exchangebiased

Exchange bias, sometimes described as exchange biasing, is a magnetic phenomenon that occurs at the interface between a ferromagnet and an antiferromagnet. After cooling the bilayer in an external magnetic field through the antiferromagnet’s Néel temperature, the ferromagnet’s hysteresis loop is shifted horizontally along the field axis. The horizontal shift is characterized by the exchange bias field H_E, and the loop may also become more or less square with a larger coercivity H_C.

The effect arises from interfacial exchange coupling between the ferromagnetic and antiferromagnetic spins. Interfacial AFM spins pinned by anisotropy favor a particular orientation of the ferromagnetic spins; when the external field is reversed, it requires a larger opposing field to reverse the magnetization in one direction, producing a unidirectional anisotropy. The sign and magnitude of H_E depend on the materials, interface quality, and cooling conditions. Training effects may reduce the bias with successive magnetization reversals.

Temperature plays a crucial role: the bias disappears above the blocking temperature and below the Néel temperature of the antiferromagnet. Measurements are commonly performed on magnetization versus field (M–H) loops at low temperatures after field cooling. The bias direction can be positive or negative depending on the system.

Exchange bias is widely used in spintronic devices such as spin valves and magnetic tunnel junctions to pin a reference layer, as well as in magnetic sensors and read heads. Common FM/AFM combinations include CoFe/IrMn, NiFe/FeMn, and NiMn-based systems.