Structural and magnetic properties of Ni-O/Ni-Fe bilayer films

Investigations on the relationship between the magnetic properties and structure of Ni–O/Ni–Fe bilayer sputtered films were conducted. The Ni–O/Ni–Fe films were prepared under various Ar gas pressures during Ni–O layer deposition and various distances between targets and substrates. The exchange coupling field decreased as the Ar gas pressure was increased, although the coercive force remained constant. The distance between the target and the substrate does not affect the magnetic properties. Structural analysis revealed that the Ni–O layer exhibits a rhombohedral structure distorted in the direction of from the cubic NaCl structure. The interaxial angle α, which is the angle between two crystallographic axes in the rhombohedral unit cell, gradually decreases as the Ar gas pressure increases until finally the cubic NaCl structure is formed. We found that there is a linear relationship between the values of the exchange coupling field and α which corresponds to the degree of distortion from the NaCl structure. It is considered that the structural distortion of the Ni–O layer affects its antiferromagnetic state, then the exchange coupling between Ni–O and Ni–Fe layers is affected as a result.