Magnetic Resonance Force Microscopy With Vacuum-Packaged Magnetic Cantilever Towards Free Radical Detection

Magnetic resonance force microscopy is becoming increasingly important for three-dimensional image reconstruction inside a sample with non-invasive measurements. In this study, a magnetic resonance force sensor with a vacuum-packaged magnetic cantilever was developed for the detection of free radicals in an atmospheric environment. A 1.5- $\mu \text{m}$ -thick silicon-based cantilever, which was mounted with a 25- $\mu \text{m}$ -diameter Nd-Fe-B magnet at the end of the cantilever, was hermetically packaged in a vacuum cavity using an anodic-bonding technique. The evaluation of the pressure dependence on the quality factor of the mechanical magnet-based cantilever revealed that the packaged-cavity pressure was approximately in the range of $7.3\,\,\times10$ 2– $1.0\,\,\times10$ 3 Pa after vacuum packaging. The corresponding detection sensitivity of the magnet-based cantilever sensor was calculated to be $1.1\,\,\times10$ -13 N/ $\sqrt {Hz} $ . Based on the magnetic force interaction, the magnetic field gradients were detected by mapping the variations in the resonant frequency and resonant amplitude of the magnetic cantilever. Finally, the magnetic resonance force, caused by the electron spin resonance in a standard radical of 1, 1-Diphenyl-2-picrylhydrazyl, was detected with a spin density of $\sim 1.5\,\,\times10$ 15 spins/cm3 in an atmospheric environment. The proposed vacuum-packaged magnet-based cantilever confirmed the high feasibility of magnetic resonance force detection within a non-hermetic environment.