Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D16: Magnetic Imaging and Characterization |
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Sponsoring Units: GMAG Chair: Mi-Young Im, Lawrence Berkeley National Laboratory Room: D173 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D16.00001: Single Shot Nanoscale Magnetic Imaging at the Linac Coherent Light Source Benny Wu, Tianhan Wang, Catherine Graves, Diling Zhu, William Schlotter, Joshua Turner, Joachim Stohr, Andreas Scherz One of the major challenges of modern magnetism research is the manipulation and control of the magnetization on ultrafast timescales. Using the unprecedented brightness of the Linac Coherent Light Source (LCLS), we have been able to image the nanoscale magnetic worm domain structures in [Co/Pd] multilayer systems with a single x-ray pulse through x-ray Fourier transform holography on the Co L3 absorption edge. We established the threshold fluences for both non-destructive imaging and sample damage. In combination with the femtosecond pulses of LCLS, single shot coherent imaging will enable the observation of nanoscale magnetization dynamics on the sub-picosecond timescale for problems such as ultrafast demagnetization and all-optical magnetization reversal. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D16.00002: Magnetic soft X-ray microscopy at 10nm spatial resolution Peter Fischer, Weilun Chao, Mi-Young Im, Erik Anderson Magnetic soft X-ray microscopy, which combines high spatial and temporal resolution with elemental specificity by utilizing the specific features of X-ray magnetic circular dichroism effects is a unique and powerful analytical technique to image fast spin dynamics of nanoscale magnetism [1]. The spatial resolution is determined by Fresnel zone plate lenses used as diffractive optics. FZPs are fabricated by state-of-the-art lithography techniques and the challenge is to produce a dense, circular line pattern with a high aspect ratio to achieve high efficiency. Using an overlay technique [2-3], which requires high position accuracy of the e-beam writer, FZPs with 12nm outermost zone width could be fabricated. Implementing this optic at BL 6.1.2 at the ALS in Berkeley CA, we have demonstrated that a 10nm line and space test pattern can be clearly resolved. First magnetic images of a PtCo film with a pronounced perpendicular anisotropy will be presented. Further progress to below 10nm can be anticipated in the near future. \\[4pt] [1] P. Fischer, IEEE Transactions on Magnetics, 44(7) 1900 (2008) \\[0pt] [2] W. Chao, et al. Nature 435, 1210 (2005) \\[0pt] [3] W. Chao, et al., Optics Express 17(20) 17669 (2009) [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D16.00003: New Developments in Magnetic Coherent Diffractive Imaging Ashish Tripathi, SangSoo Kim, Sebastian Dietze, Erik Shipton, Eric Fullerton, Oleg Shpyrko, Ian McNulty Magnetism at the nanoscale is central to understanding emergent complexity in transition metal oxides and engineered rare earth-transition metal multilayers, and in designing new magnetic data storage and spintronic technology. We study magnetism at the nanoscale here using coherent x-ray diffractive imaging (CXDI), which is a technique with potentially wavelength-limited spatial resolution that can probe deeply beyond surfaces, and potentially on ultrafast timescales using new x-ray laser sources. We look at the domain evolution vs. applied magnetic field over the whole hysteresis loop of a ferrimagnetic GdFe multilayer film using x-rays resonant at the Gd M5 edge for domain contrast. We explore complimentary and return point memory by imaging the multilayer over a large field of view. We lastly explore experimental and algorithmic improvements in CXDI using dichroism as contrast mechanism, as well as new opportunities for ultra-fast, single-shot imaging using a variation on the CXDI approach. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D16.00004: Design of a Self-Aligned, 300mK-300K Temperature range Magnetic Force Microscope(MFM) with $<$10nm Resolution Ozgur Karci, Ivan Knez, Hilal Atalan, Rui-Rui Du, Ahmet Oral We present the design of a self-aligned MFM, operating from 300mK to 300 K. Unique `Self-Aligned' design uses cantilever alignment chips and eliminates the alignment procedure and sustains the alignment across the full temperature range. The MFM is very compact, 23.6mm OD, and is adopted to fit into Oxford Instruments Heliox TL system. A fiber interferometer with $\sim $12fm/$\surd $Hz noise level is designed and used to detect cantilever deflection. Stick slip coarse approach mechanism is used to bring the sample in to close proximity of the sample. We can also move the sample in XY directions within 3 mm range, while we measure the position with capacitive encoder with 3$\mu $m accuracy. We can also operate the LT-MFM in high magnetic fields. The microscope has been successfully operated between 300mK-300K and we can achieve $<$10nm resolution. MFM images of 394 Gbpsi Harddisk at 1.5-300K and CoPt Multilayers at 350mK will be presented. