  We have determined the physical mechanism responsible for the plasma
instabilities, which was first found by Ramani and Laval (1978), associated
with anisotropic velocity distributions induced by the temperature gradient in
which there are growing low frequency transverse magnetic waves, even in the
absence of background magnetic fields. We have shown that the physical
mechanism responsible for the growth of one of the modes is identical to the
Weibel instability. The nonlinear saturation level of the instability is also
provided by considering the wave-particle interactions. The non-linear
evolutions of the magnetic fields after the saturation are speculated. The
results are applied to the cold fronts which is one of the newly discovered
structures in clusters of galaxies by the Chandra X-ray observatory. We predict
the existence of the magnetic field of $\sim 10\mu$G tangential to the surface
over the entire region of the cold front surface and that the heat conduction
is significantly suppressed by the trapping of the electrons by the generated
magnetic fields. The instability may provide a new possibility on the origin of
cosmic magnetic field.
