Here, we study the effect of magnetic field on thermal instability of a rotated couple stress Rivlin-Ericksen ferromagnetic fluid layer through a porous medium in the presence of a variable gravity field. Using normal mode analysis technique, the dispersion relation has been obtained and solved analytically. For stationary conventions, it is found that couple stress, magnetic field, and permeability of the medium have a stabilizing effect on thermal instability under certain conditions. In the absence of rotation, couple stress has a stabilizing effect if λ > 0 and a destabilizing effect if λ < 0. Rotation has a stabilizing effect if λ > 0 and a destabilizing effect if λ < 0.  In the absence of rotation, permeability has a stabilizing effect if λ < 0 and a destabilizing effect if λ > 0. Magnetisation always has a stabilizing effect either λ > 0 or λ < 0. It is also evident that the existence of rotation and magnetic field creates oscillatory modes and stability in the system for λ < 0. Oscillatory modes are not allowed in the absence of rotation and magnetic field, and the principle of exchange of stability is satisfied under certain conditions.