People DO Write Songs about our group's research:
"I Study Chromium" by former undergrad Ernesto Abruna:

I Study Chromium Mp3

Research Directions:

Direct Measurements of Antiferromagnetic Domain Wall Fluctuations

Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets.

We have used X-ray Photon Correlation Spectroscopy to couple to the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. Coherent X-ray diffraction produces a speckle pattern that serves as a 'fingerprint' of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometer distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it has a rate that saturates at a finite value—consistent with quantum fluctuations—on cooling below 40 K.

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X-ray Speckle Pattern

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Schematic of X-ray Photon Correlation Spectroscopy measurements of antiferromagnetic domain fluctuations in Chromium

Reference: O. G. Shpyrko, E. D. Isaacs, J. M. Logan, Yejun Feng, G. Aeppli, R. Jaramillo, H. C. Kim, T. F. Rosenbaum, P. Zschack, M. Sprung, S. Narayanan, and A. R. Sandy; "Direct measurement of antiferromagnetic domain fluctuations," Nature 447, 68 (2007).

 

X-ray Diffractive Imaging of Magnetic Nanoscale Domains

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Evolution of magnetic domains in GdFe multilayer films as a function of applied field.

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Magnetic X-ray Speckle Pattern used to reconstruct the nanoscale magnetic domain configuration.

 

We have performed lens-less microscopy of extended magnetic nanostructures, in which a scanned series of dichroic coherent diffraction patterns is recorded and numerically inverted to map its magnetic domain configuration. Unlike holographic methods, it does not require a reference wave or precision optics. In addition, it enables the imaging of samples with arbitrarily large spatial dimensions, at a spatial resolution limited solely by the coherent X-ray flux, wavelength, and stability of the sample with respect to the beam. We demonstrate this approach by imaging ferrimagnetic labyrinthine domains in a Gd/Fe multilayer with perpendicular anisotropy and follow the evolution of the domain structure through part of its magnetization hysteresis loop. In such multilayer systems, alternating layers of a transition metal and a rare-earth metal are deposited to create an artificial ferrimagnet with perpendicular magnetic anisotropy. The balance between exchange, anisotropy, and dipolar energies results in stripe domains.

This type of pattern is commonly found in a wide class of uniaxially modulated condensed matter systems, including diblock copolymers, liquid crystals, Langmuir monolayers, adsorbates at metallic surfaces, incommensurate structures, membranes, vesicles, and ferrofluids, as well as dewetting, phase separation, dealloying, and convection patterns. The characteristic length scale corresponding to the width of the stripe is generally defined by the energetic balance between short- and long-range interactions. The ability to tune the relative strength of these interactions and to control the degree of orientational or translational order in many of these systems represents a pathway for driven or guided self-assembly at nanoscale.

 

Reference:Dichroic Coherent Diffractive Imaging” Ashish Tripathi, J. Mohanty, S. Dietze, O. G. Shpyrko, E. Shipton, E. Fullerton, S.S. Kim and Ian McNulty, Proc. Natl. Acad. Sci. U S A, 108 (33), 13393-8 (2011)

 


Phase transitions in Langmuir-Blodgett films

The optical microscopy images of Au nanoparticle LB film at different phases

 

Understanding of the mechanical and dynamical properties of Langmuir-Blodgett films provides insights into the dynamics, collapse and relaxation of biological membranes such as lung surfactant and nanoparticle self-assembled thin films. During the compression process, the self-assembled nanoparticle Langmuir-Blodgett films at the air-liquid interface exhibit rich mechanical behavior, undergoing a rapid structural evolution which is marked by the phase transition from monolayer to hash to multilayer. We have performed different synchrotron X-ray scattering techniques to study the phase transition of these systems. We explored the capability of Grazing Incidence X-ray Off-Specular (GIXOS) scattering to capture this rapid structural evolution. The detailed analysis of GIXOS data from the self-assembled Au nanoparticle films offers the quantitative, Angstrom-resolution details of electron density profile normal to the surface with a sub-minute temporal resolution that allows us to study in-situ the rapid evolution of nanoparticle films structure in response to the lateral compression.

