Journal of Magnetism and Magnetic Materials Volume 491, 1 December 2019, 165578
M.P. Arenas, C.J. Pacheco, A.B. Fonseca, F.S. Queiroz, M. Gaudencio, C.B. Eckstein, L.Nogueira, L.H. De Almeida, J.M.A. Rebello, B. Nysten, G.R. Pereira
Atomic force microscope is a complete nanoscale characterization tool that can measure a wide variety of material properties alongside three-dimensional topography. In the realm of magnetic and electrical characterization, there are no other comparable techniques that can provide similar nanoscale information on properties such as surface charge, work function, conductivity, electrostatic force and magnetic interactions as can be supplied by the myriad of electrical and magnetic modes in AFM.
Among the most popular electrical characterization AFM based techniques is Kelvin Probe Force Microscopy (KPFM). This is a surface-sensitive method that maps the surface potential or work function of the samples and is used for a diverse array of applications from 2D materials to buried insulators. Magnetic force microscopy (MFM) remains the standard-bearer to probe local magnetic domains and has been widely used to characterize magnetic storage media, superconductors, and even biological systems.
Exhibiting the power of the complementarity of these electrical and magnetic techniques, a team of materials science researchers led by G.R. Pereira at the Federal University of Rio de Janeiro in Brazil use both KPFM and MFM to evaluate heat resistant steel alloys as a function of aging. Heat resistance HP steels are used in steam reforming tubes for hydrogen production, where they are exposed to temperature gradients across their length. The integrity and consistency of these tubes’ magnetic response is essential to the reliability of nondestructive inspections during scheduled maintenance shutdowns. However, temperature changes can cause changes in the materials’ performance as can variation from different tube manufacturers. The research team used SPM based methods to probe how the magnetic response of samples changes with variations in microstructure and aging.
The various steel samples were examined with both SEM-EDS and a Nanosurf Flex-Axiom. The SEM was used to map out the microstructure and chemical composition of the various samples as a function of aging. After initial aging, the material reveals a matrix of austenite with inter dendritic chromium-rich carbides and niobium carbides. As the sample is heated and aged, the primary carbides coalesce and a chromium precipitate forms in the intra-dendritic area while niobium carbides are transformed into a G-phase. SEM-EDS, MFM and KPFM data collected on the same areas provide for complete mapping of the chemistry, structure, magnetic, and electrical properties of all the distinctive features identified from the SEM image.
MFM data was acquired in the dual pass method where a magnetized AFM tip (coated with a thin magnetic layer) first traces out the sample topography and then is lifted 100-150nm above the surface where the magnetic forces between tip and sample are measured. KPFM data was collected in a single pass method in amplitude modulation mode with an AC electrical biased applied off-resonance. The same AFM tip is used for KPFM and MFM images and so topography, MFM, and KPFM can all be collected of the same localized region for a powerful correlation of the surface properties of this material.
The steel alloy in this study is a rich multi-phasic surface. A preliminary aging state revealed magnetic behavior at carbide boundaries while samples with more advanced aging states showed no magnetic response. The authors were able to use the KPFM potential image to confirm if the magnetic signal was real. For example, in samples with advanced aging, any magnetic response observed was due to the electrostatic interactions induced by contact potential difference, as measured by KPFM. However, in the samples with only preliminary aging, no contact potential was observed in the KPFM image while sharp contrast was observed in the corresponding MFM image revealing that the magnetic response was real in those samples.
The SPM studies also show that although different chemical composition does cause microstructural changes, the magnetic response for samples was consistent as long as the aging state was the same. These results are important to the tube manufacturers as it enables them to program replacements and repair in a rational and cost-effective manner.