In recent years, numerous experimental studies have searched for magnetism in 4d and 5d transition metal adlayers on metallic substrates, but without success. In the light of this, we present here a computational study of the relationship between low dimensional magnetism and 4d metal-substrate interactions in a metal on metal oxide system, namely Rh on MgO. As a function of both the Rh-surface interaction strength (adsorption site) and the Rh-Rh interaction strength (coverage) we find regions of localized and itinerant magnetism as well as non-magnetic regions. Increasing either of these interaction strengths reduces the magnetization, while increasing the Rh-Rh interactions, i.e. the d-d mixing, drives a crossover from localized (integer moment) to delocalized (fractional moment) magnetism.
From our ab initio Density Functional Theory results at three different adsorption sites and three different coverages we find that the magnetization is non-zero for all except that of 1 Rh monolayer above the oxygen site, which is also the most stable. We examine how this magnetization changes as a function of the Rh-surface distance and relate this to changes in the real space charge density and in the density of states (DOS) as the Rh adlayer interacts with the surface. We find that neither the magnetic transition itself, nor the localized-to-itinerant crossover, is directly related to the formation of Rh-surface bonds.
On the basis of our results we suggest that a better place to look for magnetism in 4d or 5d metal adlayers might be with low coverage adlayers on metal oxide substrates, at low temperatures.
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