Emergent exchange bias with asymmetric reversal in plasma-treated hematite/permalloy

Exchange bias refers to the unidirectional pinning of a ferromagnetic layer along a certain direction when incorporated into an antiferromagnet/ferromagnet heterostructure. Although exchange bias is an interfacial effect, it relies on the anchoring of the antiferromagnet, often governed by its magnetocrystalline anisotropy. Here, we report the emergence of large exchange bias in permalloy coupled with an argon-plasma-treated hematite. The large exchange bias (~600 Oe at 5 K) and the unusually low onset temperature (~50 K) raise questions about its origin since no magnetic ordering is anticipated at this temperature. In addition, a strikingly asymmetric magnetic reversal is observed, where a smooth partial reversal appears only in one field direction. Remarkably, the magnetic hysteresis loop varies drastically in shape and in bias magnitude depending on the temperature at which the cooling field is applied. Also, a spontaneous bias appears even if cooled in a zero field. These observations indicate that the unusually large low-temperature bias in this system arises through an unconventional mechanism, warranting further investigation into possible origins such as spin glass formation at the plasma-treated hematite surface, or altermagnetism in hematite.