No abstract

Using x-ray magnetic circular dichroism we have detected the very interfacial spins that are responsible for the horizontal loop shift in three different exchange bias sandwiches, chosen because of their potential for device applications. The "pinned" uncompensated interfacial spins constitute only a fraction of a monolayer and do not rotate in an external magnetic field since they are tightly locked to the antiferromagnetic lattice. A simple extension of the Meiklejohn and Bean model is proposed to quantitatively account for the exchange bias fields in the three studied systems from the experimentally determined number of pinned moments and their sizes.

Time-resolved images of the magnetization switching process in a spin transfer structure, obtained by ultrafast x-ray microscopy, reveal the limitations of the macrospin model. Instead of a coherent magnetization reversal, we observe switching by lateral motion of a magnetic vortex across a nanoscale element. Our measurements reveal the fundamental roles played independently by the torques due to charge and spin currents in breaking the magnetic symmetry on picosecond time scales.

Ultrashort pulse laser techniques are applied to study optically induced modulation in exchange biased ferromagnetic/antiferromagnetic ( FM /AF) thin bilayer films ( NiFe͞NiO). Photoexcitation of the FM / AF interface with subpicosecond laser pulses induces large modulation in the unidirectional exchange bias field (H ex ) on an ultrashort time scale. The "unpinning" of the exchange bias leads to coherent magnetization rotation in the permalloy film which is time resolved by the experiment and corresponds to a large modulation in the magnetization component (DM Z ͞M S ϳ 0.5), on a time scale of 100 psec.[S0031-9007(99)09042-0]

For a fixed 2 μm×2 μm area of a Co/Pt-CoO perpendicular exchange bias system we image the ferromagnetic (FM) domains for various applied fields with 10-nm resolution by magnetic force microscopy (MFM). Using quantitative MFM we measure the local areal density of pinned uncompensated spins (pinUCS) in the antiferromagnetic (AFM) CoO layer and correlate the FM domain structure with the UCS density. Larger applied fields drive the receding domains to areas of proportionally higher pinUCS aligned antiparallel to FM moments. The data confirm that the evolution of the FM domains is determined by the pinUCS in the AFM layer, and also present examples of frustration in the system.