Microstructures and magnetic properties of Fe-Pt permanent magnets

We have investigated the magnetic properties of Fe-38.5Pt, Fe-39.5Pt and Fe-50.0Pt (at%) alloys after various heat treatment conditions using a vibrating sample magnetometer, and correlated these properties with the microstructures of the alloys by transmission electron microscopy. The Fe-50Pt alloy shows poor magnetic hardness regardless of the heat treatment conditions. The magnetic hardness of the Fe-39.5Pt alloy shows a maximum value after annealing for 10 h at 873 K, while it monotonically decreases after annealing at 1073 K. The alloy with the highest coercivity was composed of a single phase γ₁ with an average domain size of approximately 10 nm. The electron diffraction results indicate that the alloy is frustrated with accumulated stress, induced by a cubic → tetragonal transformation which occurs without twinning. On the other hand, when stress is relieved by twin formation after prolonged aging, the coercivity decreases. By annealing at 1073 K, the well known polytwin structure evolves. However, only poor hard magnetic properties are observed when this polytwin structure appears. Hence, the highest coercivity is attributed to the formation of nanoscale L1₀ ordered antiphase domains which is expected to be a highly anisotropic single domain magnetic particle.