Nanocomposite and nanomaterials

Formation of ordered L10-FePt phase in FePt-Ag thin films
Shamis M.N.1, Makushko P.V.1, Fihurna O.V.1, Safonova N.Y.2, Kotenko I.E.1, Verbytska T.I.1, Albrecht M.2, Makogon Iu.M.1
1 Metal Physics Department, National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Prospect Peremogy 37, 03056 Kyiv, Ukraine 
E-mail: m.n.shamis@gmail.com
2 Institute of Physics, University of Augsburg, Universitätstrasse 1, D-86135 Augsburg, Germany 

Mechanical stress in FePt thin films depend critically on phase composition, structure and deposition conditions. Addition of Ag, Au, Cu alloying elements can change the stress state in these films, due to their low surface energy, which in turn will influence structural ordering processes and can decrease the temperature onset of L10 FePt phase formation.
In this work the influence of an Ag layer, which was located at different positions of the layer stack (top, intermediate, bottom) on the stress state affecting the structure and magnetic properties of ordered L10-phase in Fe50Pt50 films was investigated.
Ag(7.5 nm)/Fe50Pt50(15 nm), Fe50Pt50(15 nm)/Ag(7.5 nm)/Fe50Pt50(15 nm) and Fe50Pt50(15 nm)/Ag(7.5 nm) heterostructures were deposited by dc magnetron sputtering on SiO2(100 nm)/Si(001) substrates at room temperature. Heat treatment was carried out in vacuum (~1.3 10-3 Pa) in the temperature range of 300 °C – 900 °C for 30 s. Phase and structural transformations were investigated by X-ray diffraction. Films separated from the substrates were in-situ annealed in a transmission electron microscope (TEM). Residual stresses in FePt layer were evaluated by X-ray tensometry sin2(ψ) method using the (111) reflection. Depth profiles of the thin films after deposition and heat treatment was investigated by secondary neutral mass spectrometry (SNMS). SQUID magnetometry was used to analyze the magnetic properties of the films.
It was found that the position of the additional Ag layer indeed affect the stress state of the as-deposited FePt films. Stresses are caused by strain induced by structural changes during deposition and differences in thermal expansion coefficients of FePt, Ag and substrate during post-annealing. Mechanical stresses affect the ordering processes during subsequent heat treatment. The higher level of compressive stress (−7.8 GPa) in FePt/Ag/FePt films results in ordered L10 phase formation at temperature of 700 °C which is about 100 °C lower than in films with Ag bottom (−3.6 GPa) and top (+0.86 GPa) layers.
Increasing of annealing temperature up to 900 °C is accompanied by an increasing degree of L10 ordering and amount of L10-FePt phase. A more pronounced oriented growth in the (001) direction of the L10 phase is observed in systems with Ag top layer compared to films with Ag bottom and intermediate layer. Interestingly, the formation of the ordered L10 phase in FePt/Ag/FePt films separated from the substrate occurs at even lower temperatures of about 500 °C.
After annealing at 700 °C the FePt/Ag/FePt film exhibits a high magnetic coercivity of about 2.9 kOe, which increases further with temperature. However, the highest values were obtained for films with Ag intermediate and bottom layer. The segregation and aggregation of Ag along FePt grain boundaries resulting in strongly exchange decoupled FePt grains is responsible for this behavior.