Sign reversal of spontaneous exchange bias in Mn0.7Fe0.3NiGe alloy

Spontaneous exchange bias can be observed at 5 K after zero-field cooling in Mn0.7Fe0.3NiGe alloy. More interestingly, its sign can reverse from positive to negative with the increase of maximum measurement field. Such results indicate a coexistence of ferromagnetic and antiferromagnetic coupling formed at the ferromagnetism/spin glass interface during the initial magnetization process. Hence, the sign reversal would be resulted from the relative strength change of these two types of exchange coupling, which can be tuned by measurement field.