arXiv subjects

H. M. Christen

Publications and source records attributed to H. M. Christen.

At least 19 recordsLinked to original sources

arXiv2017

Full Electroresistance Modulation in a Mixed-Phase Metallic Alloy

We report a giant, ~22%, electroresistance modulation for a metallic alloy above room temperature. It is achieved by a small electric field of 2 kV/cm via piezoelectric strain-mediated magnetoelectric coupling and the resulting magnetic phase transition in epitaxial FeRh/BaTiO3 heterostructures. This work presents detailed experimental evidence for an isothermal magnetic phase transition driven by tetragonality modulation in FeRh thin films, which is in contrast to the large volume expansion in the conventional temperature-driven magnetic phase transition in FeRh. Moreover, all the experimental results in this work illustrate FeRh as a mixed-phase model system well similar to phase-separated colossal magnetoresistance systems with phase instability therein.

cond-mat.mtrl-sci↗

arXiv2016

An invisible non-volatile solid-state memory

Information technologies require entangling data stability with encryption for a next generation of secure data storage. Current magnetic memories, ranging from low-density stripes up to high-density hard drives, can ultimately be detected using routinely available probes or manipulated by external magnetic perturbations. Antiferromagnetic resistors feature unrivalled robustness but the stable resistive states reported scarcely differ by more than a fraction of a percent at room temperature. Here we show that the metamagnetic (ferromagnetic to antiferromagnetic) transition in intermetallic Fe0.50Rh0.50 can be electrically controlled in a magnetoelectric heterostructure to reveal or cloak a given ferromagnetic state. From an aligned ferromagnetic phase, magnetic states are frozen into the antiferromagnetic phase by the application of an electric field, thus eliminating the stray field and likewise making it insensitive to external magnetic field. Application of a reverse electric field reverts the antiferromagnetic state to the original ferromagnetic state. Our work demonstrates the building blocks of a feasible, extremely stable, non-volatile, electrically addressable, low-energy dissipation, magnetoelectric multiferroic memory.

cond-mat.mtrl-sci↗

arXiv2013

Unit cell orientation of tetragonal-like BiFeO$_3$ thin films grown on highly miscut LaAlO$_3$ substrates

Synchrotron and lab-scale x-ray diffraction shows that tetragonal-like T'-BiFeO3 films on miscut LaAlO3 substrates (miscut < 5 deg) exhibit (00l)-planes tilted away from those of the substrate as predicted by the "Nagai model" (except for miscut <0.2 deg). Tilts as large as 1 deg are achieved even in 100 nm thick films, strikingly larger than those observed in other perovskites. We attribute this to the large c/a ratio and the high crystalline coherency of the T'-BiFeO3/LaAlO3 interface. This coherency is possible through an observed "diagonal-on-diagonal" alignment. Interestingly, the substrate miscut does not influence the relative population of monoclinic domains.

cond-mat.mtrl-sci↗

arXiv2012

Antiferromagnetic transitions in `T-like' BiFeO3

Recent studies have reported the existence of an epitaxially-stabilized tetragonal-like ('T-like') monoclinic phase in BiFeO3 thin-films with high levels of compressive strain. While their structural and ferroelectric properties are different than those of rhombohedral-like ('R-like') films with lower levels of strain, little information exists on magnetic properties. Here, we report a detailed neutron scattering study of a nearly phase-pure film of T-like BiFeO3. By tracking the temperature dependence and relative intensity of several superstructure peaks in the reciprocal lattice cell, we confirm antiferromagnetism with largely G-type character and TN = 324 K, significantly below a structural phase transition at 375 K, contrary to previous reports. Evidence for a second transition, possibly a minority magnetic phase with C-type character is also reported with TN = 260 K. The co-existence of the two magnetic phases in T-like BiFeO3 and the difference in ordering temperatures between R-like and T-like systems is explained through simple Fe-O-Fe bond distance considerations.

cond-mat.mtrl-sci↗

arXiv2010

Stress-induced R-MA-MC-T symmetry changes in BiFeO3 films

The recent discovery of a stress-induced structural phase transition in the single-component perovskite BiFeO3 has revived interest in this lead-free ferroelectric. The coexistence of different phases may lead to large piezoelectric coefficients, a property typically associated with complex solid solutions of lead-based perovskites. Here we report combined experimental and computational results showing that the stress-induced phase transitions in BiFeO3 follow the path of rhombohedral(R)-to-monoclinic(MA)-to-monoclinic(MC)-to-tetragonal(T), where both MC and T show highly enhanced c/a ratios. This R-MA-MC-T sequence is otherwise observed only near morphotropic phase boundaries (MPBs) in lead-based perovskites (i.e. near a compositionally induced phase instability), where it is controlled by electric field, temperature, or composition. Our results represent the first time that this evolution has been induced in a single component system using strain alone, and show that substrate- imposed symmetry lowering results in a similar phase instability as the proximity to a MPB in solid solutions.

cond-mat.mtrl-sci↗

arXiv2009

Parameter-free extraction of Thin-Film Dielectric Constants from Scanning Near Field Microwave Microscope Measurements

