Ex vivo freeze-dried rat muscle tissues, collected at different times t after a single dose of subcutaneously injected iron dextran, have been magnetically characterised. The AC susceptibility of the tissues shows an overall superparamagnetic behaviour and the dependence on t of, especially, the out-of-phase component is remarkably systematic despite the fact that each tissue originates in a different rat individual. The experiments show that the akaganéite (beta-FeOOH) nanoparticles contained in the injected drug are progressively degraded in the living tissue and, at times of the order of 1 month and for all the analysed rat individuals, converge to a magnetically well-defined species with much narrower magnetic activation energy distribution than iron dextran. Thorough transmission electron microscopy experiments of the same tissues indicate the presence of oxyhydroxide particles, whose size decreases for increasing t in agreement with the interpretation of the magnetic susceptibility. The conclusions drawn from the magnetic study do well correspond to the properties of the whole tissue since no biochemical extraction work has been done. The AC susceptibility appears to be a valuable and complementary tool in pharmacological studies of iron-containing drugs.
On the R 2 Fe 14 B ͑Rϭrare earth͒ pure and hydrogenated compounds a systematic ac magnetic susceptibility study has been performed, to determine the origin of a frequently observed anomaly, nonrelated to the spin reorientation transitions. We have been able to induce the anomaly by thermal annealing the ingots, and to increase its height increasing both the annealing time and temperature. This susceptibility anomaly has been correlated with a similar one found, for the same samples, at the same temperature in magnetic aftereffect ͑MAE͒ measurements. We conclude that the anomaly is caused by magnetic disaccommodation of point defects coupled to the domain walls. Based on this mechanism a simple model is proposed, which interprets the phenomenology of this type of anomalies. For all samples measured and with activation energy and relaxation time parameters for the domain wall motion ͑coupled to defects͒ obtained from MAE experiments, the real and imaginary components of the susceptibility data have been fitted to the appropriate analytical expressions. ͓S0163-1829͑96͒09645-2͔
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