Grain size effects in polycrystalline exchange biased systems

The capability of a novel sputtering technology known as HiTUS to produce thin films
with controlled grain size and distribution has been evaluated. This has been achieved
for different materials chosen due to their importance in industrial applications. The
grain size of the deposited films was measured from TEM images in bright field mode.
All the distributions were lognormal and over five hundred particles were measured to
recompose each lognormal function. For a range of materials close control of grain size
between 4nm and 30nm was achieved.
The grain size was controlled through the deposition rate. This has allowed the
investigation of the effect of the antiferromagnetic grain size on exchange bias without
the use of underlayers, additives or substrate heating. It was found that the magnitude of
the loop shift strongly correlated to the distribution of grain volumes in the
antiferromagnetic layer. Using detailed measurements of the grain size distribution the
features of exchange bias in these systems can be explained on the basis of a grain
volume model with coherent rotation.
For example using measurement protocols that ensure reproducibility of data has
allowed the development of a new measurement technique for the anisotropy constant
of metallic antiferromagnets. This is a key result since the anisotropy is used in all
theoretical models of exchange bias as a fitting parameter. The anisotropy was
determined only from experimental features and only its temperature dependence had to
be assumed to be of the form K(OK)(1-T/TN)3 in order to calculate the values. For IrMn
K(293K) =4.14xlO6erg/cc and for FeMn K(293K) = 1.36x106erg