  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.
