Temperature dependence of transport properties and exchange field of NiMn based spin valves

We studied ion beam deposited NiFe/CoFe/Cu/CoFe/NiMn spin valves (SVs) subjected to different thermal annealing to establish the exchange field (HE) of the CoFe pinned layer. The magnetoresistive (MR) properties of the SVs were characterized by magnetotransport measurements. We found MR values of 7.5%, 12% and 14% at T=20K for SVs annealed at 453, 533 and 553K, and observed a linear decrease of MR with increasing temperature for all SVs due to the excitation of spin waves. Our results also show that the exchange field HE is approximately constant over the measured temperature range (20-300K) for SVs annealed at 453 and 553K (HE∼250 and 400Oe, respectively), while the sample annealed at 533 K shows the usual decrease of HE with temperature. These behaviors will be discussed in terms of structural differences between the measured SVs. An anomalous increase of the magnetoresistance at free layer switching in the ascending H-branch will be related with the enhancement of the pinned layer coercive field with decreasing temperature