Investigation of superparamagnetic fine particles of ordered (B2+D03) structure and ordered B2 structure with monoclinic α′-Mn

This investigation reports on optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the superconducting quantum interference device (SQUID) magnetometer study of ferromagnetic fine particles formed in Fe–9Al–30Mn–x(C,Si) alloys overaged at 823 K from 2 days to 140 days. The results reveal that the ferromagnetic fine particles of the alloy overaged at 823 K for 140 days come as a result of the lamellar perovskite κ-phase owing to a phase transition. Two structures are contained in the ferromagnetic fine particles, one is the the ferromagnetic spinel ordered (B2+D03) structure, the other is the ordered B2 structure with monoclinic α′-Mn. The superconducting quantum interference device (SQUID) magnetometer test shows that a superparamagnetic behavior appears in the alloy if it is overaged at 823 K for 140 days. The zero-field-cooled (ZFC) magnetization measurement reveals that a spontaneous saturation magnetization at TS=0 K as well as a blocking temperature TB=30 K exists in the same sample. The results of the temperature dependence of susceptibility for the ZFC and FC (field-cooled under 100 Oe) data, with hysteresis saturation magnetization behavior, follow a superparamagnetic property.