We present a novel microfluidic mixer that is easy to fabricate and simple to use. The mixer features a single mixing chamber in which particles are driven by a moving magnet. We show that the mixing efficiency is markedly increased by addition of particles to the chip, as a result of the diffusive motion of the sheared particles. The mixer is efficient for a range of driving rates and features a low pressure drop due to its simple design, making the mixer design compatible with soft chip material. It is therefore ideal to incorporate in lab-on-a-chip devices.