Electrodeposited multilayer nanowires exhibiting giant magnetoresistance (GMR) represent a significant advancement in nanomagnetic materials, particularly for high-sensitivity sensor applications. These structures, typically composed of alternating ferromagnetic and non-magnetic layers (e.g., Co/Cu or NiFe/Cu), rely on spin-dependent electron scattering and interfacial effects to produce large resistance changes under applied magnetic fields. Unlike thin-film GMR systems, nanowires offer unique advantages due to their one-dimensional geometry, high surface-to-volume ratio, and the ability to tailor their properties through precise control of layer thickness and composition during electrodeposition.

The fundamental mechanism behind GMR in multilayer nanowires is spin-dependent scattering, where electrons with spins aligned parallel to the magnetization of a ferromagnetic layer experience lower resistance than those with antiparallel spins. In a multilayer stack, the relative orientation of magnetization between adjacent ferromagnetic layers determines the overall resistance. When the layers are magnetized antiparallel (antiferromagnetic coupling), spin-dependent scattering is maximized, resulting in high resistance. Conversely, parallel alignment (ferromagnetic coupling) minimizes scattering, reducing resistance. The non-magnetic spacer layer (e.g., Cu) thickness is critical, as it mediates the interlayer coupling via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. For Co/Cu systems, the coupling oscillates between ferromagnetic and antiferromagnetic as the Cu thickness varies, with the first antiferromagnetic peak typically occurring around 0.9–1.0 nm.

Current-perpendicular-to-plane (CPP) geometry is the dominant configuration for nanowire GMR measurements, where the current flows along the wire's longitudinal axis, perpendicular to the layer interfaces. This geometry enhances the GMR ratio compared to current-in-plane (CIP) configurations because the electron mean free path becomes comparable to the layer thicknesses, increasing the contribution of interfacial scattering. CPP-GMR ratios in electrodeposited Co/Cu nanowires have been reported in the range of 15–30% at room temperature, with higher values achievable at cryogenic temperatures due to reduced thermal fluctuations and enhanced spin coherence.

Layer thickness optimization is crucial for maximizing GMR performance. The ferromagnetic layer thickness must balance two competing effects: thicker layers increase the magnetic moment but may reduce spin-polarized electron transport due to bulk scattering, while thinner layers enhance interfacial effects but may suffer from incomplete magnetization alignment. For Co/Cu nanowires, optimal Co layer thicknesses typically range between 2–5 nm. The non-magnetic spacer thickness must be tuned to maintain antiferromagnetic coupling, with Cu layers in Co/Cu systems often kept near 1 nm for the first antiferromagnetic peak. Deviations from these values can lead to reduced GMR ratios due to weakened coupling or shunting effects.

Characterization of GMR in nanowires relies heavily on magnetotransport measurements, often conducted at cryogenic temperatures to suppress thermal noise and isolate intrinsic magnetic effects. Four-probe resistivity measurements are performed while sweeping an external magnetic field applied parallel or perpendicular to the wire axis. The resulting magnetoresistance curves reveal hysteresis loops with distinct features corresponding to magnetization reversal processes. At low temperatures (e.g., 4.2 K), sharp transitions between high- and low-resistance states are observed, reflecting coherent rotation or domain wall propagation in the ferromagnetic layers. Temperature-dependent studies also provide insights into spin-flip scattering mechanisms, with resistivity typically following a logarithmic increase at low temperatures due to Kondo-like interactions.

Sensor applications of GMR nanowires leverage their high sensitivity to magnetic fields, fast response times, and miniaturization potential. In magnetic field sensors, nanowire arrays can detect sub-Oersted field changes, making them suitable for read heads, current sensors, and biomedical detection systems. The linear response region of the GMR curve is particularly important for sensor design, achieved by tuning the interlayer coupling or applying bias fields. For example, NiFe/Cu multilayers often exhibit softer magnetic switching than Co/Cu, providing broader linear ranges suitable for low-field sensing. Additionally, the nanowire geometry enables directional sensitivity, as the shape anisotropy confines magnetization along the wire axis, reducing crosstalk in dense arrays.

Fabrication of GMR nanowires typically involves template-assisted electrodeposition, where porous alumina or polycarbonate membranes serve as scaffolds. The layer sequence is controlled by modulating the deposition potential or electrolyte composition during plating. This method allows for high aspect ratios (length/diameter > 100) and uniform layer thicknesses down to atomic scales. Post-growth annealing can further enhance GMR performance by improving crystallinity and interfacial sharpness, though excessive heating may lead to interdiffusion and degraded coupling.

Challenges in nanowire GMR systems include minimizing interfacial roughness, which can disrupt spin-polarized transport, and controlling defects that act as scattering centers. Advances in pulsed electrodeposition techniques have addressed some of these issues by enabling smoother layer growth and sharper interfaces. Another consideration is the influence of wire diameter on magnetic properties, with smaller diameters (< 50 nm) exhibiting stronger shape anisotropy but potentially increased surface scattering.

Future developments may explore alternative material combinations, such as Heusler alloy-based nanowires, which promise higher spin polarization and thus larger GMR effects. Integration with semiconductor platforms for on-chip sensing and the exploration of spin-orbit coupling effects in heavy-metal-containing multilayers are also active research directions. The combination of scalable fabrication, tunable properties, and robust performance ensures that electrodeposited GMR nanowires remain a vital component in the advancement of nanomagnetic sensors.