exchangebias

Exchange bias is a phenomenon observed at the interface between a ferromagnet and an antiferromagnet. It manifests as a shift in the magnetic hysteresis loop of the ferromagnetic layer to higher or lower magnetic fields compared to the zero-field position. This shift is a result of the interfacial exchange interaction between the uncompensated magnetic moments of the antiferromagnet and the ferromagnet.

The effect was first reported by Meeks in 1956, though it was formally described and explained by Malozemoff in 1957. The key to understanding exchange bias lies in the difference between ferromagnetic and antiferromagnetic ordering. In a ferromagnet, all atomic magnetic moments align parallel, resulting in a net magnetization. In an antiferromagnet, neighboring moments align antiparallel, leading to zero net magnetization. However, at the interface, some antiferromagnetic moments may not have a perfectly compensating neighbor, leading to a net interfacial magnetization.

When the ferromagnet is cooled in the presence of a magnetic field through the Néel temperature of the antiferromagnet, the interfacial exchange coupling "freezes" the orientation of the ferromagnet's magnetization. This stable interfacial alignment then exerts a unidirectional anisotropy on the ferromagnet, causing the hysteresis loop to shift. The magnitude and direction of the shift depend on the strength of the interfacial coupling, the thickness of the ferromagnetic and antiferromagnetic layers, and the cooling field.

Exchange bias is crucial in many spintronic devices, particularly magnetic random-access memory (MRAM) and read heads for hard disk drives. It is used to create pinned layers, which provide a stable reference magnetization direction against which the free layer's magnetization can be read. The stability and tunability of exchange bias are essential for the performance and miniaturization of these technologies.