Negative differential magnetization for Ni nanoparticles in Al

Abstract

Using a layer-by-layer deposition technique, Ni thin films were sandwiched between AlOx layers in an Al matrix. By using x-ray-absorption spectroscopy, grazing incidence small-angle x-ray scattering, and highresolution transmission electron microscopy, we showed that the Ni layers are discontinuous and that pure Ni clusters are formed with sizes in the nanometer range. Depending on the preparation conditions, different magnetic behaviours are observed. When only one discontinuous Ni layer is grown, the hysteresis loops at low temperature display a conventional shape with a reduced magnetic moment per atom due to a magnetically dead layer at the cluster surface. When three discontinuous Ni layers are grown, a negative differential magnetization is observed. This phenomenon is interpreted as due to an antiferromagnetic coupling between two populations of magnetic grains having different average diameters in the different Ni layers. The coupling is indirect through the Al spacer.