J. Wolfman

Suppression of structural and magnetotransport transitions in compressed Pr0.5Sr0.5MnO3 thin films resulting in colossal magnetoresistance effect

Thin films of Pr0.5Sr0.5MnO3 have been deposited on [100]-LaAlO3 using laser ablation. In contrast to the bulk compounds, such films do not exhibit any structural and magnetotransport transitions versus temperature; more particularly the A-type antiferromagnetic phase with the Fmmm structure which exists in the bulk below TN=135 K is suppressed, and the film is an insulator in the absence of a magnetic field. However a colossal magnetoresistance effect is observed, with resistance ratios much larger than in the bulk. These differences with respect to the bulk, are explained by the presence of substrate-induced strains.

Phonon and magnon excitations in Raman spectra of an epitaxial bismuth ferrite film

An epitaxial film of bismuth ferrite BiFeO3 on a MgO(001) single-crystal substrate has been prepared by pulsed laser deposition using SrTiO3 and SrRuO3 buffer layers. At room temperature, the polarization characteristics of the Raman spectra of the BiFeO3 film under study suggest a monoclinic symmetry. The high-temperature (295–1100 K) investigations of the Raman spectra have been performed in the frequency range 20 cm−1 < ν < 1600 cm−1. Particular attention has been paid to the high-frequency region with a band observed at 610 cm−1, which corresponds to the maximum density of states of the magnon branch at the Brillouin zone boundary, and an intense band in the second-order Raman spectra with the maximum at ∼1250 cm−1, which corresponds to the density of states of two-magnon excitations. It has been found that the intensity of the band at ∼1250 cm−1 decreases linearly with an increase in the temperature and, above 650 K, this band is absent. The extrapolation of the temperature dependence of the integrated intensity of the band at 1250 cm−1 suggests that this film undergoes an antiferromagnetic phase transition at a temperature of ∼670 K.

Enhancement of piezoelectric response in Ga doped BiFeO3 epitaxial thin films

The piezoelectric properties of compositional spread (1 − x)BiFeO3-xGaFeO3 epitaxial thin films are investigated where Ga3+ substitution for Bi3+ is attempted in Bi1−xGaxFeO3 compounds. Ga content x was varied from 0 to 12% (atomic). Ferroelectric characterizations are reported at various length scales. Around 6.5% of Ga content, an enhancement of the effective piezoelectric coefficient d33eff is observed together with a change of symmetry of the film. Measured d33eff values in 135 nm thick films increased from 25 pm/V for undoped BiFeO3 to 55 pm/V for 6.5% Ga with no extrinsic contribution from ferroelastic domain rearrangement.

Tunability, dielectric, and piezoelectric properties of Ba(1−x)CaxTi(1−y)ZryO3 ferroelectric thin films

Tunable ferroelectric capacitors, which exhibit a decrease of the dielectric permittivity (ϵ) under electric field, are widely used in electronics for RF tunable applications (e.g., antenna impedance matching). Current devices use barium strontium titanate as the tunable dielectric, and the need for performance enhancement of the tunable element is the key for device improvement. We report here on libraries of Ba0.97Ca0.03Ti1−xZrxO3thin films (0 ≤ x ≤ 27%) with a thickness of about 130 nm deposited on IrO2/SiO2/Si substrates using combinatorial pulsed laser deposition allowing for gradients of composition on one sample. A total of 600 capacitors on a single sample were characterized in order to statistically investigate the dielectric properties. We show that the tunabilty is maximum at intermediate compositions, reaching values up to 60% for an electric field of about 400 kV cm−1. We attribute the high tunability in the intermediate compositions to the paraelectric-ferroelectric phase transition, which is brought down to room temperature by the addition of Zr. In addition, the piezoelectric coefficient is found to be decreasing with increasing Zr content.

Time-resolved spatial distribution of plasma in the ablation of a Ba0.6Sr0.4TiO3 target by 25 ns KrF ultraviolet laser

We performed radially and longitudinally time-resolved plasma analysis during pulsed laser deposition of Ba0.6Sr0.4TiO3 thin films. The plasma is shown to be optically thick and strongly non-uniform during the early expansion phase and the resonance line Ba II (455.4 nm) is strongly self-reversed during this time. Plasma temperature and electron density were obtained by comparing experimental emission spectra with the spectral radiance computed for a non-uniform plasma in local thermal equilibrium.

