The Bloch equations in high-gradient magnetic resonance force microscopy: theory and experiment

Area of Science:

• Physics
• Materials Science
• Spectroscopy

Background:

• Paramagnetic resonance is crucial for understanding electron spin dynamics.
• Magnetic Resonance Force Microscopy (MRFM) offers high sensitivity for detecting magnetic signals.
• High magnetic field gradients are essential for advanced MRFM applications.

Purpose of the Study:

• To report theoretical and observational results of paramagnetic resonance using MRFM.
• To investigate resonance behavior under a measured field gradient of 44,000 T/m.
• To analyze spin dynamics and compare experimental data with theoretical models.

Main Methods:

• Utilized magnetic resonance force microscopy (MRFM) with a SmCo magnetic tip.
• Induced resonance in diphenylpicrylhydrazyl (DPPH) in polystyrene at 77 K and 10 K.
• Employed an amplitude-modulated microwave field and detected force signals in the femtonewton (fN) range.

Main Results:

• Successfully detected paramagnetic resonance signals with high sensitivity, exceeding a thermal noise floor of 80 aN/√Hz.
• Observed resonance saturation and readily measured it.
• Found good agreement between experimental data and Bloch equation predictions at low microwave power levels, deviating above saturation.

Conclusions:

• The study demonstrates the capability of MRFM for detecting paramagnetic resonance under high field gradients.
• The findings validate theoretical predictions based on Bloch equations for low power regimes.
• Deviations above saturation suggest potential non-Bloch dynamics warranting further investigation in the MRFM environment.