Magnetic and transport properties of Ni81Fe19∕Al2O3 granular multilayers approaching the superparamagnetic limit

The magnetic and transport properties of Ni81Fe19∕Al2O3 granular multilayer films were studied in relation to their structural properties as the nominal thickness t of the permalloy (Ni81Fe19) layer was varied near the percolation limit: in the range of 8⩽t⩽16Å while keeping the nominal thickness of the Al2O3 layers constant at 16Å. A good structural quality of the multilayers was demonstrated by low angle x-ray reflectivity measurements, and transmission electron microscopy showed the transition from continuous permalloy layers separated by aluminium oxide layers for t=16Å to metal grains dispersed in the insulator at t=8Å. Magnetization measurements showed the gradual transition from ferromagnetic layers to superparamagnetic clusters and grains that successively become blocked as the temperature decreases. A strong correlation between transport and structural properties was observed in the temperature (T) dependence of the electrical resistance measured with the current in the plane in the range of 2⩽T⩽300K: a gradual change of behavior from continuous permalloy layers with conducting interlayer connections for t=16Å to isolated permalloy grains in a dielectric for the film with t=10Å. The percolation occurs between 12 and 10Å, as deduced both from the magnetic and resistive properties. The discontinuous metal films were analyzed within models for thermally assisted tunneling, yielding estimates of the tunneling barrier for intralayer conduction of about 20meV for t=10Å. A significant magnetic field dependence of the resistance increasing with decreasing temperature was observed in all samples.