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D16.00005: Monolithic diamond probes for nanoscale magnetic imaging using single spins in diamond Patrick Maletinsky, Sungkun Hong, Michael Grinolds, Birgit Hausmann, Ron Walsworth, Mikhail Lukin, Marko Loncar, Amir Yacoby Sensitive detection of magnetic fields at the nanoscale is a challenging problem in biological and physical sciences with great relevance to technological applications. Recent experimental demonstrations have shown the outstanding performance of diamond nitrogen-vacancy (NV) centers in magnetic field sensing [1, 2]. Here, we present a robust experimental realization of a scanning NV-magnetometer that exploits the full coherence properties of the NV-center for magnetic imaging. Our apparatus consists of a combined atomic force (AFM) and optical microscope, where the AFM tip is formed by a high purity diamond nanopillar containing a single NV center at its end. This geometry ensures high spatial resolution, long NV coherence times and waveguiding of NV fluorescence through the pillar, which combine to give maximal magnetic field sensitivity. We demonstrate the performance of our nanoscale magnetometer by imaging various magnetic field sources, including few tens of nm wide domains of a magnetic memory.\\[4pt] [1] Nature 455, 648 (2008)\\[0pt] [2] Nature 455, 644 (2008) [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D16.00006: A stroboscopic approach to combining diamond magnetometry with Atomic Force Microscopy Sungkun Hong, Michael Grinolds, Patrick Maletinsky, Mikhail Lukin, Ronald Walsworth, Amir Yacoby Nitrogen-Vacancy (NV) defect centers in diamond have been recently considered as a promising candidate for sensitive magnetic field detection[1, 2] with nanometric spatial resolution. Most applications requiring high spatial resolution necessitate stabilizing the NV center in close proximity to the sample of interest, which can be accomplished using standard Atomic Force Microscopy (AFM) techniques[2]. However, the fluctuations associated with AFM tip oscillation set limits to both spatial resolution and magnetic field sensitivity[2]. Here we demonstrate a stroboscopic approach that locks the magnetic signal acquisition to a particular position of the tip. Our approach allows us to reach the sensitivity and spatial resolution given by the intrinsic properties of NV centers. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D16.00007: High sensitivity SQUID susceptibility measurements B. Kalisky, J.R. Kirtley, L.C. Qian, B.L. Dwyer, K.A. Moler, J. Ngai, Y. Segal, J. Reiner, F. Walker, C. Ahn, A.M. Hamilton, B. Rutt, A.C. Matin, O.M. Auslaender, D.A. Bonn, R. Liang, W.N. Hardy, J.G. Analytis, J.-H. Chu, I.R. Fisher Scanning superconducting quantum interference device (SQUID) sensors have high sensitivity to magnetic flux ($ 10^{-6}\Phi_{0}/\sqrt{Hz} $) and magnetic moment ($\sim$ 100 electron spins) under reasonable scanning conditions. In addition, a single turn field coil co-centered with the SQUID sensing loop provides excitation for simultaneous measurement of low field susceptibility, with sensitivity of $\chi \sim 10^{-6}$ at a spatial resolution of a few microns. I will present our recent measurements on several systems which exhibit weak susceptometry signals: thin film paramagnetic LaNiO3 that are (hopefully) the precursors to engineered superconducting films; individual magnetotactic bacteria, which are used as MRI contrast agents; and twinned high critical temperature cuprate and pnictide superconducting samples that may experience variations in the superfluid density at the twin boundary. