High moment coupled rare earth (Gd, Dy, Tb, and GdDy) and transition metal thin film laminates

• Main Author: Ward, Christopher
• Published: Queen's University Belfast 2016
• Subjects: 621.3815
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709856

This thesis evaluates the possibility of combining the high magnetic moment of the rare earth metals with the high Curie temperature transition metals to produce a material with a greater saturating magnetisation than FeCo at room temperature. Thin films of FeCo and Gd, Dy, GdDy, and Tb separated by a Cr spacer layer were fabricated by magnetron sputtering and characterised for magnetic and crystal structure. Magnetic measurements were carried out over a temperature range of 3-330K by SQUID magnetometry and VSM measurements with the emphasis on saturating magnetisation, while crystal structure was investigated by XRD and TEM analysis. It was found that while Gd exhibited a modulating magnetic moment when coupled with FeCo, this moment was significantly supressed when compared with either FeCo or Gd. Through XRD analysis the origin of the lowered moment was indicated by the nucleation of the ultrathin Gd films into the paramagnetic FCC phase. This is explained in terms of the lattice mismatch of Gd and a possible solution lay in terms of a lattice match seed material to promote growth of HCP Gd. Yttrium, an almost perfect lattice match to Gd proved promising, reducing the FCC portion for thin layers of Gd and producing saturation values in excess of 2.4T for 6 nm films, a significant result achieved via magnetron sputtering. The study continued with the other rare earths and they all exhibited a suppressed magnetic moment, most likely due to uncoupled material in the laminate. After a move to sub nm layers this pushed deposition methods to their limits producing unreliable thicknesses. At present this combination of rare earth and transition metal stack has not shown any saturating magnetisation in excess of FeCo at room temperature.