Large exchange bias after zero-field cooling from an unmagnetized state*

Exchange bias (EB) is usually observed in systems with interface between different magnetic phases after \textit{field cooling}. Here, we report an unexpected finding that a \textit{large} EB can be realized in Ni-Mn-In bulk alloys after \textit{zero-field cooling from an unmagnetized state}. We propose that the size of superparamagnetic domains in the alloys can grow up under external magnetic fields, which induces a transition from a superspin glass to a superferromagnetic (SFM) state. The SFM unidirectional anisotropy, which is the origin of EB effect, can be created at the \textit{newly} formed SFM-antiferromgantic interface during the initial magnetizing process.