Amorphous Fe–B alloys in B–Fe–Ag multilayers studied by magnetization and Mössbauer measurements

Abstract Bulk and local magnetic properties were studied in [1 nm B + 1 nm 57 Fe +  x  nm Ag] 5 , x  = 1, 2, 4, 5 and 10, multilayer samples. Although Ag does not mix with either of the other two elements the magnetic properties of the multilayers are strongly influenced by the Ag thickness below x  = 5, whereas no such effect is observed above this value. The Mossbauer measurements indicate a complete amorphization of the thin Fe layers in each sample, as a result of intermixing with the B layers. The variation of the magnetic properties is explained by the variation of the average B concentration of the amorphous Fe–B layers, which depends on the thickness of the Ag barrier layers. The magnetization measurements indicate ferromagnetic behaviour of the ultra-thin amorphous layers with the presence of less than 10% superparamagnetic moments for x  = 5 and 10. The average B concentration of the amorphous Fe–B alloy, as estimated from the Fe hyperfine fields, is around 40 at%. It is significantly lower than the 60 at% nominal B concentration, suggesting the presence of an unalloyed B layer, as well. This picture is supported by transmission electron microscopy investigations which reveal two amorphous layers of different B concentration in between the crystalline Ag layers.