arXiv subjects

Marcelo Knobel

Publications and source records attributed to Marcelo Knobel.

5 recordsLinked to original sources

arXiv2025

Local Irreversible phase reconfiguration and thermal-memory effects in a highly-correlated manganite

Phase-separated manganites provide a unique platform to study the dynamics of competing electronic and structural orders in correlated systems. In La0.275Pr0.35Ca0.375MnO3 (LPCMO), we use temperature-cycling Raman spectroscopy to uncover a previously unidentified regime of structural irreversibility, emerging from the interplay between lattice distortions and phase competition across the phase-separation and charge-orbital ordering temperatures. This irreversible behavior encodes a thermal-memory effect reflecting the system's history-dependent energy landscape. Correlated magnetic and transport responses confirm the coupling between lattice and electronic degrees of freedom, revealing a new form of nonequilibrium phase dynamics in mixed-valence oxides. These results advance the understanding of metastability and memory phenomena in strongly correlated materials.

cond-mat.str-el↗

arXiv2022

Artificial kagome spin ice magnetic phase recognition from the initial magnetization curve

Artificial spin ices (ASIs) are designable arrays of interacting nanomagnets that span a wide range of magnetic phases associated with a number of spin lattice models. Here, we demonstrate that the phase of an artificial kagome spin ice can be determined from its initial magnetization curve. As a proof of concept, micromagnetic simulations of these curves were performed starting from representative microstates of different phases of the system. We show that the curves are characterized by phase-specific features in such a way that a pattern recognition algorithm predicts the phase of the initial microstate with good reliability. This achievement represents a new strategy to identify phases in ASIs, easier and more accessible than magnetic imaging techniques normally used for this task.

cond-mat.mes-hall↗

arXiv2013

Macroscopic quantum tunneling of magnetization explored by quantum-first-order reversal curves (QFORC)

A novel method to study the fundamental problem of quantum double well potential systems that display magnetic hysteresis is proposed. The method, coined quantum-first-order reversal curve (QFORC) analysis, is inspired by the conventional first-order reversal curve (FORC) protocol, based on the Preisach model for hysteretic phenomena. We successfully tested the QFORC method in the peculiar hysteresis of the Mn12Ac molecular magnet, which is governed by macroscopic quantum tunneling of magnetization. The QFORC approach allows one to quickly reproduce well the experimental magnetization behavior, and more importantly to acquire information that is difficult to infer from the usual methods based on matrix diagonalization. It is possible to separate the thermal activation and tunneling contributions from the magnetization variation, as well as understand each experimentally observed jump of the magnetization curve and associate them with specific quantum transitions.

cond-mat.mes-hall↗

arXiv2011

Hysteretic giant magnetoimpedance effect analyzed by first-order reversal curves

Hysteretic giant magnetoimpedance (GMI) of amorphous ribbons with a well-defined transversal domain structure is investigated by means of first-order reversal curves (FORC) analysis. The FORCs are not confined to the hysteretic area, exceeding the major curve amplitude. Irreversible switches of the transverse permeability, caused by domain wall structure transitions, may be the origin of the observed FORC distribution. An interlinked hysteron/anti-hysteron model is proposed to interpret it, which allows analyzing the influence of frequency and magnetostriction upon the hysteretic GMI effect.

cond-mat.mes-hall↗

arXiv2009

Electron Dynamics in Films Made of Transition Metal Nanograins Embedded in SiO2:Infrared Reflectivity and Nanoplasma Infrared Resonance

We report on near normal infrared reflectivity spectra of ~550 nm thick films made of cosputtered transition metal nanograins and SiO2 in a wide range of metal fractions. Co0.85(SiO2)0.15,with conductivity well above the percolation threshold has a frequency and temperature behavior according to what it is find in conducting metal oxides. The electron scattering rate displays an unique relaxation time characteristic of single type of carriers experiencing strong electron-phonon interactions. Using small polaron fits we identify those phonons as glass vibrational modes. Ni0.61(SiO2)0.39, with a metal fraction closer to the percolation threshold, undergoes a metal-non metal transition at ~77 K. Here, as it is suggested by the scattering rate nearly quadratic dependence, we broadly identify two relaxation times (two carrier contributions) associated to a Drude mode and a mid-infrared overdamped band, respectively. Disorder induced, the mid-infrared contribution drives the phase transition by thermal electron localization. Co0.51(SiO2)0.49 has the reflectivity of an insulator with a distinctive band at ~1450cm\^{-1} originating in electron promotion, localization, and defect induced polaron formation. Angle dependent oblique reflectivity of globally insulating Co0.38(SiO2)0.62, Fe0.34(SiO2)0.66, and Ni0.28(SiO2)0.72, reveals a remarkable resonance at that band threshold. We understand this as due to the excitation by normal to the film electric fields of defect localized electrons in the metallic nanoparticles

cond-mat.mtrl-sci↗