Magnetization measurements by magneto-optical methods on films with Fe particles in an Al2O3 amorphous matrix

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Abstract
We made magneto‐optical measurements (Faraday Rotation) on four different films with an approximate thickness of 0.5 μm obtained by cosputtering. The films are composed by Fe particles, with different average diameter in the range 50<Φ2O3. At the surface of each particle the presence of Fe2+ and a smaller quantity of Fe3+ ions were previously detected. Fe3+ ions were also found in large quantities at the surface of the whole sample. Our measurements are able to distinguish signals coming from the two different ions. We followed these signals by varying the temperature down to 4 K. We observed that the magneto‐optical response of Fe3+ ions remains constant, while that of Fe2+ increases with decreasing temperature (T) for T above a characteristic temperature Tc. Below Tc the Fe2+ response remains constant. Measured Tc values were 23, 29, 36, and 250 K for the four samples containing 45, 55, 60, and 70–85 wt. % iron, respectively. These results are comparable with those obtained by other techniques (Mössbauer and susceptibility). We conclude that the blocking process concerns only the magnetic moments of the particles located in the bulk of the film.