Anisotropia inducida por campo magnético en perovskitas de cobalto

En este trabajo se examina la anisotropía magnética de la perovskita de cobalto Lao.7Sro.3Co03, material de alto interés tecnológico. Las medidas convencionales, hechas en la zona de alto campo, arrojan un valor de anisotropía magnética muy elevado, que contrasta con los bajos valores de campo coercitivo medidos en los ciclos de histéresis del material. De los datos de campo coercitivo se calcula la anisotropía magnética en la región de bajo campo, evidenciando una sorprendente reducción de esta magnitud, lo que demuestra su dependencia con el campo externo aplicado. El efecto es explicado en términos de una transición, inducida por campo magnético, del estado de espín de los iones de cobalto, desde una configuración de bajo espín a una de espín intermedio, lo que covierte al ión de cobalto en un ión Jahn-TellerIn this work we examine the magnetic anisotropy of the cobalt perovskite Lao.7Sro.3Co03, a material of high technological interest. Conventional measurements, done in the high-field regime, yield a very high value of magnetic anisotropy, which is in clear contrast with the low coercive field values observed in hysteresis loops. From such values, magnetic anisotropy in the low-field region is calculated, evidencing a surprising reduction of this magnitude, demonstrating its dependence on the external applied field. This anomalous eff ect is explained in terms of a magnetic field induced spin state transition of the cobalt ions, from a low-spin to an intermediate-spin configuration, which transforms them into Jahn-Teller ionsS

A simple solvothermal synthesis of MFe2O4 (M=Mn, Co and Ni) nanoparticles

This is the accepted manuscript of the following article: Yáñez-Vilar, S., Sánchez-Andújar, M., Gómez-Aguirre, C., Mira, J., Señarís-Rodríguez, M., & Castro-García, S. (2009). A simple solvothermal synthesis of MFe2O4 (M=Mn, Co and Ni) nanoparticles. Journal Of Solid State Chemistry, 182(10), 2685-2690. doi: 10.1016/j.jssc.2009.07.028Nanoparticles of MFe2O4 (M=Mn, Co and Ni), with diameters ranging from 5 to 10 nm, have been obtained through a solvothermal method. In this synthesis, an alcohol (benzyl alcohol or hexanol) is used as both a solvent and a ligand; it is not necessary, therefore, to add a surfactant, simplifying the preparation of the dispersed particles. We have studied the influence of the synthetic conditions (temperature, time of synthesis and nature of solvent) on the quality of the obtained ferrites and on their particle size. In this last aspect, we have to highlight that the solvent plays an important role on the particle size, obtaining the smallest diameters when hexanol was used as a solvent. In addition, the magnetic properties of the obtained compounds have been studied at room temperature (RT). These compounds show a superparamagnetic behaviour, as was expected for single domain nanoparticles, and good magnetization values. The maxima magnetization values of the MFe2O4 samples are quite high for such small nanoparticles; this is closely related to the high crystallinity of the particles obtained by the solvothermal methodThe authors are grateful for financial support from the MEC of Spain (Project CSD2006-00012 of Consolider-Ingenio 2010 Programme and FPI fellowship to S. Yáñez-Vilar), from the Xunta de Galicia (Project PGIDIT06PXIB103298PR, Rede Galega de Nanomedicina and Parga Pondal Programme) and from the EU (FEDER)S