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D16.00008: Magnetic Characterization of Individual Magnetotactic Bacteria Lisa Qian, Beena Kalisky, Amanda Hamilton, Bo Dwyer, A.C. Matin, Kathryn Moler Magnetic nanoparticles 5-50nm in size are of wide interest in the biological and medical fields. In particular, magnetotatic bacteria containing chains of nanoscale magnetite particles show potential for MRI contrast agents and targeting tumors. Magnetic characterization is typically done in large ensembles, where variations in shape and structure cannot be determined and interparticle coupling may cause bulk properties from those of isolated particles. We report the detection and magnetic characterization of individual magnetotatic bacteria using a variable temperature scanning SQUID microscope (SSM). SSM is ideal for this challenge due to its high spin sensitivity, $\sim $100 mu{\_}B/sqrt(Hz). AC and DC modes of operation allow for direct probing of susceptibility and magnetic moment. We will also discuss calculation techniques used to obtain values for the magnetic moment, anisotropy energy and magnetosome chain length of individual bacteria. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D16.00009: Magneto-resistance based First Order Reversal Curve (MR-FORC) analysis of MgO based MTJs Joshua Pomeroy, John Read MR-FORC utilizes partial magneto-resistance hysteresis loops to reveal the coercive and interaction field distributions in the free layer of MgO based magnetic tunnel junctions. The interpretation of the FORC diagrams will be discussed with emphasis on the identification of coercive values, the interlayer exchange properties, the magnetic after-effect and a reversible magnetic instability under study. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D16.00010: Stress and Depth Dependence of Stochastic Processes in the Barkhausen Effect David Jiles, Lukasz Mierczak, Eugene Melikhov Magnetic Barkhausen Noise (MBN) consists of discontinuous stochastic changes in flux density caused by sudden irreversible changes in magnetization as the magnetic field H changes continuously. These changes can be detected at the surface by a magnetometer, which in its simplest form can be in the form of voltage pulses caused in a pickup coil. The amplitude of such pulses has been shown to depend on the microstructure and stress in the material. Propagation of Magnetic Barkhausen emissions in magnetic materials is frequency dependent and therefore information from different depths inside the material is contained in the frequency spectrum of the detected Barkhausen signal at the surface. However the depth dependent information, although present, is difficult to extract from the measurements. Despite this extracting the depth dependent information about material conditions, such as variations in microstructure and/or the presence of residual stress is of a great interest. This work presents a new method for extracting this information from measured MBN signals. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D16.00011: Cluster-deposited high-anisotropy magnetic nanoparticles Balamurugan Balasubramanian, Ralph Skomski, Xingzhong Li, David Sellmyer Magnetic nanoparticles of size less than 10 nm with high magnetocrystalline anisotropy are highly desirable to understand the nanoscale effects on their magnetic properties and create building blocks for modern applications such as ultra-high-density recording media and high-performance permanent magnets. In the present study, monodisperse Co-based nanoparticles with an average particle size of 3 -10 nm, such as YCo$_{5}$ and Co$_{1-x}$Pt$_{x}$ (x $<$ 0.2), were produced using an inert-gas-condensation cluster-deposition system and characterized using XRD, TEM, EDX and SQUID magnetometer. These nanoparticles were directly ordered into high-anisotropy crystal structures during the cluster-aggregation process and exhibit high anisotropic constant of order 10$^{7}$ ergs/cm$^{3}$. Size-effects on the structural and magnetic properties of YCo$_{5}$ and Co$_{x}$Pt$_{1-x}$ nanoparticles will be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D16.00012: Improved evaluation of magnetic nanoparticle susceptibility in hyperthermia, spectroscopy, and imaging Yong Wu, Zhen Yao, Timothy Atherton, Lisa Bauer, Mark Griswold, Robert Brown Magnetic nanoparticles are becoming increasingly important for both diagnosis (through applications such as MRI and magnetic particle imaging (MPI), which comes from the nonlinear magnetization of nanoparticles and provides images with both high spatial and temporal resolutions) as well as for therapy (through focal heating). Thus understanding and modeling of the magnetic susceptibility of the nanoparticles is critical. In hyperthermia calculations, a constant chord susceptibility approximation is used to get a lower bound estimate of the power dissipated. In later work, this approximation has been adapted to examine the decay effects due to nanoparticle relaxation under oscillating magnetic fields in the MPI modality. We provide in the present paper both analytical and numerical work to understand where it is, or is not, appropriate to make this approximation. In addition, we provide a more general approach that does not rely on the above approximations, and may provide new insight to manufacture optimal nanoparticles for applications. