Leonid A. Bendersky

Monolithic multichannel ultraviolet detector arrays and continuous phase evolution in MgxZn1−xO composition spreads

We have fabricated MgxZn1−xO epitaxial composition spreads where the composition across the chip is linearly varied from ZnO to MgO. By using a scanning x-ray microdiffractometer and transmission electron microscopy, we have mapped the phase evolution across the spread. We have discovered a unique growth relationship between cubic and hexagonal MgxZn1−xO where their basal planes are coincident in the phase-separated region of the phase diagram where 0.37⩽x⩽0.6. The continuously changing band gap across the spread is used as a basis for compact broadband photodetector arrays with a range of detection wavelengths separately active at different locations on the spread film. The composition-spread photodetector is demonstrated in the wavelength range of 290–380 nm using the ZnO to Mg0.4Zn0.6O region of the spread.

The formation of ordered ω-related phases in alloys of composition Ti4Al3Nb

During cooling of an alloy of composition Ti4Al3Nb from a B2 phase field above 1100°C, a metastable trigonal (P3m1) ω-related phase, designated ω″, forms along with small amounts of D019 and L10 phases. The ω″ phase exhibits partial collapse of 111 planes and reordering relative to its B2 parent. An apparently equilibrium low temperature phase with the B82 structure was found after 26 days of annealing at 700°C. Both ω″ and B82 structures were verified by means of transmission electron microscopy and by single crystal X-ray diffraction. The latter permitted detailed analysis of the collapse parameters and site occupancies. The observed transformation path, B2(Pm3m)→ω″(P3m1)→B82 (P63/mmc), occurs in two steps. The first involveds a subgroup transition during cooling that is primarily displacive with reordering consistent with the trigonal symmetry imposed by the ω-collapse. The second involves a supergroup transition during prolonged annealing that is primarily replacive and constitutes a chemical disordering. The direct equilibrium transformation, B2→B82, without the formation of an intermediate trigonal phase, can only occur by a reconstructive transformation.

Giant magnetostriction in annealed Co 1−x Fe x thin-films

Chemical and structural heterogeneity and the resulting interaction of coexisting phases can lead to extraordinary behaviours in oxides, as observed in piezoelectric materials at morphotropic phase boundaries and relaxor ferroelectrics. However, such phenomena are rare in metallic alloys. Here we show that, by tuning the presence of structural heterogeneity in textured Co(1-x)Fe(x) thin films, effective magnetostriction λ(eff) as large as 260 p.p.m. can be achieved at low-saturation field of ~10 mT. Assuming λ(100) is the dominant component, this number translates to an upper limit of magnetostriction of λ(100)≈5λ(eff) >1,000 p.p.m. Microstructural analyses of Co(1-x)Fe(x) films indicate that maximal magnetostriction occurs at compositions near the (fcc+bcc)/bcc phase boundary and originates from precipitation of an equilibrium Co-rich fcc phase embedded in a Fe-rich bcc matrix. The results indicate that the recently proposed heterogeneous magnetostriction mechanism can be used to guide exploration of compounds with unusual magnetoelastic properties.

Exploration of artificial multiferroic thin-film heterostructures using composition spreads

We have fabricated a series of composition spreads consisting of ferroelectric BaTiO3 and piezomagnetic CoFe2O4 layers of varying thicknesses modulated at nanometer level in order to explore artificial magnetoelectric thin-film heterostructures. Scanning microwave microscopy and scanning superconducting quantum interference device microscopy were used to map the dielectric and magnetic properties as a function of continuously changing average composition across the spreads, respectively. Compositions in the middle of the spreads were found to exhibit ferromagnetism while displaying a dielectric constant as high as ≈120.

Effect of the electrode layer on the polydomain structure of epitaxial PbZr0.2Ti0.8O3 thin films

PbZr0.2Ti0.8O3(PZT) thin films with and without La0.5Sr0.5CoO3(LSCO) electrodes were grown epitaxially on (001) SrTiO3 at 650 °C by pulsed laser deposition. The domain structure of the 400 nm thick PZT films with different electrode layer configurations was investigated by x-ray diffraction and transmission electron microscopy. The c-domain fractions of the PZT films with no electrode layer, with a 50 nm electrode layer between the film and the substrate, and with 50 nm electrode layers on top and bottom of the PZT film were found to be equal. Theoretical estimation of the c-domain fraction based on the minimization of the energy of internal stresses in films is in good agreement with experimental results. This means that depolarizing fields do not affect the polydomain structure of the film. Calculations of the in-plane strains based on the lattice parameters of the LSCO layer in the above configurations led to the conclusion that the bottom electrode layer is coherently strained to match the substrate.

