Exchange bias in semiconductor-ferromagnet bilayer systems is a critical phenomenon enabling advanced spintronic applications, particularly in pinned spin valves and magnetic sensor devices. The effect arises from interfacial spin coupling between a ferromagnetic layer and an adjacent antiferromagnetic or ferromagnetic layer grown on a semiconductor substrate. This coupling induces a unidirectional magnetic anisotropy, shifting the hysteresis loop of the ferromagnet along the field axis. The underlying mechanisms involve complex interfacial spin interactions, including direct exchange, spin-orbit coupling, and magnetic proximity effects.

The interfacial spin coupling in semiconductor-ferromagnet bilayers is highly sensitive to the atomic-scale structure and chemical composition of the interface. For example, in Co/FeMn bilayers grown on silicon, the exchange bias field can range from 50 to 300 Oe depending on the deposition conditions, layer thickness, and post-growth annealing. The Co layer typically exhibits a face-centered cubic (FCC) structure, while the FeMn layer forms a chemically disordered antiferromagnetic phase. The exchange coupling at the interface is mediated by uncompensated spins in the FeMn layer, which pin the magnetization of the Co layer. The magnitude of the exchange bias is influenced by the interfacial spin density, which can be modulated by introducing ultrathin oxide layers or by varying the growth temperature.

Thermal stability is a major challenge in these systems. The exchange bias effect is highly temperature-dependent, with the blocking temperature—the temperature above which the effect vanishes—typically lying between 150°C and 250°C for Co/FeMn on Si. This limitation arises from the thermal energy overcoming the interfacial spin coupling energy. To enhance thermal stability, researchers have explored alternative material combinations, such as replacing FeMn with IrMn or PtMn, which exhibit higher Néel temperatures and stronger interfacial exchange coupling. Additionally, inserting a thin NiFe (permalloy) layer between the Co and FeMn has been shown to improve thermal endurance by reducing interfacial diffusion.

Pinned spin valves leverage exchange bias to fix the magnetization direction of a reference layer, enabling highly sensitive magnetic field detection. A typical spin valve structure consists of a synthetic antiferromagnet (SAF) pinned by an exchange-biased layer, a non-magnetic spacer, and a free ferromagnetic layer. The exchange bias ensures that the reference layer remains stable under external magnetic fields, while the free layer rotates in response to the applied field, producing a measurable resistance change due to the giant magnetoresistance (GMR) effect. Such devices are widely used in hard disk drive read heads and automotive sensors.

Magnetic sensor applications benefit from the tunability of exchange bias in semiconductor-integrated systems. For instance, anisotropic magnetoresistance (AMR) sensors based on exchange-biased NiFe/FeMn bilayers on GaAs substrates exhibit linear response characteristics with sensitivities exceeding 1 mV/V/Oe. The semiconductor substrate allows for monolithic integration with electronic circuits, enabling compact and low-power sensor systems. The interfacial spin coupling also plays a role in reducing magnetic noise, a critical factor for high-resolution sensing in biomedical and industrial applications.

Material combinations beyond Co/FeMn have been investigated to optimize performance. For example, Fe3O4/CoFe2O4 bilayers on Si exhibit exchange bias due to ferrimagnetic coupling, with potential applications in high-frequency devices. Similarly, Mn-doped ZnO interfaced with Co has shown room-temperature exchange bias, attributed to defect-mediated magnetic interactions. These systems highlight the role of semiconductor substrates in tailoring interfacial properties for specific device requirements.

A key challenge in these systems is minimizing interfacial intermixing, which can degrade the exchange bias effect. Techniques such as low-temperature deposition and in-situ capping layers have been employed to preserve sharp interfaces. Additionally, post-growth annealing must be carefully controlled to optimize the spin coupling without inducing excessive diffusion. Advanced characterization methods, including polarized neutron reflectometry and X-ray magnetic circular dichroism (XMCD), are essential for probing the interfacial magnetic structure and guiding material optimization.

Future developments in exchange-biased semiconductor-ferromagnet bilayers will likely focus on enhancing interfacial spin coupling through atomic layer engineering and exploring new material systems with higher blocking temperatures. The integration of these structures with semiconductor manufacturing processes will further expand their applications in next-generation spintronic and sensor technologies. The continued refinement of interfacial control and thermal stability will be critical for meeting the demands of emerging high-performance and energy-efficient devices.