Nanostructure and Superparamagnetic Property of Overaged Fe-Al-Mn-C Alloys

Abstract

The phase transition and superparamagnetic property of nanostructures, in the Fe-9A1-30Mn- X(C, Si) alloys overaged at 823 K for 13 h to 120 d, have been studied by transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). The results reveal that the nanostructures of the alloys overaged at 823 K for 120 d, including the ferromagnetic spinel ordered (B2+D03) structures and ordered B2 structures with monoclinic a'-Mn, result from the lamellar perovskite κ-phase owing to a phase transition. The crystal orientation relationships between the ferromagnetic spinel ordered (B2+D03) structure and κ-phase can be described as (110)(subscript B2)//(220)(subscript D03)//(200)(subscript κ); (010) (subscript B2)//(020) (subscript D03)//(110) (subscript κ);and [110](subscript (B2+D03))//[10 (average)1] (subscript κ); [001](subscript (B2+D03))//[00l] (subscript κ). The crystal orientation relationships between the monoclinic a'-Mn with ordered B2 structures can be determined to be (1 (average)1 3) (subscript a'-Mn)//(1 (average)1 3) (subscript B2); and [141] (subscript a'-Mn)//[(average)152] (subscript B2). A superparamagnetic property exists in the alloys overaged at 823 K for 60 d to 120 d as confirmed by using the vibrating sample magnetometer (VSM). Through transmission electron microscopy observation, it is found that the super-soft magnetic property of the alloy overaging at 823 K for 120 d results mainly from the nanostructures, which are ferromagnetic nanostructures precipitated out of a non-magnetic matrix. However, the ferromagnetism is owing essentially to electron spins, and the electron spin imbalance in a ferromagnetic atom is just the exchange forces. Here, the ferromagnetic nanoparticles are inferred to have two mechanisms; the ferromagnetic spinel ordered (B2+D03) structures resulted from a nearest-neighbour (NN) exchange force and the ordered B2 structures with monoclinic a'-Mn ascribed to a double exchange force.