Crystal chemistry of the compound Sr 2 Bi 2 CuO 6

The compound Sr 2 Bi 2 CuO 6 should nominally be the phase with n = 1 of the high T c superconducting series Sr 2 Bi 2 Ca n O 4+2 n . However, the superconducting phase with n = 1 (with no CaO) occurs only with a gross deficiency in SrO content. Instead, at the composition Sr 2 Bi 2 CuO 6 , a different phase is formed with an x-ray diffraction pattern considerably different from that expected for the n −1 member of the series. This phase has been found, by a combination of electron diffraction and single crystal and powder x-ray diffraction, to have a commensurate lattice with monoclinic symmetry, space group C 2/ m or Cm, a = 24.473 (2), b = 5.4223 (5), c = 21.959 (2)A, and β = 105.40 (1)°. The actual composition of this phase may be deficient in CuO by as much as 1.0 mole %.

Threading dislocations in epitaxial InN thin films grown on (0001) sapphire with a GaN buffer layer

The density and types of threading dislocations (TDs) in InN thin films grown on (0001) sapphire with a GaN buffer layer were characterized by transmission electron microscopy. Perfect edge TDs with 13〈1120〉 Burgers vectors are predominant defects which penetrate the GaN and InN layers. Pure screw and mixed TDs were also observed. Overall the TD density decreases during film growth due to annihilation and fusion. The TD density in GaN is as high as ≈1.5×1011 cm−2, and it drops rapidly to ≈2.2×1010 cm−2 in InN films. Most half-loops in GaN are connected with misfit dislocation segments at the InN/GaN interface and formed loops, while some TD segments threaded the interface. Half-loops were also generated during the initial stages of InN growth.

Stable and metastable phase equilibria in the Al-Mn system

The aim of the present investigation was resolution of certain obscure features of the Al-Mn phase diagram. The experimental approach was guided by assessment of the previous literature and modeling of the thermodynamics of the system. It has been shown that two phases of approximate stoichiometry “Al4Mn” (λ and μ) are present in stable equilibrium, λ forming by a peritectoid reaction at 693 ± 2 °C. The liquidus and stable equilibrium invariant reactions as proposed by Goedecke and Koester have been verified. A map has been made of the successive nonequilibrium phase transformations of as-splat-quenched alloys. Finally, the thermodynamic calculation of the phase diagram allows interpretation of complex reaction sequences during cooling in terms of a catalogue of all the metastable invariant reactions involving (Al), Al6Mn, λ, μ, ϕ, and Al11Mn4 phases.

Three-domain architecture of stress-free epitaxial ferroelectric films

Epitaxial ferroelectric films undergoing a cubic-tetragonal phase transformation relax internal stresses due to the structural phase transformation and the difference in the thermal expansion coefficients of the film and the substrate by forming polydomain structures. The most commonly observed polydomain structure is the c/a/c/a polytwin which only partially relieves the internal stresses. Relatively thicker films may completely reduce internal stresses if all three variants of the ferroelectric phase are brought together such that the film has the same in-plane size as the substrate. In this article, we provide experimental evidence on the formation of the three-domain structure based on transmission electron microscopy in 450 nm thick (001) PbZr0.2Ti0.8O3 films on (001) SrTiO3 grown by pulsed laser deposition. X-ray diffraction studies show that the film is fully relaxed. Experimental data is analyzed in terms of a domain stability map. It is shown that the observed structure in epitaxial ferroelectric...

Ferroelectricity and ferrimagnetism in iron-doped BaTiO3

The structural and physical properties of pseudocubic thin-film BaTi1−xFexO3(0.5⩽x⩽0.75) grown by pulsed-laser deposition are reported. This material is of interest because the corresponding bulk compounds have hexagonal structure for comparable x, and because the films are both ferroelectric and ferrimagnetic well above room temperature. A substantial increase of the ferroelectric transition temperature relative to that of bulk BaTiO3 is attributed to lattice expansion induced by Fe doping.