martin Sirena | Balseiro Institute (original) (raw)

Papers by martin Sirena

Materials Chemistry and Physics, 2021

La0·8Ba0·2MnO3, Ba0·25Sr0·75TiO3 and BaTiO3 superlattices were grown to study the influence of st... more La0·8Ba0·2MnO3, Ba0·25Sr0·75TiO3 and BaTiO3 superlattices were grown to study the influence of structural disorder on the physical properties of multiferroic multilayers. Controlling the lattice mismatch of the superlattices allowed growing structures with different growth mechanisms. The manganite layers in the samples were used as “sensor layers”, that respond to the structural changes in the superlattices, induced by changing the thickness and nature of the ferroelectric layers. Stress has a weak influence on the magnetic properties of these systems. Transport properties are characterized by a high temperature thermally activated regime and a low temperature variable hopping one. The strain and structural disorder in the samples increases the localization energy of the current carriers for both regimes. Important interface effects can be achieved controlling the strain and disorder in the interfaces, allowing tuning the metal-insulator transition temperature. These results help t...

Journal of Alloys and Compounds, 2021

Artificially engineered superlattices were designed and fabricated to induce different growth mec... more 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 orBaTiO 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.

Materials Science and Engineering: B, 2020

We study the electrical transport of vertically-stacked Josephson tunnel junctions using GdBa2Cu3... more We study the electrical transport of vertically-stacked Josephson tunnel junctions using GdBa2Cu3O7−δ electrodes and a BaTiO3 barrier with thicknesses between 1 nm and 3 nm. The junctions with an area of 20 µm x 20 µm were fabricated combining optical lithography and ion etching using GdBa2Cu3O7−δ (16 nm) / BaTiO3 (1-3 nm) / GdBa2Cu3O7−δ (16 nm) trilayers growth by sputtering on (100) SrTiO3. Current-voltage measurements at low temperatures show a Josephson coupling for junctions with BaTiO3 barriers of 1 nm and 2 nm. Reducing the barrier thickness bellow a critical thickness seems to suppress the ferroelectric nature of the BaTiO3. 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 GdBa2Cu3O7−δ due to stress. The Josephson energies at 12 K are of ≈ 1.5 mV and ≈ 7.5 mV for BaTiO3 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-Tc electrodes with energy gaps much higher than those usually present in conventional low-temperature superconductors.

Journal of Applied Physics, 2016

Physical Review Applied, 2019

We report the precise control of tunneling magnetoresistance (TMR) in devices of self-assembled c... more We report the precise control of tunneling magnetoresistance (TMR) in devices of self-assembled core/shell Fe 3 O 4 /Co 1−x Zn x Fe 2 O 4 nanoparticles (0 ≤ x ≤ 1). Adjusting the magnetic anisotropy through the content of Co 2+ in the shell, provides an accurate tool to control the switching field between the bistable states of the TMR. In this way, different combinations of soft/hard and hard/soft core/shell configurations can be envisaged for optimizing devices with the required magnetotransport response.

Nanotechnology, 2019

We report the electrical transport in vertical Josephson tunnel junctions (area 400 µm2) using Gd... more We report the electrical transport in vertical Josephson tunnel junctions (area 400 µm2) 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.

Materials Research Express, 2018

We report the electrical transport properties of conducting / insulator / conducting heterostruct... more We report the electrical transport properties of conducting / insulator / conducting heterostructures by studying current-voltage IV curves at room temperature. The measurements were obtained on tunnel junctions with different areas (900 µm 2 , 400 µm 2 and 100µm 2) using a conducting atomic force microscope. Trilayers with GdBa 2 Cu 3 O 7 (GBCO) as the bottom electrode, SrTiO 3 or BaTiO 3 (thicknesses between 1.6 nm and 4 nm) as the insulator barrier, and GBCO or Nb as the top electrode were grown by DC sputtering on (100) SrTiO 3 substrates For SrTiO 3 and BaTiO 3 barriers, asymmetric IV curves at positive and negative polarization can be obtained using electrodes with different work function. In addition, hysteretic IV curves are obtained for BaTiO 3 barriers, which can be ascribed to a combined effect of the FE reversal switching polarization and an oxygen vacancy migration. For GBCO/ BaTiO 3 / GBCO heterostructures, the IV curves correspond to that expected for asymmetric interfaces, which indicates that the disorder affects differently the properties at the bottom and top interfaces. Our results show the role of the interface disorder on the electrical transport of conducting/ insulator/ conduction heterostructures, which is relevant for different applications, going from resistive switching memories (at room temperature) to Josephson junctions (at low temperatures).

