Integration of optical isolators remains one the main technological issues of photonic circuits despite several decades of research. We propose a radically new concept which enables performing broad-band isolation even in the case of low-gyrotropy material, opening the road to a new class of non-reciprocal devices using easy-to-integrate composite materials. The principle explores the separation of back-and-forth light paths, induced by the coupled mode asymmetry in magnetoplasmonic slot waveguides. We show numerically that such a structure combined with suitable absorbers gives more than a 18 dB isolation ratio on several tens of nanometers bandwidth, with 2 dB insertion losses.