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J. Rivas

Publications and source records attributed to J. Rivas.

At least 19 recordsLinked to original sources
arXiv2011

Increase in the magnitude of the energy barrier distribution in Ni nanoparticles due to dipolar interactions

The energy barrier distribution Eb of five samples with different concentrations x of Ni nanoparticles using scaling plots from ac magnetic susceptibility data has been determined. The scaling of the imaginary part of the susceptibility Chi"(nu, T) vs. Tln(t/tau_0) remains valid for all samples, which display Ni nanoparticles with similar shape and size. The mean value increases appreciably with increasing x, or more appropriately with increasing dipolar interactions between Ni nanoparticles. We argue that such an increase in constitutes a powerful tool for quality control in magnetic recording media technology where the dipolar interaction plays an important role.

cond-mat.str-el↗
arXiv2010

Nonmonotonic Evolution of the Blocking Temperature in Dispersions of Superparamagnetic Nanoparticles

We use a Monte Carlo approach to simulate the influence of the dipolar interaction on assemblies of monodisperse superparamagnetic $γ-Fe_{2}O_{3}$ nanoparticles. We have identified a critical concentration c*, that marks the transition between two different regimes in the evolution of the blocking temperature ($T_{B}$) with interparticle interactions. At low concentrations (c < c*) magnetic particles behave as an ideal non-interacting system with a constant $T_{B}$. At concentrations c > c* the dipolar energy enhances the anisotropic energy barrier and $T_{B}$ increases with increasing c, so that a larger temperature is required to reach the superparamagnetic state. The fitting of our results with classical particle models and experiments supports the existence of two differentiated regimes. Our data could help to understand apparently contradictory results from the literature.

cond-mat.mes-hall↗
arXiv2010

Rapidly fluctuating orbital occupancy above the orbital ordering transition in spin-gap compounds

Several spin systems with low dimensionality develop a spin-dimer phase within a molecular orbital below TS, competing with long-range antiferromagnetic order. Very often, preferential orbital occupancy and ordering are the actual driving force for dimerization, as in the so-called orbitally-driven spin-Peierls compounds (MgTi2O4, CuIr2S4, La4Ru2O10, NaTiSi2O6, etc.). Through a microscopic analysis of the thermal conductivity k (T) in La4Ru2O10, we show that the orbital occupancy fluctuates rapidly above TS, resulting in an orbital-liquid state. The strong orbital-lattice coupling introduces dynamic bond-length fluctuations that scatter the phonons to produce a k (T) proportional to T (i.e. glass-like) above TS. This phonon-glass to phonon-crystal transition is shown to occur in other spin-dimer systems, like NaTiSi2O6, pointing to a general phenomenon.

cond-mat.str-el↗
arXiv2009

Role of dipolar interactions in a system of Ni nanoparticles studied by magnetic susceptibility measurements

The role of dipolar interactions among Ni nanoparticles (NP) embedded in an amorphous SiO2/C matrix with different concentrations has been studied performing ac magnetic susceptibility Chi_ac measurements. For very diluted samples, with Ni concentrations < 4 wt % Ni or very weak dipolar interactions, the data are well described by the Neel-Arrhenius law. Increasing Ni concentration to values up to 12.8 wt % Ni results in changes in the Neel-Arrhenius behavior, the dipolar interactions become important, and need to be considered to describe the magnetic response of the NPs system. We have found no evidence of a spin-glasslike behavior in our Ni NP systems even when dipolar interactions are clearly present.

cond-mat.mes-hall↗
arXiv2009

Study of the pressure effects in TiOCl by ab initio calculations

Electronic structure calculations on the low dimensional spin-1/2 compound TiOCl were performed at several pressures in the orthorhombic phase, finding that the structure is quasi-one-dimensional. The Ti3+ (d1) ions have one t2g orbital occupied (dyz) with a large hopping integral along the b direction of the crystal. The most important magnetic coupling is Ti-Ti along the b axis. The transition temperature (Tc) has a linear evolution with pressure, and at about 10 GPa this Tc is close to room temperature, leading to a room temperature spin-Peierls insulator-insulator transition, with an important reduction of the charge gap in agreement with the experiment. On the high-pressure monoclinic phase, TiOCl presents two possible dimerized structures, with a long or short dimerization. Long dimerized state occurs above 15 GPa, and below this pressure the short dimerized structure is the more stable phase.

cond-mat.str-el↗
arXiv2008

Identification of a Marginal Fermi-Liquid in Itinerant Ferromagnet CoS2

We report specific heat, resistivity and susceptibility measurements at different temperatures, magnetic fields, and pressures to provide solid evidence of CoS2 being a marginal Fermi liquid. The presence of a tricritical point in the phase diagram of the system provides an opportunity to test the spin fluctuation theory with a high limit of accuracy. A magnetic field suppresses the amplitude of the spin fluctuations and recovers conventional Fermi liquid behavior, connecting both states continuously.

cond-mat.str-el↗
arXiv2008

Enhanced dimerization of TiOCl under pressure: spin-Peierls - to - Peierls transition

We report high-pressure x-ray diffraction and magnetization measurements combined with ab-initio calculations to demonstrate that the high-pressure optical and transport transitions recently reported in TiOCl, correspond in fact to an enhanced Ti3+-Ti3+ dimerization existing already at room temperature. Our results confirm the formation of a metal-metal bond between Ti3+ ions along the b-axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.