Thin Solid Films, 2018

We report on the synthesis and characterization of nanocrystalline δ-MoN by crystallization of am... more We report on the synthesis and characterization of nanocrystalline δ-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+N 2)=0.5. The as-grown films display mostly amorphous structure. Nanocrystalline δ-MoN phase is obtained after annealing at temperatures above 600 °C. The superconducting critical temperature T c depends on film thickness. Thick films (170 nm) annealed at 700 °C for 30 min display a T c = 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 δ-MoN films. Our results provide a simple method to synthesize superconducting nitride thin films on silicon wafers with T c above the ones observed for conventional superconductors such as Nb.

Materials Letters, 2018

We report on the superconducting properties of molybdenum nitride thin films grown by reactive DC... more We report on the superconducting properties of molybdenum nitride thin films grown by reactive DC sputtering at room temperature with a N 2 :Ar mixture. Thin films grown using 5 % N 2 concentration display T c = 8 K, which is gradually reduced and abruptly disappears for 40 % N 2 concentration. This suppression can be associated with changes in the nitrogen stoichiometry from Mo 2 N to MoN. Our results provide an effective and simple path to prepare Mo 2 N x thin films with tunable T c , which is relevant for the investigation of the fundamental properties and for technological applications.

Materials Letters, 2019

Effect of thermal annealing and irradiation damage on the superconducting critical temperature of... more Effect of thermal annealing and irradiation damage on the superconducting critical temperature of nanocrystalline γ-Mo 2 N 1+y (0.0≤ y ≤0.33) thin films,

Applied Physics Letters, 2016

Journal of Applied Physics, 2015

The optimization of the superconducting properties in a bottom electrode and the quality of an in... more The optimization of the superconducting properties in a bottom electrode and the quality of an insulator barrier are the first steps in the development of superconductor/insulator/superconductor tunnel junctions. Here, we study the quality of a BaTiO 3 tunnel barrier deposited on a 16 nm thick GdBa 2 Cu 3 O 7Àd thin film by using conductive atomic force microscopy. We find that the tunnel current is systematically reduced (for equal applied voltage) by increasing the BaTiO 3 barrier thickness between 1.6 and 4 nm. The BaTiO 3 layers present an energy barrier of %1.2 eV and an attenuation length of 0.35-0.5 nm (depending on the applied voltage). The GdBa 2 Cu 3 O 7Àd electrode is totally covered by a BaTiO 3 thickness above 3 nm. The presence of ferroelectricity was verified by piezoresponse force microscopy for a 4 nm thick BaTiO 3 top layer. The superconducting transition temperature of the bilayers is systematically suppressed by increasing the BaTiO 3 thickness. This fact can be associated with stress at the interface and a reduction of the orthorhombicity of the GdBa 2 Cu 3 O 7Àd. The reduction in the orthorhombicity is expected by considering the interface mismatch and it can also be affected by reduced oxygen stoichiometry (poor oxygen diffusion across the BaTiO 3 barrier). V

We report on an experimental and theoretical study of the high-frequency mixing properties of ion... more We report on an experimental and theoretical study of the high-frequency mixing properties of ion-irradiated YBa2Cu3O7 Josephson junctions embedded in THz antennas. We investigated the influence of the local oscillator power and frequency on the device performances. The experimental data are compared with theoretical predictions of the general three-port model for mixers, in which the junction is described by the resistively shunted junction model. A good agreement is obtained for the conversion efficiency in different frequency ranges, spanning above and below the characteristic frequencies fc of the junctions.

