arXiv subjects

J. F. Fernández

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

8 recordsLinked to original sources

arXiv2026

Transit timing and starspot-induced transit depth variations in the $\sim$17 Myr old HIP 67522 system

HIP 67522 is a one of the youngest multi-planetary systems discovered to date. The 17 Myr-old, Sun-like star is part of the Scorpius-Centarus OB association and is known to host two Saturn-sized planets in near 2:1 mean motion resonance. We analysed photometric transits observed with the CHaracterising ExOPlanet Satellite (CHEOPS), the Transiting Exoplanets Survey Satellite (TESS) and multiple ground-based facilities to search for transit timing variations induced by planet-planet gravitational interactions, as well as transit depth variations, linked with the changing coverage of active regions on the stellar surface. We do not detect any transit timing variations for HIP 67522 b exceeding two minutes, contrary to previous results from the James Webb Space Telescope. In addition, we found that the observed transit depth of planet b changes strongly over time, with a >30\% variation in amplitude. The high sensitivity and blue bandpass of the CHEOPS satellite also enabled us to identify multiple starspot crossings and measure their properties, including spot temperatures and sizes. We also measured the shear of the host star's differential rotation by modelling the out-of-transit photometric variability and concluded that HIP 67522 exhibits a supersolar differential rotation. Based on internal structure models and planet formation simulations, we find that HIP 67522 b likely have more than 20\% of their mass made of gaseous envelopes at their current age. Evolutionary simulations further indicate that both planets in the system will experience significant photoevaporation, evolving into either Earth size planets with completely stripped atmospheres or sub-Neptune-sized planets.

astro-ph.EP↗

arXiv2023

Greener processing of SrFe$_{12}$O$_{19}$ ceramic permanent magnets by two-step sintering

With an annual production amounting to 800 kilotons, ferrite magnets constitute the largest family of permanent magnets in volume, a demand that will only increase as a consequence of the rare-earth crisis. With the global goal of building a climate-resilient future, strategies towards a greener manufacturing of ferrite magnets are of great interest. A new ceramic processing route for obtaining dense Sr-ferrite sintered magnets is presented here. Instead of the usual sintering process employed nowadays in ferrite magnet manufacturing that demands long dwell times, a shorter two-step sintering is designed to densify the ferrite ceramics. As a result of these processes, dense SrFe$_{12}$O$_{19}$ ceramic magnets with properties comparable to state-of-the-art ferrite magnets are obtained. In particular, the SrFe$_{12}$O$_{19}$ magnet containing 0.2% PVA and 0.6% wt SiO$_2$ reaches a coercivity of 164 kA/m along with a 93% relative density. A reduction of 31% in energy consumption is achieved in the thermal treatment with respect to conventional sintering, which could lead to energy savings for the industry of the order of 7.109 kWh per year.

cond-mat.mtrl-sci↗

arXiv2023

Quantifying Li-content for compositional tailoring of lithium ferrite ceramics

Owing to their multiple applications, lithium ferrites are relevant materials for several emerging technologies. For instance, LiFeO2 has been spotted as an alternative cathode material in Li-ion batteries, while LiFe5O8 is the lowest damping ferrite, holding promise in the field of spintronics. The Li-content in lithium ferrites has been shown to greatly affect the physical properties, and in turn, the performance of functional devices based on these materials. Despite this, lithium content is rarely accurately quantified, as a result of the low number of electrons in Li hindering its identification by means of routine materials characterization methods. In the present work, magnetic lithium ferrite powders with Li:Fe ratios of 1:1, 1:3 and 1:5 have been synthesized, successfully obtaining phase-pure materials (LiFeO2 and LiFe5O8), as well as a controlled mixture of both phases. The powders have been compacted and subsequently sintered by thermal treatment (Tmax = 1100 °C) to fabricate dense pellets which preserve the original Li:Fe ratios. Li-content on both powders and pellets has been determined by two independent methods: (i) Rutherford backscattering spectroscopy combined with nuclear reaction analysis and (ii) Rietveld analysis of powder X-ray diffraction data. With good agreement between both techniques, it has been confirmed that the Li:Fe ratios employed in the synthesis are maintained in the sintered ceramics. The same conclusion is drawn from spatially-resolved confocal Raman microscopy experiments on regions of a few microns. Field emission scanning electron microscopy has evidenced the substantial grain growth taking place during the sintering process - mean particle sizes rise from about 600 nm in the powders up to 3.8(6) um for dense LiFeO2 and 10(2) um for LiFe5O8 ceramics.