We present a method for extracting high-spatial resolution dielectric constant data at microwave frequencies. A scanning near field microwave microscope probes a sample and acquires data in the form of the frequency and quality factor shifts of a resonant cavity coupled to the sample. The approach reported here is to calculate the electromagnetic fields by the finite element method in both static and time-dependent modes. Cavity perturbation theory connects the measured frequency shifts to changes in the computed energy stored in the electromagnetic field. In this way, the complex permittivity of the sample is found. Of particular interest are thin-film materials, for which a method is reported here to determine the dielectric constant without the need to use any fitting parameters.

cond-mat.mtrl-sci↗

arXiv2008

Recent advances in pulsed-laser deposition of complex-oxides

Pulsed-laser deposition (PLD) is one of the most promising techniques for the formation of complex-oxide heterostructures, superlattices, and well-controlled interfaces. The first part of this paper presents a review of several useful modifications of the process, including methods inspired by combinatorial approaches. We then discuss detailed growth kinetics results, which illustrate that 'true' layer-by-layer (LBL) growth can only be approached, but not fully met, even though many characterization techniques reveal interfaces with unexpected sharpness. Time-resolved surface x-ray diffraction measurements show that crystallization and the majority of interlayer mass transport occur on time scales that are comparable to those of the plume/substrate interaction, providing direct experimental evidence that a growth regime exists in which non-thermal processes dominate PLD. This understanding shows how kinetic growth manipulation can bring PLD closer to ideal LBL than any other growth method available today.

cond-mat.other↗

arXiv2004

Spectroscopic imaging of single atoms within a bulk solid

The ability to localize, identify and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information is spatially confined around the scattering atom. Furthermore we show how the depth of the atom within the crystal may be estimated.

cond-mat.other↗

arXiv2003

Superconducting gap structure and pinning in disordered MgB2 films

We have performed a comparative study of two thin films of magnesium diboride (MgB2) grown by different techniques. The critical current density at different temperatures and magnetic fields was evaluated from magnetisation curves, the structure of superconducting order parameter was obtained from point-contact spectroscopy, and the scattering rates were evaluated by fitting the temperature dependent normal-state resistivity to the two-band model. The films have similar critical temperatures close to 39 K, but the upper critical fields were different by a factor of 2 (5.2T and 2.5 T at 20 K). We have found that the film with higher Hc2 also had stronger scattering in the sigma band and smaller value of the superconducting gap in this band. As the scattering in sigma band is primarily due to the defects in boron plane, our results are consistent with the assumption that disordering the boron planes leads to enhanced Hc2 and better pinning properties in magnetic field.

cond-mat.supr-con↗

arXiv2003

An improved continuous compositional-spread technique based on pulsed-laser deposition and applicable to large substrate areas

A new method for continuous compositional-spread (CCS) thin-film fabrication based on pulsed-laser deposition (PLD) is introduced. This approach is based on a translation of the substrate heater and the synchronized firing of the excimer laser, with the deposition occurring through a slit-shaped aperture. Alloying is achieved during film growth (possible at elevated temperature) by the repeated sequential deposition of sub-monolayer amounts. Our approach overcomes serious shortcomings in previous in-situ implementations of CCS based on sputtering or PLD, in particular the variations of thickness across the compositional spread and the differing deposition energetics as function of position. While moving-shutter techniques are appropriate for PLD-approaches yielding complete spreads on small substrates (i.e. small as compared to distances over which the deposition parameters in PLD vary, typically about 1 cm), our method can be used to fabricate samples that are large enough for individual compositions to be analyzed by conventional techniques, including temperature-dependent measurements of resistivity and dielectric and magnetic and properties (i.e. SQUID magnetometry). Initial results are shown for spreads of (Sr,Ca)RuO$_3$.

cond-mat.mtrl-sci↗

arXiv2001

The structure of the superconducting gap in MgB2 from point-contact spectroscopy

We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 +- 0.3 K and resistivity of about 20 microOhm cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, DELTA1=2.3+-0.3 meV and DELTA2=6.2+-0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder-Tinkham-Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic co-existence of two values of the gap are discussed.

cond-mat.supr-con↗

arXiv2001

Microwave Power, DC Magnetic Field, Frequency and Temperature Dependence of the Surface Resistance of MgB2

The microwave power, dc magnetic field, frequency and temperature dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by a number of characteristics, the most clear being the breakdown in the omega squared law for poor quality samples. Analysis of the experimental data suggests the most attractive procedure for high quality film growth for technical applications.

cond-mat.supr-con↗

arXiv2001

Far-infrared optical conductivity gap in superconducting MgB2 films

We report the first study of the optical conductivity of MgB2 covering the range of its superconducting energy gap. Terahertz time-domain spectroscopy is utilized to determine the complex, frequency-dependent conductivity of thin films. The imaginary part reveals an inductive reponse due to the emergence of the superconducting condensate. The real part exhibits a strong depletion of oscillator strength near 5 meV resulting from the opening of a superconducting energy gap. The gap ratio of 2Delta/kTc = 1.9 is well below the weak-coupling value, pointing to complex behavior in this novel superconductor

cond-mat.supr-con↗

arXiv2001

Unusual Hall Effect in Superconducting MgB$_2$ Films: Analogy to High-T$_c$ Cuprates

