Anisotropic Magnetoresistive (AMR) technology stands as one of the most prominent magnetic sensing methods, enabling the measurement of low-level magnetic fields with high sensitivity. Based on the direction-dependent resistance change observed in ferromagnetic materials, this technology is extensively utilized in navigation, automotive systems, industrial automation, space technologies, and non-destructive testing applications. Today, AMR-based sensors hold a critical intermediate position between Hall-effect and fluxgate sensors due to their low power consumption, high accuracy, and compact structures. The fundamental mechanism of the AMR effect relies on electron scattering processes, which vary depending on the angle between the current direction and the magnetization vector. This characteristic facilitates the measurement of weak magnetic fields, comparable to the Earth’s magnetic field, at microtesla and nanotesla levels. In this study, the KMZ51 magnetic field sensor, widely recognized as one of the most mature and commercially successful embodiments of AMR technology, is comprehensively reviewed from theory to application. The KMZ51 is a high-sensitivity sensing device comprising a thin-film permalloy-based magnetoresistive Wheatstone bridge, an internal set/reset (flipping) coil, and a compensation coil. This paper details the physical foundations of the sensor, including Stoner-Wohlfarth energy models, Barber-Pole linearization geometry, and Wheatstone bridge mathematical derivations. Furthermore, fundamental field-level error sources such as temperature drift, cross-axis sensitivity, and hysteresis are investigated through mathematical models, and corresponding closed-loop feedback and synchronous lock-in detection circuit architectures are presented. A comprehensive performance matrix is established to benchmark the KMZ51 against alternative technologies such as GMR, TMR, and fluxgate sensors. Its functional roles in small-satellite Attitude Determination and Control Systems (ADCS), non-destructive testing via Magnetic Flux Leakage (MFL), and contactless current measurement systems for smart grids are also evaluated. In conclusion, this review demonstrates that although modern TMR sensors achieve higher absolute sensitivity levels, the KMZ51 maintains its standard in both research laboratories and field applications owing to its linear output characteristics, low 1/f noise, cost-effectiveness, and industrial reliability. It is projected that through MEMS integration, IoT-based sensor networks, and artificial intelligence-driven noise
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