AVS2015 Session SP+AS+MI+NS+SS-FrM: Probe-Sample Interactions
Friday, October 23, 2015 8:20 AM in 212A
SP+AS+MI+NS+SS-FrM-1 Direct Visualization of Magnetoelectric Domains in Hexagonal Manganites
Weida Wu (Rutgers University)
Multiferroics are materials with coexisting magnetic and ferroelectric orders, where the cross‐coupling between two ferroic orders can result in strong magnetoelectriceffects [1‐4]. Therefore, it is of both fundamental and technological interest to visualize cross‐coupled magnetoelectric domains and domain walls in multiferroics. Recently, intriguing topological defects with six interlocked structural antiphase and ferroelectric domains merging into a vortex core were revealed in multiferroic hexagonal REMnO3 (R=rare earths) [5, 6]. Many emergent phenomena, such as enhanced conduction and unusual piezoelectric response, were observed in charged ferroelectric domain walls protected by these topological defects [7‐9]. More interestingly, alternating uncompensated magnetic moments were discovered at coupled structural antiphase and ferroelectric domain walls in hexagonal manganites using cryogenic magnetic force microscopy (MFM) , which demonstrates the cross‐coupling between ferroelectric and magnetic orders. Here we present the application of a magnetoelectric force microscopy (MeFM) technique that combines MFM with in situ modulating high electric fields. This new microscopy technique allows us to image the magnetoelectric response of the domain patterns in hexagonal manganites directly [11, 12]. We find that this response changes sign at each structural domain wall, a result that is corroborated by symmetry analysis and phenomenological modelling , and provides compelling evidence for a lattice-mediated magnetoelectric coupling. The direct visualization of magnetoelectric domains at mesoscopic scales opens up explorations of emergent phenomena in multifunctional materials with multiple coupled orders.
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SP+AS+MI+NS+SS-FrM-3 Kelvin Probe Force Microscopy Studies of Magnetic Atoms on Ultrathin Insulating MgO Film
Taeyoung Choi, William Paul, Susanne Baumann, Christopher Lutz, Andreas Heinrich (IBM Almaden Research Center)
The interplay of single atoms and their local environment on surfaces influences the atoms’ spin excitations and dynamics, which can be utilized in progress toward atomic-scale memory and quantum information processing. We find that spin-excitation energy of Fe atoms on an insulating MgO film shifts depending on the tip-to-atom separation. This may be attributed to the electric field across the tunneling junction, as well as to local charge and structural changes around the atom. The Kelvin Probe Force Microscopy (KPFM) has been very useful tool to measure changes of local contact potential differences between a tip and a sample at the atomic level . In this talk, we employ tuning fork KPFM/STM and show preliminary results on the charge character and spin excitations of Fe atoms.
This work is supported by grants from IBM.
 Leo Gross et al., Phys. Rev. B 90, 155455 (2014).
SP+AS+MI+NS+SS-FrM-4 Nanoscale Schottky Barrier Height Mapping Utilizing Ballistic Electron Emission Microscopy
Christopher Durcan, Westly Nolting (College of Nanoscale Science and Engineering); Vincent LaBella (SUNY Polytechnic Institute)
The Schottky barrier is the electrostatic barrier between a metal and a semiconductor that results in rectification and is found in many types of devices such as source drain contacts to sub 20-nm-node transistors. Naturally, the Schottky barrier height can fluctuate across the interface due to variations in bonding, compositional fluctuations in the materials, and the presence of defects. However measuring and mapping these electrostatic fluctuations is impossible with bulk IV or CV techniques. This presentation will demonstrate how the Schottky barrier height can be mapped to nanoscale dimensions using an STM based technique called ballistic electron emission microscopy (BEEM). The STM tip is positioned on a regularly spaced grid and BEEM spectra are acquired from which the barrier height can be extracted. A map and histogram is then generated by measuring and fitting thousands of these spectra. These maps provide detailed insight into the electrostatic fluctuations occurring at the buried interface with nanoscale resolution that cannot be accomplished with other bulk measurements.
SP+AS+MI+NS+SS-FrM-5 Electron Transport Studies of Metal Films Utilizing Ballistic Electron Emission Microscopy
Christopher Durcan (SUNY College of Nanoscale Science and Engineering); Vincent LaBella (SUNY Polytechnic Institute)
Understanding scattering of electrons in nanometer thick metal films is of fundamental and technological importance. One method to study electron scattering is with ballistic electron emission microscopy (BEEM), which is a three terminal STM based technique that measures both scattering through a metal film and the Schottky barrier height for metal-semiconductor junctions with both nanometer spatial resolution and meV energy resolution. This presentation will describe our work at understanding the relationship between the metal resistivity and the electron scattering lengths measured with BEEM by exploring metals with a range of resistivities from Ag (1.7 µΩ-cm) to Cr (12.6 µΩ-cm). In addition, nanoscale mapping of the Schottky barrier height of these metals to silicon will also be presented to understand the spatial uniformity of the transport.