cond-mat.str-el↗
arXiv2008

Homopolar bond formation in ZnV$_2$O$_4$ close to a metal-insulator transition

Electronic structure calculations for spinel vanadate ZnV$_2$O$_4$ show that partial electronic delocalization in this system leads to structural instabilities. These are a consequence of the proximity to the itinerant-electron boundary, not being related to orbital ordering. We discuss how this mechanism naturally couples charge and lattice degrees of freedom in magnetic insulators close to such a crossover. For the case of ZnV$_2$O$_4$, this leads to the formation of V-V dimers along the [011] and [101] directions that readily accounts for the intriguing magnetic structure of ZnV$_2$O$_4$.

cond-mat.str-el↗
arXiv2007

Enhanced Pressure Dependence of Magnetic Exchange in A2+[V2]O4 Spinels Approaching the Itinerant Electron Limit

We report a systematic enhancement of the pressure dependence of TN in A2+[V2]O4 spinels as the V-V separation approaches the critical separation for a transition to itinerant-electron behavior. An intermediate phase between localized and itinerant electron behavior is identified in Zn[V2]O4 and Mg[V2]O4 exhibiting mobile holes as large polarons. In Zn[V2]O4, cooperative ordering of V-V pairs below a Ts=TN does not totally suppress the V3+-ion spins at ambient pressure, but makes TN to decrease with pressure. Our results demonstrate that Zn[V2]O4 and Mg[V2]O4 are less localized than previously thought.

cond-mat.str-el↗
arXiv2007

VO: A Strongly Correlated Metal Close to a Mott- Hubbard Transition

Here we present experimental and computational evidences to support that rock-salt cubic VO is a strongly correlated metal with Non-Fermi-Liquid thermodynamics and an unusually strong spin-lattice coupling. An unexpected change of sign of metallic thermopower with composition is tentatively ascribed to the presence of a pseudogap in the density of states. These properties are discussed as signatures of the proximity to a magnetic quantum phase transition. The results are summarized in a new electronic phase diagram for the 3d monoxides, which resembles that of other strongly correlated systems. The structural and electronic simplicity of 3d monoxides make them ideal candidates to progress in the understanding of highly correlated electron systems.

cond-mat.str-el↗
arXiv2006

Scale properties as a basis of power law relaxation processes

Computer simulations of first-order relaxation processes show that the spatial configurations of the system acquire an invariant shape once the stationary regime is attained. Inspired by them we find that, in any first-order relaxation process, if the interaction that governs the system fulfils a simple scale property, then the relaxation will end up by following a stationary process described by a power law. A scaling law and some invariants are obtained for the time evolution of the system in such a case.

cond-mat.other↗
arXiv2006

Magnetic field-dependent dielectric constant in La2/3Ca1/3MnO3

We report a rather high dependence of the dielectric permittivity on the magnetic field in La2/3Ca1/3MnO3. The variation is maximum at around 270 K, little above the Curie temperature, TC, and it reaches a 30% under only 0.5 T. We attribute this phenomenon to the space-charge or interfacial polarization produced between the insulator and the metallic regions segregated intrinsically in the material above TC.

cond-mat.str-el↗
arXiv2005

Suppression of Ferromagnetic Double Exchange by Vibronic Phase Segregation

From Raman spectroscopy, magnetization, and thermal-expansion on the system La2/3(Ca1-xSrx)1/3MnO3, we have been able to provide a quantitative basis for the heterogeneous electronic model for manganites exhibiting colossal magnetoresistance (CMR). We construct a mean-field model that accounts quantitatively for the measured deviation of TC(x) from the TC predicted by de Gennes double exchange in the adiabatic approximation, and predicts the occurrence of a first order transition for a strong coupling regime, in accordance with the experiments. The existence of a temperature interval TC<T<T* where CMR may be found is discussed, in connection with the occurrence of an idealized Griffiths phase.

cond-mat.str-el↗
arXiv2005

Power-law decay in first-order relaxation processes

Starting from a simple definition of stationary regime in first-order relaxation processes, we obtain that experimental results are to be fitted to a power-law when approaching the stationary limit. On the basis of this result we propose a graphical representation that allows the discrimination between power-law and stretched exponential time decays. Examples of fittings of magnetic, dielectric and simulated relaxation data support the results.

cond-mat.mtrl-sci↗
arXiv2005

New low-temperature phases in NaxCoO2 linked to quantum criticality

New electronic phases have been identified and placed in the (T,H) phase diagram of metallic NaxCoO2. At low Na-content (x = 0.36), the magnetic susceptibility diverges with a power law T^(-n), n<1, and shows (T,H) scaling, indicating the proximity to a magnetic quantum phase transition. At high Na contents (x = 0.6) the mass of the quasiparticles does never diverge, but renormalizes and becomes strongly field dependent at low temperatures, forming a heavy Fermi-Liquid. Our results make superconducting NaxCoO2 a clear candidate for magnetically mediated pairing.

cond-mat.str-el↗
arXiv2005

Effects of Sr doping on the magnetic properties of Sr$_x$Ba$_{1-x}$CoO$_3$

Magnetic properties of the Sr-doped BaCoO$_3$ are explained on the basis of ab initio calculations and the analysis of experimental literature. Formation of magnetic clusters bigger than in the parent compound and increase in the blocking temperature are observed. Superparamagnetism remains at room temperature. The possibility of tuning the size and properties of the magnetic clusters with doping is explored.

cond-mat.str-el↗
arXiv2004

Evidence for magnetic clusters in BaCoO$_3$

Magnetic properties of the transition metal oxide BaCoO$_3$ are analyzed on the basis of the experimental and theoretical literature available via ab inito calculations. These can be explained by assuming the material to be formed by noninteracting ferromagnetic clusters of about 1.2 nm in diameter separated by about 3 diameters. Above about 50 K, the so-called blocking temperature, superparamagnetic behavior of the magnetic clusters occurs and, above 250 K, paramagnetism sets in.

cond-mat.str-el↗