Browsing by Autor "Juan Gabriel Ramírez"

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Driving magnetic domains at the nanoscale by interfacial strain-induced proximity
(Royal Society of Chemistry, 2021) Ilya Valmianski; Arantxa Fraile Rodríguez; Javier Rodríguez-Álvarez; M. Garcı́a del Muro; Christian Wolowiec; Florian Kronast; Juan Gabriel Ramírez; Iván K. Schuller; A. Labarta; X. Batlle
We investigate the local nanoscale changes of the magnetic anisotropy of a Ni film subject to an inverse magnetostrictive effect by proximity to a V2O3 layer. Using temperature-dependent photoemission electron microscopy (PEEM) combined with X-ray magnetic circular dichroism (XMCD), direct images of the Ni spin alignment across the first-order structural phase transition (SPT) of V2O3 were obtained. We find an abrupt temperature-driven reorientation of the Ni magnetic domains across the SPT, which is associated with a large increase of the coercive field. Moreover, angular dependent ferromagnetic resonance (FMR) shows a remarkable change in the magnetic anisotropy of the Ni film across the SPT of V2O3. Micromagnetic simulations based on these results are in quantitative agreement with the PEEM data. Direct measurements of the lateral correlation length of the Ni domains from XMCD images show an increase of almost one order of magnitude at the SPT compared to room temperature, as well as a broad spatial distribution of the local transition temperatures, thus corroborating the phase coexistence of Ni anisotropies caused by the V2O3 SPT. We show that the rearrangement of the Ni domains is due to strain induced by the oxide layers' structural domains across the SPT. Our results illustrate the use of alternative hybrid systems to manipulate magnetic domains at the nanoscale, which allows for engineering of coercive fields for novel data storage architectures.
Optoelectronic Properties of the Yba2cu3o7-Δ-Batio3 Hybrid System
(RELX Group (Netherlands), 2024) Sebastián Rodríguez; Nicolle Tello Diaz; Mario Fernando Quinones Penagos; John Betancourt; Juan Gabriel Ramírez; Andrea Steffania Esquivel; Milton Manotas-Albor; W. Lopera; Luis Alfredo Rodríguez; Lorena Marín Mercado
Search for New Superconductors: An Electro-Magnetic Phase Transition in an Iron Meteorite Inclusion at 117 K
(Cornell University, 2015) Stefan Guénon; Juan Gabriel Ramírez; Ali C. Basaran; Jamie Wampler; M. H. Thiemens; Iván K. Schuller
The discovery of superconductivity in pnictides and iron chalcogenides inspires the search for new iron based superconducting phases. Iron-rich meteorites present a unique opportunity for this search, because they contain a broad range of compounds produced under extreme growth conditions. We investigated a natural iron sulfide based materials (Troilite) inclusion with its associated minerals in the iron meteorite Tlacotepec. Tlacotepec formed in an asteroidal core under high pressure and at high temperature over millions of years, while insoluble sulfur rich materials segregated into inclusions during cooling along with included minerals. The search for superconductivity in these heterogeneous materials requires a technique capable of detecting minute amounts of a superconducting phase embedded in a non-superconducting matrix. We used Magnetic Field Modulated Microwave Spectroscopy (MFMMS), a very sensitive, selective, and non-destructive technique, to search for superconductivity in heterogeneous systems. Here, we report the observation of an electro-magnetic phase transition at 117 K that causes a MFMMS-response typical of a superconductor. A pronounced and reproducible peak together with isothermal magnetic field sweeps prove the appearance of a new electromagnetic phase below 117 K. This is very similar to the characteristic response due to flux trapping in a granular superconductor with a short coherence length. Although the compound responsible for the peak in the MFMMS-spectra was not identified, it is possibly an iron sulfide based phase, or another material heterogeneously distributed over the inclusion.
Tuning electronic and magnetic properties through disorder in V2O5 nanoparticles
(Nature Portfolio, 2023) Sergio Correal; Daniel Hernández-Gómez; Andrea Steffania Esquivel; Alexander Cardona-Rodríguez; Andreas Reiber; Yenny Hernández; Rafael González‐Hernández; Juan Gabriel Ramírez
We report on the synthesis and characterization of V<sub>2</sub>O<sub>5</sub> nanoparticles grown using a sol-gel method at different calcination temperatures. We observed a surprising reduction in the optical band gap from 2.20 to 1.18 eV with increasing calcination temperature from 400 to 500 °C. Raman and X-Ray diffraction measurements indicated slight changes in the lattice parameters induced by the growth process. However, density functional theory calculations of the Rietveld-refined and pristine structures revealed that the observed optical gap reduction could not be explained by structural changes alone. By introducing oxygen vacancies to the refined structures, we could reproduce the reduction of the band gap. Our calculations also showed that the inclusion of oxygen vacancies at the vanadyl position creates a spin-polarized interband state that reduces the electronic band gap and promotes a magnetic response due to unpaired electrons. This prediction was confirmed by our magnetometry measurements, which exhibited a ferromagnetic-like behavior. Our findings suggest that oxygen vacancies play a crucial role in band gap reduction and the promotion of a ferromagnetic-like response in an otherwise paramagnetic material. This provides a promising route to engineer novel devices.
Voltage-tunable spin resonance in quantum phase-separated material
(American Institute of Physics, 2025) Gabriel B. Gomide; Diego Carranza-Célis; G. Edward Kuhl; M. Knobel; Juan Gabriel Ramírez; Diego Muraca
The voltage control of spin and charge degrees of freedom in complex materials is a cornerstone for the realization of advanced electronic devices with enhanced functionalities. Here, we demonstrate in situ indirect current control via the Joule effect of the spin resonance parameters in a phase-separated La5/8−yPryCa3/8MnO3 sample while simultaneously inducing resistive switching. By employing electron paramagnetic resonance (EPR) spectroscopy under an applied bias voltage, we observe sharp, reversible modifications in the EPR spectra—linewidth, resonance field, and intensity—concurrent with voltage-driven transitions between the ferromagnetic metallic (FMM) and paramagnetic charge-ordered (PM-CO) states. This real-time probing of spin resonance during resistive switching provides crucial insights into the interplay between spin, charge, and lattice degrees of freedom, elucidating the distinct roles of the FMM and PM-CO phases in the observed behavior. These findings pave the way for the development of novel spintronic and neuromorphic devices with voltage-tunable functionalities.