect sample. The position value is defined as increasing while moving upward through the layers from the substrate.

Figure 6

The refined Co concentration per (Fe,Co) atom for the linear model (dashed) and the step model (solid). The gray regions show the location of the Pt layers in a perfect sample.

Figure 7

(Color online) Total energies of 1 ML Fe (circles) and 1 ML Co (squares), respectively, on and inside a 10 ML Pt slab. Layer index S corresponds to the surface layer. The inset shows an enlargement for monolayers deeper inside the Pt slab.

Figure 8

(Color online) Calculated total energy of 1 ML Pt on and inside 12 ML slabs of Fe (circles), Co (squares), and (Fe,Co) (triangles). Layer index S corresponds to the surface layer.

Figure 9

Specular reflectivity from sample FCP3 (circles) and refined simulation (line) using Gaussian interface profiles (Nevot-Croce factors). The data were collected with $\text{Cu}K\alpha$ radiation.

Figure 10

Rocking curves around the second (upper) and third (lower) Bragg peaks of sample FCP3 measured with $\text{Cu}K\alpha$ radiation. Circles denote the measured data and lines denote the simulations. Note that the specular component, $\theta -\omega =0$, is not included in the simulations and that the panels have different scales. The arrows indicate the positions of resonant Bragg-type peaks which occur in vertically correlated multilayers (Ref. 31).

Figure 11

The composition profile for 3 ML (Fe,Co) in Pt with the amount of interdiffusion from off-specular simulations. The line represents the continuous composition profile, whereas the bars correspond to the composition in each monolayer. The concentration is expressed as (Fe,Co)/Pt atomic ratio without distinction between Fe and Co atoms.