We have investigated the temperature and magnetic field dependence of the Hall coefficient of two well-characterized superconducting MgB$_2$ films (T$_{c0}$=38.0 K) in both the normal and superconducting states. Our results show that the normal-state Hall coefficient R$_H$ is positive and increases with decreasing temperature, independent of the applied magnetic field. Below T$_c$(H), R$_H$ decreases rapidly with temperature and changes sign before it reaches zero. The position and magnitude at which R$_H$ shows a minimum depends on the applied field. Quantitative analysis of our data indicates that the Hall response of MgB$_2$ behaves very similarly to that of high-T$_c$ cuprates: R$_H$ $\propto$ T and cot$θ_H$ $\propto$ T$^2$ in the normal state, and a sign reversal of R$_H$ in the mixed state. This suggests that the B-B layers in MgB$_2$, like the Cu-O planes in high-T$_c$ cuprates, play an important role in the electrical transport properties.

cond-mat.supr-con↗

arXiv2001

Growth mechanism of superconducting MgB2 films prepared by various methods

The growth mechanisms of MgB2 films obtained by different methods on various substrates are compared via a detailed cross-sectional scanning electron microscopy (SEM) study. The analyzed films include (a) samples obtained by an ex-situ post-anneal at 900 degree of e-beam evaporated boron in the presence of an Mg vapor (exhibiting bulk-like Tc0 about 38.8 K), (b) samples obtained by the same ex-situ 900 degree anneal of pulsed laser deposition (PLD)-grown Mg+B precursors (exhibiting Tc0 ~ 25 K), and (c) films obtained by a low-temperature (600 - 630 degree) in-situ anneal of PLD-grown Mg+B precursors (with Tc0 about 24 K). A significant oxygen contamination was also present in films obtained from a PLD-grown precursors. On the other hand, it is clearly observed that the films obtained by the high-temperature reaction of e-beam evaporated B with Mg vapor are formed by the nucleation of independent MgB2 grains at the film surface, indicating that this approach may not be suitable to obtain smooth and (possibly) epitaxial films.

cond-mat.supr-con↗

arXiv2001

A continuous compositional-spread technique based on pulsed-laser deposition and applied to the growth of epitaxial films

A novel continuous-compositional spread technique based on the non-uniformity of the deposition rate typically observed in Pulsed Laser Deposition (PLD) is introduced. The approach uses the spatial variations in the deposition-rate naturally occurring in PLD; therefore, there is no need for the masks typically used in combinatorial techniques. Consequently, combinatorial materials synthesis can be carried out under optimized film growth conditions (e.g. at high temperature). Additionally, lifting the need for post-annealing renders this method applicable to heat-sensitive materials and substrates (e.g. films of transparent oxides on polymer substrates). Composition determination across the sample and mapping of physical properties onto the ternary phase diagram is achieved via a simple algorithm using the parameters that describe the deposition-rate profiles. Results are shown for the high-temperature growth of crystalline perovskites (including (Ba,Sr)TiO3 and the formation of a metastable alloy between SrRuO3 and SrSnO3) and the room-temperature growth of transparent conducting oxides.

cond-mat.mtrl-sci↗

arXiv2001

Superconducting magnesium diboride films on Silicon with Tc0 about 24K grown via vacuum annealing from stoichiometric precursors

Superconducting magnesium diboride films with Tc0 ~ 24 K and sharp transition \~ 1 K were successfully prepared on silicon substrates by pulsed laser deposition from a stoichiometric MgB2 target. Contrary to previous reports, anneals at 630 degree and a background of 2x10^(-4) torr Ar/4%H2 were performed without the requirement of Mg vapor or an Mg cap layer. This integration of superconducting MgB2 films on silicon may thus prove enabling in superconductor-semiconductor device applications. Images of surface morphology and cross-section profiles by scanning electron microscopy (SEM) show that the films have a uniform surface morphology and thickness. Energy dispersive spectroscopy (EDS) reveals these films were contaminated with oxygen, originating either from the growth environment or from sample exposure to air. The oxygen contamination may account for the low Tc for those in-situ annealed films, while the use of Si as the substrate does not result in a decrease in Tc as compared to other substrates.

cond-mat.supr-con↗

arXiv2001

Superconducting MgB(2) films via precursor post-processing approach

Superconducting MgB(2) films with Tc = 38.6 K were prepared using a precursor-deposition, ex-situ post-processing approach. Precursor films of boron, ~0.5 micrometer thick, were deposited onto Al(2)O(3) (102) substrates by e-beam evaporation; a post-anneal at 890 deg C in the presence of bulk MgB(2) and Mg metal produced highly crystalline MgB(2) films. X-ray diffraction indicated that the films exhibit some degree of c-axis alignment, but are randomly oriented in-plane. Transport current measurements of the superconducting properties show high values of the critical current density and yield an irreversibility line that exceeds that determined by magnetic measurements on bulk polycrystalline materials.

cond-mat.supr-con↗