  Iron manganese trioxide (Fe0.25Mn0.75)2O3 nanocrystals were synthesized by
the sol-gel method. The 80 K Mossbauer spectrum was well-fitted using two
doublets representing the 8b and 24d crystallographic sites of the
(FexMn1-x)2O3 phase and two weak extra sextets which were attributed to
crystalline and amorphous hematite. Our findings showed formation of a bixbyite
primary phase. The Raman spectrum exhibits six Raman active modes, typical of
(Fe,Mn)2O3, and two extra Raman modes associated with the secondary hematite
phase. X-ray photoelectron spectroscopy analysis confirmed the presence of
oxygen vacancy onto the (FexMn1-x)2O3 particle surface, with varying oxidation
states. X-band magnetic resonance data revealed a single broad resonance line
in the whole temperature range (3.8 K - 300 K). The temperature dependence of
both resonance field and resonance linewidth shows a remarkable change in the
range of 40 - 50 K, herein credited to surface spin glass behavior. The model
picture used assumes (FexMn1-x)2O3 nanoparticles with a core-shell structure.
Results indicate that below about 50 K the spin system of shell reveals a
paramagnetic to spin glass-like transition upon cooling, with a critical
temperature estimated at 43 K. In the higher temperature range, the
superparamagnetic hematite (secondary) phase contributes remarkably to the
temperature dependence of the resonance linewidth. Zero-field-cooled (ZFC) and
fieldcooled (FC) data show strong irreversibility and a peak in the ZFC curve
at 33 K, attributed to a paramagnetic-ferrimagnetic transition of the main
phase. Hysteresis curve at 5 K shows a low coercive field of 4 kOe, with the
magnetization not reaching saturation at 70 kOe, suggesting the occurrence of a
ferrimagnetic core with a magnetic disorder at surface, characteristic of
core-shell spin-glass-like behavior.
