Session X34: Magnetic Characterization and Imaging

Scanning Coherent X-ray Diffraction Imaging of GdFe Magnetic Multilayers

Crucial to understanding fundamental physics puzzles such as colossal magnetoresistance and developing future technologies for magnetic data storage is an understanding of the nanoscale behavior of magnetism. Probes with the ability to see beyond surfaces on this length scale, ultimately on ultrafast time scales, would greatly enhance this understanding. Coherent X-ray Diffraction Imaging (CXDI) is a promising new technique with wavelength-limited resolution that can probe deeply beyond surfaces. We studied the evolution of ``worm'' domains in a ferrimagnetic GdFe multilayer film as a function of applied field using CXDI. Ferromagnetic domains in the multilayer produce high contrast speckle when the film is illuminated with linearly polarized coherent x-rays resonant at the Gd M5 edge. This diffraction is purely magnetic since the sample is electronically amorphous. We recorded coherent diffraction data by scanning overlapping regions on the sample. These diverse diffraction patterns are then numerically inverted using ptychographic iterative algorithms to give a high resolution map of the ferromagnetic domain configuration.   

Microscopic Memory in Co/Pd Multilayer Films

We report measurements of microscopic memory between speckle patterns of CoPd films exhibiting perpendicular magnetic anisotropy. The speckle patterns were formed by coherent x-ray resonant scattering in a transmission geometry. Return point memory and conjugate point memory were determined as function of applied field. On the length scale probed, the results indicate that the measured memory is a statistical distribution itself, and not a fixed quantity. Memory as a function a scattered wave vector factor was also investigated.   

Diffraction magneto-optical Kerr effect of a natural ferromagnetic grating, MnAs/GaAs(001)

Diffracted optical magnetometry (DMOKE) technique was applied to the material system MnAs/GaAs(001), which forms a self-organized ferromagnetic array at room temperature. Because of the strain in MnAs when grown on GaAs(001), two structural (and magnetic) phases coexist in a range of temperatures around the $T_C $ in bulk MnAs. The two faces form of a periodic array of strain stabilized stripes running parallel to the MnAs(0001) direction. The stripe array consists of alternating ferromagnetic $\alpha $-MnAs and paramagnetic $\beta $-MnAs. The period and width are highly dependent on both the film thickness and the temperature. Light diffracted from the array has shown anomalous magneto-optical polarization changes, different from the standard magneto-optical Kerr effect, which in our case is measured from the specular reflection. The hysteresis anomalies are discussed in terms of the magnetic form factors extracted from domain structure simulations.   

Imaging the evolution of the antiferromagnetic to ferromagnetic magnetostructural first order phase transition of FeRh thin films

Iron-Rhodium films undergo an antiferromagnetic (AFM) - ferromagnetic (FM) first order transition at $\sim $380 K and this is accompanied by a 0.6~{\%} volume expansion. Because different lattice parameter of each phase result in different diffraction conditions for the AFM and FM phases, each phase can be discriminated by the diffraction condition. Using the Nano-diffraction beam line at the Advanced Photon Source, the phase separation was imaged by scanning the beam across the sample and the phase evolution with temperature was measured in the Nano-scale. It is confirmed that the nucleation sites associate with defects. The temperature dependence of the phase separation will be discussed in conjunction with the rate of the phase evolution. Work at Argonne, including the Advanced Photon, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

Spin polarized Auger electrons in Core-Valence-Valence decays of 3$d$ impurities in metals

The spin polarization of the emitted electrons from 3$d$ impurities and adatoms in simple metal hosts in a Core-Valence- Valence Auger process is analyzed in terms of a first principle density functional theory approach, by using the golden rule. The relationship between the spin dependent local density of states, the magnetic moments of the 3$d$ atoms and the energy dependent and total spin polarization of the Auger electrons is discussed. It is shown how to estimate the magnetic moment of the impurities from a measure of the total spin polarization of the Auger electrons. This can be achieved considering i) that the Auger signal is simply due to the impurities only, ii) the very locality of the Auger phenomenon, and iii) a simple and general relationship between the spin polarization and the magnetic moment of the impurity which we show to be independent of the metal host.   

