Session S44: Magnetic and Force Microscopy

Detection and Manipulation of the Statistical Fluctuations in Nuclear Spin Ensembles Using Magnetic Resonance Force Microscopy

We have detected and manipulated the naturally occurring $\sqrt N $ statistical polarization in nuclear spin ensembles using MRFM. We have studied fluorine nuclei in CaF$_{2}$, as well as protons in the polymer PMMA and the protein collagen. The ensembles studied contained of order 10$^{8}$ nuclear spins, corresponding to volumes of order (200nm)$^{3}$, which resulted in statistical polarizations of order 10$^{4}$ net spins. We have also implemented a scheme similar to one proposed by Weitekamp \textit{et al}, in which we suppressed the effect of the statistical uncertainty so as to extract meaningful information from time-averaged measurements. In this way, we have successfully made nutation and transverse spin relaxation measurements in a nominally unpolarized sample of CaF$_{2}$ at low temperatures.   

Force-detected ESR from E$^\prime$ centers

Magnetic Resonance Force Microscopy (MRFM) is a novel technique that combines magnetic resonance with scanned probe techniques. We report on low temperature force-detected electron spin resonance (ESR) signals from $E^\prime$-centers in fused silica (SiO$_2$). By utilizing the high gradients close to a micron sized SmCo ferromagnetic tip mounted on an AFM cantilever, spin resonance was observed from a sub- micron thick ``sensitive slice'' whose location can be varied with respect to the sample surface. E$^\prime$ centers at low temperatures ($<$ 10 K) are characterized by long spin-lattice relaxation times T$_1$ approaching a few seconds. The spins were adiabatically inverted at the appropriate frequency by means of microwave FM techniques and T$_1$ was studied as a function of field gradient. We also discuss the sensitivity of the microscope and ongoing efforts to improve it.   

MRFM and non-contact friction experiments under UHV conditons

In this contribuiton we present magnetic resonance force microscopy (MRFM ) experiments and non-contact friction force experiments made on $\gamma $-irradiated quarz samples under UHV condtions and low temperatures. Ultrasensitve cantilevers with integrated magnetic tips allow us to detect forces in the order of 10$^{-18 }$N/$\surd $Hz [1]. The measurements showed that the sensitivity did not change significantly in a homogenous magnetic field. In quarz samples long-range, non contact friction forces are observerd. A spin-lifetime of 137 ms was measured by cyclic inversion of the spins. The measurement was achieved with an ultrasensitve PLL using the OSCAR protocol. [1] Gysin et al., Phys. Rev. B \textbf{69}, 045403 (2004)   

Tip design and tip-sample interaction in magnetic resonance force microscopy

Magnetic resonance force microscopy (MRFM) is a three-dimensional, subsurface imaging technique which registers the presence of sample spins via the deflection, or change in mechanical resonance frequency, of a magnet-tipped cantilever. At single-nuclear-spin sensitivity, MRFM would have numerous exciting applications, such as imaging of single biomolecules or spin-state readout for solid-state quantum computing. We have previously reported unprecedented sensitivity in nuclear MRFM, and are currently improving our sensitivity by attacking two remaining technical challenges: producing usable nanomagnetic tips, and learning to control excess cantilever energy dissipation to the sample surface. We will discuss our recent results in these areas and our latest MRFM results.   

NMR Force Microscopy on an hcp Co single crystal

We discuss the implementation of NMR Force Microscope setup designed and built for NMR study of ferromagnetic thin films. The details of data collection and analysis as well as the design of RF and Magnetic Field subsystems are presented. As an example of subsurface NMR investigation we present the preliminary data obtained from an hcp Co single crystal followed by the discussion of sensitivity and spatial resolution of the method.   

Magnetic Resonance Force Microscopy and Force-Detected NMR of Microcrystals

We report our advances in nuclear magentic resonance force microscopy (NMRFM) and NMR of microcrystals using force (mechanical oscillator) detection. For each, we report microfabrication of sensitive single-crystal-silicon multiple-torsional micro-oscillators using both optical and e-beam lithography and a back-etch technique. We characterize mechanical oscillator frequency, quality factor, and spring constant from the noise spectral density of oscillator motion, detected using fiber-optic interferometry. We review past work on scanning-mode detection of the NMR response from volumes as small as 2 $\mu$m$^3$ at room temperature. We primarily discuss progress in two experiments currently underway: 1) the study of $^1$H dynamics in submicron-thick metal hydride films, where the NMRFM technique permits selective response to motion-modulated dipolar interactions with correlation times from microseconds to seconds, and 2) detection of the $^{11}$B resonance in microcrystals over the temperature range $4\ {\rm K} < T < 300{\rm K}$. We also overview measurements made in our $^3$He low-temperature NMRFM system.   

