| A sample containing an extended free radical network derived from the condensation of cyanuric chloride with p-phenylenediamine was used for obtaining different concentrations of magnetic centers (free radicals and magnetic agglomerates). The temperature dependent magnetic resonance spectra were measured in the 4€290K temperature range. At room temperature the magnetic resonance measurements showed that the EPR spectrum was the sum of two lines attributed to two different magnetic centers: a narrow line at Hr=3373.57(3) Gs (geff=2.0032(1)) with linewidth ΔHpp=8.31(2) Gs (due to free radicals) and a broad line centered at Hr=3198(5) Gs (geff=2.234(1)) with linewidth ΔHpp=1000(5) Gs (arising from magnetic iron oxide agglomerates). The sample was prepared in such a way that the narrow line was more intense. The integrated intensities decreased with decreasing temperatures in both spectra in the high temperature range. This type of behavior was similar to that of magnetic nanoparticles in nonmagnetic matrixes. The resonance field of the broad line shifted to smaller magnetic fields upon lowering the temperature with gradient δHr/ΔT=1.5(1) Gs/K, while the narrow line shifted towards higher magnetic fields with δHr/ΔT =0.020(1) Gs/K. The broader line linewidth increased with the decreasing temperature while this change was only minor for the narrow line (especially at higher temperatures). The magnetic iron oxide clusters produced an internal magnetic field which acted on free radicals and its strength depended essentially on the relative concentration of clusters. This field could force free radicals to form a magnetic ordered state. |
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