The study is devoted to experimental and theoretical investigation of thermallyinduced diffusion and ordering via various kinetic mechanisms in heterogeneous thin film stacks based on Pt/Fe with additional nanolayers of magnetic (Mn, Tb), nonmagnetic (Ag, Au) and main (Fe, Pt) elements as well as to determination of temperature ranges of hard-magnetic L10-FePt phase stability.
It is found that consistent pattern of phase formation and direction of Me atoms diffusion flux in nanosized Pt/Me/Fe systems (Me – Mn, Tb, Au) at temperatures up to 0.5 Tm are being thermodynamically determined by the ratio of oxides formation enthalpies of intermediate Me and Fe: metal of intermediate layer with high affinity to oxygen segregates on the outer surface, metal with low affinity to oxygen – near the substrate.
By the correlation of electroresistive, magnetic and structural phase characteristics, the Curie temperatures of the A1-FePt disordered phase and the L10-FePt ordered phase for film systems Pt(15 nm)/Fe(15 nm) and Fe50Pt50(30 nm) are defined. Also, it is shown that the nanoscale factor does not significantly affect the temperature of the magnetic transition, the difference between the Curie temperatures of the studied phases in film and bulk states does not exceed ~ 3%.
The possibility of low-temperature formation of kinetically stable hard-magnetic L10-FePt phase in nanosized Pt/Fe and Pt/Au/Fe film compositions at temperatures of ~ 0.2 Tm by the mechanism of reaction diffusion induced by grain boundary motion is proved. Addition of the intermediate Au layer to the Pt/Fe system leads to acceleration of Pt and Fe atoms mutual diffusion, formation of A1-FePt disordered phase after partial "cold" homogenization of the chemical composition, ordering with L10-FePt phase formation, which allows to significantly increase coercivity. Wherein, kinetics of low-temperature ordering of Pt/Fe-based film systems, due to addition of an intermediate layer of noble metal, depends on the nature of its interaction with the main components: absence of intermediate metastable phases is an additional factor accelerating the ordering process; homogenization of chemical composition of Pt/Me/Fe film systems (Me – Au; Ag) with achievement of equiatomic concentration of Fe50Pt50 for entire film thickness and long-range structural order of L10-FePt phase under conditions of type C kinetic regime of grain boundary diffusion does not occur,
but formed structural and phase state provides magnetic characteristics level that corresponds to the high-temperature annealing range.
Change of additional magnetic and non-magnetic layers configuration from symmetric to asymmetric in nanosized Pt/Fe-based compositions allows not only to accelerate the process of L10-FePt ordered phase formation with increase of coercivity,
but also to create a graded distribution of hard-magnetic and soft-magnetic phases in one volume of film material.
A complex approach, which involves change of the thermal treatment atmosphere from neutral to hydrogenous one and increase of number of ferromagnetic FePt alloy and nonmagnetic Au nanolayers with the periodic [FePt/Au/FePt]2x system formation, allows to double the coercivity compared to FePt alloy, raise the recrystallization temperature, stabilize both the grain size of the ordered L10-FePt phase and the surface roughness of the film material.
Model ideas of diffusion processes development in heterogeneous film systems with different configurations of magnetic and nonmagnetic nanolayers by the mechanisms of diffusion-induced grain boundaries migration and reaction diffusion induced by the grain boundaries motion are developed.
By means of molecular dynamics simulation, the process of Fe and Pt selfdiffusion in the ordered L10-FePt phase by the bulk mechanism was theoretically analyzed, the quantitative parameters of this process were determined, and its anisotropic nature was confirmed. Fe self-diffusion parameters were also determined experimentally in an epitaxial film system with nanolayers of different isotopic composition – 56Fe and 57Fe. Theoretical and experimental results confirm the significant contribution of grain boundaries to diffusion-controlled phase formation processes in nanoscale materials for all investigated temperature ranges.
The established physical patterns, developed model representations, as well as defined practically important characteristics create scientific bases of management in wide temperature ranges of structural-phase states and physical properties of heterogeneous film systems with various configurations of magnetic and nonmagnetic nanolayers and are of interest for development of innovative technologies for film elements, nanoelectronics and spintronics devices production.