Neutron-scattering determination of local magnetic structure in patterned thin-film devices

The performance of devices based on patterned magnetic films depends strongly on the uniformity of the magnetic state of individual magnetic elements across the whole device. Characterization with scattering techniques provides an ensemble average over a large spatial region, ideal for studies of this kind. We present off-specular neutron scattering measurements on patterned magnetic films of permalloy. Interpretation of the results involves a novel modeling technique, first determining likely magnetic configurations of the elements from micromagnetic modeling (using OOMMF software) followed by direct calculation (in a modified Born Approximation) of the expected neutron scattering from an ensemble of such elements. Two different element shapes (ring and c-shaped, $\sim 1 \mu$m long, 20nm thick) were measured, modeled and contrasted. In the closed ring structure the vortex ground state lacks preferred chirality, while in the c-shape the chirality can be controlled over the whole device. The signature of these states in the neutron scattering is distinct and has applications, e.g. for novel layered storage media, in which the magnetic structure is inaccessible to other, direct-mapping microscopy techniques.   

Scanning probe magnetometer using a nitrogen-vacancy defect center in diamond

Imaging weak magnetic signals down to the individual spin level with nanometer-scale spatial resolution has been a long standing goal due to its possible impacts in biological and physical sciences [1]. Recent experiments [2,3] have demonstrated that nitrogen-vacancy(NV) defect centers in diamond can serve as an excellent sensor, enabling above-mentioned conditions even at room temperature. Here, we report our recent progress in the development of a scanning probe magnetometer, which consists of an atomic force microscope, a single NV center at the apex of the tip, and a confocal microscope. We will present a detailed description of our setup, its operating principles and specifications. \\[4pt] [1] D. Rugar \textit{et al}.\textit{ Nature} 430, 329 (2004)\\[0pt] [2] J. Maze \textit{et al}. \textit{Nature} 455, 644 (2008)\\[0pt] [3] G. Balasubramanian \textit{et al}. \textit{Nature} 455, 648 (2008)   

Scanning magnetometry of nanomagnets via a nitrogen-vacancy center in diamond scanning microscope

Recently, nitrogen-vacancy (NV) centers in diamond have shown to be an attractive system for performing sensitive magnetic measurements on the nanoscale under ambient conditions [1,2]. Here, we demonstrate magnetic imaging of nanoscale structures using a trapped NV center in an atomic force microscope (AFM). By fixing an NV center within tens of nanometers from the end of our AFM tips, we are able to spatially map magnetic fields from nanomagnets. Due to our achieved NV-sample distance, we are able to detect smaller magnetic sources than previously demonstrated [2] consisting of ensembles of thousands of electron spins. Progress towards the mapping the dipole field of a single-electron spin is additionally presented. \\[4pt] [1] J. Maze \textit{et al}. \textit{Nature} 455, 644 (2008)\\[0pt] [2] G. Balasubramanian \textit{et al}. \textit{Nature} 455, 648 (2008)   

Micromagnetic Modeling of Magnetic Resonance Force Imaging of Individual Ferromagnetic Nanowires

Magnetic Resonance Force Microscopy (MRFM) is a scanned probe technique capable of spatially resolved Ferromagnetic Resonance Imaging (FMR) on magnetic samples via excitation of a localized FMR mode confined by the inhomogeneous field of the MRFM probe magnet. FMR imaging using MRFM is capable of spatially resolved mapping of the magnetic properties, such as saturation magnetization, of a sample under investigation. We present the results of micromagnetic modeling of magnetic resonance force imaging of individual ferromagnetic nanowires with the typical diameter of 20 nm. We will discuss issues of force sensitivity and the spatial resolution of the method.   

Nanomagnet Characterization Using Scanning SQUID Microscopy

Magnetic nanoparticles 5-50nm in size are of wide interest in the biological and medical fields. For instance, magnetotatic bacteria containing nanoscale mangnetite particles show potential for MRI contrast agents and targeting tumors. To date, characterization of nanomagnets has been 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 our progress towards the detection and magnetic characterization of individual nanomagnets 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, magnetic moment and switching times. Thermal separation between SQUID and sample allows temperature dependent studies of the transition between ferromagnetism and superparamagnetism. Results for both FePt nanomagnets and magnetosomes from magnetotactic bacteria will be shown.   

