Exchange stiffness reduction in Ta substituted NiFe alloys

In magnetic tunnel junctions (MTJs) for memory or sensor applications, insertion of Ta nanolayers at the interface of, or within magnetic layers such as CoFe or NiFe is often employed to enhance the tunneling magnetoresistance, although it also locally reduces the magnetization and modifies the magnetic properties. To properly model the magnetic behavior of such structures, it is important to be able to accurately account for these modifications. In this paper, we investigate experimentally and by means of ab initio calculations, the degradation of magnetic and magneto-dynamic properties of Ni0.81Fe0.19 films doped with small amounts of Ta. Both the calculations and the experiment confirm the expected drop of saturation magnetization parameter with increased Ta doping. It is also found that the exchange stiffness is reduced much faster than the magnetization. Calculations show that the presence of Ta impurities has the greatest effect on the magnetic properties of Ni, the dominant element. For example, the introduction of 10 atomic % Ta reduces the exchange stiffness constant by a factor of 10, while the room temperature magnetization reduces by less than a factor of 3, resulting mainly from a decrease in the Curie temperature from 870 to 420 K. Consequently, the reduction in exchange coupling across Ta nanolayers can significantly modify the detailed magnetic behavior and in particular the propagation of exchange coupling within MTJ multilayers.