Hybrid systems formed by carbon nanotubes filled with metal represent a class of nanomaterials with manifold functionality that can find applications in different fields as, for example, magnetic storage and spintronics. Here a detailed and comprehensive study is reported on the magnetic properties of template grown cobalt cluster-filled multiwall carbon nanotube arrays. The temperature and magnetic field dependence of the samples magnetization are investigated in the range 5–760 K and from −50 kOe to +50 kOe, respectively. Hysteresis cycles are slightly open and transmission electron microscopy reveals that the cobalt clusters inside the nanotubes exhibit a crystalline nature. The zero-field cooling/field-cooling magnetization temperature dependence, analyzed in the framework of a “two-state” progressive crossover model, reveals spin disorder and suggests a spin glass-like scenario. These findings show that …