Temperature dependent rotation of the uniaxial easy axis of magnetization in granular SiO2–(Co/Fe/CoFe2)–Si(1 1 1) multilayers

We have observed temperature dependent rotation of the uniaxial easy axis of magnetization and introduced the phenomenon of the superspin reorientation transition (SSRT) for the nanogranular thin films and multilayers. The effect is demonstrated on the nanogranular SiO2–M(x)–Si(1 1 1) multilayers, where M is the metal (Co, Fe or CoFe2) of the nominal thickness x = 3, 5 and 10 nm, respectively. The morphology and layer thickness have been examined using the transmission electron microscopy, X-ray reflectivity measurement and atomic force microscopy. Magnetic property measurements demonstrated that the orientation of the uniaxial in-plane easy axis of magnetization given by the film shape anisotropy was thickness independent at temperatures above 200 K. Below 50 K, the films shape anisotropy has been overcome and the uniaxial easy axis was rotated into different directions for each samples series, forming an angles from 40 up to 150° with respect to the film plane. Such effect of the low temperature magnetization rotation has been attributed to the specific arrangement of the nanoparticles in the individual layers and has been assigned to the SSRT phenomena.