Magnetoelectric behavior in the complex CaMn7O12 perovskite

This is the accepted manuscript of the following article: Sánchez-Andújar, M., Yáñez-Vilar, S., Biskup, N., Castro-García, S., Mira, J., Rivas, J., & Señarís-Rodríguez, M. (2009). Magnetoelectric behavior in the complex CaMn7O12 perovskite. Journal Of Magnetism And Magnetic Materials, 321(11), 1739-1742. doi: 10.1016/j.jmmm.2009.02.018We report a magnetoelectric effect in the double perovskite CaMn7O12, that shows a complex magnetic behavior below 90 K with two magnetic phases coexisting (one ferrimagnetic and the other modulated). A second magnetic transition, associated with changes in the magnetic modulation and magnetic ordering coherence lengths of the two magnetic phases occurs at 50 K (TN2). A detailed structural characterization of this compound, that we have carried out by means of high-resolution X-ray powder diffraction, reveals an anisotropic thermal expansion of its lattice parameters at 50 K (TN2). In addition, our study of the complex permittivity of this sample as a function of temperature, frequency and magnetic field shows very interesting results below 90 K and specially below 50 K: the dielectric constant ε′r that was decreasing continuously on cooling experiences an upturn, and even more, on application of a magnetic field it shows a moderate magnetoelectric response. We attribute such dielectric behavior to the formation of electric dipoles by magnetostriction in this charge and spin ordered system, that are sensible to the presence of an external magnetic fieldThe authors are grateful from financial support for MEC (Spain) under project FEDER MAT 2007-66696 and Xunta de Galicia under project PGIDIT06PXB103298PR. S. Yáñez-Vilar want to thank to MEC of Spain for her FPI fellowship and M. Sánchez-Andújar acknowledges to Xunta de Galicia for support under program Parga PondalS

Atypical Magnetic Behavior in the Incommensurate (CH3NH3)[Ni(HCOO)3] Hybrid Perovskite

A plethora of temperature-induced phase transitions have been observed in (CH3NH3)[M(HCOO)3] compounds, where M is Co(II) or Ni(II). Among them, the nickel compound exhibits a combination of magnetic and nuclear incommensurability below Néel temperature. Despite the fact that the zero-field behavior has been previously addressed, here we study in depth the macroscopic magnetic behavior of this compound to unveil the origin of the atypical magnetic response found in it and in its parent family of formate perovskites. In particular, they show a puzzling magnetization reversal in the curves measured starting from low temperatures, after cooling under zero field. The first atypical phenomenon is the impossibility of reaching zero magnetization, even by nullifying the applied external field and even compensating it for the influence of the Earth’s magnetic field. Relatively large magnetic fields are needed to switch the magnetization from negative to positive values or vice versa, which is compatible with a soft ferromagnetic system. The atypical path found in its first magnetization curve and hysteresis loop at low temperatures is the most noticeable feature. The magnetization curve switches from more than 1200 Oe from the first magnetization loop to the subsequent magnetization loops. A feature that cannot be explained using a model based on unbalanced pair of domains. As a result, we decipher this behavior in light of the incommensurate structure of this material. We propose, in particular, that the applied magnetic field induces a magnetic phase transition from a magnetically incommensurate structure to a magnetically modulated collinear structureThe authors thank financial support from the Ministerio de Economía y Competitividad MINECO and EU-FEDER (projects MAT2017-86453-R and PDC 2021-121076-I00). The authors are grateful to Dra. Ana Arauzo at Servicio de Medidas Físicas of the Universidad de Zaragoza for heat capacity data. O.F. acknowledges the Spanish Ministry of Universities (UNI/551/2021) and the European Union through the Funds Next GenerationS

Coexistence of magnetic and electrical order in the new perovskite-like (C3N2H5)[Mn(HCOO)3] formate

This is the accepted manuscript of the following article: Pato-Doldán, B., Gómez-Aguirre, L., Bermúdez-García, J., Sánchez-Andújar, M., Fondado, A., & Mira, J. et al. (2013). Coexistence of magnetic and electrical order in the new perovskite-like (C3N2H5)[Mn(HCOO)3] formate. RSC Advances, 3(44), 22404. doi: 10.1039/c3ra43165gIn this work we further the structural characterization of the recently discovered (C3N2H5)[Mn(HCOO)3] metal–organic framework with perovskite-like structure, and we present its magnetic and dielectric properties up to 350 K. At low temperature, the C3N2H5+ imidazolium cations, that sit oblique within the cavities of the [Mn(HCOO)3]− framework structure, show a cooperative order resulting in an antiparallel arrangement of their electrical dipole moments. Very interestingly, it is only above 220 K that thermal energy seems to be able to break this antiferroelectric order, resulting in a linear increase of its dielectric constant with temperature. In addition, this Mn(II) compound is antiferromagnetic below TN = 9 K, with a slightly non-collinear arrangement of its magnetic moments, yielding to a weak ferromagnetism. Therefore, this is a new multiferroic material which exhibits coexistence of magnetic and electric orderingThe authors are grateful for financial support from Ministerio de Economía y Competitividad MINECO (Spain) under project FEDER MAT2010-21342-C02-01 and from Xunta de Galicia under project PGIDIT10PXB103272PR. B.P.-D. also wants to thank MICINN for a FPI fellowshipS

