Determining cobalt-alloy anisotropy constants via epitaxial unicrystal films

Co-alloy thin films with Cr underlayers are currently the most popular hard disk media. Magnetocrystalline anisotropy is an important property of Co-alloy materials. While the anisotropy field determines the maximum achievable coercivity of Co-alloy thin films, the product of the anisotropy constant and the particle volume dictates the thermal stability of the recorded data. Hence, knowledge and control of the anisotropy constants are essential for the design of high density Co-alloy media.; The purpose of this study is to determine the anisotropy constants of Co alloys. For this research objective we have developed an epitaxy process to grow unicrystal Co-alloy films on Cr/Ag templates sputter deposited on hydrofluoric acid (HF)-etched Si(110) single crystal substrates. These unicrystal films exhibit a single, in-plane easy axis orientation, and thus allow the direct determination of the anisotropy constants by torque and hard axis hysteresis loop measurements. These two methods of determining the anisotropy constants are preferred to the method of the field dependence of rotational hysteresis which does not provide accurate values of the anisotropy field due to intergranular interactions in the films.; The epitaxial orientation relationship in unicrystal films was studied with x-ray theta/2theta diffraction, pole figure 4 scan, and electron diffraction, and it was determined to be Co(101¯0)[0001] ∥ Cr(112)[11¯0] ∥ Ag(110)[001] ∥ Si(110)[001]. The microstructure of unicrystal films was studied with transmission electron microscopy (TEM). The observation focused on the stacking faults as the presence of the stacking faults in these films is expected to be detrimental to achieving the potential values of the anisotropy constants of perfect single crystals. The TEM studies showed that pure Co films contain a high density of stacking faults, while applying a substrate bias during the deposition reduces the stacking fault density. In comparison to pure Co, for Co-alloy films the introduction of Pt increases the stacking fault density, while Cr results in fewer stacking faults.; Using the unicrystal film structure we first determined the anisotropy constants of pure Co and studied the dependence on the measurement temperature, deposition conditions, and underlayer materials. The anisotropy constants of unicrystal Co films are smaller than the values published for bulk Co single crystals, and decrease faster with increasing temperature. On the other hand, applying a substrate bias during the Co deposition not only increases K1, but also makes it more stable with respect to temperature. A comparison of the microstructure and the anisotropy constants between the films prepared with and without substrate bias suggests a strong correlation between the stacking faults and the anisotropy. High stacking fault density appears to result in low anisotropy constants with strong temperature dependence.; The unicrystal film structure also enabled us to determine the Co-alloy anisotropy constants as functions of the Pt and Cr concentrations and study their temperature dependence. CoPt alloys show a significant increase in K1 with increasing Pt content, while CoCr alloys suffer a small decrease as Cr concentration increases. Pt also increases the K1 of CoCr20Pt alloys. The K 2 values of Co alloys are all smaller than that of pure Co. Meanwhile, the addition of Cr and/or Pt results in weaker temperature dependences of K1 for Co alloys.