Dynamic nuclear polarization in a magnetic resonance force microscope experiment.

Area of Science:

• Physics
• Materials Science
• Chemistry

Background:

• Nuclear magnetization is typically weak at low magnetic fields (0.6 T) and cryogenic temperatures (4.2 K), limiting Nuclear Magnetic Resonance (NMR) sensitivity.
• Dynamic Nuclear Polarization (DNP) is a technique that can enhance nuclear spin polarization by transferring polarization from electron spins.

Purpose of the Study:

• To demonstrate enhanced nuclear magnetization using DNP in a magnetic resonance force microscope (MRFM) experiment.
• To investigate the spatial distribution and characteristics of DNP-enhanced nuclear magnetization at the nanoscale.

Main Methods:

• Utilized a MRFM setup with a microwire coplanar waveguide to deliver radiofrequency and microwave irradiation.
• Employed a nitroxide-doped polystyrene sample and a micron-scale nickel tip cantilever to detect spin resonance.
• Mapped the nuclear polarization enhancement factor (ε) by varying radio wave frequencies and analyzing changes in cantilever's mechanical resonance frequency.

Main Results:

• Achieved observable NMR signals at 0.6 T and 4.2 K via DNP, which were not detectable through Curie-law magnetization alone.
• Observed a bipolar spatial distribution of the nuclear polarization enhancement factor (ε), with values ranging from +10 to +20 near the magnet and -10 to -20 distal to it.
• Demonstrated that the observed polarization profile is consistent with cross-effect DNP in a high magnetic field gradient (∼10^5 T m⁻¹).

Conclusions:

• DNP effectively enhances nuclear magnetization in MRFM experiments, enabling NMR signal detection at previously inaccessible field strengths.
• The spatially resolved bipolar polarization profile provides insights into DNP mechanisms in inhomogeneous magnetic fields.
• The findings suggest potential for DNP-enhanced MRFM in nanometer-resolution magnetic resonance imaging, while acknowledging associated challenges.