Magnetic transitions and isotropic versus anisotropic magnetic behaviour of [CH3NH3][M(HCOO)3] M = Mn2+, Co2+, Ni2+, Cu2+ metal–organic perovskites

Here we present an in-depth study of the magnetic properties of a family of metal–organic perovskites ABX3, [CH3NH3][M(HCOO)3] in which A = CH3NH3+ is the methylammonium cation, B = M is a divalent metal cation (Mn2+, Co2+, Ni2+ or Cu2+), and X is the formate anion (HCOO−). The magnetic properties have been measured on powdered samples and along the different orientations of mm-sized single crystals. They display spin-canted weak ferromagnetism with Néel temperatures of 8.0 K (Mn2+), 15.7 K (Co2+) and 34 K (Ni2+), which are inversely proportional to the ionic radii of the metal cations. The Cu2+ member displays low-dimensional magnetism as a result of orbital ordering of the Cu2+ d orbitals originating from a Jahn–Teller distortion. Pulsed-field magnetization experiments (fields of up to 60 T at temperatures down to 0.6 K) show that Mn2+, Co2+ and Ni2+ formates display cation-characteristic spin flop transitions. A saturation magnetization value of 5 μB (at 12.5 T) was observed for Mn2+, meanwhile the Co2+ formate shows an orientation dependent quasi saturation (5.1 μB at 21 T along [101] vs. 5.8 μB at 26 T along [010]). The different isotropic/anisotropic behaviour can be explained by the orbital contribution to the magnetic responseThe Spanish authors are grateful for financial support from Ministerio de Economía y Competitividad (MINECO) (Spain) and EU under the project ENE2014-56237-C4-4-R, and Xunta de Galicia under the project GRC2014/042. L. C. G.-A. acknowledges UDC for a predoctoral fellowship and Fundación Barrié for the research stay grant at LANL. Work at LANL, A. P. H. and B. P.-D.'s visit to LANL were funded by the Laboratory Directed Research and Development program at LANL. The NHMFL pulsed-field facility is funded by the U.S. National Science Foundation through Cooperative Grant No. DMR-1157490, the State of Florida, and the U.S. Department of EnergyS

Multiple phase and dielectric transitions on a novel multi-sensitive [TPrA][M(dca)3] (M: Fe2+, Co2+ and Ni2+) hybrid inorganic–organic perovskite family

The hybrid inorganic–organic [TPrA][M(dca)3] (M: Fe2+, Co2+ and Ni2+) compounds, where TPrA is the tetrapropylammonium cation and dca is the dicyanamide anion, are unique multi-sensitive compounds that display multiple phases and dielectric transitions. These materials exhibit up to three first-order structural transitions (between the polymorphs I, Ia, Ib and II) associated with the same number of dielectric transitions in the temperature range of 210–360 K. The mechanisms responsible for these dielectric responses are found to be novel within the hybrid perovskites, involving ionic displacements of the A-site cations (TPrA) and order/disorder processes of the X anions (dca). In addition, the phase transitions and dielectric transition temperatures can be tuned by applying external hydrostatic pressure or by inducing internal pressure by modifying the tolerance factor through ionic substitution in the B-sites. This multi-sensitive response towards temperature, external and internal pressure opens up promising technological applications for this family of materials, such as dielectric transductors or multistimuli-sensors, whose response can be modulated in a wide range of temperatures and pressuresThe authors are grateful for the financial support from Ministerio de Economía y Competitividad MINECO (MINECO) ENE2014-56237-C4-4-R and Xunta de Galicia under the project GRC2014/042. J. M. B.-G. also wants to thank Barrié Foundation for a predoctoral fellowship and S. Y.-V. to the Xunta de Galicia for a postdoctoral grant (Plan I2C)S

