Nanometer-scale structures with high aspect ratio such as nanowires and nanotubes combine low mechanical dissipation with high resonance frequencies, making them ideal force transducers and scanning probes in applications requiring the highest sensit
ivity. Such structures promise record force sensitivities combined with ease of use in scanning probe microscopes. A wide variety of possible material compositions and functionalizations is available, allowing for the sensing of various kinds of forces with optimized sensitivity. In addition, nanowires possess quasi-degenerate mechanical mode doublets, which has allowed the demonstration of sensitive vectorial force and mass detection. These developments have driven researchers to use nanowire cantilevers in various force sensing applications, which include imaging of sample surface topography, detection of optomechanical, electrical, and magnetic forces, and magnetic resonance force microscopy. In this review, we discuss the motivation behind using nanowires as force transducers, explain the methods of force sensing with nanowire cantilevers, and give an overview of the experimental progress and future prospects of the field.
The drive to improve the sensitivity of nuclear magnetic resonance (NMR) to smaller and smaller sample volumes has led to the development of a variety of techniques distinct from conventional inductive detection. In this chapter, we focus on the tech
nique of force-detected NMR as one of the most successful in yielding sensitivity improvements. We review the rationale for the technique, its basic principles, and give a brief history of its most important results. We then cover in greater detail its application in the first demonstration of three-dimensional (3D) nuclear magnetic resonance imaging (MRI) with nanometer-scale resolution. Next we present recent developments and likely paths for improvement. Finally, the technique and its potential are discussed in the context of competing and complementary technologies.
Using dynamic cantilever magnetometry, we study the vortex lattice and its corresponding melting transition in a micrometer-size crystallite of superconducting NbSe2. Measurements of the cantilever resonance frequency as a function of magnetic field
and temperature respond to the magnetization of the vortex-lattice. The cantilever dissipation depends on thermally activated vortex creep motion, whose pinning energy barrier is found to be in good agreement with transport measurements on bulk samples. This approach reveals the phase diagram of the crystallite, and is applicable to other micro- or nanometer-scale superconducting samples.
We demonstrate nuclear double resonance for nanometer-scale volumes of spins where random fluctuations rather than Boltzmann polarization dominate. When the Hartmann-Hahn condition is met in a cross-polarization experiment, flip-flops occur between t
wo species of spins and their fluctuations become coupled. We use magnetic resonance force microscopy to measure this effect between 1H and 13C spins in 13C-enriched stearic acid. The development of a cross-polarization technique for statistical ensembles adds an important tool for generating chemical contrast in nanometer-scale magnetic resonance.
We calculate the Overhauser frequency shifts in semiconductor nanostructures resulting from the hyperfine interaction between nonequilibrium electronic spins and nuclear spins. The frequency shifts depend on the electronic local density of states and
spin polarization as well as the electronic and nuclear spin relaxation mechanisms. Unlike previous calculations, our method accounts for the electron confinement in low dimensional semiconductor nanostructures, resulting in both nuclear spin polarizations and Overhauser shifts that are strongly dependent on position. Our results explain previously puzzling measurements of Overhauser shifts in an Al$_x$Ga$_{1-x}$As parabolic quantum well by showing the connection between the electron spin lifetime and the frequency shifts.
Recent advances in the fabrication of microelectromechanical systems (MEMS) and their evolution into nanoelectromechanical systems (NEMS) have allowed researchers to measure extremely small forces, masses, and displacements. In particular, researcher
s have developed position transducers with resolution approaching the uncertainty limit set by quantum mechanics. The achievement of such resolution has implications not only for the detection of quantum behavior in mechanical systems, but also for a variety of other precision experiments including the bounding of deviations from Newtonian gravity at short distances and the measurement of single spins. Here we demonstrate the use of a quantum point contact (QPC) as a sensitive displacement detector capable of sensing the low-temperature thermal motion of a nearby micromechanical cantilever. Advantages of this approach include versatility due to its off-board design, compatibility with nanoscale oscillators, and, with further development, the potential to achieve quantum limited displacement detection.
We report on measurements of the spin lifetime of nuclear spins strongly coupled to a micromechanical cantilever as used in magnetic resonance force microscopy. We find that the rotating-frame correlation time of the statistical nuclear polarization
is set by the magneto-mechanical noise originating from the thermal motion of the cantilever. Evidence is based on the effect of three parameters: (1) the magnetic field gradient (the coupling strength), (2) the Rabi frequency of the spins (the transition energy), and (3) the temperature of the low-frequency mechanical modes. Experimental results are compared to relaxation rates calculated from the spectral density of the magneto-mechanical noise.
We use a 1.0-um-wide patterned Cu wire with an integrated nanomagnetic tip to measure the statistical nuclear polarization of 19F in CaF2 by magnetic resonance force microscopy (MRFM). With less than 350 uW of dissipated power, we achieve rf magnetic
fields over 4 mT at 115 MHz for a sample positioned within 100 nm of the microwire rf source. A 200-nm diameter FeCo tip integrated onto the wire produces field gradients greater than 10^5 T/m at the same position. The large rf fields from the broadband microwire enable long rotating-frame spin lifetimes of up to 15 s at 4 K.
