Abstract
Wedge-shaped Fe/Zr multilayers have been investigated using the magneto-optic Kerr effect at room temperature (RT). A laser spot was scanned along a wedge to obtain polar hysteresis loops for various Fe thicknesses dFe = 1.2–2.5 nm. An analysis of shapes of the hysteresis loops yields information on a complex magnetic structure of ultrathin Fe which may be conceived of as a mixture of ferromagnetic, superparamagnetic and paramagnetic states with the volume fraction of each component depending on dFe. The paramagnetic state of Fe prevails in the as-received Fe/Zr with dFe < 2.5 nm. This state is unstable and can be easily transformed on annealing to a partially ferromagnetic state due to the coarsening and coalescence of the bcc Fe grains. A simple model is proposed to describe the shape of polar hysteresis loops, and the fits to the experimental data provide a realistic picture of the transformation of paramagnetic (amorphous) Fe into crystalline ferromagnetic bcc Fe.
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