Imaging Spin-Polarized Scattering of Topological Surface States

Surface states of topological insulators are comprised of Dirac fermions protected from backscattering due to alignment of spin and momentum, a property characteristic of relativistic particles. We realize topological states on the surface of pure antimony, a fundamental parent matrix of various topological insulator compounds. With low-temperature scanning tunneling microscopy (STM), we directly visualize spin-polarized scattering of surface state electrons with sub-atomic precision around surface and subsurface defects. These results link closely to angle-resolved photoemission spectroscopy (ARPES) studies on the same materials, unambiguously measure the spin-protection of topologically ordered states, and access a striking transition to a single unpaired Dirac species.   

The Effect of Relaxation on Magnetic Particle Imaging

Magnetic particle imaging[1] is a new tomographic technique that allows fast, inexpensive imaging through the use of ferro-fluid agents leading to submillimeter resolution. Selection fields combined with oscillating driving fields can move unsaturated field-free-points so as to cover the field of view. In previous studies, the average magnetization is assumed to respond instantaneously to changes in the applied field.[1-4] Realistically, however, a finite relaxation time slows the magnetic response. The present simulation demonstrates that, for contrast agents of interest, the choice of an optimal particle size is strongly dependent on this effect. A trade-off thus exists between sensitivity and resolution. [1] B. Gleich and J. Weizenecker, Nature v.435:1214, 2005 [2] J. Weizenecker et al., Phys. Med. Biol., v.54: L1, 2009 [3] J. Rahmer et al., BMC Medical Imaging, 2009 [4] P. W. Goodwill et al., IEEE Trans. on Medical Imaging, v.28:1231, 2009   

Ferromagnetic Resonance Imaging using a submicron localized spin wave mode

Ferromagnetic Resonance Force Microscopy (FMRFM) is a highly sensitive spectroscopic tool for the study of nanoscale ferromagnets. Nanoscale imaging of buried or multi-component ferromagnetic systems requires a mechanism for defining the localized volume under study. Recently, we have discovered a new approach that employs the strong, nonuniform magnetic field of the micromagnetic probe tip aligned anti-parallel to magnetization in sample to localize FMR modes. The highest resolution obtained in our experiment is 200 nm with relatively large probe ($\sim $ 1.2 x 1.2 x 1.5 $\mu $m$^{3})$ and probe-sample separation ($\sim $ 1.3 $\mu $m), showing sub-surface scanning capability. We have imaged the non-uniform demagnetizing field of an individual 5 $\mu $m Permalloy disk and the variation of the internal magnetic field in the Permalloy film with high sensitivity ($\sim $ 1 Gauss/Hz$^{1/2})$ in the small volume $\sim $ 200 x 200 x 40 nm$^{3}$. Our method presents a technique for exploring nanoscale magnetism and spin dynamics in inhomogeneous magnetic fields.   

Demonstration of vector magnetometry using N-V center impurities in single crystal diamond

N-V centers in diamond have been shown to provide a promising approach to high-sensitivity and high spatial resolution magnetometry at room temperature. We demonstrate vector magnetometry using N-V centers in diamond under ambient conditions. Using a co-planar wave guide we have performed optically detected ESR in commercially available single-crystal diamond containing a high-density of nitrogen impurities. As a first step toward high resolution magnetic field imaging we have imaged the spatial variation of the magnetic field from a small magnet by measuring the variation of the optical ESR signal as a 532nm laser spot is scanned with respect to the magnet. Exploiting the dependence of the NV diamond ESR shift on the orientation of the magnetic field relative to the axis of the NV center we have mapped the magnitude and orientation of the spatially varying magnetic field of the fabricated magnet.   

Developing a gradient coil for spatially resolved magnetometry using nitrogen-vacancy centers in diamond

Magnetic resonance imaging and optical microscopy are major tools in many scientific disciplines, particularly in the biological sciences. In Magnetic resonance imaging, a high magnetic field gradient is used to encode spatial information of the sample into the frequency domain. This allows spatial resolution which is determined by the strength of the field gradient and the magnetic resonance linewidth. Here we present our work on developing a magnetic gradient coil for use in spatially resolved magnetometry based on optical detection of electron spin resonance of nitrogen-vacancy (NV) centers in diamond. We apply a pulsed current through microwires to provide a magnetic field gradient that allow us to distinguish spatially separated volumes of NV centers.