arXiv subjects

M. Sirena

Publications and source records attributed to M. Sirena.

12 recordsLinked to original sources

arXiv2020

Tuning the magneto-electrical properties of multiferroic multilayers through interface strain and disorder

Artificially engineered superlattices were designed and fabricated to induce different growth mechanisms and structural characteristics. DC sputtering was used to grow ferromagnetic (La$_{0.8}$Ba$_{0.2}$MnO$_3$) / ferroelectric (Ba$_{0.25}$Sr$_{0.75}$TiO$_3$ or BaTiO$_3$) superlattices. We systematically modified the thickness of the ferromagnetic layer to analyze dimensional and structural effects on the superlattices with different structural characteristics. The crystalline structure was characterized by X-Ray diffraction and transmission electron microscopy. The magnetic and electronic properties were investigated by SQUID magnetometry and resistance measurements. The results show that both strain and structural disorder can significantly affect the physical properties of the systems. Compressive strain tends to increase the competition between the magnetic interactions decreasing the ferromagnetism of the samples and the localization of the charge carrier through the electron-phonon interaction. Tensile strain reduces the charge carrier localization, increasing the ferromagnetic transition temperature. Structural defects have a stronger influence on the magnetic properties than on the transport properties, reducing the ferromagnetic transition temperature while increasing the magnetic hardness of the superlattices. These results help to further understand the role of strain and interface effects in the magnetic and transport properties of manganite based multiferroic systems.

cond-mat.mtrl-sci↗

arXiv2020

Josephson coupling in high-T$_c$ superconducting junctions using ultra-thin BaTiO$_3$ barriers

We study the electrical transport of vertically-stacked Josephson tunnel junctions using GdBa$_2$Cu$_3$O$_{7-d}$ electrodes and a BaTiO$_3$ barrier with thicknesses between 1 nm and 3 nm. The junctions with an area of 20 mm x 20 mm were fabricated combining optical lithography and ion etching using GdBa$_2$Cu$_3$O$_{7-d}$ (16 nm) / BaTiO$_3$ (1 - 3 nm) / GdBa$_2$Cu$_3$O$_{7-d}$ (16 nm) trilayers growth by sputtering on (100) SrTiO$_3$. Current-voltage measurements at low temperatures show a Josephson coupling for junctions with BaTiO$_3$ barriers of 1 nm and 2 nm. Reducing the barrier thickness bellow a critical thickness seems to suppress the ferroelectric nature of the BaTiO$_3$. The Josephson coupling temperature is strongly reduced for increasing barrier thicknesses, which may be related to the suppression of the superconducting critical temperature in the bottom GdBa$_2$Cu$_3$O$_{7-d}$ due to stress. The Josephson energies at 12 K are of $\approx$ 1.5 mV and $\approx$ 7.5 mV for BaTiO$_3$ barriers of 1 nm and 2 nm. Fraunhofer patterns are consistent with fluctuations in the critical current due to structural inhomogeneities in the barriers. Our results are promising for the development of Josephson junctions using high-T$_c$ electrodes with energy gaps much higher than those usually present in conventional low-temperature superconductors.

cond-mat.supr-con↗

arXiv2019

Electrical conductivity in extremely disordered molybdenum oxynitrides thin films

We report on the influence of the chemical composition on the electronic properties of molybdenum oxynitrides thin films grown by reactive sputtering on Si (100) substrates at room temperature. The partial pressure of Ar was fixed at 90 %, and the remaining 10 % was adjusted with mixtures N$_2$:O$_2$ (varying from pure N$_2$ to pure O$_2$). The crystalline and electronic structures and the electrical transport of the films depend on the chemical composition. Thin films grown using oxygen mixtures up 2 % have gamma-Mo$_2$N phase and display superconductivity. The superconducting critical temperature T$_c$ reduces from ~ 6.8 K to below 3.0 K as the oxygen increases. On the other hand, films grown using oxygen mixtures richer than 2 % are mostly amorphous. The electrical transport shows a semiconductor-like behavior with variable-range hopping conduction at low temperatures. The analysis of the optical properties reveals that the samples have not a defined semiconductor band gap, which can be related to the high structural disorder and the excitation of electrons in a wide range of energies

cond-mat.mtrl-sci↗

arXiv2019

Improving the Josephson energy in High-T$_c$ superconducting junctions for ultra-fast electronics

