Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H21: Focus Session: Advances in Scanned Probe Microscopy II -- High Frequencies and Optical Techniques |
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Sponsoring Units: GIMS Chair: Sebastian Loth, IBM Research Room: D161 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H21.00001: Rapid Serial Prototyping and Analysis of Nanomagnet-Tipped Attonewton-Sensitivity Cantilevers for Magnetic Resonance Force Microscopy John Marohn, Eric Moore, Jonilyn Longenecker Magnetic resonance force microscopy offers exciting possibilities for imaging protons and electrons in native and spin-labeled biomolecules. The central component of a magnetic resonance force microscope experiment is a custom-fabricated attonewton-sensitivity cantilever with an overhanging magnetic-nanorod tip. We have recently developed a method for making precision tips which involves 1) fabricating overhanging magnetic tips on shortened mock cantilevers, 2) using focused ion beam milling and deposition (FIB/FID) to cut the mock cantilever (and attached tip) free from the substrate, and then 3) attaching the released structure to a full-length high-sensitivity cantilever. The resulting magnets have been characterized by cantilever magnetometry, high-resolution transmission electron microscopy (HR-TEM), and nanometer-resolution electron energy loss spectroscopy (EELS). This approach to fabrication and analysis is allowing us to optimize tips for proposed single-electron-spin imaging experiments in a very short time. Rapid access to such high-quality tips will significantly advance our ability to image individual biomolecules and macromolecular complexes. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H21.00002: Development of Magnet-on-Oscillator Low Temperature NMR Force Microscopy J.W. Paster, J.T. Markert We report recent advances for our nuclear magnetic resonance force microscopy (NMRFM) experiment. Force detection of nuclear spins is made possible by coupling NMR spin flip sequences to a mechanical oscillator. Periodic inversion of the spins in a magnetic field gradient provides the ac coupling force. The force sensitivity for NMRFM improves with decreasing distance between a small gradient magnet and the spins in a sample. Adapting a perpendicular oscillator orientation allows us to decrease the magnet-to-sample distance, providing increased sensitivity. We've also adapted a magnet-on-oscillator design. With this approach, we can perform experiments using oscillating cantilever-driven adiabatic reversal, a technique which has been used to detect single electron spins below the surface of a solid [1]. We've integrated an optical fiber interferometer to measure an oscillator's motion with sub nanometer precision. We can routinely measure the resonance frequencies, quality factors, and spring constants of various oscillators.\\[4pt] [1] Rugar D et al. Nature. 2004;430:329--332 [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H21.00003: Correlation of Noise in Multiple Cantilever Modes Doran Smith, Dimitri Alexson The ultrasensitive cantilevers utilized in frequency detection MRFM schemes are susceptible to noncontact interactions between the cantilever tip and the sample. Large efforts have been undertaken to comprehensibly understand the surface related dissipation mechanisms. We propose that sample dielectric fluctuations near DC frequencies should effect all the driven cantilever's oscillatory modes similarly. Utilizing the fundamental and second harmonic mode of the cantilever we have demonstrated this. The correlation between the frequency deviations of the two modes of a driven cantilever increases as the tip-to-sample distance decreases and the measured 1/f noise due to dielectric fluctuations becomes manifest. This result provides additional support for the theory developed by Yazdanian, Marohn and Loring, J. Chem. Phy. 128, 224706 (2008) explaining the origins of noncontact friction and frequency jitter. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 9:12AM |
H21.00004: Nanoscale scanning probe ferromagnetic resonance imaging using localized modes Invited Speaker: We report the demonstration of scanned probe ferromagnetic resonance imaging (FMRI), a new technique based on Magnetic Resonance Force Microscopy that offers a window into nanoscale properties of buried ferromagnets. Images have been obtained with a current resolution of 200 nm, and significant improvements are straightforwardly possible. Ferromagnetic Resonance (FMR) is a powerful spectroscopic tool for studying internal magnetic fields, interactions and dynamic magnetic properties of ferromagnetic systems, but conventional FMR measures global properties of an entire sample. In FMRI the ``magnetic field well'' created by the probe tip field confines the spin wave modes; these can then be scanned to obtain FMR images. This new microscope is unique in its ability to map internal magnetic fields in buried ferromagnets with spectroscopic precision and nanoscale resolution. First images in permalloy films reveal the ability to image inhomogeneities in magnetic properties with field resolution of approximately 1 Gauss/$\sqrt{\rm {\mathbf Hz}}$. We report a first application to imaging the internal exchange bias field in exchange-biased films. