Surface termination and thickness dependent magnetic coupling of Cr adlayers on Ni₂MnGa(001) surfaces: An ab initio study

Cr adlayers on magnetic and non-magnetic substrates have attracted attention in the recent years due to the interesting magnetic properties of these systems. In this work, we explore the magnetic properties of Cr adlayers on the (001) surface of a ferromagnetic Heusler alloy, namely Ni₂MnGa, using density functional theory based electronic structure calculations. The magnitudes and orientation of the magnetic moments of the Cr adlayers on both the Mn-Ga and Ni terminated Ni₂MnGa (001) surfaces have been investigated and we find that the magnetic ordering is sensitive to the substrate atoms and the adlayer thickness as well. For one monolayer of Cr adlayer on Mn-Ga (Ni) surface, the Cr atoms are ferromagnetically (anti-ferromagnetically) coupled within the layer. However, for two monolayers of Cr, the Cr adatoms are ferromagnetically coupled, irrespective of the surface termination. The ferromagnetic ordering of the Cr atoms has been explained from the Stoner criterion. The origin of different magnetic orderings of the Cr adatoms with the substrate has been understood by analyzing the Heisenberg exchange coupling parameters between the adatoms and the substrate atoms beneath the adlayer. Further, to obtain a chemical understanding of the observed magnetic behavior, we have employed the crystal orbital hamilton population bonding analyses. The results corroborate well with the results of Heisenberg exchange coupling parameters as well as spin-polarized atom-derived density of states.