Martha R. McCartney

Quantitative phase-sensitive imaging in a transmission electron microscope

This paper presents a new technique for forming quantitative phase and amplitude electron images applicable to a conventional transmission electron microscope. With magnetised cobalt microstructures used as a test object, we use electron holography to obtain an independent measurement of the phase shift. After a suitable calibration of the microscope, we obtain quantitative agreement of the phase shift imposed on the 200 keV electrons passing through the sample.

Synthesis, characterization, and modeling of high quality ferromagnetic Cr-doped AlN thin films

We report a theoretical and experimental investigation of Cr-doped AlN. Density functional calculations predict that the isolated Cr t2 defect level in AlN is 1/3 full, falls approximately at midgap, and broadens into an impurity band for concentrations over 5%. Substitutional Al1−xCrxN random alloys with 0.05⩽x⩽0.15 are predicted to have Curie temperatures over 600 K. Experimentally, we have characterized and optimized the molecular beam epitaxy thin film growth process, and observed room temperature ferromagnetism with a coercive field, Hc, of 120 Oe. The measured magnetic susceptibility indicates that over 33% of the Cr is magnetically active at room temperature and 40% at low temperature.

Magnetite morphology and life on Mars

Nanocrystals of magnetite (Fe3O4) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from the Martian meteorite. In contrast to previous accounts, we argue that the existing crystallographic and morphological evidence is inadequate to support the inference of former life on Mars.

Accurate measurements of mean inner potential of crystal wedges using digital electron holograms

Abstract The mean inner potential of a solid is a fundamental property of the material and depends on both composition and structure. By using cleaved crystal wedges of known angle, combined with dogital recording of off-axis electron holograms and with theoretical calculations of dynamical effects, the mean inner potential of Si (9.26±0.08 V), MgO (13.01±0.08 V), GaAs (14.53±0.17 V) and PbS (17.19±0.12 V) is measured with high accuracy of about 1%. Dynamical contributions to the phase of the transmitted beam are found by Bloch wave calculations to be less than 5% when the crystal wedges are titled away from zone-axis orientations and from major Kikuchi bands. The accuracy of the present method is a factor of 3 better than previously achieved by reflection high-energy electron diffraction and electron interferometry. The major causes of uncertainty were specimen imperfections and errors in phase measurement and magnification calibration.

Off-axis electron holography of magnetotactic bacteria : Magnetic microstructure of strains MV-1 and MS-1

Off-axis electron holography in the transmission electron microscope is used to characterize the magnetic microstructure of magnetotactic bacteria. The practical details of the technique are illustrated through the examination of single cells of strains MV-1 and MS-1, which contain crystals of magnetite (Fe 3 O 4 ) that are ∼50 nm in size and are arranged in chains. Electron holography allows the magnetic domain structures within the nanocrystals to be visualized directly at close to the nanometer scale. The crystals are shown to be single magnetic domains. The magnetization directions of small crystals that would be superparamagnetic if they were isolated are found to be constrained by magnetic interactions with adjacent, larger crystals in the chains. Magnetization reversal processes are followed in situ , allowing a coercive field of between 30 and 45 mT to be measured for the MV-1 cell. To within experimental error, the remanent magnetizations of the chains are found to be equal to the saturation magnetization of magnetite (0.60T). A new approach is used to determine that the magnetic moments of the chains are 7 and 5×10 −16 Am 2 for the 1600-nm long MV-1 and 1200-nm long MS-1 chains examined, respectively. The degree to which the observed magnetic domain structure is reproducible between successive measurements is also addressed.

Towards quantitative electron holography of magnetic thin films using in situ magnetization reversal

An approach based on off-axis electron holography has been developed for quantifying the magnetization in a sample of unknown local thickness with lateral variations in composition. The magnetic field of the objective lens is used to reverse the magnetization direction in the sample without altering its magnitude, thereby enabling phase changes due to magnetization to be separated from those due to thickness and compositional variations. The technique is demonstrated in applications to a lithographically patterned magnetic film on a silicon nitride membrane and a cross-sectional sample containing a magnetic tunnel junction. The importance of dynamical diffraction effects and fringing fields is discussed.

MAGNETIC ANISOTROPY AND MICROSTRUCTURE IN MOLECULAR BEAM EPITAXIAL FEPT (110)/MGO (110)

We compare the growth, structural, and magnetic properties of FexPt1−x(x≃0.6) films grown by molecular beam epitaxy on MgO (110) substrates held at 300 and 500 °C. Growth at both temperatures yields nearly single orientation epitaxial films with FePt, Pt [001]//MgO[001] and FePt, Pt (110)//MgO (110). For growth at 300 °C, chemical ordering to the L10 (CuAu I) tetragonal phase is incomplete (long-range order parameter, S=0.38) in contrast with growth at 500 °C for which ordering is nearly complete (S=0.81). Despite incomplete chemical ordering, a large two-fold, in-plane magnetic anisotropy (Ku=1.5×107 erg/cm3) is present for the film grown at 300 °C. Transmission electron microscopy for the 300 °C film confirms epitaxial growth with few extended defects but also reveals that chemical ordering is distributed inhomogeneously within the film. The direction of ordering is generally uniform (c axis along MgO [001]) but variations in the degree of ordering are evident on a length scale of 1–10 nm.

Strain mapping in nanowires

A method for obtaining detailed two-dimensional strain maps in nanowires and related nanoscale structures has been developed. The approach relies on a combination of lattice imaging by high-resolution transmission electron microscopy and geometric phase analysis of the resulting micrographs using Fourier transform routines. We demonstrate the method for a germanium nanowire grown epitaxially on Si(111) by obtaining the strain components epsilon(xx), epsilon(yy), epsilon(xy), the mean dilatation, and the rotation of the lattice planes. The resulting strain maps are demonstrated to allow detailed evaluation of the strains and loading on nanowires.

Magnetic Microstructure of Bacterial Magnetite by Electron Holography

A brackish lagoon at Itaipu, Brazil, contains magnetotactic bacteria with unusually large magnetite magnetosomes (lengths 100–200 nm). The micromagnetic structures of the magnetosomes from two different coccoid organisms from the lagoon have been determined by electron holography. The results are consistent with single-magnetic-domain structure in the elongated magnetosomes from one organism and metastable, single-magnetic-domain structure in the larger, more equi-axed, magnetosomes from the other organism. The results are consistent with theoretical predictions of the transition dimension between stable and metastable single-domain structure in magnetite.

Quantitative analysis of one-dimensional dopant profile by electron holography

The one-dimensional dopant profile of a silicon p–n junction was characterized using off-axis electron holography in a transmission electron microscope (TEM). Quantitative comparisons were made with simulated voltage profiles based on data obtained from secondary-ion mass spectroscopy. Close agreement was obtained over a range of sample thicknesses, and a spatial resolution of 5 nm and sensitivity of 0.1 V were established. By using a sample that had been wedge polished and briefly ion milled, depleted surface layers did not need to be taken into account, and beam-induced charging was removed by carbon coating one exposed surface of the TEM specimen.