Structural and magnetic properties of strongly carbon doped Fe–Co thin films
Introduction
Fe–Co is proposed as a possible candidate alloy for permanent magnet applications due to its high intrinsic magnetic moment. Apart from high magnetic moment, high magneto-crystalline anisotropy is required for permanent magnets. In this work the effect of carbon doping on the stabilization of metastable tetragonal phases, with strained unit cells and high magneto-crystalline anisotropyin sputtered Fe–Co thin films is studied. According to theoretical calculations FeCo alloy can support large uniaxial magnetic anisotropy energy (MAE), saturation magnetization Ms, while chemical disorder affects MAE significantly [1], [2], [3]. Tetragonal distortion can be induced via coherent growth of Fe–Co layers on different substrate or buffer materials. This was shown in various experimental studies. However, only ultrathin Fe–Co layers with a maximum thickness of 15 monolayers exhibit a perpendicular magnetic easy axis [4], [5], [6]. This strong magneto-crystalline anisotropy (MCA) was attributed to the tetragonal strain, expressed by the (c/a)bcc ratio of lattice distortion in the range between 1.1 and 1.25 according to Burkert et al. [1]. The reason why a strong MCA was not observed in thicker films is the lattice relaxation, which leads to a reduction of c/a to its equilibrium value of 1, as demonstrated with in situ measurements [7]. From these experimental studies, the chosen buffer layer was always considered being the key parameter to distort the Fe–Co lattice. For this purpose, a variable composition AuxCu1−x alloy library has been prepared based on combinatorial sputtering methods, in order to achieve Taylor made lattice parameter values [8]. Several single crystal substrates have also been employed to create an appropriate lattice mismatch [9], [10], [11. Adding a third element, e.g. carbon, was recently proposed to stabilize the strain in Fe–Co and thus to hamper a complete lattice relaxation [12]. Remarkable magneto-crystalline anisotropy values were predicted for ternary Fe–Co–C system, according to these DFT calculations [9]. Carbon atoms occupying the interstitial positions in the FeCo lattice cause a tetragonal distortion. These suggestions from DFT theory were already confirmed for Fe–Co with low additions of C of approximately 2 at% by Reichel et al. [7]. In their (Fe0.4Co0.6)0.98C0.02 films, a strain of c/a=1.03 was observed which led to Ku of 0.44 MJ/m³. In this study, we introduce Fe–Co films with higher additions of C and demonstrate the consequences of the lattice strain, towards achieving a high tetragonal distortion of the unit cell.
Section snippets
Experimental details
An ATC 2200-V high vacuum magnetron sputtering system supplied from AJA Inc. was used to prepare the samples, with a base pressure of 4×10−9 Torr. The depositions were performed on single crystalline MgO (100) substrates. The layer structure consists of a 3 nm Cr seed layer, a 30 nm Au–Cu buffer and the Fe–Co(C) magnetic layer. Two different stoichiometry buffers were deposited at 300 °C, Au30Cu70 and L10 ordered Au50Cu50 with cubic and tetragonal symmetry respectively. Fe–Co–C films of different
Results and discussion
Based on previous work on AuxCu1−x combinatorial sputtering depositions [8], [15], the Fe–Co magnetic layer was deposited on different crystallographic buffer layers, Au50Cu50 with a=0.396 nm and Au30Cu70 with a=0.375 nm. The Au–Cu layers grow epitaxially on MgO (100), as indicated by the diffraction patterns (Fig. 1) where only high intensity (001), (002), (003), (004) reflections are visible. It also allows an epitaxial growth of Fe–Co [7]. The epitaxial relation for the whole layer setup,
Conclusions
Based on promising theoretical and experimental results on C doped tetragonally strained Fe–Co alloy films system, we extended the studies to higher amounts of carbon (up to 20%). An increased strain for the heavily alloyed films was not observed, which leads to the conclusion that not the entire C amount does enter into interstitial sites of the Fe–Co phase. Only a low fraction of C occupies interstitial positions along the c axis and causes a strain of approximately 1%, at film thicknesses
Aknowledgments
Work is supported by the EU Program REFREEPERMAG (GA 280670). We thank Juliane Scheiter and Christine Damm for FIB lamellae preparation and TEM investigations.
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