  When submitted to a magnetic field, micron size wires with superparamagnetic
properties behave as embedded rheometers and represent interesting sensors for
micro-rheology. Here we use rotational magnetic spectroscopy to measure the
shear viscosity of the cytoplasm of living cells. We address the question of
whether the cytoplasm is a viscoelastic liquid or a gel. The main result of the
study is the observation of a rotational instability between a synchronous and
an asynchronous regime of rotation, found for murine fibroblasts and human
cancer cells. For wires of susceptibility 3.6, the transition occurs in the
range 0.01...1 rad s-1. The determination of the shear viscosity (10...100 Pa
s) and elastic modulus (5...20 Pa) confirms the viscoelastic character of the
cytoplasm. In contrast to earlier studies, it is concluded that the interior of
living cells can be described as a viscoelastic liquid, and not as an elastic
gel.
