Magnetorheological fluids (MRFs) include suspensions of ferromagnetic particles, medium oils, and several types of additives. Their rheological properties change rapidly, stably, and repeatedly when magnetic fields are applied. In this present study, we focus on a haptic device using MRFs. Though particle sedimentation is one of the drawbacks of MRFs, we developed an MRF using iron particles of size 10-100 μm and silicon oil (Nano-MRF); more specifically, we use this material as a working material for the haptic device. We developed a prototype of an MRF brake as a core device for haptic control. In this paper, we describe our design method for the corresponding magnetic circuit. In addition, we designed a detailed structure for the unit. We investigated the basic properties of this unit by using a test machine with two types of MRFs, i.e., our Nano-MRF and a commercial MRF. From our results, the Nano-MRF showed better stability and response times versus that of conventional materials.