Magnetic and Magnetotransport Properties of Exchange-Biased NiFe/NiO Bilayers

The magnetism of a series of NiFe (20 nm)/NiO (10 nm) bilayers with different Ni oxides has been studied. Ni oxides were made using ratios of O2 to Ar ions ranging from 7 to 33% during ion-beam deposition. Transmission electron microscopy (TEM) has shown that with 7% O2/Ar used during bottom layer deposition, the film produced consisted of fcc Ni (a=3.52 A) and rock-salt NiO (a=4.21 A) phases. A bottom film layer prepared with 33% O2/Ar consisted of a pure NiO phase with an expanded lattice constant (a=4.32 A). A strong temperature dependence of the coercivity (Hc) and a 20 kOe field-cooled loop shift exchange bias field (Hex) were observed below 100 K. In addition, Hex increased with increasing O2/Ar ratio for the bottom layer antiferromagnetic (AF) component. At 10 K, a NiFe/NiO (33% O2/Ar) bilayer exhibited the higest Hc (110 Oe) and Hex (-60 Oe). This indicated that the more expanded the NiO lattice, the stronger the exchange coupling with the NiFe. The magnetotransport studies have shown that these NiFe/NiO bilayers exhibit anisotropic magnetoresistance (AMR) behavior. In addition, the total MR ratio of these NiFe/NiO bilayers increases with increasing O2/Ar ratio owing to strong anisotropic scattering at the NiFe/NiO interface.