Radio-frequency Heating of Magnetic Nanoparticles

In the present study, a power supply capable of converting a direct current into an alternating current was built. The frequency of oscillation of the output current could be varied from 174.8 kHz to 726.0 kHz by setting a set of capacitors in resonance. To this power supply is attached a 20-turns copper coil in the shape of a spiral. Because of the high heat generated in the coil, the latter has to be permanently water-cooled. A vacuum pump removes the air between the sample holder and the coil. A fiber optic temperature sensor with an accuracy of 0.001 K was used to measure the temperature of the nanoparticles.Four ferromagnetic nanoparticles (CoFe_2O_4, NiFe_2O_4, Ni_0.5Zn_0.5Fe_2O_4, Co_0.4Ni_0.4Zn_0.2Fe_2O_4) with different magnetic properties were subjected to heating. The heating performance is given by the specific power loss (SPL) which was calculated from the initial slope of the heating curve whereby there is minimum heat loss to the surroundings and the highest SPL was obtained when magnetic field and frequency were highest at 20.6 µT and 348.0 kHz, respectively. When the frequency was changed from 174.8 to 348.0 kHz, the SPL value was double indicating a one-to-one relationship between the frequency and the SPL. No such relationships were found when the magnetic field was increased from 13.8 µT to 20.6 µT. There is also a coercivity and remanent magnetization dependence on the SPL and both of them obey an inverse law. We found that the SPL for Ni_0.5Zn_0.5Fe_2O_4 (GPZ4) was 1.09 ± 0.02 W/g2. Moreover, the sample CoFe_2O_4 (GPZ11) with the highest power loss density (12250 J/m3) measured by the variable sample magnetometer was found to have the lowest SPL (0.135 ± 0.002 W/g2) in the lot. The magnetic field inside the coil is only a fraction the VSM, so when the low magnetic field was turned on, the hysteresis loop opened only to the extent of the minor loop.