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J. F. Fernandez

Publications and source records attributed to J. F. Fernandez.

At least 19 recordsLinked to original sources
arXiv2020

Experimental behaviour of a three-stage metal hydride hydrogen compressor

A three-stage metal hydride hydrogen compressor (MHHC) system based in AB2-type alloys has been set-up. Every stage can be considered as a Sieverts-type apparatus. The MHHC system can work in the pressure and temperature ranges comprised from vacuum to 250 bar and from RT to 200C, respectively. An efficient thermal management system was set up for the operational ranges of temperature designed. It dumps temperature shifts due to hydrogen expansion during stage coupling and hydrogen absorption/desorption in the alloys. Each reactor consists of a single and thin stainless-steel tube to maximize heat transfer. They are filled with similar amount of AB2 alloy. The MHHC system was able to produce a compression ratio (CR) as high as of 84.7 for inlet and outlet hydrogen pressures of 1.44 and 122 bar for a temperature span of 23 to 120C.

physics.app-ph↗
arXiv2009

Surface magnetism in ZnO/Co3O4 mixtures

We recently reported the observation of room temperature ferromagnetism in mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic character of these oxides respectively. Here we present a detailed study on the electronic structure of this material in order to account for this unexpected ferromagnetism. Electrostatic interactions between both oxides lead to a dispersion of Co3O4 particles over the surface of ZnO larger ones. As a consequence, the reduction of Co+3 to Co2+ at the particle surface takes place as evidenced by XAS measurements and optical spectrocopy. This reduction allows to xplain the observed ferromagnetic signal within the well established theories of magnetism.

cond-mat.mtrl-sci↗
arXiv2009

Sources of experimental errors in the observation of nanoscale magnetism

It has been recently reported that some non-magnetic materials in bulk state, exhibit magnetic behavior at the nanscale due to surface and size effects. The experimental observation of these effects is based on the measurement of very small magnetic signals. Thus, some spurious effects that are not critical for bulk materials with large magnetic signals may become important when measuring small signals (typically below 0.0001 emu). Here, we summarize some sources of these small magnetic signals that should be considered when studying this new nanomagnetism

cond-mat.mtrl-sci↗
arXiv2009

Room-temperature ferromagnetism in the mixtures of the TiO2 and Co3O4 powders

We report here the observation of ferromagnetism (FM) at 300 K in mixtures of TiO2 and Co3O4 powders despite the antiferromagnetic and diamagnetic character of both oxides respectively. The ferromagnetic behavior is found in the early stages of reaction and only for TiO2 in anatase structure; no FM is found for identical samples prepared with rutile-TiO2. Optical spectroscopy and X-ray absorption spectra confirm a surface reduction of octahedral Co+3->Co+2 in the mixtures which is in the origin of the observed magnetism

cond-mat.mtrl-sci↗
arXiv2009

Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes on a site diluted lattice

We study partially occupied lattice systems of classical magnetic dipoles which point along randomly oriented axes. Only dipolar interactions are taken into account. The aim of the model is to mimic collective effects in disordered assemblies of magnetic nanoparticles. From tempered Monte Carlo simulations, we obtain the following equilibrium results. The zero temperature entropy approximately vanishes. Below a temperature T_c, given by k_B T_c= (0.95 +- 0.1)x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy and x is the site occupancy rate, we find a spin glass phase. In it, (1) the mean value <|q|>, where q is the spin overlap, decreases algebraically with system size N as N increases, and (2) D|q| = 0.5 <|q|> (T/x)^1/2, independently of N, where D|q| is the root mean square deviation of |q|.

cond-mat.stat-mech↗
arXiv2009

Insights into the room temperature magnetism of ZnO/Co3O4 mixtures

The origin of room temperature (RT) ferromagneticlike behavior in ZnO-based diluted magnetic semiconductors is still an unclear topic. The present work concentrates on the appearance of RT magnetic moments in just mixed ZnO/Co3O4 mixtures without thermal treatment. In this study, it is shown that the magnetism seems to be related to surface reduction of the Co3O4 nanoparticles, in which, an antiferromagnetic Co3O4 nanoparticle (core) is surrounded by a CoO-like shell. This singular superficial magnetism has also been found in other mixtures with semiconductors such as TiO2 and insulators such as Al2O3.

