arXiv subjects

A. G. Leyva

Publications and source records attributed to A. G. Leyva.

10 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↗

arXiv2014

Correlation between electrical and magnetic properties of phase separated manganites studied with a General Effective Medium model

We have performed electrical resistivity and DC magnetization measurements as a function of temperature, on polycrystalline samples of phase separated LaPrCaMnO. We have used the General Effective Medium Theory to obtain theoretical resistivity vs. temperature curves corresponding to different fixed ferromagnetic volume fraction values, assuming that the sample is a mixture of typical metallic like and insulating manganites. By comparing this data with our experimental resistivity curves we have obtained the relative ferromagnetic volume fraction of our sample as a function of temperature. This result matches with the corresponding magnetization data in excellent agreement, showing that a mixed phase scenario is the key element to explain both the magnetic and transport properties in the present compound.

cond-mat.mtrl-sci↗

arXiv2014

Hydrostatic pressure to trigger and assist magnetic transitions: baromagnetic refrigeration

The possible application of the barocaloric effect to produce solid state refrigerators is a topic of interest in the field of applied physics. In this work, we present experimental data about the influence of external pressure on the magnetic properties of a manganite with phase separation. Using the Jahn Teller effect associated with the presence of the charge ordering we were able to follow the transition to the ferromagnetic state induced by pressure. We also demonstrated that external pressure can assist the ferromagnetic state, decreasing the magnetic field necessary to generate the magnetic transition.

cond-mat.str-el↗

arXiv2013

Non-volatile multilevel resistive switching memory cell: A transition metal oxide-based circuit

We study the resistive switching (RS) mechanism as way to obtain multi-level memory cell (MLC) devices. In a MLC more than one bit of information can be stored in each cell. Here we identify one of the main conceptual difficulties that prevented the implementation of RS-based MLCs. We present a method to overcome these difficulties and to implement a 6-bit MLC device with a manganite-based RS device. This is done by precisely setting the remnant resistance of the RS-device to an arbitrary value. Our MLC system demonstrates that transition metal oxide non-volatile memories may compete with the currently available MLCs.

cond-mat.other↗

arXiv2012

Reversible switching of room temperature ferromagnetism in CeO2-Co nanoparticles

We investigated the reversible ferromagnetic (FM) behavior of pure and Co doped CeO2 nanopowders. The as-sintered samples displayed an increasing paramagnetic contribution upon Co doping. Room temperature FM is obtained simply by performing thermal treatments in vacuum at temperatures as low as 500^{\circ}C and it can be switched off by performing thermal treatments in oxidizing conditions. The FM contribution is enhanced as we increase the time of the thermal treatment in vacuum. Those systematic experiments establish a direct relation between ferromagnetism and oxygen vacancies and open a path for developing materials with tailored properties.

cond-mat.mtrl-sci↗

arXiv2012

Electrical transport properties of manganite powders under pressure

We have measured the electrical resistance of micrometric to nanometric powders of the La$_{5/8-y}$Pr$_y$Ca$_{3/8}$MnO$_3$ (LPCMO with y=0.3) manganite for hydrostatic pressures up to 4 kbar. By applying different final thermal treatments to samples synthesized by a microwave assisted denitration process, we obtained two particular grain characteristic dimensions (40 nm and 1000 nm) which allowed us to analyze the grain size sensitivity of the electrical conduction properties of both the metal electrode interface with manganite (Pt / LPCMO) as well as the intrinsic intergranular interfaces formed by the LPCMO powder, conglomerate under the only effect of external pressure. We also analyzed the effects of pressure on the phase diagram of these powders. Our results indicate that different magnetic phases coexist at low temperatures and that the electrical transport properties are related to the intrinsic interfaces, as we observe evidences of a granular behavior and an electronic transport dominated by the Space Charge limited Current mechanism.

cond-mat.mtrl-sci↗

arXiv2010

Magnetocaloric effect in manganites: metamagnetic transitions for magnetic refrigeration

We present a study of the magnetocaloric effect in La5/8-yPryCa3/8MnO3 (y=0.3) and Pr0.5Ca0.09Sr0.41MnO3 manganites. The low temperature state of both ystems is the result of a competition between the antiferromagnetic and ferromagnetic phases. The samples display magnetocaloric effect evidenced in an adiabatic temperature change during a metamagnetic transition from an antiferromagnetic to a ferromagnetic phase . As additional features, La5/8-yPryCa3/8MnO3 exhibits phase separation characterized by the coexistence of antiferromagnetic and ferromagnetic phases and Pr0.5Ca0.09Sr0.41MnO3 displays inverse magnetocaloric effect in which temperature decreases while applying an external magnetic field. In both cases, a significant part of the magnetocaloric effect appears from non-reversible processes. As the traditional thermodynamic description of the effect usually deals with reversible transitions, we developed an alternative way to calculate the adiabatic temperature change in terms of the change of the relative ferromagnetic fraction induced by magnetic field. To evaluate our model, we performed direct measurement of the sample's adiabatic temperature change by means of a differential thermal analysis. An excellent agreement has been obtained between experimental and calculated data. These results show that metamagnetic transition in manganites play an important role in the study of magnetic refrigeration.

cond-mat.str-el↗

arXiv2008

Magnetic Behavior of Single La$_{0.67}$Ca$_{0.33}$MnO$_3$ Nanotubes: Surface and Shape Effects

We report magnetization experiments in two magnetically isolated ferromagnetic nanotubes of perovskite La$_{0.67}$Ca$_{0.33}$MnO$_3$. The results show that the magnetic anisotropy is determined by the sample shape although the coercive field is reduced by incoherent magnetization reversal modes. The temperature dependence of the magnetization reveals that the magnetic behavior is dominated by grain surface properties. These measurements were acquired using a Silicon micro-mechanical oscillator working in its resonant mode. The sensitivity was enough to measure the magnetic properties of these two samples with a mass lower than 14 picograms and to obtain for the first time the magnetization loop for one isolated nanotube.

cond-mat.mes-hall↗

arXiv2005

The electrical current effect in phase separated La5/8-yPryCa3/8MnO3: Charge order melting vs. Joule heating

We have studied the effect of electric field on transport properties of the prototypical phase separated manganite La5/8-yPryCa3/8MnO3 with y=0.34. Our results show that the suggested image in which the charge ordered state is melted by the appliance of an electric current and/or voltage has to be revised. We were able to explain the observed resistivity drop in terms of an artifact related to Joule heating and the particular hysteresis that the system under study display, common to many other phase separated manganites.

cond-mat.str-el↗

arXiv2005

Simultaneous electric and magnetic field induced nonvolatile memory

We investigate the electric field induced resistive switching effect and magnetic field induced fraction enlargement on a polycrystalline sample of a colossal magnetoresistive compound displaying intrinsic phase coexistence. Our data show that the electric effect (presumably related to the presence of inhomogeinities) is present in a broad temperature range(300 to 20 K), being observable even in a mostly homogeneous ferromagnetic state. In the temperature range in which low magnetic field determines the phase coexistence fraction, both effects, though related to different mechanisms, are found to determine multilevel nonvolatile memory capabilities simultaneously.

cond-mat.str-el↗