Development of a New Generation of Piezoresistive Cantilevers Designed for Torque Magnetometry

Torque magnetometry is known to be a very sensitive technique to measure the magnetization of anisotropic magnetic materials. Applying the piezoresistive technology pushed forward by atomic force microscopy made extremely small and powerful torquemeters possible. However, commercially available cantilevers, which are optimized for force measurements, are not well suited for torque measurements. Cantilevers specially designed for torque magnetometry greatly improved the performance of such sensors and sensitivities of the order of $10^{-14}$\,Nm are obtained in a dynamic operation mode [1]. Based on this work we present here a new generation of torque sensors with greatly improved performance [2]. Together with our recently improved software for the automatic control of our torque magnetometer this offers new possibilities of performing systematic studies of magnetic phenomena with high resolution. The power of this new device will be demonstrated by magnetization studies in cuprate superconductors. [1] M. Willemin et al., J. Appl. Phys. 83, 1163 (1998) [2] S. Kohout et al., in preparation   

Ultra-Sensitive Micromechanical Cantilevers with Integrated Magnetic Structures

We report on the design, fabrication, and implementation of ultra-sensitive micromechanical oscillators. These novel devices have been developed for use as cantilever magnetometers and as force sensors in nuclear magnetic resonance force microscopy. Our single-crystal silicon cantilevers with integrated magnetic structures are fabricated using a novel process in which magnetic film patterning and deposition are combined in a nondestructive manner with cantilever fabrication. Current magnetic moment sensitivity achieved for the devices, when used as magnetometers, is 10$^{-15}$ J/T at room temperature. Finite element modeling was used for several different cantilever geometries to improve design parameters, ensure that the devices meet experimental demands, and correlate mode shape with observed results. Post-fabrication focused-ion-beam milling was used to further pattern the integrated magnetic structures when nanometer scale dimensions were required.   

Zeptogram Scale Nanomechanical Mass Sensing

We show very high frequency nanoelectromechanical systems (NEMS) that provide a profound sensitivity increase for inertial mass sensing into zeptogram-scale. Measurement and analysis from our still unoptimized experiments already demonstrate mass sensitivity at the level of 7 zg, the mass of an individual 4 kDa molecule or 30 xenon atoms. Implication of the detailed analysis of the ultimate sensitivity of such devices based on experimental results is especially compelling: they indicate NEMS can ultimately provide inertial mass sensing of individual electrically neutral macromolecules with single Dalton sensitivity. The scheme has been employed to study noise arising from adsorption desorption of xenon on the NEMS surface. We also anticipate this will offer an unprecedented opportunity for many interesting applications in surface science, atomic physics and biology.   

Toward single-molecule nanomechanical mass spectrometry

Nanoelectromechanical systems (NEMS) offer immense potential for high-sensitivity applications in sensor technology. In mass sensitivity, recent reports have logged dramatic progress with milestones at the level of, first, single femtogram, then single attogram\footnote{ M. L. Roukes and K. L. Ekinci, U. S. Patent 6,722,200 (filed: 4 May 2001, granted: 20 April 2004); see also Appl.Phys.Lett. \textbf{84}, 4469 (2004).}, and most recently few zeptogram\footnote{ Y. T. Yang, Carlo Callegari, X. L. Feng, Kamil L. Ekinci, and M. L. Roukes, ``Zeptogram Scale Nanomechanical Mass Sensing,'' this meeting.} -- pushing the state of the art to over a billion times the sensitivity of commercially-available mass sensors. It is now conceivable ``to weigh'' single macromolecules of viruses and proteins, simply by accreting them one-by-one onto a NEMS device\footnote{ K. L. Ekinci, Y. T. Yang, and M. L. Roukes, ``Ultimate limits to inertial mass sensing based upon nanoelectromechanical systems,'' J. Appl. Phys. \textbf{95}, 2682 (2004).}. When achieved, the ability to weigh single molecules may provide a tranformationally different core sensing mechanism and a new niche platform for mass spectrometry. The experimental approach underway at California Institute of Technology to achieve this measurement milestone will be discussed.   

