Evolution of surface antiferromagnetic Neel temperature with film coverage in ultrathin MnO films on Ag(001)

The surface antiferromagnetic ordering of MnO(001) ultrathin films has been probed using Low-energy Electron Diffraction (LEED) and Angle-Resolved Photoemission spectroscopy (ARPES) techniques. We observe coherent exchange scattered half-order spot in LEED which is only visible at low electron beam energy (<40 eV) and arises below a certain temperature with a periodicity of the magnetic unit cell of MnO(001) surface, confirms its magnetic origin. A thickness dependent evolution of surface antiferromagnetic (AFM) Neel temperature has been observed for the MnO films. Interestingly, we found the Neel temperature of the MnO films exceed its bulk Neel temperature even in the ultrathin limit (similar to 2 ML). Detailed LEED analysis reveals that the increase of the Neel temperature is due to the presence of strain and the image charge screening effect in the film while, the competition between the finite size effects, surface strain, and substrate polarizability ultimately determine the observed Neel temperature. The surface magnetic critical exponent (beta(1)) varies from 0.16 +/- .03 to 0.81 +/- .03, upon changing film thickness from 3 to 10 ML, indicates the transition from nearly 2D Ising system to 3D Ising/Heisenberg system. Furthermore, upon paramagnetic (PM)-AFM transition, ARPES energy distribution curves (EDCs) show, opening an energy gap of 150 meV with an increased occupation in the e(g) levels is good agreement with the theoretical prediction of the presence of AFM-II type ordering in the film.