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

We studied the effects of Mn concentration (C_Mn) in PtMn antiferromagnets (AFs) and the deposition temperature (T_s) of PtMn in bottom PtMn spin valves. The exchange coupling field (H_ua) and the critical thickness of PtMn were found to depend strongly on C_Mn. Spin valves made with the optimum C_Mn (51%) show an H_ua 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 T_s dependence study, the NiFeCr-NiFe underlayer, PtMn AF layer, and part of the CoFe pinned layer were deposited at T_s 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 T _s>250°C and with >150-Å-thick PtMn show H_ua of around 250 Oe, as deposited (without postannealing). The H_ua values increase upon postannealing for the spin valves deposited at all the T_s studied (25°C-300°C). However, the ferromagnetic coupling field (H_f) also increases with repetitive postannealing, the rate of H_f increase being lower for the spin valves deposited at higher T_s. The sheet resistance of PtMn spin valves was found to increase both after postannealing and with increasing T_s, consistent with the higher resistivity observed for thin PtMn films deposited at higher T _s. 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 T_s, as deduced from X-ray diffraction