Effect of Ta underlayer on magnetic properties of FeMn/NiFe films

Effect of Ta underlayer on the magnetic properties of sputter-prepared NiFe(5 nm)/FeMn(20 nm) bilayer films have been studied. The magnetic properties of studied films are optimized by modification of working Ar pressure deposition of Ta (PTa) in the range of 2-12 mTorr and thickness of Ta (tTa) in the range of 0-25 nm. X-ray diffraction results show that the crystallinity of the FeMn(111) strongly depends on the PTa and tTa. All studied films exhibit smooth and flat surface with root-mean-square roughness below 1 nm due to deposition at RT. Large EB field (Heb) of 65-123 Oe with small coercivity (Hc) of 5-16 Oe is obtained. Besides, the change of Heb with various PTa and tTa are related to the crystallinity of FeMn(111) layer, interfacial roughness, and also strain/stress. Correlation between magnetic properties and microstructure is also discussed. This study suggests that proper Ta underlayer is crucial in the exchange bias for NiFe/FeMn bilayer system.