An array of four 87 Rb vector magnetometers are used to detect nuclear quadrupole resonance (NQR) signals in an unshielded environment at 1 MHz. With a baseline of 25 cm, the length of the array, radio-frequency interference mitigation (RFIM) is also demonstrated; a radio-station signal is suppressed by a factor of 20 without degradation to the signal of interest. With these compact sensors, in which the probe beam passes through twice, the fundamental limit to detection sensitivity is found to be photon shot noise. More passes of the probe beam overcome this limitation. With a sensor of similar effective volume, 0.25 cm 3 , but 25 times more passes, the sensitivity is improved by an order of magnitude to 1.7 ± 0.2 fT/ √ Hz.

Magnetic force microscopy was performed on 300 nm thick magnetite films grown epitaxially on MgO (001) at temperatures ranging from well below to well above the Verwey transition temperature, TV. Frequency shift images were acquired at different locations on the sample as temperature was increased through the Verwey transition. The magnetic domain features are persistent at all temperatures, which indicates that the domains are pinned across the phase transition, probably due to antiphase boundaries. An enhancement of magnetic contrast below TV indicates the moments tilt out of the plane below TV, which is corroborated by superconducting quantum interference device magnetometry.

3000 Å Fe3O4 (magnetite) thin films were simultaneously grown on (001) MgO single crystal substrates with and without 30 Å buffer layers of Fe, Cr, Mo, and Nb. For all samples, the Verwey transition temperature (TV) occurs between 119 and 125 K, indicating good oxygen stoichiometry. We observe highly oriented (001) Fe3O4 with Mo and no buffer layer, reduced (001) texture with Nb and Fe, and polycrystalline growth with Cr. Mo, Cr, and unbuffered magnetite show typical magnetic behavior, whereas Nb and Fe buffers lead to anomalous magnetic properties that may be due to interfacial reactivity.

Nuclear Quadrupole Resonance (NQR) has been demonstrated for the detection of 14-N in explosive compounds. Application of a material specific radio-frequency (RF) pulse excites a response typically detected with a wirewound antenna. NQR is non-contact and material specific, however fields produced by NQR are typically very weak, making demonstration of practical utility challenging. For certain materials, the NQR signal can be increased by transferring polarization from hydrogen nuclei to nitrogen nuclei using external magnetic fields. This polarization enhancement (PE) can enhance the NQR signal by an order of magnitude or more. Atomic magnetometers (AM) have been shown to improve detection sensitivity beyond a conventional antenna by a similar amount. AM sensors are immune to piezo-electric effects that hamper conventional NQR, and can be combined to form a gradiometer for effective RF noise cancellation. In principle, combining polarization enhancement with atomic magnetometer detection should yield improvement in signal-to-noise ratio that is the product of the two methods, 100-fold or more over conventional NQR. However both methods are even more exotic than traditional NQR, and have never been combined due to challenges in operating a large magnetic field and ultra-sensitive magnetic field sensor in proximity. Here we present NQR with and without PE with an atomic magnetometer, demonstrating signal enhancement greater than 20-fold for ammonium nitrate. We also demonstrate PE for PETN using a traditional coil for detection with an enhancement factor of 10. Experimental methods and future applications are discussed.

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.