Insights on the Heating Characteristics of Mn and Co-doped Fe3O4

Abstract

Fe3O4 (FeF), MnxFe3-xO4 (Mn1, Mn3, Mn5 for x = 0.1, 0.3, 0.5) and CoxFe3-xO4 (Co1, Co3, Co5 for x = 0.1, 0.3, 0.5) nanoparticles (NPs) were synthesized using co-precipitation technique to study the effect of factors that influence the effective specific absorption rate (ESAR). The samples were characterized using XRD, TEM, VSM, TMA and ESR. The heating efficiency was probed employing infrared thermography at ac field of 4 kA/m and frequency 500 kHz [1, 2]. The particle sizes are in the range of 10-13 nm, except Co5 which is 40 nm. The saturation magnetization (Ms) of the samples at 300 K is between 55-66 emu/g. The coercivity (Hc) of the Mn-doped samples is below 33 Oe where it increased up to 548 Oe with Co-doping. The low-temperature (15 K) hysteresis show a decrease in Hc with Mn-doping and enhancement with Co from 326 to 180 and 4345 Oe, respectively. Fig. 1 shows the ESR spectra from which the effective magnetic anisotropy (Keff) was computed. While the values are between 15.3-16.5 kJ/m3 for the Mn-doped samples, an increase in Keff of 20.9 (Co1) and 31.3 kJ/m3 (Co3) is observed with Co-doping reflecting the effect of particle size and doping. ESAR vs. magnetic anisotropy energy (KeffV) was theoretically and experimentally estimated (Fig. 2), where V is the volume of the particle. The highest and lowest ESAR (theoretical) of 1124 and 89 × 10-9 Wg-1Oe-2Hz-1 was obtained for Mn5 and Mn3, respectively. In contrast, the experimental ESAR (right axis) was the highest for Mn3 with a value of 21 × 10-9 Wg-1Oe-2Hz-1 and decreased with KeffV. The ESAR of FeF is 20 × 10-9 Wg-1Oe-2Hz-1. The deviation in the experimental from the theoretical values of the Mn-doped samples is mainly due to the distribution in particle sizes, whereas the decrease in ESAR with Co-doping from 20 to 18 × 10-9 Wg-1Oe-2Hz1 was as per the linear response theory (LRT) [3]. Our superparamagnetic NPs produce heat in accordance with LRT. The decreasing trend in ESAR with Co-doping in Fe3O4 is correlated with anisotropy, whereas with Mn-doping, particle size distribution plays a significant role rather than anisotropy.