In a previous paper, a device for the characterization of MagnetoRheological Fluids (MRF) has been described. The MRF consist of micro-sized, magnetically active particles dispersed in a carrier medium. When exposed to a magnetic field, MRF change their own consistency turning from fluid to near-solid state responding to the applied field within milliseconds. This interesting property, suggests the possibility to use magnetorheological fluids to mimic the compliance of biological tissues in order to realize a haptic display, such as in surgical training for minimally invasive surgery and/or open surgery simulations . In this scenario the operator could interact with a virtual object which simulate several biological tissues by magnetically tuning the rheological properties of the fluid. In this paper an accurate analysis of an immersive device for the magnetic excitation of the fluid is presented. In particular such analysis is focused on a system of ferromagnetic "pistons", that, properly positioned in the device, can address the magnetic flux in the MRF.