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D16.00013: The Spin-Lattice Relaxation of Hyperpolarized $^{89}$Y Complexes Ashish Jindal, Lloyd Lumata, Yixun Xing, Matthew Merritt, Piyu Zhao, Craig Malloy, Dean Sherry, Zoltan Kovacs The low sensitivity of NMR can be overcome by dynamic nuclear polarization (DNP). However, a limitation to the use of hyperpolarized materials is the signal decay due to $T_{1}$ relaxation. Among NMR-active nuclei, $^{89}$Y is potentially valuable in medical imaging because in chelated form, pH-sensitive agents can be developed. $^{89}$Y also offers many attractive features -- 100 \% abundance, a 1/2 spin, and a long $T_{1}$, up to 10 min. Yet, developing new $^{89}$Y complexes with even longer $T_{1}$ values is desirable. Designing such complexes relies upon understanding the mechanism(s) responsible for $T_{1}$ relaxation. We report an approach to hyperpolarized $T_{1}$ measurements that enabled an analysis of relaxation mechanisms by selective deuteration of the ligand backbone, the solvent or both. Hyperpolarized $^{89}$Y -- DTPA, DOTA, EDTA, and deuterated EDTA complexes were studied. Results suggest that substitution of low-gamma nuclei on the ligand backbone as opposed to that of the solvent most effectively increase the $^{89}$Y $T_{1}$. These results are encouraging for in vivo applications as the presence of bound water may not dramatically affect the $T_{1}$. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D16.00014: Magnetic Thermal Hysteresis in Dy nanolayers Ajani Ross, Ali Koymen Magnetic thermal hysteresis is observed when the temperature dependent magnetic properties of a material are reliant on the starting point of the measurement. Trilayer samples of pure Dysprosium (Dy) and Gadolinium (Gd) were grown on substrates of glass (Gd$_{n}$/Dy$_{m}$/Gd$_{n})$, n and m constitutes the number of layers. We observed magnetic thermal hysteresis in these thin films at low values of constant external magnetic field strengths. The temperature is swept from 20K to 300K at constant field, then back (300K to 20K) under the same field. In these temperature sweeps differences in magnetic moment were observed near the low end of the temperature range. Experiments are being done, currently, to confirm the existence of alternate helicity (AH-state) and helical (H-state) states in these trilayer films, which are believed to be the cause of the observed thermal hysteresis according to theoretical calculations. In addition, the temperature dependence of entropy change for these samples is calculated. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D16.00015: Chain formation in a magnetic fluid under the influence magnetic fields. Matt Barrett, Andreas Deschner, Jan Embs, An-Chang Shi, Maikel Rheinstadter We studied the aggregation of magnetic particles into simple chainlike structures in a Cobalt-based magnetic fluid, exposed to external magnetic fields [1]. The length of chain segments in very strong magnetic fields of up to 2 T was measured using small angle neutron scattering in-situ. Although it was predicted that the chain length can be described by a Langevin function, leading to chains several hundred particles in length, we observe a maximum correlation length of $\sim $ 650 {\AA}, or 4-5 particles. To gain insight into the molecular mechanisms involved, our experiments were complemented by Monte Carlo simulations. We observed that the chains which formed increased in length as the magnetic field increased until reaching equilibrium at 4 particles, in excellent agreement with our experimental findings. We speculate that the interplay between the entropy and energy of the system combined with the particular properties of the magnetic dipole-dipole interaction ultimately decide the length of the particle chains. We observed attractive or repulsive interaction between chain segments depending on their relative position. [1] \textit{``Chain formation in a magnetic fluid under the influence of magnetic fields'', }M. Barrett, A. Deschner, J.P. Embs, A.-C. Shi, M.C. Rheinst\"{a}dter, submitted to Physical Review Letters. [Preview Abstract] |
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