Magnetic properties of $Fe∕MgO$ granular multilayers prepared by pulsed laser deposition

Granular multilayers $[Fe(tnm)∕MgO(3nm)]N$ with $0.4nm⩽t⩽1.5nm$ were prepared by sequential pulsed laser deposition. Transmission electron microscopy (TEM) images show that increasing $t$ causes the growth of the sizes of Fe nanoparticles and broadening of the particle size distribution. For $t>0.81nm$⁠, continuous Fe layers are formed. The evolution of the shapes and sizes of the particles is reflected in the magnetic properties of the investigated films. A crossover from superparamagnetic to ferromagnetic behavior upon formation of a continuous Fe layer is observed. The fit of zero field cooled and field cooled susceptibility measurements and magnetization curves using Curie–Weiss law and a weighted sum of Langevin functions, respectively, allows the estimation of the average granule size for the films with $t<0.61nm$⁠. The results of the estimations correlate with the data obtained from TEM images. Reduction of saturation magnetization for Fe nanoparticles and an increase of the coercivity up to $1200Oe$ at low temperatures were found. It is attributed to the formation of Fe-core∕$FeOx$-shell structured nanocrystals. The oxide shell gives rise to a strong contribution of surface anisotropy. Isotropic tunneling magnetoresistance up to $∼3%$ at room temperature and in magnetic field up to $18kOe$ was found for the film with $t=0.61nm$⁠. For higher $t$⁠, an anisotropic magnetoresistance typical for continuous ferromagnetic films was observed.