  A theory is developed of the time-dependent magnetization of the metacrystal
composed of magnetoferritin macromolecules. Such superstructures, comprising up
to several millions of superparamagnetic nanoparticles encapsulated in protein
shells, can be created artificially using biochemical assembling technologies.
They have been also shown to occur naturally in sensitive cells of the inner
ear of birds, which suggests their possible involvement in the detection of the
geomagnetic field for orientation and navigation of migratory animals. The
dynamics of the magnetic system of the magnetoferritin metacrystal, comprising
a very large number of magnetic moments coupled by long-range dipole forces, is
exceedingly complex. In order to find the response of the metacrystal to
high-frequency magnetic fields, we used a thermodynamic approach borrowed from
the theory of nuclear spin systems of solids. The resulted theory yields the
time-dependent superspin temperature and magnetization induced by oscillating
magnetic fields of arbitrary strength. The predicted dependence of the
high-frequency response on the static magnetic field can be used for
experimental detection and characterization of magnetoferritin metacrystals in
biological tissues.

Mesoscale and Nanoscale Physics;Biological Physics