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The temperature dependence of the static magnetic susceptibility (5 to 700 K) and that of the electron paramagnetic resonance linewidth (300 to 700 K) measured at 9.25 GHz is reported for bulk polycrystalline samples of and with negative giant magnetoresistance. For both systems, and the product increase linearly with temperature above . Following the analysis of Huber and Seehra for , it is argued that this linear temperature dependence of is most probably due to contributions from spin - phonon interaction. This result supports the recent suggestions of the importance of lattice effects in the magnetism of these oxides.

The nature of magnetism and electron paramagnetic resonance (EPR) spin dynamics in 30 Å particles of FeOOH have been investigated in the range 5–350 K. X-ray diffraction shows that the particles are neither α-Fe2O3 nor α-FeOOH based and they convert to Fe3O4 (α-Fe2O3) on heating to 800 K in vacuum (air). Magnetization M vs T data show a blocking temperature TB in the range 40 K<TB<100 K so that for T≥100 K, scaling of M with H/T for H up to 50 kOe verifies superparamagnetism. In EPR, both the linewidth ΔH and the g value decreases upon increasing T so that near 300 K, g≂2.05 (characteristic of Fe3+). A model of motional narrowing is used to explain the ΔH vs T behavior. The dramatic changes in M, ΔH, g value, and the recoilless fraction of Mössbauer spectroscopy observed near 50 K are all related to the thermal motion of the superparamagnetic nanoscale particles.

In this paper, the techniques of SQUID magnetometry and line broadening in x-ray diffraction are employed for determining an important parameter for catalysts, viz. the particle size distribution. Magnetization versus temperature (5 K–400 K) and magnetization versus field (up to 55 kOe) data are reported for an α–Fe2O3 based catalyst. After determining the region of superparamagnetism, the distribution function f(r) is determined assuming a log normal distribution and Langevin paramagnetism of superparamagnetic particles. The distribution is found to be fairly symmetric with center near 65 Å and range of 35 to 115 Å. From line-broadening of Bragg peaks in x-ray diffraction, particle radii varying between 75 Å and 110 Å are obtained. These results are compared with the reported Mössbauer measurements of Huffman et al. on the same sample.

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