High magnetic saturation holmium-terbium thin films alloys: application in high-T_c machines

Full text: The heavy rare-earth metals, holmium and terbium, are promising materials for application in high-Tc superconducting (HTS) machines due to their high magnetic saturation (μ₀M_s)¹ within the machine operating range (20-50 K). Bulk single-crystal holmium is particularly interesting as it is predicted to have a magnetic saturation as high as ~3.8 T although exhibiting a low Curie temperature of ~20K². On the other hand, single-crystal terbium offers a much higher curie temperature of ~230K and a similar μ₀M_s (3.4 T) to Holmium. The complexity arises due to competing non-ferromagnetic phases and the significant crystallographic and magnetic anisotropy in the predicted magnetic properties. We report the optimization of the growth of thin film alloys of these rare-earth metals with the purpose of identifying a composition exhibiting both a Tc in the 20-50 K range with a maximum μ₀M_s. Thin films of holmium, terbium and their alloys were grown DC magnetron sputtering. Microstructure and magnetic properties of holmium and terbium metal were measured as a function of nominal deposition rate (ζ= 1,2,3 Ås^-1), and deposition and ex situ annealing temperatures. XRD measurements of both holmium and terbium films revealed that higher deposition rates promote the ferromagnetic hexagonal-close-packed (hcp) phase required to achieve high Ms, instead of the paramagnetic fcc phase. Ho_xTb_1-x alloys x= (0.25, 0.5, 0.75) were co-sputtered at ζ= 2Ås^-1, 350°C. Ho_0.5Tb_0.5 exhibits a μ₀M_s ≈ 2.5T at 10K and showed a strong hcp c-axis texturing characteristic of ferromagnetic rare-earth phase. The results clearly indicated that increasing nominal growth deposition rate and substrate temperature optimize the crystallographic microstructure required to achieve a high μ₀M_s in Ho_0.5Tb_0.5.