Karl B. Schliep

Communication: Effects of thermionic-gun parameters on operating modes in ultrafast electron microscopy

Ultrafast electron microscopes with thermionic guns and LaB6 sources can be operated in both the nanosecond, single-shot and femtosecond, single-electron modes. This has been demonstrated with conventional Wehnelt electrodes and absent any applied bias. Here, by conducting simulations using the General Particle Tracer code, we define the electron-gun parameter space within which various modes may be optimized. The properties of interest include electron collection efficiency, temporal and energy spreads, and effects of laser-pulse duration incident on the LaB6 source. We find that collection efficiencies can reach 100% for all modes, despite there being no bias applied to the electrode.

Picosecond Fresnel transmission electron microscopy

We report the demonstration of picosecond Fresnel imaging with an ultrafast transmission electron microscope (UEM). By operating with a low instrument repetition rate (5 kHz) and without objective-lens excitation, the picosecond demagnetization of an FePt film, via in situ, femtosecond laser excitation, is directly imaged. The dynamics are quantified and monitored as a time-dependent change in the degree of electron coherence within the magnetic domain walls. The relative coherence of conventional (thermionic) Fresnel transmission electron microscopy is also directly compared to that of Fresnel UEM through the domain-wall size. Further, the robustness and reversibility of the domain-wall dynamics are illustrated by repeating the picosecond image scans at defocus values having the same magnitude but different signs (e.g., +25 mm vs. −25 mm). Control experiments and approaches to identifying and isolating systematic errors and sources of artifacts are also described. This work, and continued future developm...

Characterizing Physical Properties of Superparamagnetic Nanoparticles in Liquid Phase Using Brownian Relaxation

Superparamagnetic iron oxide nanoparticles (SPIONs) have been extensively used as bioimaging contrast agents, heating sources for tumor therapy, and carriers for controlled drug delivery and release to target organs and tissues. These applications require elaborate tuning of the physical and magnetic properties of the SPIONs. The authors present here a search-coil-based method to characterize these properties. The nonlinear magnetic response of SPIONs to alternating current magnetic fields induces harmonic signals that contain information of these nanoparticles. By analyzing the phase lag and harmonic ratios in the SPIONs, the authors can predict the saturation magnetization, the average hydrodynamic size, the dominating relaxation processes of SPIONs, and the distinction between single- and multicore particles. The numerical simulations reveal that the harmonic ratios are inversely proportional to saturation magnetizations and core diameters of SPIONs, and that the phase lag is dependent on the hydrodynamic volumes of SPIONs, which corroborate the experimental results. Herein, the authors stress the feasibility of using search coils as a method to characterize physical and magnetic properties of SPIONs, which may be applied as building blocks in nanoparticle characterization devices.

Enhancement of tunneling magnetoresistance by inserting a diffusion barrier in L10-FePd perpendicular magnetic tunnel junctions

We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in realizing high TMR ratios in bulk p-MTJs such as those based on FePd and Mn-based perpendicular magnetic anisotropy materials for spintronic applications.We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in re...

Diffraction contrast as a sensitive indicator of femtosecond sub-nanoscale motion in ultrafast transmission electron microscopy

With ultrafast transmission electron microscopy (UTEM), access can be gained to the spatiotemporal scales required to directly visualize rapid, non-equilibrium structural dynamics of materials. This is achieved by operating a transmission electron microscope (TEM) in a stroboscopic pump-probe fashion by photoelectrically generating coherent, well-timed electron packets in the gun region of the TEM. These probe photoelectrons are accelerated down the TEM column where they travel through the specimen before reaching a standard, commercially-available CCD detector. A second laser pulse is used to excite (pump) the specimen in situ. Structural changes are visualized by varying the arrival time of the pump laser pulse relative to the probe electron packet at the specimen. Here, we discuss how ultrafast nanoscale motions of crystalline materials can be visualized and precisely quantified using diffraction contrast in UTEM. Because diffraction contrast sensitively depends upon both crystal lattice orientation as well as incoming electron wavevector, minor spatial/directional variations in either will produce dynamic and often complex patterns in real-space images. This is because sections of the crystalline material that satisfy the Laue conditions may be heterogeneously distributed such that electron scattering vectors vary over nanoscale regions. Thus, minor changes in either crystal grain orientation, as occurs during specimen tilting, warping, or anisotropic expansion, or in the electron wavevector result in dramatic changes in the observed diffraction contrast. In this way, dynamic contrast patterns observed in UTEM images can be used as sensitive indicators of ultrafast specimen motion. Further, these motions can be spatiotemporally mapped such that direction and amplitude can be determined.

Effect of capping layer on formation and magnetic properties of MnBi thin films

We report on the effect of varied capping layers on the formation of thin film MnBi, and the associated magnetic and crystalline properties for use in magnetic memory. MnBi thin films with a capping layer of either Ta, SiO2, Cr, or Au were grown, and it was observed that the magnetic properties vary significantly depending on the capping layer. Continuous 20 nm MnBi thin films capped with Ta and SiO2 show ferromagnetism with large perpendicular magnetocrystalline anisotropy, however, films capped with Cr and Au show no ferromagnetic behavior. In this work, MnBi thin films have been characterized utilizing magnetization vs. field, x-ray diffraction, cross-section transmission electron microscopy, and optical microscopy. We show that the capping layer plays a significant role in the formation of the low temperature phase MnBi structure and propose that the underlying cause is due to a surface energy difference for the MnBi//Au and MnBi//Cr interface, which allows for Mn oxidation, and prevents the formation...

Effects of Electron-Gun and Laser Parameters on Collection Efficiency and Packet Duration in Ultrafast Electron Microscopy

Real-space imaging of full-morphological, angstrom-scale structural dynamics occurring on the femtosecond (fs) timescale is possible with ultrafast electron microscopy (UEM) via the stroboscopic pump-probe methodology enabled by interfacing an otherwise standard TEM with a short-pulsed laser system [1-3]. The technology has matured to the point that commercial systems, such as the FEI Tecnai Femto, are now available [4]. The base platform for the Tecnai Femto is FEI’s Tecnai G2 20 200 kV instrument with a thermionic electron gun. In UEM mode, the LaB6 is operated cold such that electron emission occurs only during photo-illumination. Despite this, nanosecond single-shot imaging and diffraction – wherein >10 7 electrons per packet reach the specimen – are possible with a Tecnai-based UEM equipped with a conventional thermionic electron gun [5,6]. While this illustrates that sufficient photoelectrons for such experiments are generated in an otherwise unmodified Tecnai TEM, the parameter space for application-specific optimal electron-gun and photoelectron-generating laser properties remains ill-defined. Quantitatively and systematically mapping such parameter space is challenging due to the large number of variables affecting photoelectron packet properties [7-9].