Peculiarities in the electrical and magnetic properties of cobalt perovskites Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+)

This is the accepted manuscript of the following article: Señarís-Rodríguez, M. et al. (1999). Peculiarities in the electrical and magnetic properties of cobalt perovskites Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+). International Journal Of Inorganic Materials, 1(3-4), 281-287. doi: 10.1016/s1466-6049(99)00042-2We refer here to the electrical and magnetic properties of the Ln1−xMxCoO3 systems (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+), paying special attention to those ferromagnetic compounds that display M–I transitions as temperature rises: La1−xMxCoO3 (M2+: Ca2+, Sr2+, Ba2+) in the compositional interval x=0.2–0.3, and Nd1−xSrxCoO3, with x=0.40. The magneto-transport properties of such materials are peculiar and interesting: they show diodic behavior and large relaxation effects — these latter being specially important in the Nd compound — they display magnetoresistive effects specially at the M–I transition temperatures, and they age with time. All these results are discussed on the basis of the inhomogeneous electronic structure of these doped cobalt perovskites and taking into account the influence of the lanthanide ion on their magnetic and electrical propertiesWe thank the Spanish DGICYT for financial support under project MAT98-0416-C03-02S

Hybrid organic-inorganic perovskites: a spin-off of oxidic perovskites

ABX3 compounds with perovskite structure have been intensively and extensively studied in the last decades in view of their structural richness and amazing variety of interesting properties, such as piezoelectricity, ferroelectricity, ferromagnetism, superconductivity, magnetoresistance, multiferroicity, etc. In this chapter, we recompile well established chemical and structural concepts in pure inorganic perovskites (mainly oxidic perovskites), and extend them to the young family of hybrid organic-inorganic perovskites. Our final goal is to help understanding the relationships among composition, crystal structure and properties in this new family of compounds, for inspiring further the design of novel materials.This work was financially supported by the Ministerio de Economía y Competitividad (MINECO) and EU-FEDER under the project MAT2017 -86453-R and Xunta de Galicia under the project ED431 G/09. A. García-Fernández acknowledges Carl Tryggers Stiftelse for a Postdoctoral Fellowship.Peer reviewe

New properties in old systems: cooperative electric order in ferrocene and ammonia-borane

After half a century of intensive and extensive studies on ferrocene [Fe(C5H5)2] and ammonia-borane (H3N·BH3), it is very exciting to observe that these “classical” compounds still hide interesting properties never described before. Despite it being well-known that they both experience phase transitions as a function of temperature, here, for the first time, we give experimental evidence for the dielectric transitions they experience associated to the former, results that we support with DFT calculations. In that context, we report that ferrocene displays a temperature-induced paraelectric to antiferroelectric transition associated with the monoclinic–triclinic transition, which implies order–disorder of the cyclopentadienyl (Cp) ligands and displacement of the Fe atoms within the ferrocene molecules. As for the ammonia-borane, we report a sharp dielectric transition at T ≈ 232 K, associated with a structural transition that combines ordering and atomic displacement of the H3N·BH3 molecules. In this case, we attribute such behaviour to the temperature dependent displacement of the H3N·BH3 molecules out-of the crystal polar c-axisThe authors are grateful for financial support from Ministerio de Economía y Competitividad (MINECO) ENE2014-56237-C4-4-R and Xunta de Galicia under the project GRC2014/042. S. Y.-V. acknowledges Xunta de Galicia for a Postdoctoral fellowshipS