Morphology-induced magnetic phase transitions in Fe deposits on MgO films investigated withXMCD and STM

The interplay between magnetic properties and morphology of thin iron deposits on MgO films grown onAg001 was investigated by means of x-ray magnetic circular dichroism and scanning tunneling microscopySTM measurements as a function of the Fe thickness and after postgrowth thermal treatments. The as-grownFe deposits display a sharp transition as a function of the Fe thickness, corresponding to the development offerromagnetism at around 4.5 ML. The ferromagnetic phase can be turned into a superparamagnetic phase bymeans of a thermal treatment. STM measurements allowed us to ascribe the onset of ferromagnetism to thetransition from a three-dimensional to a two-dimensional growth mode of the iron deposit and to explain thesuperparamagnetic behavior in the annealed film as due to the formation of a collection of well-separatedsquarediron particles. Moreover, using the particle shape and size distribution measured by STM, we calculateda value of the magnetic anisotropy of the Fe particles, which is 1 order of magnitude larger than the bulkiron one. This increase is mainly ascribed to the role of surface anisotropy in the Fe nanoparticles and also therole of the dipolar interactions between particles is discussed.