Long-range chemical interactions in solid-state reactions: effect of an inert Ag interlayer on the formation of L10-FePd in epitaxial Pd(0 0 1)/Ag(0 0 1)/Fe(0 0 1) and Fe(0 0 1)/Ag(0 0 1)/Pd(0 0 1) trilayers

Abstract

The effect of 0, 0.5, and 1 μm-thick Ag interlayers on the chemical interaction between Pd and Fe in epitaxial Pd(0 0 1)/Ag(0 0 1)/Fe(0 0 1)/MgO(0 0 1) and Fe(0 0 1)/Ag(0 0 1)/Pd(0 0 1)/MgO(0 0 1) trilayers has been studied using X-ray diffraction, 57Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, and magnetic structural measurements. No mixing of Pd and Fe occurs via the chemically inert Ag layer at annealing temperatures up to 400 °C. As the annealing temperature is increased above 400 °C, a solid-state synthesis of an ordered L10-FePd phase begins in the Pd(0 0 1)/Ag(0 0 1)/Fe(0 0 1) and Fe(0 0 1)/Ag(0 0 1)/Pd(0 0 1) film trilayers regardless of the thickness of the buffer Ag layer. In all samples, annealing above 500 °C leads to the formation of a disordered FexPd1−x(0 0 1) phase; however, in samples lacking the Ag layer, the synthesis of FexPd1−x is preceded by the formation of an ordered L12-FePd3 phase. An analysis of the X-ray photoelectron spectroscopy results shows that Pd is the dominant moving species in the reaction between Pd and Fe. According to the preliminary results, the 2.2 μm-thick Ag film does not prevent the synthesis of the L10-FePd phase and only slightly increases the phase’s initiation temperature. Data showing the ultra-fast transport of Pd atoms via thick inert Ag layers are interpreted as direct evidence of the long-range character of the chemical interaction between Pd and Fe. Thus, in the reaction state, Pd and Fe interact chemically even though the distance between them is about 104 times greater than an ordinary chemical bond length.