Temperature dependence of magnetoresistance in spin valves with different thicknesses of NiO

Spin valves with structure X Å NiO/60 Å NiFe/30 Å Cu/80 Å NiFe/50 Å Ta, with X from 250 to 1450, were fabricated by dc magnetron sputtering, and their magnetoresistances and NiO/NiFe exchange fields were measured over the temperature range from 20 to 200 °C. ΔR/R decreased almost linearly with increasing temperature from 20 °C up to a transition temperature, and then decreased more steeply to zero at the blocking temperature. A broad distribution of NiO/NiFe exchange paths was observed peaking around 70 °C. The ΔR/R transition temperatures and the exchange path distributions were dependent on NiO thickness. Part of the reduction of ΔR/R at high temperatures may be attributed to fluctuations in NiO spin orientation which cause imperfect antiparallel alignment of the magnetizations of the NiFe layers.