Schematic view of the scattering geometry with the Langmuir trough. An inset shows a typical intensity profile from Au nanoparticle film, plotted as function of wavevector transfer normal to the surface, qz.

 

 

 

Morphological instabilities in Nanoimprinted Polymer nanostructures

 

The stability of the imprinted polymer pattern is crucial to the Nanoimprint Lithography technique. The development of a novel collective type of zig-zag capillary instability has been recently observed upon annealing of line-space polymer patterns formed with nanoimprint lithography.  We performed synchrotron based, time-resolved Grazing Incidence Small Angle X-ray Scattering (GISAXS) studies of the onset and development of this zig-zag instability that complement earlier Atomic Force Microscope (AFM) based studies.  Time resolved GISAXS provides the in-situ measurement of the rapid formation of instability during the annealing process over large sample area, a task that remains challenging for local scanning probes such as AFM.  A relatively simple unit cell model has been applied to quantitatively interpret the GISAXS data and identify the onset of the Rayleigh-Taylor morphological instabilities.

 

 

 

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AFM data of as-imprinted polystyrene with fluorosurfactant line-space grating.  SEM (inset) shows the lines rectangular cross-section at a fractured edge.  (B)  After annealing the sample a lateral zig-zag instablilty forms in the pattern.  (C-E)  Nanoimprint Lithography schematic.

 

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Time evolution of the GISAXS pattern following the evolution of the Rayleigh-Taylor morphological instabilities

 

 

 

Lateral Diffusion in 2D Nanoparticle Self-Assembled Films

The recent trend towards smaller and lighter technological devices has created a substantial demand for nanoscale films engineered to meet specific optical, electrical, and magnetic properties.  Examples include the implementation of thin films into antireflective coatings, computer memory, and lightweight, flexible energy-storage devices.  Despite the number of potential applications, which will only continue to grow, methods for characterizing the mechanical properties and stability of ultrathin films have not been well established.  The drastic increase in spatial and time resolution achieved by coherent x-ray synchrotron sources and beamlines in recent years has begun to allow for direct coupling of x-rays to monitor interparticle motions.

 

 

 

 

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Langmuir-Blodgett trough setup used for compressing nanoparticle monolayers self-assembled on the surface of water

 

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Micron-sized wrinkles in a compressed 5nm iron oxide nanoparticle monolayer

 

 

We study self-assembled monolayers, e.g. iron oxide nanoparticles self-assembled at the water-air interface using X-Ray Photon Correlation Spectroscopy (XPCS).  This technique effectively compares “snapshots” of the diffraction pattern produced by the monolayer at different times in order to reproduce the film structure and interparticle dynamics.  These interparticle motions depend directly upon parameters such as particle size, and evolve as the surface pressure of the system is increased, ultimately leading to “buckling” or “wrinkling” at points of local instabilities.

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The first-order Grazing Incidence Diffraction (GID) peak for a self-assembled monolayer of 20nm iron oxide nanoparticles

 

 

Coherent X-ray diffraction imaging of strained nanowires

Highly strained nanoscale wires have been intensively studied because a structural deviation of the crystal from the ideal arrangement can enhance dramatically electronic, magnetic and optical properties, and its high-surface-to-volume ratio is useful in a wide range of applications such as catalysis, sensors, batteries, fuel cells, and magnetic devices.

We use Coherent X-ray Diffraction in the Bragg geometry highly sensitive to lattice distortion caused by deformation fields within a small crystal. This technique allows us to image not only the shape in 3D but also the projection of the crystal lattice distortion on to the Q vector of the measured Bragg spot.

Upon successful inversion to real space, the amplitude and phase of the complex density represents physical density and lattice deformation, as projected onto the Q vector of the Bragg peak chosen, respectively.

 

 

 

 

Scanning electron micrograph of the specimen (left) and 3D reconstruction of the phases and magnitude of the displacement field of a single crystal Ni nanowire (right)

 

 

 

However, since the detector only measures intensities, a direct reconstruction is impossible, unless a phase problem is solved via special phasing procedure, an iterative process using back and forth Fourier transforms between real and reciprocal space.

