  We have investigated the magnetoresistance of Permalloy (Ni80Fe20) films with
thicknesses ranging from a single monolayer to 12 nm, grown on Al2O3, MgO and
SiO2 substrates. Growth and transport measurements were carried out under
cryogenic conditions in UHV. Applying in-plane magnetic vector fields up to 100
mT, the magnetotransport properties are ascertained during growth. With
increasing thickness the films exhibit a gradual transition from tunneling
magnetoresistance to anisotropic magnetoresistance. This corresponds to the
evolution of the film structure from separated small islands to a network of
interconnected grains as well as the transition from superparamagnetic to
ferromagnetic behavior of the film. Using an analysis based on a theoretical
model of the island growth, we find that the observed evolution of the
magnetoresistance in the tunneling regime originates from the changes in the
island size distribution during growth. Depending on the substrate material,
significant differences in the magnetoresistance response in the transition
regime between tunneling magnetoresistance and anisotropic magnetoresistance
were found. We attribute this to an increasingly pronounced island growth and
slower percolation process of Permalloy when comparing growth on SiO2, MgO and
Al2O3 substrates. The different growth characteristics result in a markedly
earlier onset of both tunneling magnetoresistance and anisotropic
magnetoresistance for SiO2. For Al2O3 in particular the growth mode results in
a structure of the film containing two different contributions to the
ferromagnetism which lead to two distinct coercive fields in the high thickness
regime.
