engleski

Magnetic properties of CoxMg1-xFe2O4 compounds

Magnetic hysteresis curves for CoxMg1-xFe2O4 compounds with x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1 were recorded using the Vibrating-Sample Magnetometer (VSM) model PAR/EG&G 4500. Samples in powder form were pressed in the gelatin ampoule which was tightly fastened to the sample rod, in order to avoid the rotation of the powder. Measurements were performed at room temperature, with maximum magnetic field of 10 000 Oe (1T). Constant magnetic field sweep rate of 1000 Oe/min was used and for each measured curve 1 000 data points were collected. Driving frequency of the sample and integration time were 82 Hz and 100 ms, respectively. For every sample, we also measured ZFC and FC curves in constant magnetic field of 100 Oe, in temperature range from 80 °C to 600 °C. Because of high temperatures, samples were placed in ampoule made of boron nitride, which is thermally stable and non-magnetic. Measured magnetic hysteresis show monotonous development with Mg-Co substitution. Increase of magnetization happens with substitution of diamagnetic Mg2+ ions with cobalt ions. Results show almost linear dependence of remanence magnetization, coercive field and magnetization in field of 10 kOe on concentration of Co (Mg). Slightly larger deviation for sample with x=0.9 is in agreement with other anomalies observed for this concentration, like density. Values for pure Co and Mg ferrites are in good agreement with existing data, for example, saturation magnetic moments listed in [1] are 1.1μB for MgFe2O4 and 3.7μB for CoFe2O4 . Coercive field increases with cobalt content, which can be understood as a consequence of stronger spin-orbit coupling leading to larger anisotropy and therefore to harder reversal of their magnetic moments, causing stronger pinning of the domain walls, i.e. more difficult change of magnetic domain structure. The same reasoning applies to the remanence magnetization. Recorded ZFC and FC curves allowed for accurate determination of phase transition temperatures, as well as of critical exponents which are in a good agreement with theoretical values. For some of the samples additional hysteresis curves were recorded at the temperatures slightly below and above the transition temperature, demonstrating a clear difference between ferrimagnetic and paramagnetic phase.

magnetism, ferrites

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