Observation of Phase Separation in Fe3Al Alloys

Impression of magnetic clusters, critical behavior and magnetocaloric effect in Fe3Al alloys

Herein, we report unusual magnetic behavior in arc melted bulk stoichiometric Fe₃Al alloy with a D0₃ structure. The temperature variation in the magnetization measurements revealed two transitions, i.e. one at ∼763 K and another at ∼830 K. Below 763 K, it exhibits ferromagnetic ordering and the nature of the transition is second order. However, the second transition is more complex and detailed analysis of the magnetic data suggested the coexistence of ferromagnetic and paramagnetic phases (two-phase: α + D0₃/α + B2) and structural transitions triggered by temperature. We observed dual peaks in the magnetic entropy change curve, in accordance with the magnetization results, which corroborate the occurrence of a two-phase system. We believe that the concurrent magnetic ordering and the complex two-phase are associated with the evolution of short-range ordered magnetic clusters having a magnetic moment of ∼10³μ_B in the host matrix. A cluster hypothesis is proposed to explain the observed intricate magnetic behavior of this alloy at high temperature. The estimated critical exponents using a modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis lie in between the 3D-Heisenberg and 3D-Ising model, indicating a short-range interaction and magnetic inhomogeneity, which again are consistent with the magnetization results. The obtained critical exponents follow the universal scaling behavior, which indicates the renormalization of interactions around the magnetic ordering transition (T_C). Despite the obvious larger thermal entropy at very high temperature, our synthesized Fe₃Al alloy showed enhanced magnetic entropy changes. The obtained magnetic entropy change for binary Fe₃Al alloy shows twice the value of that of other binary/ternary Fe-based alloys.

The Mutual Influence of Magnetic and Chemical Ordering

It can be concluded from experiments that atomic (also called “chemical”) and magnetic ordering reactions exert a mutual influence on each other. In bcc Fe-Al alloys a strong depression of the Curie temperature is observed with the onset of atomic long range order (lro) as compared to the extrapolation of Tc from the random alloys, see Fig. 1. In fcc Co-Pt alloys a depression of the critical temperature of atomic lro L12 →A1 is observed in the Co-rich alloys which are ferromagnetic, as compared to the Pt-rich counterparts which are paramagnetic, see Fig. 2. In this instance it should be noticed that this assymetry is not observed in Ni-Pt allovs where the Curie temperature is inferior to the critical temperatures of atomic lro [1].