We report on the magnetic actuation of superparamagnetic particles in a microfluidic channel. The formation of a highly confined and dynamic plug of particles extending over the cross section of the microchannel is enabled by superposing a static and a time-varying magnetic field. The latter is generated by an electromagnet and focused across the microchannel using microstructured soft magnetic tips. Finite element calculations of the magnetic force distribution do compare well with experiments. Our system allows maximizing the exposure of superparamagnetic particles to a microfluidic flow of interest for integrated lab-on-a-chip applications.