Ni thin-films on Pd surfaces and effects of oxygen adsorption: Ab-initio study of structures, electronic properties, magnetic anisotropy

Abstract

We report first-principles electronic structure calculations of the structural, electronic, and magnetic properties of model epitaxial layers consisting of nickel (Ni) atomic layers deposited on palladium (Pd) substrate, i.e., Ni(001)${}_m$ $\mid$Pd${\left(001\right)}_n$ where $m=1,2,6$ and $n=3,10,$ are layer thicknesses. We also investigate the effect of oxygen adsorption on the calculated properties. We found variation in magnetization of between $\approx 0.6{\mu }_B$ to 1.00 ${\mu }_B$ across the nickel layers. Also, finite magnetic moments albeit of small values of between 0.2 ${\mu }_B$ and 0.3 ${\mu }_B$ is found on the Pd at the interface. This magnetic moment on an otherwise non-magnetic Pd atoms has been adduced to interfacial strain due to lattice mismatch between the Ni and Pd layers at the Ni$|$Pd interface. The effect of adsorbed oxygen on the Ni${}_m$ $\mid$Pd${}_n$ is that it increases the magnetic moment on the nickel layers. Also, regarding the magnitude of magnetic anisotropy energy (MAE), we found a high perpendicular values of 1.63 meV and 1.37 meV per unit cell respectively for Ni${}_m$ $\mid$Pd₁₀ ($m=2,6$) which are relatively higher than those reported for other transition metal epitaxial layers. However, the presence of oxygen atom on the Ni$\mid$Pd changes the direction and magnitude of MAE. Indeed, O adsorption favours or enhances in-plane magnetization direction depending on the thickness of the Ni layers for a fixed Pd thickness. Plots of local density of states (LDOS) which include the effect of spin–orbit coupling (SOC), show that in the case of Ni$\mid$Pd having perpendicular MAE, there appears a new SOC-induced electronic states below and above the Fermi level. These states appears to stabilize this type of magnetic anisotropy. On the other hand, in-plane MAE is characterized by SOC-induced localized states below the Fermi level (E${}_F$) as well as the lowering of the DOS at the E${}_F$. Our work explores the physical, magnetic and electronic properties that may be useful in designing Ni$\mid$Pd-based superlattices for magnetic or spintronic applications.