We have developed a delicate method for the determination of stress and magnetostriction coefficient in magnetic multilayer films, to clarify the contribution of magnetoelastic anisotropy to the perpendicular magnetic anisotropy(PMA) observed in these systems.
In the fields of a study on ultrathin magnetic films, magnetic multilayers showing room-temperature perpendicular magnetic anisotropy have been of particular interests because of their novel properties and potential technical applications. Various explanations for the PMA have been suggested for the magnetic multilayers, such as interface anisotropy as a consequence of the reduced symmetry and enhanced magnetocrystalline anisotropy due to altered electronic structure. Magnetoelastic anisotropy, which is expressed by the product of stress and magnetostriction, was often refered as a possible source of PMA, but it was seldom measured accurately because of the difficulty of the measurements, nor was it referred as a main origin of PMA.
For the determination of stress, we have developed a ultrahigh sensitive in situ stress-measurement apparatus of thin films using an optical non-contact displacement detector. A change of the gap distance between the detector and the substrate, caused by stress of a deposited film, was detected by a corresponding change of the reflectivity. The highest sensitivity of the displacement detector was reached to 134mV/㎛ with the aid of computer simulation. The apparatus was applied to in situ stress measurements of several multilayers of Co/Pt, Ni/Pd, and Ni/Pt prepared on the glass substrates by dc magnetron sputtering. The detector turned out to be sensitive enough to observe the coherent-to-incoherent transition within submonolayer deposition in the matching planes of the multilayers. The coherent-to-incoherent transition was confirmed by the cross-section TEM pictures for Co/Pt multilayers. We also found that this powerful tool could demonstrate the incipient behavior of stress due to the difference of surface free energies between two dissimilar materials at the interface.
With the same apparatus, we achieved a high resolution of $3\times10^{-7}$ in the measurement of magnetostriction coefficient, which is sensitive enough to observe the surface magnetostriction effect in Co/Pd, Co/Pt, and Ni/Pt multilayers.
By applying this method to several multilayer systems, we could explain the role of magnetoelastic anisotropy in these systems. In particular, we found that the magnetoelastic anisotropy was the major origin of the observed PMA in Ni/Pt multilayers for the first time. Futhermore, by changing the stress state in Ni sublayer with changing the Ar sputtering pressure, we firstly demonstrate that the magnetic anisotropy can be artificially engineered for the Ni/Pt multilayers with the same composition.
In summary, we have studied the magnetoelastic anisotropy in Co- and Ni-based multilayers for clarifying its contribution to PMA in these system. Using the high sensitive optical detector, we could observe the variation of stress and magnetostriction with an atomic-layer sensitivity. In Ni/Pt multilayers we found that the magnetoelastic anisotropy was the main origin of PMA, and using this fact the magnetic anisotropy could be manipulated via stress-engineering for the first time.
