Effect of substrate and antiferromagnetic film’s thickness on exchange-bias field (invited)

Abstract

In this work, we studied the effect of different substrates and the thickness dependence of the antiferromagnetic layer on the exchange-bias field, created through interfacial exchange coupling between NiFe and FeMn. The substrates considered were glass and a metallic underlayer, such as Ta, on glass. We noticed different behaviors of the two types of films when annealed in a magnetic field at 240 °C and we carefully looked at their temperature dependence, observing that the blocking temperature, at which the exchange-bias field goes to zero, decreases with increasing FeMn thickness. We also found that, when reducing the antiferromagnetic film’s thickness, the exchange-bias field of the annealed NiFe/FeMn structure improves on both types of substrate. Since a low blocking temperature is undesirable, we developed a method to take advantage of the thinner FeMn film’s properties and to compensate the decrease in blocking temperature. The FeMn was deposited in two separate layers. The first layer, at the NiFe interface, was sputtered in pure Ar, and the second in an Ar/O2 or Ar/N2 mixture. The advantage of this technique was particularly impressive for films deposited on Ta over glass, where a factor of 2 improvement in exchange bias field was obtained at both room temperature and 80 °C, when compared to the structure with the same total antiferromagnetic material’s thickness deposited in one single layer. Blocking temperatures were above 150 °C.