Resistive Switching Hysteresis in Thin Films of Bismuth Ferrite

We have studied the resistive switching (RS) phenomenon in series of BiFeO3 thin films of thickness of 40 – 154 nm deposited by PLD technique on conducting Nb-doped substrate of SrTiO3 and with Pt top electrodes. It was found that 154 nm film demonstrates the interface-provided I–V characteristic of Schottky diode when the applied voltage does not exceed the threshold value Vd = 1.3 V. The RS phenomenon appears as the current hysteresis loop during the 0 → Vm → 0 → −Vm → 0 voltage sweep cycle, provided that the maximal stop-voltage Vm is larger than Vd. For thinner films neither diode-like I–V behavior nor substantial RS effect were observed. The results are interpreted in terms of the filamentary model of the mobile oxygen vacancies.

Interlayer strain effects on the structural behavior of BiFeO3/LaFeO3 superlattices

Artificial (BiFeO3)0.5Λ/(LaFeO3)0.5Λ superlattices have been grown by pulsed laser deposition. The periodicity Λ was varied from 150 A to 25 A and the relative ratio between BiFeO3 (BFO) and LaFeO3 (LFO) is kept constant in each period. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy investigations indicate antiferroelectric-like structures for large periodicity (Λ ≥ 76 A), while Pnma LaFeO3-like structures are observed for small periodicity Λ ≤ 50 A. Room temperature magnetic measurements were obtained by vibrating sample magnetometry and suggest antiferromagnetic ordering with weak ferromagnetism. Temperature dependent x-ray diffraction studies show an important shift of paraelectric-antiferroelectric phase transition scaling with BFO thickness. Strain and size effects explain this behavior and discussion is also made on the possible role of the oxygen octahedral rotation/tilt degree of freedom.Artificial (BiFeO3)0.5Λ/(LaFeO3)0.5Λ superlattices have been grown by pulsed laser deposition. The periodicity Λ was varied from 150 A to 25 A and the relative ratio between BiFeO3 (BFO) and LaFeO3 (LFO) is kept constant in each period. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy investigations indicate antiferroelectric-like structures for large periodicity (Λ ≥ 76 A), while Pnma LaFeO3-like structures are observed for small periodicity Λ ≤ 50 A. Room temperature magnetic measurements were obtained by vibrating sample magnetometry and suggest antiferromagnetic ordering with weak ferromagnetism. Temperature dependent x-ray diffraction studies show an important shift of paraelectric-antiferroelectric phase transition scaling with BFO thickness. Strain and size effects explain this behavior and discussion is also made on the possible role of the oxygen octahedral rotation/tilt degree of freedom.

Cyclable and non-volatile electric field control of magnetism in BiFeO3 based magnetoelectric heterostructures

Room temperature manipulation of the ferromagnetic state via an electric field is investigated in Ni/BiFe0.95Mn0.05O3 thin film heterostructures. A 600% increase in the magnetic coercive field of the Ni layer is observed at the initial DC electrical poling of the ferroelectric BiFe0.95Mn0.05O3 layer. The magnetoelectric effect is remanent, and the magnetic coercive field can be modulated between a low value and a high value by successively switching the ferroelectric polarization. After the initial poling, the coercive field difference is decreased by subsequent back and forth switching. However, the magnetic bi-stability is preserved at least up to 250 cycles, which is promising for spintronic applications.

Modelling and experimental measurements of the mechanical response of piezoelectric structures from millimetre to micrometre

ABSTRACT Laser interferometry techniques have shown their ability to assess the mechanical response of a piezoelectric thin film. To support these investigations, a numerical study based on the finite element method is carried out: three-dimensional modelling of piezoelectric samples from bulk materials to thin films is examined. For each considered sample, a time-dependent analysis and a frequency domain study are performed. By performing time-dependent analysis, we obtain effective piezoelectric coefficient, d33, of the samples. The frequency domain study helps to calculate the frequency response of these samples. The calculated d33 values and the first frequency resonance values are then compared with the experimental data.

Cryogenic plasma-processed silicon microspikes as a high-performance anode material for lithium ion-batteries

Micro- or nano-structuring is essential in order to use Si as an anode material for lithium ion batteries. In the present study, we attempted to use Si wafers with a spiky microstructure (SMS), the so-called black-Si, prepared by a cryogenic reactive ion etching process with an SF6/O2 gas mixture, for Li half-cells. The SMS with various sizes of spikes from 2.0 μm (height) × 0.2 μm (width) to 21 μm × 1.0 μm was etched by varying the SF6/O2 gas flow ratio. An anode of SMS of 11 μm-height in average showed stable charge/discharge capacity and Coulombic efficiency higher than 99% for more than 300 cycles, causing no destruction to any part of the Si wafer. The spiky structure turned columnar after cycles, suggesting graded lithiation levels along the length. The present results suggest a strategy to utilize a wafer-based Si material for an anode of a lithium ion battery durable against repetitive lithiation/delithiation cycles.