Structure of Cr monolayer on Ag(001): A buried two-dimensional c(2 x 2) antiferromagnet

Abstract

The growth, morphology, and magnetic structure of ultrathin Cr films grown on a Ag(001) substrate are studied using low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), and ab initio density functional theory (DFT) calculations. The presence and temperature dependence of c(2 x 2) half-order spots in the LEED pattern, for low electron energies, along with the presence of characteristic Cr 3d bands in the ARPES spectra, confirm the existence of antiferromagnetic ordering for the Cr monolayer case. Our experiments are also consistent with the presence of a p(1 x 1) Ag overlayer on top of the Cr layer, suggesting the existence of a Ag/Cr/Ag(001) sandwich structure at the surface. Our DFT calculations confirm that this is the most favored geometric and magnetic structure of the system. The Cr layer is found to retain a "two-dimensional" character with enhanced Cr 3d magnetic moments, despite being buried below a Ag monolayer, due to the absence of significant hybridization between Cr 3d and Ag 4d electronic states. The coverage dependence of the magnetic ordering indicates a maximum ordering above the expected monolayer coverage, possibly due to intermixing between Ag and Cr atoms in the overlayer.