Thermal convection in magnetic fluids can be driven by buoyancy or by magne
tic forces (due to the thermomagnetic effect). Depending on the direction o
f the applied temperature gradient, buoyancy effects: can be stabilizing (h
eating from above) or destabilzing (heating from below), whereas the magnet
ic forces always play a destabilizing role for magnetic fields perpendicula
r to the interface. We investigate the influence of rotations using both li
near and weakly non linear analyses of the governing hydrodynamic equations
in the Boussinesq approximation. With a linear stability analysis we deter
mine the values of the wavelength and the temperature gradient at the onset
of convection (critical values). These are calculated analytically in the
case of stress free boundaries and numerically for rigid boundaries. We dis
cuss the validity of the assumptions entering the calculations for stress f
ree boundaries. In the case of free boundary conditions, asymptotic express
ions of the critical values for high rotation rates are derived. When the s
ystem is heated from above and the magnetic forces only slightly exceed the
buoyancy forces, linear results show that both the critical wavelength and
the critical temperature gradient diverge. Again, this behavior is describ
ed by asymptotic expressions. We derive envelope equations for convection p
atterns characterized by both: one wave vector and two competing wave vecto
rs of equal length but different directions. These equations show that the
system always exhibits a forward bifurcation. The well-known Kuppers-Lortz
instability is also present in magnetic fluids. This instability sets in at
critical values for a sufficiently high rotation rate. In simple fluids th
e angle alpha depends only on the Prandtl number of the fluid. We show that
for magnetic fluids this angle can be changed by changing the ratio of the
buoyancy forces to the magnetic forces (i.e. by changing the magnetic fiel
d). There is also a weak dependence on the other magnetic parameters of the
system. For a commercially available magnetic fluid this angle can be incr
eased by approximately 10 degrees - 15 degrees compared to the simple fluid
case.