Magnetic and energetic properties of low-index Cr surfaces and Fe/Cr interfaces: A first-principles study

Density functional theory calculations are performed to investigate the impact of magnetism on the energetics of low-index Cr surfaces and Fe/Cr interfaces, that is, Cr(100), Cr(110), Fe/Cr(100), and Fe/Cr(110). We have also determined the stability of various Cr magnetic structures, particularly the spin-density waves, in the presence of these surfaces and interfaces. We show that the most stable structure of the spin-density wave is mainly dictated by the subtle balance between bulk and surface/interface influences, and strongly dependent on the surface/interface orientation. Regarding the Cr surfaces, we confirm the role of magnetism to lower the surface energy of Cr(100) with respect to Cr(110). Among all the possible orientations of the wave vector, only the out-of-plane wave is found to be stable near Cr(100) surfaces with the high-moment sites located at the surface layer. At variance, the in-plane wave is shown to be the most stable one, consistent with experimental data for very thin Cr(110) films. Concerning the Fe/Cr interfaces, magnetic frustrations are identified to be responsible for a higher formation energy of Fe/Cr(110) compared to that of Fe/Cr(100). This unusual anisotropy of interface energies is clearly different from the corresponding interfaces between Cr and a nonmagnetic element, Cu. Two ways are suggested to relax partially the magnetic frustrations at the (110) interface and to lower its formation energy. Noncollinear magnetic configurations can be developed where local moments of Fe and Cr atoms are perpendicular to each other. Also, in order to preserve phase coherence, in-plane spin-density waves show a very stable magnetic structure with the nodes at the interface layer. The presence of low-moment sites at Fe/Cr(110) offer another way to relax the magnetic frustrations and lower the interfacial energy.