Ferromagnetic Relaxation in Manganese—Iron Spinels

Abstract

We have extended our previous measurements on the ferrimagnetic manganese—iron spinel system MnxFe3−xO4(x=1) to include the concentrations x=1.25, 0.80, and 0.60. The FMR linewidth and field for resonance were measured over the temperature range 4°–300°K at various X-band frequencies. For each concentration, relaxation mechanisms involving ``lossy ions'' (Mn3+ or Mn4+ and Fe2+) are found to be the dominant source of linewidth. A ``slow relaxation'' mechanism resulting in a linewidth maximum (or shoulder in the x=0.85 and 0.60 cases) near 20°K and an effective field for resonance minimum near 10°K is operable in each concentration studied. The responsible ``lossy ions'' are most likely manganese. For the x<1 concentrations, a ``slow relaxation'' mechanism due to itinerant electrons associated with the Fe2+–Fe3+ conduction yields a linewidth maximum near 70°K. An ``eddy current'' contribution to the linewidth due to the presence of Fe2+ ions is also observed in the x<1 samples. These same phenomena have been observed previously in excess-iron nickel ferrites. Finally in manganese ferrite (x=1), a linewidth peak at higher temperatures (100–300°K) is observed whose position and magnitude is very sensitive to the microwave frequency used. As the driving frequency is swept from 8.7 to 12 GHz, the linewidth maximum shifts from 200 Oe at 250°K to 40 Oe at 160°K. This sensitive frequency (or field) dependence rules out the ``fast relaxation'' mechanism previously proposed to account for this phenomenon and a ``slow relaxation'' mechanism which predicts an increase in the temperature of the linewidth maximum with increasing frequency. Mechanisms involving thermally produced Fe2+ ions are proposed to account for this behavior. A complete account of this work will be submitted as a regular article to this Journal.