Aperiodical oscillation of interlayer coupling in epitaxial Co/Ir(001) superlattices

High quality epitaxial cobalt-iridium superlattices were successfully grown on MgO(001) substrates via molecular-beam epitaxy and were found to exhibit anomalous oscillatory interlayer exchange coupling with a strong coupling constant. The aperiodical antiferromagnetic (AF) peaks appear at Ir layer thicknesses of 5 {}, 15 {}, and 33 {}. Maximum magnetoresistance (MR) ratios of ≈1.1% are obtained and are almost the same for the first and the second AF peaks. The interlayer coupling corresponding to the first AF peak is so strong that the magnetization did not saturate even in an applied field up to 90 kOe. The estimated value of the interlayer exchange constant by MR curves at room temperature is more than 2.2 erg/cm² close to the first AF peak. A distinction of the in-plane magnetization process between [100] and [110] directions is observed and the easy axis of magnetization is found to be along the [110] axis in plane. The aperiodic oscillations are thought to indicate multiperiodicity of the interlayer exchange constants.