Nuclear Magnetic Resonance Force Microscopy Research Articles

Feasibility of imaging in nuclear magnetic resonance force microscopy using Boltzmann polarization

We report on magnetic resonance force microscopy measurements of the Boltzmann polarization of nuclear spins in copper by detecting the frequency shift of a soft cantilever. We use the time-dependent solution of the Bloch equations to derive a concise equation describing the effect of radio-frequent (RF) magnetic fields on both on- and off-resonant spins in high magnetic field gradients. We then apply this theory to saturation experiments performed on a 100 nm thick layer of copper, where we use the higher modes of the cantilever as a source of the RF field. We demonstrate a detection volume sensitivity of only (40nm)3, corresponding to about 1.6×104 polarized copper nuclear spins. We propose an experiment on protons where, with the appropriate technical improvements, frequency-shift based magnetic resonance imaging with a resolution better than (10nm)3 could be possible. Achieving this resolution would make imaging based on the Boltzmann polarization competitive with the more traditional stochastic spin-fluctuation based imaging, with the possibility to work at millikelvin temperatures.

Dynamical measurements with a nuclear magnetic resonance force microscope

We report imaging and dynamical measurements using a H3e nuclear magnetic resonance force microscopy probe. Relaxation-time measurements and a one-dimensional image were obtained for H1 nuclei in a micron-scale crystal of (NH4)2SO4. The force detection was made possible by a small Permalloy magnet, which supplied a field gradient of 500 T/m. These experiments were performed in the sample-on-oscillator configuration at room temperature, where the oscillator had a resonance frequency of 1.5 kHz and a spring constant of 0.03 N/m. The proton magnetic moments underwent cyclic adiabatic inversions (CAIs) under the influence of a frequency-modulated rf field. Scanning the position of the magnet with respect to the sample provided a micron-scale image with a signal-to-noise ratio of 4.3. A spin nutation signal was also obtained; those data imply a rotating rf field of 14 G. Using a 90°-τ-180°-t-90°-CAI sequence, a spin echo was mapped out, with a full width at half maximum of 8 μs. We also discuss future applications of this instrument toward relaxation measurements of single-crystal MgB2 at low temperatures.

Nuclear magnetic resonance force microscopy with a microwire rf source

The authors use a 1.0μm wide patterned Cu wire with an integrated nanomagnetic tip to measure the statistical nuclear polarization of F19 in CaF2 by magnetic resonance force microscopy. With less than 350μW of dissipated power, the authors achieve rf magnetic fields over 4mT at 115MHz for a sample positioned within 100nm of the “microwire” rf source. A 200nm diameter FeCo tip integrated onto the wire produces field gradients greater than 105T∕m at the same position. The large rf fields from the broadband microwire enable long rotating-frame spin lifetimes of up to 15s at 4K.

Quantitative determination of the adiabatic condition using force-detected nuclear magnetic resonance

The adiabatic condition governing cyclic adiabatic inversion of proton spins in a micron-sized ammonium chloride crystal was studied using room temperature nuclear magnetic resonance force microscopy. A systematic degradation of signal to noise was observed as the adiabatic condition became violated. A theory of adiabatic following applicable to cyclic adiabatic inversion is reviewed and implemented to quantitatively determine an adiabaticity threshold ${(\ensuremath{\gamma}{H}_{1})}^{2}∕({\ensuremath{\omega}}_{osc}\ensuremath{\Omega})=6.0$ from our experimental results.

Novel fabrication of micromechanical oscillators with nanoscale sensitivity at room temperature

In this paper, we report on the design, fabrication, and implementation of ultrasensitive micromechanical oscillators. Our ultrathin single-crystal silicon cantilevers with integrated magnetic structures are the first of their kind: They are fabricated using a novel high-yield process in which magnetic film patterning and deposition are combined with cantilever fabrication. These novel devices have been developed for use as cantilever magnetometers and as force sensors in nuclear magnetic resonance force microscopy (MRFM). These two applications have achieved nanometer-scale resolution using the cantilevers described in this work. Current magnetic moment sensitivity achieved for the devices, when used as magnetometers, is 10/sup -15/ J/T at room temperature, which is more than a 1000-fold improvement in sensitivity, compared to conventional magnetometers. Current room temperature force sensitivity of MRFM cantilevers is /spl sim/10/sup -16/ N//spl radic/Hz, which is comparable to the room temperature sensitivities of similar devices of its type. Finite element modeling was used to improve design parameters, ensure that the devices meet experimental demands, and correlate mode shape with observed results. The photolithographic fabrication process was optimized, yielding an average of /spl sim/85% and alignment better than 1 /spl mu/m. Postfabrication-focused ion-beam milling was used to further pattern the integrated magnetic structures when nanometer scale dimensions were required.

External field effects on the resonant frequency of magnetically capped oscillators for magnetic resonance force microscopy

We study the resonant frequency shift of CoPt-capped single-crystal-silicon micro-oscillators when a magnetic field is applied perpendicular to the magnetic film, as required for application to nuclear magnetic resonance force microscopy. The oscillator resonant frequencies show two distinct regimes of behavior. At low fields, when the magnetic moment is nearly perpendicular to the external field, the frequency decreases sharply with field, while at high fields, when the moment and field are nearly aligned, the frequency increases. We present models that accurately describe both behaviors. The transition point between these two regimes scales with the volume of the micromagnets.