Fabrication and growth of c -axis textured Nd₂Fe₁₄B thin films by high-rate sputtering

To apply Nd-Fe-B thin films for mass-produced heat-assisted magnetic recording media, we investigated the high-rate sputtering conditions required to obtain c-axis textured Nd₂Fe₁₄B thin films and analyzed the growth mechanism. Magnetization curves indicated that higher substrate temperatures and sputtering rates resulted in a higher degree of perpendicular magnetic anisotropy; a Nd-Fe-B layer deposited at a substrate temperature (T_{sub_0}) of 600 °C and a sputtering rate (R_sp) of 2.6 nm/s had the easy axis perpendicular to the film plane. The dependence of the magnetic properties on the sputtering rate was due to a decrease in the substrate temperature during sputtering; there was a threshold for obtaining a high squareness ratio. X-ray diffraction analysis and transmission electron microscopy (TEM) images showed that the c-axis textured Nd₂Fe₁₄B crystal phase was formed in the Nd-Fe-B layer deposited at T_{sub_0} = 600 °C and R_sp = 2.6 nm/s, which resulted in the highly perpendicular magnetic anisotropy. In addition, the TEM images showed a layer of Nd₂Fe₁₄B with no obvious lattice fringes near the interface between the Nd-Fe-B layer and Mo underlayer, while the lattice fringes of the Nd₂Fe₁₄B phase were not parallel to this interface but gently curved along the Mo cap layer. We propose that the c-axis orientation was achieved by the rotation of the c plane, which has the lowest surface energy in the Nd₂Fe₁₄B phase, toward the vacuum-side surface during sputtering.