Magnetic properties of ultrathin layers composed of Fe and MgO
Anna Kozioł-Rachwał (University of Science and Technology, Krakau, Polen)

In this talk I will review our work on magnetic properties of the ultrathin layers composed of Fe and MgO; subnanometer Fe films in MgO/Fe/MgO stack, Fe/MgO/Fe trilayers and Fe/MgO multilayers. All the layers were grown using molecular beam epitaxy under ultra high vacuum conditions on MgO(001) crystal substrates. Magnetic properties of the studied systems were examined by various techniques, including methods based on hyperfine interaction spectroscopy (CEMS and NRS).
A study of sub-nanometer iron films in MgO/Fe/MgO stack reveals existence of perpendicular magnetic anisotropy in the system below the critical thickness tc. Because of the three - dimensional cluster growth of Fe on MgO(001) the superparamagnetic fluctuations of Fe magnetic moments are observed at room temperature with characteristic frequencies ranging over tens of MHz.
The superparamagnetism in MgO/Fe/MgO can be suppressed by applying low-temperature deposition. For the iron deposited at 140 K continuous layer was obtained for the Fe thickness of 6Å. Furthermore, the perpendicular magnetization was found at room temperature below tc = 9Å. In the vicinity of tc we observed the spin reorientation transition (SRT) with the easy magnetization axis switching from the out-of-plane to in-plane direction. For the Fe thickness close to tc the temperature driven SRT was reported.
Studies of the magnetic properties of Fe/MgO/Fe trilayers proved the existence of antiferromagnetic (AFM) interlayer exchange coupling (IEC) between the Fe layers for the subnanometer thickness of MgO spacer. We showed that when the trilayer is grown on homoepitaxial MgO buffer layer the strength of IEC is increased by a factor of two in comparison with the un-buffered sample. Moreover, the range of MgO thickness for which AFM coupling occurs, is shifted toward thinner spacers for the buffered sample.
Finally, I will characterize the magnetic properties of Fe/MgO multilayers. I will show that in the course of optimization a stable ferromagnetic state of the multilayers can be obtained for the thicknesses of both Fe and MgO sublayers equal to 6Å and 10 repetitions of Fe/MgO stack. Mössbauer measurements demonstrated that the out-of-plane magnetization component was enhanced when the repetition number was increased from 10 to 20. Correlations between the CEMS and MOKE measurements suggest a complex vortex-like magnetization distribution. Plausibility of this domain structure was confirmed by the micromagnetic simulations.