  The dielectric properties of MnFe$_2$O$_4$ and $\gamma$-Fe$_2$O$_3$ magnetic
nanoparticles embedded in insulating matrices were investigated. The samples
showed frequency dependent dielectric anomalies coincident with the magnetic
blocking temperature and significant magnetocapacitance above this blocking
temperature, as large as 0.4% at H = 10kOe. For both samples the magnetic field
induced change in dielectric constant was proportional to the square of the
sample magnetization. These measurements suggest that the dielectric properties
of magnetic nanoparticles are closely related to the disposition of magnetic
moments in the system. As neither bulk gamma-Fe2O3 nor MnFe2O3 are
magnetoelectric materials, this magnetodielectric coupling is believed to arise
from extrinsic effects which are discussed in light of recent work relating
magnetoresistive and magnetocapacitive behavior.
