José Luis García-Muñoz

Bio: José Luis García-Muñoz is an academic researcher from Institute of Cost and Management Accountants of Bangladesh. The author has contributed to research in topics: Neutron diffraction & Magnetization. The author has an hindex of 37, co-authored 195 publications receiving 5410 citations. Previous affiliations of José Luis García-Muñoz include European Synchrotron Radiation Facility & Autonomous University of Barcelona.

Colossal magnetoresistance of ferromagnetic manganites: Structural tuning and mechanisms.

Abstract: The amplitude $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$ and temperature ${T}_{M}$, where the colossal magnetoresistance (CMR) response of ${L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ manganites are maximum, are found to be controlled by the radius of the lanthanide $({L}^{3+})$ which modifies the bending of the Mn-O-Mn bond. Increasing the bond distortion lowers ${T}_{M}$ and enhances $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$. Enhanced CMR arises from (1) a shift to lower temperatures of ${T}_{M}$, (2) a reduced mobility of the doping holes, and (3) an increase of the coupling between itinerant and localized electrons. The resistivity $\ensuremath{\rho}\left(H\right)$ follows an $\ensuremath{\approx}{\mathrm{BM}}^{2}\left(H\right)$ law and the parameter $B$ is also tuned by the Mn-O-Mn bond angle. The narrowing of the electronic bandwidth is the fundamental parameter controlling the observed CMR.

Neutron-diffraction study of RNiO3 (R=La,Pr,Nd,Sm): Electronically induced structural changes across the metal-insulator transition.

Abstract: In the R${\mathrm{NiO}}_{3}$ series (R=La,Pr,Nd,Sm), the metal-insulator (M-I) transition temperature rises systematically as the size of the rare earth decreases and as the subsequent distortion from the ideal cubic perovskite increases. For R=La the system keeps its metallic character down to 1.5 K, while for R=Pr, Nd, and Sm electronic localization occurs at 135, 200, and 400 K, respectively. High-resolution neutron-powder-diffraction experiments have been performed to investigate the structural anomalies across the first-order M-I transition in the orthorhombic ${\mathrm{PrNiO}}_{3}$ and ${\mathrm{NdNiO}}_{3}$ compounds. The cell volume undergoes a subtle increase when the compounds become insulating, due to a slight increase of the Ni-O distances. This effect is accompanied by coupled tilts of ${\mathrm{NiO}}_{6}$ octahedra, which imply changes in the Ni-O-Ni angles (\ensuremath{\Delta}${\mathrm{\ensuremath{\Theta}}}_{\mathrm{O}\mathrm{\ensuremath{-}}\mathrm{Ni}\mathrm{\ensuremath{-}}\mathrm{O}}$\ensuremath{\approxeq}-0.5\ifmmode^\circ\else\textdegree\fi{}) governing the transfer integral between Ni ${\mathit{e}}_{\mathit{g}}$ and O 2p orbitals. These changes are sterically driven by the observed increase of the nickel-oxygen distances (\ensuremath{\Delta}${\mathit{d}}_{\mathrm{Ni}\mathrm{\ensuremath{-}}\mathrm{O}}$\ensuremath{\approxeq}+0.004 \AA{}) in the insulating (low-temperature) phase. The results of valence-bond calculations suggest the existence of ${\mathrm{Ni}}^{3+}$(${\mathit{d}}^{7}$) and ${\mathit{R}}^{3+}$ states for nickel and rare earth.

Charge Disproportionation in R NiO 3 Perovskites: Simultaneous Metal-Insulator and Structural Transition in YNiO 3

Abstract: Neutron and synchrotron diffraction data provide the first observation of changes in the crystal symmetry at the metal-insulator transition in $R{\mathrm{NiO}}_{3}$ perovskites. At high temperatures, ${\mathrm{YNiO}}_{3}$ is orthorhombic and metallic but below ${T}_{\mathrm{MI}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}582\mathrm{K}$ it changes to a monoclinic insulator due to a charge disproportionation $(2{\mathrm{Ni}}^{3+}\ensuremath{\rightarrow}{\mathrm{Ni}}^{3+\ensuremath{\delta}}+{\mathrm{Ni}}^{3\ensuremath{-}\ensuremath{\delta}})$ that develops at the opening of the gap. We report the presence of two alternating ${\mathrm{NiO}}_{6}$ octahedra with expanded (Ni1) and contracted (Ni2) Ni-O bonds and a magnetic structure $[\mathbf{k}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(1/2,0,1/2)]$ with unequal moments at Ni1 and Ni2 octahedra.

Two-dimensional mapping of chemical information at atomic resolution.

Abstract: The simultaneous measurement of structural and chemical information at the atomic scale provides fundamental insights into the connection between form and function in materials science and nanotechnology. We demonstrate structural and chemical mapping in ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ using an aberration-corrected scanning transmission electron microscope. Two-dimensional mapping is made possible by an adapted method for fast acquisition of electron energy-loss spectra. The experimental data are supported by simulations, which help to explain the less intuitive features.

Multiferroics: a magnetic twist for ferroelectricity

Abstract: Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites

Abstract: Colossal magnetoresistance1—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure2,3,4, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.