Magnetic viscosity and microstructure of GePt/FePt films

We found various GePt/FePt microstructures at different post-annealing temperatures. The Ge(2)Pt(3) compound was formed when the annealing temperature was 800 °C and the particle-like structure was able to relax the growth stress between Ge(2)Pt(3) and quartz. After deposition of FePt film, a discontinuous L1(0) FePt layer was formed when it was post-annealed at 400 °C. However, isolated L1(0) FePt particles were observed at 800 °C post-annealing temperature, and each particle contains many grains. Furthermore, the magnetic viscosity was measured to investigate the different GePt/FePt morphology effects on thermal activation behaviour. When the applied field was less than the coercivity field H(c), we found smaller activation volume (V(a) = 0.5 × 10(-18) cm(3)) for a film with 400 °C post-annealing. This is because smaller FePt grains are in between GePt islands and the moments in these grains are hard to reverse. In contrast, a larger V(a) (= 1.9 × 10(-18) cm(3)) was found in samples post-annealed at 600-800 °C. This is because the GePt islands agglomerate to become particle-like at high temperature and larger FePt grains were distributed on each GePt particle.