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 833 K for 140 days come as a result of the lamellar perovskite kappa-phase owing to a phase transition. Two structures are contained in the ferromagnetic fine particles, one is the the ferromagnetic spinel ordered (B2 + DO3) structure, the other is the ordered B2 structure with monoclinic alpha'-Mn. The superconducting quantum interference device (SQUiD) magnetometer test shows that a superparamagnetic behavior appears in the alloy if it is overaged at 823 It for 140 days. The zero-field-cooled (ZFC) magnetization measurement reveals that a spontaneous saturation magnetization at T-S = 0 K as well as a blocking temperature T-B = 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. (C) 2000 Elsevier Science S.A. All rights reserved.