Structural and magnetic properties of epitaxial rare-earth cobalt thin films

With the ever increasing demand for beyond-state-of-the magnets, which could also serve in extreme conditions, it is crucial to reduce the dependence on the critical rare-earths and explore new materials or material designs. The cobalt rich compositions of the rare-earth (R) cobalt intermetallic systems offer the combined advantages of a strong magnetocrystalline anisotropy and a high saturation magnetization, well above room temperature. This work focuses on investigating thin films of R-Co intermetallics as model systems for designing the materials at the nanoscale, understanding the magnetic hardness mechanisms and developing more sustainable magnetic systems. The technique of molecular beam epitaxy (MBE) is utilized to investigate the thin film phase diagrams of lesser critical (Y and Sm), and abundant (Ce) rare-earth cobalt based systems. The growth window of buffer-free (00l) oriented R${}_2$Co${}_{17}$ and RCo${}_5$ thin films onto (001)-Al${}_2$O${}_3$ substrate are explored. With the manipulation of individual atomic beams, MBE enables a fine tuning of the stoichiometry, whereby it was possible to stabilize not only the individual phases of Y${}_2$Co${}_{17}$ and YCo${}_5$ but also a nanocomposite of these phases. The Y${}_2$Co${}_{17}$ film has an easy-plane anisotropy matching well to bulk single crystal while the YCo${}_5$ film exhibits a perpendicular anisotropy. As a result of exchange coupling of the Y${}_2$Co${}_{17}$ phase to the YCo${}_5$ phase, the nanocomposite films also shows perpendicular anisotropy. The outcome of a uniaxial anisotropy induced in an easy-plane material of Y${}_2$Co${}_{17}$, and hence, the resulting coercivity, shows a potential way of broadening the class of materials useful for permanent magnets. Furthermore, we were able to fabricate CeCo${}_5$ thin films with a saturation magnetization of 500 emu/cm${}^3$, a perpendicular anisotropy of 0.44 MJ/m${}^3$ and coercivity of 2.74 kOe, which are the highest reported so far for the thin films. An extremely large perpendicular anisotropy of 1.67 MJ/m${}^3$ is achieved SmCo${}_5$ thin film without the use of any buffer layers. The atomic scale resolution of the film revealed that the SmCo${}_5$ phase grows perfectly c-axis oriented on the (001)-Al${}_2$O${}_3$ substrate, however, with possible traces of the Sm${}_2$Co${}_{17}$. The result of a perpendicular anisotropy obtained in single layer of RCo${}_5$ phase make them interesting for magnetic recording and spintronic applications. In view of tuning the magnetocrystalline anisotropy of a compound by strain introduced by chemical substitution, thin films of yttrium substituted cerium (Y,Ce)Co${}_5$ intermetallics are investigated. We observed that the structural and magnetic properties of (Y${}_{1-x}$Ce${}_x$)Co${}_5$ films show a non-linear dependence on Ce content, x which is supposedly due to a varying chemical valence the Ce ion. The absolute values vary, but a similar trend in the lattice parameters and magnetization is also observed in the bulk single crystals.