Magnetic hardening and coercivity mechanisms in L10 ordered FePd ferromagnets

The tetragonal L1₀ family of ferromagnets including the Co-Pt, Fe-Pt, Fe-Pd and Mn-Al alloy systems exhibits high, uniaxial magnetocrystalline anisotropies with first anisotropy constants K₁ ~ 10⁷–10⁸ ergs/cm³. The domain parameters of these materials are similar to the rare earth permanent magnets and the defect structure is dominated by planar faults such as APB's, stacking faults, and twins. In this study the micromagnetic analysis of Kronmüller has been applied to elucidate the mechanism of coercivity controlling the hysteresis of the polytwinned L1₀ FePd equiatomic alloy and the results are consistent with pinning control involving atomically thick, planar defects. Also, Lorentz microscopy has been used to observe the interaction of magnetic domain walls with the planar faults characteristic of these materials. Finally, a thermomechanical treatment involving concomitant ordering and recrystallization is shown to eliminate the well-known polytwinned structures which generally evolve during the formation of the L1₀ phase leading to enhanced coercivities. The fundamental structure-property relationships involved are discussed.