A magnetic sensing device for use in a magnetic head includes a sensor stack structure having a sensing layer structure and an insulator structure formed adjacent the sensing layer structure. The insulator structure includes a plurality of oxidized metallic sublayers, a plurality of nitrided metallic sublayers, or a plurality of oxynitrided metallic sublayers. The insulator structure may be a capping layer structure of a giant magnetoresistance sensor or, alternatively, a tunnel barrier layer structure of a tunneling magnetoresistance sensor or a magnetic random access memory. Advantageously, each treated metallic sublayer is sufficiently uniformly treated so as to increase the magnetoresistive effect and improve soft magnetic properties of the magnetic sensing device. A method for use in forming the magnetic sensing device of the present application includes the steps of forming a sensor stack structure which includes a sensing layer structure; depositing a metallic layer; performing, on the metallic layer, either an oxidation, nitridation, or oxynitridation process; and repeating the steps of depositing and performing one or more times to thereby form an insulator structure.