We report the electrical transport in vertical Josephson tunnel junctions (area 400 $μm$$^2$) using GdBa$_2$Cu$_3$O$_7$$_{-δ}$ electrodes and SrTiO$_3$ as an insulating barrier (with thicknesses between 1 nm and 4 nm). The results show Josephson coupling for junctions with SrTiO$_3$ barriers of 1 nm and 2 nm. The latter displays a Josephson of 8.9 mV at 12 K. This value is larger than the usually observed in planar arrays of junctions. Our results are promising for the development of superconducting electronic devices in the terahertz regime.

cond-mat.supr-con↗

arXiv2018

Synthesis of nanocrystalline d-MoN by thermal annealing of amorphous thin films grown on (100) Si by reactive DC sputtering at room temperature

We report on the synthesis and characterization of nanocrystalline delta-MoN by crystallization of amorphous thin films grown on (100) Si by reactive sputtering at room temperature. Films with chemical composition MoN were grown using a deposition pressure of 5mTorr with a reactive mixture of Ar/(Ar+N2)=0.5. The as-grown films display mostly amorphous structure. Nanocrystalline delta-MoN phase is obtained after annealing at temperatures above 600 °C. The superconducting critical temperature Tc depends on film thickness. Thick films (170 nm) annealed at 700 °C for 30 min display a Tc = 11.2 K (close to the one reported for bulk specimens: 13 K), which is gradually suppressed to 7.2 K for 40 nm thick delta-MoN films. Our results provide a simple method to synthesize superconducting nitride thin films on silicon wafers with Tc above the ones observed for conventional superconductors such as Nb.

cond-mat.mtrl-sci↗

arXiv2017

Effect of the nitrogen-argon gas mixtures on the superconductivity properties of reactively sputtered molybdenum nitride thin films

We report on the superconducting properties of molybdenum nitride thin films grown by reactive DC sputtering at room temperature with a N2:Ar mixture. Thin films grown using 5 % N2 concentration display Tc = 8 K, which is gradually reduced and abruptly disappears for 40 % N2 concentration. This suppression can be associated with changes in the nitrogen stoichiometry from Mo2N to MoN. Our results provide an effective and simple path to prepare Mo2Nx thin films with tunable Tc, which is relevant for the investigation of the fundamental properties and for technological applications.

cond-mat.mtrl-sci↗

arXiv2009

Direct observation of electronic inhomogeneities induced by point defect disorder in manganite films

We have investigated the influence of point defect disorder in the electronic properties of manganite films. Real-time mapping of ion irradiated samples conductivity was performed though conductive atomic force microscopy (CAFM). CAFM images show electronic inhomogeneities in the samples with different physical properties due to spatial fluctuations in the point defect distribution. As disorder increases, the distance between conducting regions increases and the metal-insulator transition shifts to lower temperatures. Transport properties in these systems can be interpreted in terms of a percolative model. The samples saturation magnetization decreases as the irradiation dose increases whereas the Curie temperature remains unchanged.

cond-mat.mtrl-sci↗

arXiv2009

Influence of ion implantation on the magnetic and transport properties of manganite films