Physica C: Superconductivity and its Applications, 2015

This paper provides a systematic analysis of the morphology and the superconducting critical temp... more This paper provides a systematic analysis of the morphology and the superconducting critical temperature obtained in very thin GdBa 2 Cu 3 O 7Àd films grown on (0 0 1) SrTiO 3 substrates by DC sputtering. We find that the use of a very thin SrTiO 3 buffer layer (%2 nm) modify the nucleation of GdBa 2 Cu 3 O 7Àd on the surface of the substrate reducing the formation of 3 dimensional clusters. Our results demonstrate that 16 nm thick GdBa 2 Cu 3 O 7Àd films with an average root-mean-square (RMS) smaller than 1 nm and large surface areas (up 10 lm 2) free of 3 dimensional topological defects can be obtained. In films thinner than 24 nm the onset (zero resistance) of superconducting transition of the films is reduced, being close to liquid nitrogen. This fact can be associated with stress reducing the orthorhombicity and slightly drop in oxygen stoichiometry.

Physical Review B, 2010

We have used oxygen ions irradiation to generate controlled structural disorder in thin manganite... more 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 T MI. 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 2 magnetic and the metal-insulator transition is found for ion irradiated films as opposed to the classical double exchange model scenario.

Journal of Magnetism and Magnetic Materials, 2004

The structural and physical properties of La 0:55 Sr 0:45 MnO 3 =SrTiO 3 superlattices grown by m... more The structural and physical properties of La 0:55 Sr 0:45 MnO 3 =SrTiO 3 superlattices grown by magnetron sputtering are studied. Two deposition temperatures and different mismatched substrates and buffer layers were used. The structure was determined by refinement through X-ray diffraction pattern fitting. The results indicate 1 unit cell (u.c.) interdiffusion at the interfaces and a 1 u.c. layer thickness fluctuation, i.e. roughness. In-plane hysteresis loops show the expected ferromagnetic behavior, while the perpendicular-to-plane hysteresis loops show anomalies which could evidence magnetic domain size inhomogeneities and stress.

Journal of Magnetism and Magnetic Materials, 2001

ABSTRACT The time dependence of the magnetic and transport properties on La0.6Sr0.4MnO3 films and... more ABSTRACT The time dependence of the magnetic and transport properties on La0.6Sr0.4MnO3 films and bulk samples has been studied through magnetization and resistivity measurements. A magnetic after-effect has been observed in all samples. At low temperatures, the low-field magnetization, can be described by the function M(t)=Mc+Mdexp(−t/τ)+S(H,T)ln(t). The resistivity increases logarithmically in the same temperature range, indicating the evolution of the sample to a more disordered state. Above a characteristic temperature, this behaviour is reversed and an increase of the magnetization with time is observed. The relaxation parameters depend on the bulk or films character of the samples. In the latter case, a dependence on the film thickness was found. A direct correlation between the time dependence of the resistivity and magnetization curves in manganite compounds was found.

Journal of Applied Physics, 2000

Journal of Applied Physics, 2003

Structural and magnetic measurements have been performed on the FINEMET type of ribbons with nomi... more Structural and magnetic measurements have been performed on the FINEMET type of ribbons with nominal composition of Fe 74 Cu 0.5 Nb 3 Si 13.5 B 9 synthesized by rapid solidification technique. The crystallization behavior and the nanocrystal formation have been studied by differential thermal analysis (DTA) and X-ray diffraction (XRD). The crystallization onset temperatures determined by XRD are in good agreement with DTA results. Magnetic permeability and magnetization measurements have been carried out using inductance analyzer and vibrating sample magnetometer (VSM). Magnetic permeability sensitively depends on the annealing temperature which increases sharply with the increase of annealing temperature. Maximum permeability corresponding to optimum annealing temperature (T a) was observed at T a = 575°C. Saturation magnetization, M s, increases with T a for the sample and finally decreases for annealing at a temperature much higher than peak crystallization temperature. The results show that the amounts of Cu and Nb are very important for the soft magnetic properties of FINEMET alloys.

Journal of Applied Physics, 2007

ABSTRACT In this article we describe the effect of ion irradiation on high-Tc superconductor thin... more ABSTRACT In this article we describe the effect of ion irradiation on high-Tc superconductor thin film and its interest for the fabrication of Josephson junctions. In particular, we show that these alternative techniques allow to go beyond most of the limitations encountered in standard junction fabrication methods, both in the case of fundamental and technological purposes. Two different geometries are presented: a planar one using a single high-Tc film and a mesa one defined in a trilayer structure.