cond-mat.mtrl-sci↗
arXiv2008

Importance of the crystalline symmetry in the piezoelectric properties of (K0.44+xNa0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3+x/2 lead-free ceramics

Lead-free ceramics (K0.44+xNa0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3+x/2 (x=-0,06) were prepared by conventional solid state sintering. The results indicate a correlation between crystalline symmetry and electrical properties. Higher tetragonality was observed in samples with higher piezoelectric response. As the tetragonality increase the presence of polymorphic phase decrease associated with a reduction of the relaxor behaviour response. The ceramics with c/a = 1.011 ratio exhibit enhanced electrical properties. The Raman spectroscopy showed a certain asymmetry in the paraelectric phase above the Curie temperature Tc that provokes retention of the polarization. The crystalline symmetry plays a crucial role in the piezoelectric properties of the system.

cond-mat.mtrl-sci↗
arXiv2006

Continuous spin reorientation in antiferromagnetic films

We study anisotropic antiferromagnetic one-layer films with dipolar and nearest-neighbor exchange interactions. We obtain a unified phase diagram as a function of effective uniaxial D_e and quadrupolar C anisotropy constants. We study in some detail how spins reorient continuously below a temperature T_s as T and D_e vary.

cond-mat.stat-mech↗
arXiv2006

Theoretical simulation of the anisotropic phases of antiferromagnetic thin films

We simulate antiferromagnetic thin films. Dipole-dipole and antiferromagnetic exchange interactions as well as uniaxial and quadrupolar anisotropies are taken into account. Various phases unfold as the corresponding parameters, J, D and C, as well as the temperature T and the number n of film layers vary. We find (1) how the strength Delta_m of the anisotropy arising from dipole-dipole interactions varies with the number of layers m away from the film's surface, with J and with n; (2) a unified phase diagram for all n-layer films and bulk systems; (3) a layer dependent spin reorientation (SR) phase in which spins rotate continuously as T, D, C and n vary; (4) that the ratio of the SR to the ordering temperature depends (approximately) on n only through (D+Delta/n)/C, and hardly on J; (5) a phase transformation between two different magnetic orderings, in which spin orientations may or may not change, for some values of J, by varying n.

cond-mat.stat-mech↗
arXiv2006

Magnetic dipolar ordering and relaxation in the high-spin molecular cluster compound Mn6

Few examples of magnetic systems displaying a transition to pure dipolar magnetic order are known to date, and single-molecule magnets can provide an interesting example. The molecular cluster spins and thus their dipolar interaction energy can be quite high, leading to reasonably accessible ordering temperatures, provided the crystal field anisotropy is sufficiently small. This condition can be met for molecular clusters of sufficiently high symmetry, as for the Mn6 compound studied here. Magnetic specific heat and susceptibility experiments show a transition to ferromagnetic dipolar order at T_{c} = 0.16 K. Classical Monte-Carlo calculations indeed predict ferromagnetic ordering and account for the correct value of T_{c}. In high magnetic fields we detected the contribution of the ^{55}Mn nuclei to the specific heat, and the characteristic timescale of nuclear relaxation. This was compared with results obtained directly from pulse-NMR experiments. The data are in good mutual agreement and can be well described by the theory for magnetic relaxation in highly polarized paramagnetic crystals and for dynamic nuclear polarization, which we extensively review. The experiments provide an interesting comparison with the recently investigated nuclear spin dynamics in the anisotropic single molecule magnet Mn12-ac.

cond-mat.mes-hall↗
arXiv2005

Phases of anisotropic dipolar antiferromagnets

We study systems of classical magnetic dipoles on simple cubic lattices with dipolar and antiferromagnetic exchange interactions. By analysis and Monte Carlo (MC) simulations, we find how the antiferromagnetic phases vary with uniaxial and fourfold anisotropy constants, C and D, as well as with exchange strength J. We pay special attention to the spin reorientation (SR) phase, and exhibit in detail the nature of its broken symmetries. By mean field theory and by MC, we also obtain the ratio of the higher ordering temperature to the SR transition temperature, and show that it depends mainly on D/C, and rather weakly on J. We find a reverse SR transition.

