Metastability of ultrathin Fe in Fe/Zr multilayers investigated by the Kerr effect

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 d(Fe) - 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 d(Fe). The paramagnetic state of Fe prevails in the as-received Fe/Zr with d(Fe) < 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.