Tuning of Exchange-Biased Ni-Mn/Fe-Ni Films With High Blocking Temperatures

Effect of a reduction in an antiferromagnetic layers' thickness and insertion of a Ta spacer on thermal behavior of exchange bias field, coercivity, and on blocking temperature of Ni-Mn-based exchange-biased films is studied. Reduction of a Ni-Mn layer's thickness from 20 to 17.5 nm to 14 and 12 nm gradually decreases the exchange bias field. The dependence of the blocking temperature on the thickness of a Ni-Mn layer shows a finite-size scaling effect with a maximum blocking temperature of 650 K for a 20 nm thick antiferromagnetic layer. Despite that, an unusually stable or even increasing exchange bias is present for some thicknesses in the temperature range up to 400 K. Addition of a Fe-Ni buffer with an increase in Ni concentration in Ni-Mn layer yields a greater exchange-bias-field-to-coercivity ratio and an even better thermal stability of exchange bias, which remains steady in the range from 300 to 475 K. Addition of a very thin Ta spacer (0.1 nm, 0.2 nm, or 0.3 nm thick) significantly decreases the exchange bias field, while only slightly decreasing the blocking temperature. A peak in the coercivity at around 700 K is found for all of the studied films, and is likely due to structural transformations.