The effect of magnetic field dependent (MFD) viscosity on the onset of convection in a ferromagnetic fluid layer heated from below and cooled from above in the presence of vertical magnetic field is investigated theoretically. The bounding surfaces are considered to be either rigid-ferromagnetic or stress free with constant heat flux conditions. The resulting eigenvalue problem is solved using the Galerkin technique and also by regular perturbation technique. It is found that increase in MFD viscosity and decrease in magnetic number is to delay the onset of ferroconvection, while the nonlinearity of fluid magnetization has no influence on the stability of the system.

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The onset of ferromagnetic convection in a micropolar ferromagnetic fluid layer heated from below in the presence of a uniform applied vertical magnetic field has been investigated. The rigid-isothermal boundaries of the fluid layer are considered to be either paramagnetic or ferromagnetic and the eigenvalue problem is solved numerically using the Galerkin method. It is noted that the paramagnetic boundaries with large magnetic susceptibility χ delays the onset of ferromagnetic convection the most when compared to very low magnetic susceptibility as well as ferromagnetic boundaries. Increase in the value of magnetic parameter M₁ and spin diffusion (couple stress) parameter N₃ is to hasten, while increase in the value of coupling parameter N₁ and micropolar heat conduction parameter N₅ is to delay the onset of ferromagnetic convection. Further, increase in the value of M₁, N₁, N₅ and χ as well as decrease in N₃ is to diminish the size of convection cells.