SP+AS+MI+NS+SS-FrM-6 Utilizing Ballistic Electron Emission Microscopy to Study Sidewall Scattering of Electrons
Westly Nolting, Christopher Durcan, Robert Balsano (College of Nanoscale Science and Engineering, University of Albany); Vincent LaBella (College of Nanoscale Science and Engineering, SUNY Polytechnic Institute)
Sidewall scattering of electrons within aggressively scaled metallic interconnects increases the resistance since the mean free path (~40 nm) is larger than the dimensions of the material. One method to study hot-electron scattering in nm-thick metallic films is Ballistic Electron Emission Microscopy (BEEM), which is an STM based technique. In this work, we perform BEEM scattering measurements on lithographically patterned fin structures with a Schottky diode interface to determine its ability to measure sidewall scattering. This is accomplished by acquiring BEEM spectra on a regularly spaced grid and fitting the results to determine both the Schottky barrier height and the slope of the spectra. The slope of the spectra is related to the scattering in the film and interface. The position of fin structures are then determined by mapping both the Schottky height and slope over a square micron to observe scattering at the interface caused by the patterned structures. The poster will discuss the fabrication of the patterned 50-nm-pitched sidewall structures that are used for mapping the sidewall scattering. In addition, it will present the preliminary BEEM measurements on these structures.
SP+AS+MI+NS+SS-FrM-7 Progress in Nanoscale Magnetic Resonance Imaging
Daniel Rugar (IBM Research Division)
Nuclear magnetic resonance (NMR) is the basis of powerful spectroscopic and imaging techniques, but extension to nanoscale samples has been a longstanding challenge due to the insensitivity of conventional detection methods. We are exploring the use of individual, near-surface nitrogen-vacancy (NV) centers in diamond as atomic-size magnetometers to detect proton NMR in organic material located external to the diamond. Using a combination of electron spin echoes and proton spin manipulation, the NV center senses the nanotesla field fluctuations from the protons, enabling both time-domain and spectroscopic NMR measurements on the nanometer scale. By scanning a small polymer test object past a near-surface NV center, we have recently demonstrated proton magnetic resonance imaging (MRI) with spatial resolution on the order of 10 nm.
One key issue in NV-NMR experiments is the loss of spin coherence when the NV center is located near the diamond surface. Although this loss of coherence is frequently attributed to the effect of magnetic noise emanating from unpaired spins on the diamond surface, we will show evidence that electric field noise from fluctuating surface charge may be the dominant factor.
Work performed in collaboration with M. Kim, H. J. Mamin, M. H. Sherwood, C. T. Rettner, K. Ohno, and D. D. Awschalom
SP+AS+MI+NS+SS-FrM-9 Reactive Intermediates Created and Analyzed by Scanning Probe Microscopy
Bruno Schuler (IBM Research - Zurich, Switzerland); Niko Pavliček (IBM Research - Zurich); Sara Collazos (CIQUS, Universidade de Santiago de Compostela); Nikolaj Moll, Shadi Fatayer (IBM Research - Zurich); Dolores Pérez, Enrique Guitán (CIQUS, Universidade de Santiago de Compostela); Gerhard Meyer (IBM Research - Zurich); Diego Peña (CIQUS, Universidade de Santiago de Compostela); Leo Gross (IBM Research - Zurich)
Reactive intermediates are involved in most chemical transformations. However, their characterization is a great challenge because of their short lifetime and high reactivity.
Here we report on the creation of single radicals and diradicals on a thin insulating surface by means of atomic manipulation. Importantly, the thin insulating film facilitates the stabilization of these reactive intermediates at cryogenic temperatures. The molecules were characterized by atomic-resolution atomic force microscopy (AFM) imaging with a CO functionalized tip  and scanning tunneling microscopy (STM) orbital imaging . We show that the molecules’ reactivity is preserved even at low temperatures by performing different on-surface reactions by atomic manipulation. As an example, the generation of aryne is discussed, a very reactive intermediate caught for the first time .
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 N. Pavliček et al. On-surface generation and imaging of arynes by atomic force microscopy. (submitted)
SP+AS+MI+NS+SS-FrM-10 The Negative Stiffness and Positive Damping of Squeezed Air in Dynamic Atomic Force Microscopy
Xiaokong Yu, Mingjiang Tao, Nancy Burnham (Worcester Polytechnic Institute)
By oscillating a micro-sized cantilever beam at a certain frequency and observing its interaction with the sample surface, dynamic mode atomic force microscopy (AFM) has gained attention for characterizing mechanical properties of a variety of materials at the micro and nano scales. The thin air film, confined between the oscillating cantilever beam and the stationary sample surface, causes the so-called “squeeze-film effect” when the gap between the two boundaries is less than a hundred microns. Although studies have shown that the squeeze film can act as a spring and a damper in accelerometers and microelecromechanical systems , the influence of the squeeze-film effect on the dynamics of an AFM cantilever has not been previously explored, to the authors’ knowledge. In this project, the stiffness and damping properties of the squeeze film between an oscillating AFM cantilever and a glass slide were calculated from the cantilevers’ amplitude and phase responses as recorded by the AFM digital system. The smaller the cantilever-sample gap, the larger the absolute values of the stiffness and the damping of the squeeze film. Results from different cantilevers (consequently having different spring constants and resonant frequencies) indicated that the air film exhibited negative stiffness and positive damping, with normalized changes from free values of up to 40%. Theoretical analysis was conducted using an equivalent-circuit model  along with the phasor diagram, and the derived stiffness and damping values were in excellent agreement with the experimental ones. Interestingly, a rotation angle between 20o and 30o in the fit of the data to the model reveals a phase lead of the squeeze-film damping before the usual air damping when the cantilever is far from a surface: the maximum squeeze-film damping occurs before the maximum velocity of the cantilever because air becomes less dense as it rushes out of the tip-sample gap. The surprising sign of the stiffness is thus explained by the phase lead. Future work includes incorporating the squeeze-film effect into more accurate measurements of a material’s stiffness and damping properties using dynamic AFM.
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