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H21.00005: MRFM based spectroscopy of GaAs Dimitri Alexson, Doran Smith The apparent contradiction of how to perform NMR spectroscopy given the large magnetic field gradients present in MRFM is resolved by removing the magnetic field gradient while RF based NMR spectroscopic pulses are applied to the sample. This is accomplished by 1) shuttling (move) the sample away from the magnetic particle mounted on the cantilever, 2) apply RF spectroscopic pulse sequences to the sample, 3) store a component of the free induction decay along the z-axis, 4) shuttle the sample back to the cantilever, and 5) read out the magnetization stored on the z-axis with MRFM using an adiabatic rapid passage protocol (ARP). We will describe our progress on performing shuttle based spectroscopy of GaAs using MRFM. We will describe our measurements of T1 of Ga69 in GaAs with an inversion-recovery experiment. Using a single ARP sweep, the polarization is inverted and its recovery is monitored with a driven cantilever using the CERMIT protocol. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H21.00006: Development of Variable Temperature NMR Force Microscopy Isaac V. Manzanera Esteve, John T. Markert We report our progress on the construction of a variable temperature NMR force microscopy probe and the development of its control system for three dimensional nanoscale scanning. The probe contains two 3-axis piezo-driven slip-stick motion stages for fiber interferometer and for gradient magnet positioning. The control station is a LabView software based control system capable to perform signal generation and data acquisition. Preliminary scan position dependent NMR Force measurements on ammonium sulfate (NH$_{4})_{2}$SO$_{4}$ were performed at room temperature in a sample-on-oscillator configuration. Both piezo-driven and thermal noise cantilever motion have been analyzed to determine resonant frequencies $\omega _{c}$, quality factor Q, and spring constants k; a typical cantilever yielded $\omega _c =1494.40\pm 0.10$Hz, $k=0.039\pm 0.004N/m$, $Q=93.$ RF frequency-modulation-driven artifact effects have been observed and measured during analysis. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H21.00007: Nitrogen Impurities in Diamond Studied using Magnetic Resonance Force Microscopy Michael Herman, Palash Banerjee, Denis Pelekhov, P. Chris Hammel Spin-bearing defects and impurities in diamond have attracted much attention in recent years, with the N-V center defect being a good example. A related defect in the diamond lattice is comprised of a substitutional nitrogen alone and is known as the P1 center with an electron spin S = 1/2 localized on a N-C bond with a strong hyperfine coupling to the $^{14}$N nuclear spin I = 1. We have used Magnetic Resonance Force Microscopy (MRFM) to study the properties of a small collection of P1 centers in diamond. By operating with large field gradients approaching a few Gauss per nanometer, we are able to couple fewer than 100 spins and probe their relaxation properties with a sensitivity approaching a few spins. We have seen that spin lifetimes in the rotating frame are dependent on impurity concentration. We'll show long spin lifetimes ($>$2 s) while undergoing tens of thousands adiabatic spin flips. We also show that spin lifetimes are shorter in diamond implanted with nitrogen ions to create P1 centers. This work was supported by The Army Research Office under W911NF-07-1-0305 and the National Science Foundation under DMR-0807093. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H21.00008: Parametric Amplification Protocol for Frequency-Modulated Magnetic Resonance Force Microscopy Signals Lee Harrell, Eric Moore, SangGap Lee, Steven Hickman, John Marohn We present data and theoretical signal and noise calculations for a protocol using parametric amplification to evade the inherent tradeoff between signal and detector frequency noise in force-gradient magnetic resonance force microscopy signals, which are manifested as a modulated frequency shift of a high- $Q$ microcantilever. Substrate-induced frequency noise has a $1/f$ frequency dependence, while detector noise exhibits an $f^2 $ dependence on modulation frequency $f$. Modulation of sample spins at a frequency that minimizes these two contributions typically results in a surface frequency noise power an order of magnitude or more above the thermal limit and may prove incompatible with sample spin relaxation times as well. We show that the frequency modulated force-gradient signal can be used to excite the fundamental resonant mode of the cantilever, resulting in an audio frequency amplitude signal that is readily detected with a low-noise fiber optic interferometer. This technique allows us to modulate the force-gradient signal at a sufficiently high frequency so that substrate-induced frequency noise is evaded without subjecting the signal to the normal $f^2 $ detector noise of conventional demodulation. