Temperature dependence of magnetic anisotropy for single domain L10 FePd crystal and role of the ordering defects

The ordered alloys of Fe with nominally non-magnetic 4d/5d (Rh, Pd, Pt) elements demonstrate an array of an attractive magnetic properties. The high magnetic anisotropy energy (MAE) of the ordered L1₀ FePt is considered as particularly important property for achieving small thermally stable magnetic grains. Recent progress in fabrication and characterization of nano-granular and nano-particulate FePt films puts emphasize on understanding of the temperature dependence of MAE. Theoretical framework for the temperature dependence of the MAE was summarized by Callen and Callen. This theory predicts universal parametric dependence between K₁(T) and M_S(T) which for the uniaxial magnet simply given by the K₁(M_S(T)) ~ M_S³ in the low temperature region. Measurements on epitaxial FePt films, however, demonstrate K(T) ~ M_S² scaling in the wide temperature range. The origin of this behavior was found to be in the magnetic interactions mediated by the Pt induced spin moment. In FePt anisotropic exchange mediated by the induced Pt spin moment appears to be dominating over the Fe contribution and thus leading to the K₁ ~ M_s^2.1 low temperature scaling behavior. In this work we present results of the magnetic measurements for a single variant highly ordered L1₀ FePd crystals. This chemical analog of the FePt is known for its significantly lower ordering temperature (950 vs. 1910 K) and somewhat smaller MAE K₁ of about 2´10⁷ erg/cc. We present results of K₁(T) measurements on single ordering domain FePd samples which clearly demonstrate different than reported for the FePt thin films scaling behavior K(T) ~ M^3.8. We discuss initial theoretical interpretation provided on the basis of recently measured ordering defects in the form of intermediate disordered phase.