We report an approach that extends the applicability of ultrasensitive force-gradient detection of magnetic resonance to samples with spin-lattice relaxation times (T 1 ) as short as a single cantilever period. To demonstrate the generality of the approach, which relies on detecting either cantilever frequency or phase, we used it to detect electron spin resonance from a T 1 = 1 ms nitroxide spin probe in a thin film at 4.2 K and 0.6 T. By using a custom-fabricated cantilever with a 4 μm-diameter nickel tip, we achieve a magnetic resonance sensitivity of 400 Bohr magnetons in a 1 Hz bandwidth. A theory is presented that quantitatively predicts both the lineshape and the magnitude of the observed cantilever frequency shift as a function of field and cantilever-sample separation. Good agreement was found between nitroxide T 1 's measured mechanically and inductively, indicating that the cantilever magnet is not an appreciable source of spin-lattice relaxation here. We suggest that the new approach has a number of advantages that make it well suited to push magnetic resonance detection and imaging of nitroxide spin labels in an individual macromolecule to single-spin sensitivity.MRFM | ESR | TEMPAMINE | mechanically detected magnetic resonance | molecular structure imaging A generally applicable approach for determining the tertiary structure of an individual macromolecule in vitro at angstrom or subangstrom resolution would create exciting opportunities for answering many longstanding questions in molecular biology. For macromolecules too large to characterize by NMR or X-ray diffraction, the tertiary structure of proteins (1-3), nucleic acids (4, 5), and biomolecular assemblies (6, 7) can be explored by using inductively-detected electron spin resonance (ESR) to measure distances between pairs of attached spin labels (2-5, 7, 8). These studies, however, require bulk quantities of sample (9) and demand multiple experiments with spin labels attached to different locations in the target macromolecule. Mechanical detection and imaging of single-electron spins has been demonstrated, in E centers in gamma-irradiated quartz (10), and it is natural to explore applying magnetic resonance force microscopy (MRFM) (11-15) to map the locations of individual spin labels attached to a single biomacromolecule.The ultimate limit of imaging resolution in MRFM is set by the intrinsic linewidth of the resonance and the applied magnetic field gradient. For a 0.1 mT homogeneous linewidth, typical of the organic radical studied here, a gradient of 4 × 10 6 T/m allows selective excitation of individual spin labels only 0.025 nm apart. A magnetic field gradient this large has recently been demonstrated in an MRFM experiment by using ferromagnetic pillars fabricated by electron-beam lithography (15). The force sensitivity required to detect single electrons in this gradient is 40 aN, above the minimum detectable force (in 1 Hz bandwidth) of 5 − 10 aN reported for a high-compliance cantilever operated with its metalized leading edge above ...

The invention and initial demonstration of magnetic resonance force microscopy (MRFM) in the early 1990s launched a renaissance of mechanical approaches to detecting magnetic resonance. This article reviews progress made in MRFM in the last decade, including the demonstration of scanned probe detection of magnetic resonance (electron spin resonance, ferromagnetic resonance, and nuclear magnetic resonance) and the mechanical detection of electron spin resonance from a single spin. Force and force-gradient approaches to mechanical detection are reviewed and recent related work using attonewton sensitivity cantilevers to probe minute fluctuating electric fields near surfaces is discussed. Given recent progress, pushing MRFM to single proton sensitivity remains an exciting possibility. We will survey some practical and fundamental issues that must be resolved to meet this challenge.

We have batch-fabricated cantilevers with ~100 nm diameter nickel nanorod tips and force sensitivities of a few attonewtons at 4.2 kelvin. The magnetic nanorods were engineered to overhang the leading edge of the cantilever and, consequently, the cantilevers experience what we believe is the lowest surface noise ever achieved in a scanned probe experiment. Cantilever magnetometry indicated that the tips were well magnetized, with a ≤ 20 nm dead layer; the composition of the dead layer was studied by electron microscopy and electron energy loss spectroscopy. In what we believe is the first demonstration of scanned probe detection of electron-spin resonance from a batch fabricated tip, the cantilevers were used to observe electron-spin resonance from nitroxide spin labels in a film via force-gradient-induced shifts in cantilever resonance frequency. The magnetic field dependence of the magnetic resonance signal suggests a non-uniform tip magnetization at an applied field near 0.6 T.

There are few materials that are broadly used for fabricating optical metasurfaces for visible light applications. Gallium phosphide (GaP) is a material that, due to its optical properties, has the potential to become a primary choice but due to the difficulties in fabrication, GaP thin films deposited on transparent substrates have never been exploited. In this article we report the design, fabrication, and characterization of three different amorphous GaP metasurfaces obtained through sputtering. Although the material properties can be further optimized, our results show the potential of this material for visible applications making it a viable alternative in the material selection for optical metasurfaces.

We measure the spin-lattice relaxation time as a function of sample temperature in GaAs in a real-time single-shot inversion recovery experiment using spin force gradients acting on a magnetic tipped cantilever. After inverting 69 Ga spins localized near the magnet with a single 20 ms adiabatic rapid passage sweep, the spins' magnetization recovery was passively tracked by recording the cantilever's frequency change, which is proportional to the longitudinal component of the spins' magnetization. The cantilever's frequency was recorded for a time 3*T 1 for sample temperatures ranging from 4.8 to 25 K. The temperature dependence was observed for the Magnetic resonance force microscopy (MRFM) combines the benefits of the two mature fields of magnetic resonance imaging and cantilever based scanning probe microscopy.1 The technology is pressing towards three dimensional imaging at the nanometer scale while providing chemically selective information, where a wide arsenal of nuclear magnetic resonance (NMR) spectroscopic techniques can be used. MRFM is an attractive tool, as it can provide both spatial imaging and chemical characterization on samples such as biological organisms, organic molecules, superconductors, 2,3 and the expanding class of engineered nanomaterials. Mechanically detected NMR has been previously reported on inorganic materials including GaAs, 4,5 CaF, 6 and imaging on an organic specimen. 7A basic measurement in NMR is that of the spin-lattice relaxation time (T 1 ) with an inversion recovery experiment. An understanding of the spin relaxation mechanisms for a sample is essential for interpretation of NMR data and proper identification of a material's structure. Knowledge of T 1 finds use in many areas, i.e., superconductors, 2 semiconductors, 8 and organic materials. 9 Extensive studies have been carried out for the spin 3/2 quadrupolar nuclei in GaAs semiconductors, over wide temperature ranges 10 and doping properties, 11,12 where the relative contributions from quadrupolar mechanisms of relaxation and the roles of acoustic and optical phonons on relaxation rates have been neatly sorted out. The modern inductively detected NMR system is sensitive only to the transverse magnetization. Using inductively detected NMR to determine T 1 requires applying a polarizing magnetic field along the z axis and measuring the precession of magnetization in the xy plane. In solids, the transverse magnetization typically dephases in a time T 2 which is on the order of only 100 microseconds (typically much less than T 1 ). A standard NMR inversion recovery experiment is performed by inverting the magnetization, waiting some fraction of the T 1 time, laying the remaining polarization in the xy plane, and recording its magnitude by measuring the size of the free induction decay (FID). One must then wait a time the order of T 1 for the magnetization to recover before performing the experiment over again. To obtain N points on the T 1 curve takes a time the order of N*T 1 . For samples with a long T 1 , this prese...