A magnetoresistive sensor having a substrate that has been treated with
nitrogen (nitrogenated). The nitrogenated substrate includes an alumina
base layer and a thin top layer of crystalline alumina that has had a
very small amount of nitrogen deposited on top. The amount of nitrogen
deposited on top of the alumina is less than or equal to two monolayer,
and is preferably less than on monolayer. The amount of nitrogen
deposited on top of the alumina substrate is not enough to constitute a
layer of nitrogen, but affects the structure of the alumina to cause the
alumina to have a desired crystalline structure and an extremely smooth
surface.