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

The interplay between magnetic properties and morphology of thin iron deposits on MgO films grown on Ag001 was investigated by means of x-ray magnetic circular dichroism and scanning tunneling microscopy STM measurements as a function of the Fe thickness and after postgrowth thermal treatments. The as-grown Fe deposits display a sharp transition as a function of the Fe thickness, corresponding to the development of ferromagnetism at around 4.5 ML. The ferromagnetic phase can be turned into a superparamagnetic phase by means of a thermal treatment. STM measurements allowed us to ascribe the onset of ferromagnetism to the transition from a three-dimensional to a two-dimensional growth mode of the iron deposit and to explain the superparamagnetic behavior in the annealed film as due to the formation of a collection of well-separatedsquared iron particles. Moreover, using the particle shape and size distribution measured by STM, we calculated a value of the magnetic anisotropy of the Fe particles, which is 1 order of magnitude larger than the bulk iron one. This increase is mainly ascribed to the role of surface anisotropy in the Fe nanoparticles and also the role of the dipolar interactions between particles is discussed.