Reference: E. Fohtung, J. W. Kim, Keith T. Chan, Ross Harder, Eric E. Fullerton, and O. G. Shpyrko; " Probing the 3D strain inhomogeneity and equilibrium elastic properties of single crystal Ni nanowires", submitted to Applied Physics Letters (May 2012).

 

 

Strain Mediated Magnetoelectric Coupling as a route to Novel Hybrid Multiferroic Devices

Smart materials for sensor technology, non-volatile device memories for information technology, and ultrasound generators in medical imaging have one thing in common, their active elements consist of ferroelectrics (FE) driven by voltages or ferromagnetics (FM) driven by magnetization. In the quest to design high functionality devices to meet today's consumer technological demands, high focus has been given to multiferroic. However, the coexistence of magnetic order and ferroelectric polarization combined in a single-phase material has proven to be rear as most of these materials tend to have low magnetic ordering temperatures and are often antiferromagnets, in which the magnetoelectric (ME) coupling effect is intrinsically small. Field effect concepts can be applied to complex oxides, such as high-TC superconductors and manganites, in order to create new electronic and magnetic phase

 Schematic view of hybrid (a) FE-FM device system. LSMO is in the metallic phase at room temperature and serves as a bottom electrode for poling the ferroelectric domains in PZT with the Pt contacts used as top electrode. (b) Shows sharp atomic terraces in the PZT film that confines the periodicity of the ferroelectric domains. (c) Shows 3D diffraction pattern in the vicinity of the 101 reciprocal lattice point collected using a spiral Ptychographic scan. Clear periodicity of the superstructure arising from the PZT domains are seen. (d) Shows a 2D slice of the 3D diffraction pattern. (e) Shows reconstructed periodic domains from the diffraction pattern.

 

In magneto-electronic materials such as multiferroics an external electric field displaces ions from their equilibrium positions, which alters the magnetostatic and exchange interactions yielding magnetoelectric coupling.

 

We utilize an alternative approach to design multiferroic-hybrid devices (see fig 1a) of FE-FM heterostructures where the magnetoelectric coupling emerges from strain-mediated interaction between individual phases. Specific examples of systems we are presently studying and intend to pursue further include but not limited to heterostructures consisting of magnetic (LSMO, transition-metal alloy) films and highly piezoelectric (PMN-PT and PZT) layers.  The devices schematic structures are shown in Fig. 1a.  The sample structure is Si (001) substrates, STO (SrTiO3), LSMO (La1-x Sr xMnO3) bottom electrodes, PMN-PT((1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3] or PZT (PbZrTi3) and a top electrode that is either a non-magnetic metal (e.g. Pt) or a magnetic transition metal film (e.g. Co). In the presence of an external applied electric field, Ti4+(Zr4+ ) cations are formed as the Ti- atoms are displaced from their equilibrium position. This alters the magnetostatic and exchange interactions yielding magnetoelectric coupling. Strain is observed as displacement derivatives.

 

This strain mediation is qualitatively studied in the form of a nonlinear thermodynamic theory for strain-mediated direct Magnetoelectric effect. We utilize our in-house atomic force microscopy (AFM) and Piezoelectric Force Microscopy (PFM) to understand the switching nature of the Ferroelectric domains. To understand how it couples to the lattices and its depth dependence, Bragg Ptychographic Coherent Diffraction Imaging (BCDI) is invoked. The interfacial coupling and enhancement of magnetization is probed using neutron scattering techniques.

 

Our complementary use of various probing techniques enables us to nondestructively obtain insights to the coupling phenomena of the order parameters required for device functionality. We use Bragg Ptychographic Coherent Diffraction Imaging (BCDI) and neutron scattering as the unique tools of choice for sub-nanometer resolution and nondestructive probing of the order parameters in these materials and devices.

 

 

 


Old(er) Research Highlights:

Our research summarized in a song ("I Study Chromium") by REU student Ernesto Abruna:

I Study Chromium Mp3