We have used oxygen ions irradiation to generate controlled structural disorder in thin manganite films. Conductive atomic force microscopy CAFM), transport and magnetic measurements were performed to analyze the influence of the implantation process in the physical properties of the films. CAFM images show regions with different conductivity values, probably due to the random distribution of point defect or inhomogeneous changes of the local Mn3+/4+ ratio to reduce lattice strains of the irradiated areas. The transport and magnetic properties of these systems are interpreted in this context. Metal-insulator transition can be described in the frame of a percolative model. Disorder increases the distance between conducting regions, lowering the observed TMI. Point defect disorder increases localization of the carriers due to increased disorder and locally enhanced strain field. Remarkably, even with the inhomogeneous nature of the samples, no sign of low field magnetoresistance was found. Point defect disorder decreases the system magnetization but doesn t seem to change the magnetic transition temperature. As a consequence, an important decoupling between the magnetic and the metal-insulator transition is found for ion irradiated films as opposed to the classical double exchange model scenario.

cond-mat.mtrl-sci↗

arXiv2007

Improving HTc Josephson Junctions (HTc JJ) by annealing: the role of vacancy-interstitial annihilation

We have studied the annealing effect in transport properties of High temperature Josephson Junctions (HTc JJ) made by ion irradiation. Low temperature annealing (80 degrees Celsius) increases the JJ transition temperature (TJ) and the Ic.Rn product, where Ic is the critical current and Rn the normal resistance. We found that the spread in JJ characteristics can be lowered by sufficient long annealing times. Using random walk numerical simulations, we showed that the characteristic annealing time and the evolution of the spread in JJ characteristics can be explained by a vacancy-interstitial annihilation process rather than by an oxygen diffusion one.

cond-mat.supr-con↗

arXiv2007

Study and optimization of ion-irradiated High-Tc Josephson nanoJunctions by Monte Carlo simulations

High Tc Josephson nanoJunctions (HTc JnJ) made by ion irradiation have remarkable properties for technological applications. However, the spread in their electrical characteristics increases with the ion dose. We present a simple model to explain the JnJ inhomogeneities, which accounts quantitatively for experimental data. The spread in the slit's width of the irradiation mask is the limiting factor.Monte Carlo simulations have been performed using different irradiation conditions to study their influence on the spread of the JnJ charcateristics. A "universal" behavior has been evidenced, which allows to propose new strategies to optimize JnJ reproducibility.

cond-mat.supr-con↗

arXiv2005

Combining half-metals and multiferroics into epitaxial heterostructures for spintronics

We report on the growth of epitaxial bilayers of the La2/3Sr1/3MnO3 (LSMO) half-metallic ferromagnet and the BiFeO3 (BFO) multiferroic, on SrTiO3(001) by pulsed laser deposition. The growth mode of both layers is two-dimensional, which results in unit-cell smooth surfaces. We show that both materials keep their properties inside the heterostructures, i.e. the LSMO layer (11 nm thick) is ferromagnetic with a Curie temperature of ~330K, while the BFO films shows ferroelectricity down to very low thicknesses (5 nm). Conductive-tip atomic force microscope mappings of BFO/LSMO bilayers for different BFO thicknesses reveal a high and homogeneous resistive state for the BFO film that can thus be used as a ferroelectric tunnel barrier in tunnel junctions based on a half-metal.

cond-mat.mtrl-sci↗

arXiv2004

Antiferromagnetism at the YBa2Cu3O7 / La2/3Ca1/3MnO3 interface

The magnetic properties of a series of YBa2Cu3O7-x/La2/3Ca1/3MnO3 (YBCO/LC1/3MO) superlattices grown by dc sputtering at high oxygen pressures (3.5 mbar) show the expected ferromagnetic behaviour. However, field cooled hysteresis loops at low temperature show the unexpected existence of exchange bias, effect associated with the existence of ferromagnetic/antiferromagnetic (F/AF) interfaces. The blocking temperature (TB) is found thickness dependent and the exchange bias field (HEB) is found inversely proportional to the FM layer thickness, as expected. The presence of an AF material is probably associated to interface disorder and Mn valence shift towards Mn4+.

cond-mat.mtrl-sci↗