Development and Application of Alternating Magnetic Force Microscopy (A-MFM)

Alternating magnetic force microscopy (A-MFM) is a modulation-based magnetic imaging technique derived from frequency-modulation magnetic force microscopy (FM-MFM). In A-MFM, an alternating magnetic interaction generated by external magnetic field modulation induces frequency modulation of the cantilever oscillation, and the modulated frequency component is selectively detected using lock-in techniques. This approach enables highly sensitive magnetic imaging near the sample surface while suppressing non-magnetic interactions such as electrostatic and van der Waals forces. In this review, the fundamental principles of A-MFM are systematically summarized, including the frequency-modulation detection mechanism, sideband generation, and magnetic force gradient detection. The magnetic response characteristics of hard magnetic, soft magnetic, and superparamagnetic tips are compared in relation to signal formation and measurement capability. Representative applications are also reviewed, including low-frequency and high-frequency magnetic field imaging, vector-resolved magnetic field detection, magnetic energy imaging, and quantitative magnetic field reconstruction based on transfer-function approaches. These developments demonstrate that A-MFM provides a versatile platform for nanoscale magnetic field imaging with high sensitivity, directional selectivity, and broadband frequency response.