cond-mat.stat-mech↗
arXiv2005

Time relaxation of interacting single--molecule magnets

We study the relaxation of interacting single--molecule magnets (SMMs) in both spatially ordered and disordered systems. The tunneling window is assumed to be, as in Fe8, much narrower than the dipolar field spread. We show that relaxation in disordered systems differs qualitatively from relaxation in fully occupied cubic and Fe_8 lattices. We also study how line shapes that develop in ''hole--digging'' experiments evolve with time t in these fully occupied lattices. We show (1) that the dipolar field h scales as t^p in these hole line shapes and show (2) how p varies with lattice structure. Line shapes are not, in general, Lorentzian. More specifically, in the lower portion of the hole, they behave as (h/t^p)^{(1/p)-1} if h is outside the tunnel window. This is in agreement with experiment and with our own Monte Carlo results.

cond-mat.stat-mech↗
arXiv2005

Comment on "Hole digging in ensembles of tunneling molecular magnets"

Tupitsyn et al. [Phys. Rev. B v69, 132406 (2004)] have recently reported results for the relaxation of crystalline systems of single--molecule magnets, such as Fe_8. They claim that, quite generally, (1) the magnetization and hole widths of field--distributions evolve with time t as sqrt{t}, and (2) the holes' line shapes are Lorentzian. We give a counter--example to these conclusions, and show that the main assumption on which they rest is invalid.

cond-mat.other↗
arXiv2003

How systems of single-molecule magnets magnetize at low temperatures

We model magnetization processes that take place through tunneling in crystals of single-molecule magnets, such as Mn_12 and Fe_8. These processes take place when a field H is applied after quenching to very low temperatures. Magnetic dipolar interactions and spin flipping rules are essential ingredients of the model. The results obtained follow from Monte Carlo simulations and from the stochastic model we propose for dipole field diffusion. Correlations established before quenching are shown to later drive the magnetization process. We also show that in simple cubic lattices, m \propto sqrt(t) at time t after H is applied, as observed in Fe_8, but only for 1+2\log_10(h_d/h_w) time decades, where h_d is some near-neighbor magnetic dipolar field and a spin reversal can occur only if the magnetic field acting on it is within some field window (-h_w,h_w). However, the sqrt(t) behavior is not universal. For BCC and FCC lattices, m \propto t^p, but p \simeq 0.7 . An expression for p in terms of lattice parameters is derived. At later times the magnetization levels off to a constant value. All these processes take place at approximately constant magnetic energy if the annealing energy epsilon_a is larger than the tunneling window's energy width (i.e., if epsilon_a \gtrsim gμ_B h_w S). Thermal processes come in only later on to drive further magnetization growth.

cond-mat.stat-mech↗
arXiv2003

Magnetic Tunnel Window's Imprint and Beyond

We report results from Monte Carlo simulations of systems of magnetic dipoles that relax through quantum tunneling, much as Fe_8 crystals at very low temperature.For short times, a hole develops in suitably defined magnetic field pseudo--distributions, which matches the shape of the tunnel window (TW). Much later, ordinary field distributions P(h) develop similar holes if thermal energies are not much larger than the TW's energy. Still later, below the long--range ordering (LRO) temperature, P(h) exhibits the signature of LRO.

cond-mat↗
arXiv2003

Magnetization Process of Single Molecule Magnets at Low Temperatures

We show that correlations established before quenching to very low temperatures, later drive the magnetization process of systems of single molecule magnets, after a magnetic field is applied at t=0. We also show that in SC lattices, m \propto sqrt(t), as observed in Fe_8, but only for 1+2*log_10(h_d/h_w) time decades, where h_d is a nearest neighbor dipolar magnetic field and a spin reversal can occur only if the field on it is within (-h_w,h_w). However, the sqrt(t) behavior is not universal. For BCC and FCC lattices, m \propto t^p, but p \simeq 0.7. The value to which m finally levels off is also given.

cond-mat↗
arXiv2002

Long-range ferromagnetic dipolar ordering of high-spin molecular clusters

We report the first example of a transition to long-range magnetic order in a purely dipolarly interacting molecular magnet. For the magnetic cluster compound Mn6O4Br4(Et2dbm)6, the anisotropy experienced by the total spin S=12 of each cluster is so small that spin-lattice relaxation remains fast down to the lowest temperatures, thus enabling dipolar order to occur within experimental times at Tc = 0.16 K. In high magnetic fields, the relaxation rate becomes drastically reduced and the interplay between nuclear- and electron-spin lattice relaxation is revealed.

cond-mat.mes-hall↗