Significant exchange bias (EB) is induced across the spin reorientation (Morin) transition (${\mathrm{T}}_{\mathrm{M}}$) in epitaxial $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3\left(001\right)/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (hematite/permalloy) heterostructures, far below the antiferromagnetic transition temperature. In contrast to typical exchange coupled systems, these heterostructures exhibit two distinct features: (i) exchange bias is established at the Morin transition, rather than the Néel ordering, and (ii) a qualitatively different temperature dependence where EB is confined to a narrow temperature window near ${\mathrm{T}}_{\mathrm{M}}$ instead of increasing monotonically upon cooling as in conventional systems. The temperature- and angular-dependent measurements imply that this thermal behavior is caused by separate transitions of the interfacial and bulk hematite spins. As a consequence, interdiffusion drastically alters the thermal dependence of EB, which highlights the role of the interfacial spins. Our study reveals that the spin reorientation transition breaks the magnetic isotropy at the antiferromagnetic hematite (001) surface and controls the interfacial exchange interactions. These results demonstrate that the spin reorientation transition in hematite, including its widely available doped variants, can be exploited to realize thermally programmable and temperature-selective exchange bias. With the growing interest in antiferromagnetic spintronics, they further show that spin reorientation transitions in antiferromagnets provide a tunable window independent of the Néel temperature for manipulating AFM spins and may assist Néel vector switching in spin torque devices.

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• Antiferromagnetism
• Exchange bias
• Magnetic phase transitions
• Spintronics
• Heterostructures
• Thin films
• Transition metal oxides
• Magnetization measurements
• X-ray diffraction