cond-mat.mtrl-sci↗

arXiv2023

FeCo Nanowire-Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products

Due to the issues associated with rare-earth elements, there arises a strong need for magnets with properties between those of ferrites and rare-earth magnets that could substitute the latter in selected applications. Here, we produce a high remanent magnetization composite bonded magnet by mixing FeCo nanowire powders with hexaferrite particles. In the first step, metallic nanowires with diameters between 30 and 100 nm and length of at least 2 μm are fabricated by electrodeposition. The oriented as-synthesized nanowires show remanence ratios above 0.76 and coercivities above 199 kA/m and resist core oxidation up to 300 °C due to the existence of a > 8 nm thin oxide passivating shell. In the second step, a composite powder is fabricated by mixing the nanowires with hexaferrite particles. After the optimal nanowire diameter and composite composition are selected, a bonded magnet is produced. The resulting magnet presents a 20% increase in remanence and an enhancement of the energy product of 48% with respect to a pure hexaferrite (strontium ferrite) magnet. These results put nanowire-ferrite composites at the forefront as candidate materials for alternative magnets for substitution of rare earths in applications that operate with moderate magnet performance.

cond-mat.mtrl-sci↗

arXiv2013

Numerical results for the Edwards-Anderson spin-glass model at low temperature

We have simulated Edwards-Anderson (EA) as well as Sherrington-Kirkpatrick systems of L^3 spins. After averaging over large sets of EA system samples of 3 =< L =< 10, we obtain accurate numbers for distributions p(q) of the overlap parameter q at very low temperature T. We find p(0)/T --> 0.233(4) as T --> 0. This is in contrast with the droplet scenario of spin glasses. We also study the number of mismatched links --between replica pairs-- that come with large scale excitations. Contributions from small scale excitations are discarded. We thus obtain for the fractal dimension of outer surfaces of q~0 excitations in the EA model d_s --> 2.59(3) as T tends to 0. This is in contrast with d_s --> 3 as T --> 0 that is predicted by mean field theory for the macroscopic limit.

cond-mat.dis-nn↗

arXiv2012

Pair correlation function for spin glasses

We extract a pair correlation function (PCF) from probability distributions of the spin-overlap parameter q. The distributions come from Monte Carlo simulations. A measure, w, of the thermal fluctuations of magnetic patterns follows from the PCFs. We also obtain rms deviations (over different system samples) delta_p away from average probabilities for q. For the linear system-sizes $L$ we have studied, (i) w and delta_p are independent of L in the Edwards-Anderson model but scale as 1/L and sqrt{L}, respectively, in the Sherrington-Kirkpatrik model.

cond-mat.dis-nn↗

arXiv2010

Monte Carlo study of the spin-glass phase of the site-diluted dipolar Ising model

By tempered Monte Carlo simulations, we study site-diluted Ising systems of magnetic dipoles. All dipoles are randomly placed on a fraction x of all L^3 sites of a simple cubic lattice, and point along a given crystalline axis. For x_c< x<=1, where x_c = 0.65, we find an antiferromagnetic phase below a temperature which vanishes as x tends to x_c from above. At lower values of x, we find an equilibrium spin-glass (SG) phase below a temperature given by k_B T_{sg} = x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy. We study (a) the relative mean square deviation D_q^2 of |q|, where q is the SG overlap parameter, and (b) xi_L/L, where xi_L is a correlation length. From their variation with temperature and system size, we determine T_{sg}. In the SG phase, we find (i) the mean values <|q|> and decrease algebraically with L as L increases, (ii) double peaked, but wide, distributions of q/<|q|> appear to be independent of L, and (iii) xi_L/L rises with L at constant T, but extrapolations to 1/L -> 0 give finite values. All of this is consistent with quasi-long-range order in the SG phase.

cond-mat.stat-mech↗

arXiv2009

Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes

We study fully occupied lattice systems of classical magnetic dipoles which point along random axes. Only dipolar interactions are considered. From tempered Monte Carlo simulations, we obtain numerical evidence that supports the following conclusions: in three dimensions, (a) there is an equilibrium spin glass phase at temperatures below $T_c$, where $k_BT_c= (0.86\pm 0.07)\varepsilon_d $ and $\varepsilon_d$ is a nearest neighbor dipole-dipole interaction energy, (b) in the spin glass phase the overlap parameter is approximately given by $\sqrt{1-T/T_c}$, and (c) the correlation length $ξ$ diverges at $T_c$ with a critical exponent $ν=1.5\pm0.5$; in two dimensions $ξ$ diverges at or near T=0 and $ν=3\pm1$.

cond-mat.mtrl-sci↗