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Final Activity Report Summary - NESSMIN (Neutron Scattering Studies of Magnetic Interactions in Nanostructures)

Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density magnetic recording and medicine. Giant magnetoresistance (GMR) in nanogranular samples arises from spin-dependent electron scattering, yielding reduced resistivity when magnetic nanoparticles are aligned. Soon after the discovery of GMR in nanogranualr Co/Cu, similar effects were reported for various systems, and the maximum 24% change in resistivity at room temperature was achieved using Co/Ag, and that remained the highest GMR for a nanogranular system for more than one decade. We have achieved a record 40% room temperature GMR in a nanogranular alloy by optimizing the concentration, sputtering conditions and cumulative short thermal treatments. Upon annealing, the GMR effect exhibits a local minimum at 230 C before reaching a maximum at 300 C. Assuming the presence of both dipolar and RKKY-like exchange interactions between the particles, these features can be accounted for by considering that the latter correlations are progressively inhibited as the matrix, a supersaturated Ag-Co solid solution, segregates the solute Co atoms, and this was directly observed via x-ray diffraction from the Co nanoparticles in an Ag matrix.

Recently, exchange bias, in which magnetic hysteresis loops are shifted from the zero-field axis, has been suggested as a source of anisotropy in order to stabilise the magnetic moment of ferromagnetic nanoparticles, therefore helping to beat the superparamagnetic limit for high-density recording. We have grown Co(core)CoO(shell) particles embedded in a nonmagnetic Ag matrix by a simple magnetron sputtering technique using various oxygen pressures in the chamber. The observed hysteresis loops reveal large exchange-bias fields for high oxygen pressures. As the oxygen pressure increases, there is an initial decrease in the blocking temperature, which we attribute to the inhibition of RKKY-like interactions by the formation of insulating oxide layer around the ferromagnetic cores. For higher oxygen pressures the blocking temperature and the exchange-bias fields increase due to the exchange bias between the Co core and the CoO shell.

The microscopic magnetic ordering in the ferromagnetic and antiferromagnetic layers in Fe/Mn multilayers have been studied to illuminate the mechanism behind the exchange-bias phenomenon. The antiferromagnetic ordering in the Mn is completely unexpected, suggesting that the models assumed for exchange bias over the last decade are probably wrong. We have commenced studies of NiFe/FeMn and NiFe/PtMn multilayers, which are used in commercial spin valves, and preliminary results indicate that these systems are comparable to Fe/Mn.

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