Magnetic force microscopy (MFM) operates as a specialized mode of atomic force microscopy (AFM) designed to map magnetic interactions between a sample surface and a magnetized probe. Unlike conventional AFM, which measures topography through mechanical forces, MFM detects long-range magnetic forces by leveraging a cantilever with a magnetic coating. The technique provides high-resolution imaging of magnetic domains and structures at the nanoscale, making it indispensable for studying materials such as ferromagnetic thin films, data storage media, and spintronic devices.

The core principle of MFM relies on detecting shifts in the resonant frequency or oscillation amplitude of the cantilever caused by magnetic forces. These forces arise from the interaction between the magnetic tip and the stray fields emanating from the sample. To separate magnetic signals from topographic artifacts, MFM employs a two-pass technique known as lift mode. In the first pass, the tip scans the surface in standard AFM mode to record topography. During the second pass, the tip retraces the same path at a fixed lift height (typically 10–100 nm above the surface), where van der Waals and electrostatic forces diminish, leaving magnetic forces dominant. The phase or frequency shift in the cantilever oscillation during this second pass correlates with the magnetic field gradient.

Lift mode operation is critical for minimizing crosstalk between topography and magnetic signals. The lift height must be carefully selected—too low, and topographic interference persists; too high, and magnetic signal strength drops. Optimal lift heights vary with sample properties but generally fall between 20 nm and 50 nm for most domain imaging applications. The phase contrast in MFM images reflects variations in the magnetic field gradient, with bright and dark regions indicating attractive or repulsive interactions, respectively.

The magnetic tip is a decisive factor in MFM performance. Tips are typically coated with ferromagnetic or hard magnetic materials such as cobalt-chromium, nickel, or iron-platinum alloys to ensure sufficient magnetization. Key requirements include high coercivity to prevent tip magnetization reversal during scanning, uniform coating to avoid spurious signals, and sharp apex geometry for spatial resolution. Commercially available MFM probes often feature a coercivity exceeding 500 Oe to maintain stable magnetization under typical operating conditions. The tip’s magnetic moment and geometry influence sensitivity; for instance, a thinner coating reduces stray fields but may compromise signal strength.

Domain imaging with MFM reveals features such as domain walls, vortices, and skyrmions in ferromagnetic materials. The technique can resolve domains as small as 20–30 nm in width, depending on tip sharpness and sample properties. In thin films, MFM visualizes stripe domains, where alternating up and down magnetizations create periodic contrast patterns. For soft magnetic materials, external magnetic fields may be applied in situ to observe domain dynamics, though care must be taken to avoid perturbing the tip’s magnetization.

Quantitative interpretation of MFM data requires caution, as the signal depends on both tip and sample magnetization distributions. While MFM provides relative contrast, extracting absolute field values demands rigorous modeling of tip-sample interactions. For example, the point dipole approximation treats the tip as a single magnetic dipole, simplifying calculations but neglecting the tip’s finite size. More advanced models use micromagnetic simulations to account for the tip’s complex magnetization structure.

MFM faces limitations in imaging conductive or electrically biased samples due to competing electrostatic interactions. Proper grounding and non-conductive coatings mitigate these effects. Additionally, MFM cannot directly measure magnetization magnitude or orientation; complementary techniques like Kerr microscopy or spin-polarized scanning tunneling microscopy may be required for full vectorial analysis.

Despite these constraints, MFM remains a powerful tool for nanoscale magnetic characterization. Its non-destructive nature and compatibility with ambient conditions make it preferable for routine domain imaging. Advances in tip fabrication, such as focused ion beam milling for sharper geometries, continue to push resolution limits. Meanwhile, hybrid techniques combining MFM with electrical or thermal probes expand its utility in emerging fields like magnonics and topological magnetism.

In summary, MFM’s lift mode operation and specialized probe design enable precise mapping of magnetic domains with minimal topographic interference. The technique’s versatility and nanoscale resolution sustain its relevance in both fundamental research and industrial applications, from analyzing magnetic recording media to optimizing spintronic devices. Future refinements in probe technology and quantitative modeling promise to further enhance its capabilities.