On recent developments for high-speed AFM imaging

This contribution discusses some key challenges for the next generation of high-speed atomic force microscopes (AFM). For high-speed imaging all AFM components have to be optimized in performance, i.e. the scanning unit, the force sensor and the AFM electronics. i) The three dimensional positioner (scanner) needs not only sub-nanometer resolution and a high bandwidth but also must not show any oscillatory behavior in order to achieve the required position accuracy. To this end a new mechanical design that uses stack piezos for the spatial movement is combined with new control schemes [1] that are designed for highest positioning bandwidth combined with high control performance. ii) The force sensor has to be soft and fast in order to minimize the imaging forces and force variations together with a higher sensor bandwidth. This is achieved by using small cantilevers [2]. iii) For high-speed imaging also the feedback and piezo drive electronics as well as the data acquisition system have to fulfill high bandwidth and timing requirements. Combining all these improvements, the next generation of AFMs will enabling imaging speeds two orders of magnitudes faster than current commercial AFM systems. [1] G. Schitter, F. Allgoewer, A. Stemmer, Nanotechnology 15(1), p.108 (2004) [2] T.E Schaeffer, M. Viani, D.A. Walters, B. Drake, E.K. Runge, J.P. Cleveland, M.A. Wendman, P.K. Hansma, SPIE 3009, p.48 (1997)   

Measuring AFM Cantilever Spring Constants

Josephson-junctions are prime candidates for the realization of quantum bits. Understanding the properties of Josephson-junction materials is crucial to building a functional qubit. Conducting atomic force microscopy (CAFM), which can simultaneously measure local topography and conductance, is a promising tool for these purposes. Previous results suggest that control of the imaging force in CAFM is vital to achieve reproducible conductance images. In this talk, we discuss how control of the imaging force can be achieved by measuring the CAFM cantilever spring constant, and how this results in reproducible conductance images. The theoretical background and the experimental technique for measuring the spring constant by two distinct methods will be discussed. We present spring constant measurements from these two methods for comparison. Finally, we present our user-friendly program which measures the spring constant in situ on a Digital Instruments atomic force microscope.   

Force Microscopy with Light Atom Probes

The charge distribution in atoms with closed electron shells is spherically symmetric, while atoms with partially filled shells can form covalent bonds with pointed lobes of increased charge density. Covalent bonding in the bulk can also affect surface atoms, leading to four tiny humps spaced by less than 100 pm in the charge density of adatoms on a (001) tungsten surface. We image these charge distributions via atomic force microscopy by using a light-atom probe (a graphite atom) to directly measure high-order force derivatives of its interaction with a tungsten tip. Features with a lateral distance of only 77 pm are revealed (Science 305, 380, 2004).   

Coupled ion - nanomechanical systems

The nanomechanical modes can be manipulated and probed via their coupling with effective quantum two level systems. Here we study a coupled ion - nanomechanical system where the ion is in a nanotrap with the electrodes being nanomechanical resonators. The motion of the ion and that of the nanomechanical modes can be described as coupled harmonical oscillators. The ions play the role of a quantum optical system that acts as a probe and control, and allows entanglement with or between nanomechanical resonators. We show as examples the laser cooling and the entanglement generation between the resonators [1] L. Tian and P. Zoller, quant-ph/0407020   

Nanomagnetic Planar Magnetic Resonance Microscopy ``Lens''

The achievement of three-dimensional atomic resolution magnetic resonance microscopy remains one of the main challenges in visualization of biological molecules. The prospects for single spin microscopy have come tantalizingly close due to the recent developments in sensitive instrumentation. Despite the single spin detection capability in systems of spatially well-isolated spins, the challenge that remains is the creation of conditions in space where only a single spin is resonant and detected in the presence of other spins in its natural dense spin environment. We present a nanomagnetic planar design where a localized Angstrom-scale point in three-dimensional space is created above the nanostructure with a non-zero minimum of the magnetic field magnitude. The design thereby represents a magnetic resonance microscopy ``lens'' where potentially only a single spin located in the ``focus'' spot of structure is resonant. Despite the presence of other spins in the Angstrom-scale vicinity of the resonant spin, high gradient magnetic field of the ``lens'' renders those spins inactive in the detection process.