PROPERTIES OF FERROMAGNETIC MNAL THIN-FILMS WITH ADDITIVES

MnAl thin films with additives were prepared by dc magnetron sputtering. The ferromagnetic tau phase was obtained in the composition of Mn60Al40, which is Mn rich by 6 at. % as compared with the tau-phase bulk one. Excess Mn atoms are considered to occupy the Al site and cause the reduction in saturation magnetization M(s) to one-fourth of the bulk one because of their antiparallel spin orientation. We tried to add Fe, Co, Ni, Cu, Zn, and Ag separately to replace the antiparallelly oriented Mn atoms in order to increase M(s). A few atomic percent additions of Fe, Co, Ni, and Cu brought 1.7-2.5 times the increase in M(s) (i.e., 200-300 G), with the increase of H(c) as well except that with the Cu addition. Further addition brought a sharp decrease in H(c). Different types of crystal orientation were observed regarding the film thickness and the kind of additives. For a tau-phase film thinner than 0.3-mu-m, (111) orientation was observed in every additive. In this case the easy magnetization axis, which is the c axis in the tetragonal structure, is neither in the film plane nor normal to the film plane. Consequently, magnetically isotropic films were obtained. For thicker ones than 0.3-mu-m, the addition of elements with an atomic radius smaller than that of Mn (i.e., Ni, Co, Fe) directed (110) to the normal plane, and therefore longitudinally magnetically oriented films were obtained. The crystallographic and magnetic properties as a function of anneal temperature were also evaluated and found practically to be invariable with the temperature under 250-degrees-C.