Enhanced and tuneable orbital moment and magnetocrystalline anisotropy in tetragonally strained Fe-Co-B films

Ruslan Salikhov, University of Duisburg-Essen, Duisburg, Germany
Ludwig Reichel, IFW Dresden, Dresden, Germany
Sebastian Fähler, IFW Dresden, Dresden, Germany
Michael Farle, University of Duisburg-Essen, Duisburg, Germany
Ulf Wiedwald, University of Duisburg-Essen, Duisburg, Germany

Spin-orbit coupling (SOC) is responsible for the coupling of the spin system to the crystal lattice and of tremendous importance for magnetic phenomena that are key to most applications of magnetic materials. SOC gives rise to the magnetocrystalline anisotropy, magnetostriction, anisotropic magnetoresistance, spin and anomalous Hall effects, and magnetic resonance damping [1]. In the cubic crystal symmetry of 3D metals and their alloys, the orbital contribution to the magnetic moment is small and SOC-associated phenomena like the magnetocrystalline anisotropy are also commonly small. However, if the cubic symmetry is broken, the magnetocrystalline anisotropy energy (MAE) can be enhanced by more than three orders of magnitude, which has been demonstrated for Fe-Co alloys [2-5].

Here, we present an approach for tuning the orbital magnetic moment and the MAE. 20 nm tetragonally strained Fe-Co-B alloys were epitaxially grown on Au0.55Cu0.45 buffer layers. The B atoms preferentially occupy specific octahedral interstitial sites in the Fe-Co lattice, leading to a local distortion of Fe and Co atoms around the dopant, which results in lattice strain. The strain axis is perpendicular to the film plane and the corresponding lattice constant c is enlarged with respect to the in-plane lattice parameter a. The tetragonal strain was stabilized in Fe-Co films with different c/a ratios by adding B with concentrations up to 10 at.%.

Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment and MAE increases with increasing c/a ratio, and reaches maximum of 0.17 μB/atom and 0.4 MJ/m3 at c/a = 1.034. Thus, the reduced crystal symmetry leads to a noticeable enhancement of SOC in the Fe-Co-B alloys. These results offer an approach to tune SOC effects in bulk Fe-Co films by controlling the c/a ratio via B doping.

[1] J. Stöhr, and H. C. Siegmann, Magnetism. From fundamentals to nanoscale dynamics. Springer Berlin Heidelberg 2006; pp 203-220 and 294-310.
[2] T. Burkert, et al., Phys. Rev. Lett. 93, 027203 (2004).
[3] E. K. Delczeg-Czirjak, et al., Phys. Rev. B 89, 144403 (2014).
[4] L. Reichel, et al., J. Appl. Phys. 116, 213901 (2014).
[5] L. Reichel, et al., J. Phys.: Condens. Matter 27, 476002 (2015).

« back