Effects of Mn concentration and deposition temperature on the giant magnetoresistive properties of ion-beam-deposited PtMn spin valves

Abstract

We studied the effects of Mn concentration (CMn) in PtMn antiferromagnets (AFs) and the deposition temperature (Ts) of PtMn in bottom PtMn spin valves. The exchange coupling field (Hua) and the critical thickness of PtMn were found to depend strongly on CMn. Spin valves made with the optimum CMn (51%) show an Hua value of around 410 Oe even with 125-Å-thick PtMn, compared to the saturation value of about 500 Oe for the spin valves with much thicker PtMn. For the Ts dependence study, the NiFeCr-NiFe underlayer, PtMn AF layer, and part of the CoFe pinned layer were deposited at Ts up to 300 °C, and the rest of the layers were deposited after cooling the spin valve down to ambient temperature. The spin valves deposited at Ts > 250 °C and with > 150-Å-thick PtMn show Hua of around 250 Oe, as deposited (without postannealing). The Hua values increase upon postannealing for the spin valves deposited at all the Ts studied (25 °C-300 °C). However, the ferromagnetic coupling field (Hf) also increases with repetitive postannealing, the rate of Hf increase being lower for the spin valves deposited at higher Ts. The sheet resistance of PtMn spin valves was found to increase both after postannealing and with increasing Ts, consistent with the higher resistivity observed for thin PtMn films deposited at higher Ts. These results are attributed to the smaller grain size and the higher degree of chemical ordering for PtMn in the postannealed spin valves deposited initially at higher Ts, as deduced from X-ray diffraction.