A magnetoresistive sensor having a substrate that has been treated with nitrogen (nitrogenated) and having a Ta cap layer with nitrogen added in situ during deposition. 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 riot 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. The nitrogen in the cap layer can be formed by depositing a Ta cap layer in a sputter deposition chamber having a small amount of nitrogen in an Ar atmosphere.