Improved exchange coupling between ferromagnetic Ni-Fe and antiferromagnetic Ni-Mn-based films

Dc magnetron sputtered Ni-Mn and Ni-Mn-Cr films are demonstrated to exhibit strong and thermally stable antiferromagnetism, as well as high corrosion resistance. For a 25.2 nm thick 53.3 Ni-46.7 Mn (in atomic percent) film deposited on top of a 28.5 nm thick 81 Ni-19 Fe film, a unidirectional anisotropy field (HUA) of 120.6 Oe is obtained at room temperature after annealing in vacuum. The equivalent interfacial exchange coupling energy (JK) is 0.27 erg/cm2, three times higher than that of bilayer Ni-Fe/50Fe-50Mn films. This strong exchange coupling appears correlated with the presence of an antiferromagnetic θ (NiMn) phase with a CuAu-I-type ordered face-centered-tetragonal structure. The blocking temperature, at which the exchange coupling disappears, is higher than 400°C. The Cr addition to the Ni-Mn film dilutes the exchange coupling, but the JK for the Cr content ≤10.7 at. %, is still higher than that of the Ni-Fe/Fe-Mn films. Both Ni-Mn and Ni-Mn-Cr films exhibit corrosion behaviors much better than the Fe-Mn film and comparable to the Ni-Fe film. The films are proposed as longitudinal bias layers for the stabilization of magnetoresistive read sensors.