Yuri Obukhov

Nanoscale scanning probe ferromagnetic resonance imaging using localized modes

The discovery of new phenomena in layered and nanostructured magnetic devices is driving rapid growth in nanomagnetics research. Resulting applications such as giant magnetoresistive field sensors and spin torque devices are fuelling advances in information and communications technology, magnetoelectronic sensing and biomedicine. There is an urgent need for high-resolution magnetic-imaging tools capable of characterizing these complex, often buried, nanoscale structures. Conventional ferromagnetic resonance (FMR) provides quantitative information about ferromagnetic materials and interacting multicomponent magnetic structures with spectroscopic precision and can distinguish components of complex bulk samples through their distinctive spectroscopic features. However, it lacks the sensitivity to probe nanoscale volumes and has no imaging capabilities. Here we demonstrate FMR imaging through spin-wave localization. Although the strong interactions in a ferromagnet favour the excitation of extended collective modes, we show that the intense, spatially confined magnetic field of the micromagnetic probe tip used in FMR force microscopy can be used to localize the FMR mode immediately beneath the probe. We demonstrate FMR modes localized within volumes having 200 nm lateral dimensions, and improvements of the approach may allow these dimensions to be decreased to tens of nanometres. Our study shows that this approach is capable of providing the microscopic detail required for the characterization of ferromagnets used in fields ranging from spintronics to biomagnetism. This method is applicable to buried and surface magnets, and, being a resonance technique, measures local internal fields and other magnetic properties with spectroscopic precision.

Nanoscale confined mode ferromagnetic resonance imaging of an individual Ni81Fe19 disk using magnetic resonance force microscopy (invited)

We demonstrate and characterize the localized Ferromagnetic Resonance (FMR) modes created in an individual micron-sized Ni81Fe19 disk by means of a strong inhomogeneous probe field applied anti-parallel to the saturated magnetization of the sample. Our variational calculation accurately predicts the frequencies of the localized modes in FMR spectra, and characterizes the sizes and all related internal magnetic fields of the Bessel function modes in a simple model analogous to particle wavefunctions in a quantum well. The localized modes enable FMR imaging of the nonuniform demagnetizing field inside a Py disk demonstrating a novel magnetic resonance imaging technique able to map internal fields in ferromagnets with spectroscopic accuracy.

Magnetic force microscopy in the presence of a strong probe field

We describe a magnetic force microscopy (MFM) imaging approach in which we take advantage of the strong, localized magnetic field of the MFM probe to deterministically modify the magnetization of the sample. This technique enables quantitative mapping of sample magnetic properties including saturation magnetization and anisotropy, a capability not generally available using conventional MFM methods. This approach yields a fruitful theoretical analysis that accurately describes representative experimental data we obtain from an isolated permalloy disk.