Abstract
The epoxy/magnetite nanocomposites express superparamagnetism under a static or low-frequency electromagnetic field. At the microwave frequency, said the X-band, the nanocomposites reveal an unexpected diamagnetism. To explain the intriguing phenomenon, we revisit the Debye relaxation law with the memory effect. The magnetization vector of the magnetite is unable to synchronize with the rapidly changing magnetic field, and it contributes to diamagnetism, a negative magnetic susceptibility for nanoparticles. The model just developed and the fitting result can not only be used to explain the experimental data in the X-band but also can be used to estimate the transition frequency between paramagnetism and diamagnetism.
Highlights
Nanocomposites have variable applications in medicine and engineering, as both nanoparticles and host media are changeable to meet current needs
It is of interest to analyze the volume fraction and frequency dependence because it can help us understand physics inside the materials and can predict electromagnetic characteristics outside the measured ranges, which may reduce the cost of measurements and help us approximate the needed composite composition
The superparamagnetism is from the thermal fluctuations between stable states, while the diamagnetism originates from the out of phase response with the external driven field (Supplement Part III)
Summary
Nanocomposites have variable applications in medicine and engineering, as both nanoparticles and host media are changeable to meet current needs. The combination of magnetite and liquid media, called ferrofluid, has medical applications in MRI images and hyperthermia therapy[1] Another example is the mixing of nanoparticles and polymer matrix, which can be microwave absorbers used in military and communication engineering[2]. We use the transmission/reflection method to extract the permittivity and permeability of epoxy/magnetite nanocomposites in the X-band and analyze their dependence on frequencies and volume fractions. The work of Klik et al.[20] shows that the transition of states may contribute to the negative magnetic susceptibility by considering the memory effect of nanoparticles They considered the correction of the master equation of the spins by an exponential memory kernel. This theory is not the only possible explanation and needs more experimental e xploration[21,22], it can explain the coexistence of superparamagnetism at low frequencies and diamagnetism at high frequencies as to why we adopt it to explain our results
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
