METASTABILITY OF ULTRATHIN FE IN FE ZR MULTILAYERS INVESTIGATED BY THE KERR-EFFECT/

Wedge-shaped Fe/Zr multilayers have been investigated using the magnet o-optic Kerr effect at room temperature (RT). A laser spot was scanned along a wedge to obtain polar hysteresis loops for various Fe thickne sses 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 wh ich may be conceived of as a mixture of ferromagnetic, superparamagnet ic and paramagnetic states with the volume fraction of each component depending on d(Fe). The paramagnetic state of Fe prevails in the as-re ceived Fe/Zr with d(Fe) < 2.5 nm. This state is unstable and can be ea sily transformed on annealing to a partially ferromagnetic state due t o the coarsening and coalescence of the bcc Fe grains. A simple model is proposed to describe the shape of polar hysteresis loops, and the f its to the experimental data provide a realistic picture of the transf ormation of paramagnetic (amorphous) Fe into crystalline ferromagnetic bcc Fe.