arXiv subjects

José Luis García-Muñoz

Publications and source records attributed to José Luis García-Muñoz.

6 recordsLinked to original sources

arXiv2021

Magnetic properties of a highly ordered single crystal of the layered perovskite YBaCuFe$_{0.95}$Mn$_{0.05}$O$_5$

The layered perovskite YBaCuFeO5 (YBCFO) is able to adopt chiral magnetic order up to unexpectedly high temperatures, paving the way to strong magnetoelectric coupling at room temperature. In this perovskite A-site cations are fully ordered whereas the occupancy of the B-sites strongly depend on the preparation process. Though the structure is not geometrically frustrated, the presence of partial Fe3+/Cu2+ disorder at the B-sites produces magnetic frustration. In an effort to increase the spin-orbit coupling in the system, we have synthesized and studied YBaCuFe0.95Mn0.05O5 in single crystal form, where the highly symmetric Fe3+ ions (3d5) are partially substituted with Jahn-Teller active 3d4 Mn3+ ions. We report the structural and magnetic properties of a highly ordered single crystal of this layered perovskite, which are presented in comparison with a polycrystalline specimen (three times more disordered). Single-crystal neutron diffraction measurements reveal two collinear magnetic phases and the lack of incommensurate spiral order. The magnetic phases and transitions found in the crystal grown by the traveling solvent floating zone (TSFZ) method are fully described and analyzed in the light of its high level of Fe/Cu cationic order (~90%).

cond-mat.mtrl-sci↗

arXiv2021

Ferromagnetic metallic Sr-rich Ln$_{1/2}$A$_{1/2}$CoO$_3$ cobaltites with spontaneous spin rotation

The Pr$_{0.50}$Sr$_{0.5}$0CoO$_3$ perovskite exhibits unique magnetostructural properties among the rest of ferromagnetic/metallic Ln$_{0.50}$Sr$_{0.50}$CoO$_3$ compounds. The sudden orthorhombic-tetragonal (Imma $\to$ I4/mcm) structural transition produces an unusual magnetic behavior versus temperature and external magnetic fields. In particular, the symmetry change is responsible for a spontaneous spin rotation in this metallic oxide. We have studied half-doped Ln$_{0.50}$(Sr$_{1-x}$A$_x$)$_{0.50}$CoO$_3$ cobaltites varying the ionic radius rA of A-site cations (divalent cations and lanthanides) in order to complete the T-rA phase diagram. The influence of the structural distortion and the A-cations size for the occurrence of a spontaneous spin reorientation in the metallic state has been investigated. As the reorientation of the magnetization is driven by the temperature induced collapse of the orthorhombic distortion, a careful investigation of the structural symmetry is presented varying the structural distortion of the Sr-rich half-doped cobaltites by means of both compositional and temperature changes. The region in the phase diagram of these perovskites where the phase of magnetic symmetry Fm'm'm replaces that of Im'm'a symmetry was determined in this family of ferromagnetic/metallic cobaltites. In that region the magnetization direction has rotated 45 degrees within the a-b plane with respect to the second.

cond-mat.mtrl-sci↗

arXiv2016

Magnetostructural coupling, magnetic ordering and cobalt spin reorientation in metallic Pr0.50Sr0.50CoO3 cobaltite

In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr at temperatures close to 100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating below the transition. This work investigates and describes the relationship between the Imma-to-I4/mcm symmetry change [Padilla-Pantoja et al, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSCO versus temperature and external magnetic fields. The FM1 and FM2 ferromagnetic phases, above and below the magnetostructural transition (TS1=120 K) have been investigated. The FM2 phase of PSCO is composed of [100] FM domains, with magnetic symmetry Im'm'a (mz=0). The magnetic space group of the FM1 phase is Fm'm'm (with mx=my). Neutron data analyses in combination with magnetometry and earlier reports results agrees with a reorientation of the magnetization axis by 45 deg within the a-b plane across the transition, in which the system retains its metallic character. The presence below TS1 of conjugated magnetic domains, both of Fm'm'm symmetry but having perpendicular spin orientations along the diagonals in the xy-plane of the tetragonal unit cell, is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field m0H > 30 mT is able to reorient the magnetization within the a-b plane, whereas a higher field (m0H,[H//z] >1.2 T at 2K) is necessary to align the Co moments perpendicular to a-b plane. Such a spin reorientation, in which the orbital and spin components of the Co moment rotate joined by 45deg, was not observed with other lanthanides different to praseodymium.

cond-mat.mtrl-sci↗

arXiv2011

Observation of a valence transition in (Pr,Ca)CoO3 cobaltites: charge migration at the metal-insulator transition

X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f 2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f 1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr0.5Ca0.5CoO3 is an homogeneous Co3.5-d state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3+ + Co3.5 \to 1/2Pr3+2d + Co3.5-d, with 2d \approx 0.26 e-.

cond-mat.mtrl-sci↗

arXiv2003

Effect of band-filling and structural distortions on the Curie temperature of Fe-Mo double perovkites

By means of high resolution neutron powder diffraction at low temperature we have characterized the structural details of $\rm La_{x}Sr_{2-x}FeMoO_6$ ($0\leq {\rm x}\leq 0.5$) and $\rm Ca_{x}Sr_{2-x}FeMoO_6$ ($0\leq {\rm x}\leq 0.6$) series of compounds. This study reveals a similar variation of the mean bond-angle \FeOMo in both series. In contrast, the mean bond-distance \FeMoO\ increases with La but not with Ca substitution. Both series also present a different evolution of the Curie temperature ($T_C$), which raises in the La series and slightly decreases in the Ca one. We thus conclude that the enhancement of $T_C$ in the La series is due to the electron filling of the conduction band and a concomitant rising of the density of states at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2000

Dependence of the physical properties of $Nd_{0.5}Ca_{0.5}MnO_{3+δ}$ on the oxydation state of $Mn$

We present a comparative study of the magnetic, transport and structural properties of $Nd_{0.5}Ca_{0.5}MnO_{3+δ}$ [$δ=$ 0.02(1) and 0.04(1)]. We have found significant differences between the low temperature magnetic orders and the magnetization curves below $T_{CO}\approx 250 K$ of the two samples. In particular one component of the magnetic moment presents a ferromagnetic coupling between the $(0 0 1)$ planes ($P bnm$ setting) deviating the angle between neighboring $Mn$ ions from $180^o$ (perfect CE order) to $150^o$ [$δ=0.02(1)$] and $130^o$ [$δ= 0.04(1)$]. These results imply a remarkable $δ$ dependence which is discussed in the light of a non-random spatial distribution of defects in the perfect charge order scheme.

cond-mat↗