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H21.00009: Nonlinear Near-Field Microwave Microscopy for RF Defect Localization in Nb-Based Superconducting Radio Frequency Cavities Tamin Tai Niobium Superconducting Radio Frequency (SRF) cavities are very sensitive to localized defects that give rise to quenches at high accelerating gradients. In order to identify these defects via scanning microscopy, and to further understand the origins of the quench under high radio frequency excitation (1-3 GHz), a scanning probe with localized and up to $\sim $200 mT RF magnetic field is required for low temperature microscopy to achieve sub-micron resolution. For this purpose, we developed a micro loop probe on silicon substrate with outer diameter 20 $\mu $m and inner diameter 17 $\mu $m and successfully fabricated it by lithography. The probe has been used to identify a signal arising from the nonlinear Meissner effect in a Nb thin film. In addition, a magnetic write head is another promising candidate to achieve this goal of understanding localized defect behavior under high RF magnetic field at low temperatures [1]. We will discuss and compare both types of probe for nonlinear scanning microscopy and RF defect localization in superconductors. \\[4pt] [1] Tamin Tai, X. X. Xi, C. G. Zhuang, Dragos I. Mircea and Steven M. Anlage, ``Nonlinear Near-Field Microwave Microscope For RF Defect Localization in Superconductors'' (http://arxiv.org/abs/1008.2948) [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H21.00010: Super-rolloff electron tunneling transduction of nanomechanical motion using frequency downmixing Meng Kan, Mark Freeman, Wayne Hiebert Electron tunneling transduction has high sensitivity for detecting the motion of nanomechanical devices, but the relatively low detection bandwidth of a few 10's of kHz has limited its development. Here we demonstrate a novel downmixing transduction scheme which eliminates the detection bandwidth problem of electron tunneling transduction. With this technique, the high frequency vibration modes ($\sim $ 1 MHz) of a MEMS doubly clamped beam are measured. This measurement is 2 orders of magnitude above the electronic bandwidth of our readout circuitry with no fundamental limitation anticipated up to microwave frequencies. The displacement sensitivity is 40 fm/Hz$^{1/2}$ comparable to state-of-the-art low finesse free-space optical interferometry. The back-action force induced by the STM tip on the MEMS device is also explored and is shown to have a small effect on the measurement resonance frequency, causing slight resonance frequency shifts of order 1{\%}. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H21.00011: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H21.00012: Toward Atomic-Scale Optical Probes with UHV STM \"Ozg\"un S\"uzer, Li Gao, Joseph A. Smerdon, Jongweon Cho, Nathan P. Guisinger, Jeffrey R. Guest We present the details of a variable temperature ultra-high vacuum (UHV) scanning tunneling microscopy (STM) apparatus with optical access for the investigation of optically active materials at the atomic scale. Local field enhancement in close proximity to the ultra-sharp STM tip enables the observation of optical signals from a very small number of surface adsorbates and even single molecules, which, combined with the electronic sensitivity and high spatial resolution of STM, allows the simultaneous optical, electronic, and topographic analysis of nanoscale systems. A high-numerical-aperture (NA) optic is integrated into the STM to achieve sharp and stable focusing of the laser excitation while maintaining polarization integrity and high collection efficiency. The initial findings of investigations carried out on epitaxial graphene grown on SiC and operational characteristics of the apparatus are discussed. A next-generation optically accessible 4K UHV STM apparatus under development is also introduced. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H21.00013: Protecting TERS Probes While Keeping Extreme Enhancement Rebecca Agapov, Andrey Malkovskiy, Alexei Sokolov, Mark Foster Protecting the probes used for tip enhanced Raman spectroscopy (TERS) with alumina coatings reduces chemical, mechanical and thermal degradation that otherwise limit the applicability of this emerging technique. Protected plasmonic structures are of interest for surface enhanced Raman spectroscopy (SERS) generally, especially for SERS-based sensors, but we focus particularly on the special case of TERS. Most recently we have focused on the detailed effect of the protective coating for cases in which the enhancement is particularly strong. ``Blinking,'' which is characteristic of extreme enhancement, has been observed with TERS on polymer films for the first time. An Al$_{2}$O$_{3}$ coating prolongs the duration of blinking from 20 minutes to 2 days without significant detriment to the extreme enhancement. The fact that blinking has been observed in the presence of the alumina coating allows us to eliminate chemical enhancement as a major mechanism of the extreme enhancement evidenced by blinking. [Preview Abstract] |
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