Magnetic and Calorimetric Study of the Magnetocaloric Effect in Intermetallics Exhibiting First-order Magnetostructural Transitions

En este estudio se caracteriza el efecto magnetocalórico de varias series de compuestos intermetálicos con transiciones magneto-estructurales de primer orden, tales como Gd5(Si,Ge)4, La(Fe,Si)13Hy, MnAs, (Mn,Fe)2(P,Ge) y MnCoGeBx. La histéresis térmica y magnética asociada a estas transiciones afecta a la determinación del cambio isotérmico de entropía y del cambio adiabático de temperatura inducidos por una variación del campo magnético. Se han usado medidas directas y métodos indirectos en las determinaciones magnetocalóricas, incluyendo estos últimos métodos técnicas magnéticas y calorimétricas. Se han comparado los valores resultantes para los cambios de entropía y de temperatura usando los distintos métodos. Se analiza la influencia del proceso de medida y de la irreversibilidad en los valores resultantes de los cambios de entropía y de temperatura. Esta investigación es de interés para el uso de estos materiales en refrigeración magnética a temperatura ambiente

Large magnetocaloric effect and magnetic properties of polymorphic RCrO4 (R=rare earth) oxides

Trabajo presentado al: "International Symposium on the Reactivity of Solids" celebrado en San Petersburgo (Rusia) del 9 al 13 de junio de 2014.-- et al.RCrO4 oxides, where R=rare earth, crystallize at ambient conditions with the tetragonal zircon-type structure, S.G. I41/amd. Recently, we have succeeded to synthesize the scheelite polymorphs (S.G. I41/a) for most of the R elements, by treating the zircon phases at 40 kbar and 813 K. These scheelite high pressure forms are quencheable after releasing the pressure, and TGA and X-ray diffraction data reveal a reversible transition from scheelite polymorph to zircon at 700 K. The coexistence of two paramagnetic ions in these phases, namely Cr5+ and R3+, constitutes a very interesting scenario to study 3d-4f magnetic interactions. In this sense, most of the zircon-type RCrO4 oxides behave as ferromagnetic, while the scheelite polymorphs are antiferromagnetic. The change in the sign of the magnetic interaction can be explained by considering the changes in both distances and bond angles of Cr-O-R pathway through which the superexchange interactions take place. Neutron diffraction studies have been used to determine the nuclear and magnetic structures for these zircon and scheelite RCrO4 polymorphs. The analysis of the data reveals the onset of new reflections below the estimated Néel temperature from the previous magnetic susceptiblity data corresponding to the scheelite phases. The magnetic structure for these scheelite RCrO4 oxides can be described with a propagation vector κ = [0 0 0], where the moments of R3+ and Cr5+ are aligned along the c-axis or confined in the ab-plane of the tetragonal structure depending on the nature of the R element. This behavior has been confirmed from heat capacity measurements where the onsets of λ-anomalies at the ordering temperatures are almost coincident with those determined from magnetic susceptibility and neutron diffraction analysis. A detailed analysis of the zircon and scheelite structural types is included in this work to explain the different magnetic behavior showed by these two polymorphic phases allowing establishing relationships structure-magnetic properties. Very recently it has been reported that the zircon RCrO4 (R= Ho and Dy) phases show large values of the magnetocaloric parameters that make of these oxides potential refrigerant materials to be used for the liquefaction of hydrogen. It is noting that the field induced magnetization in the case of the scheelite polymorphs yields magnetic moments larger than in the case of the homologous zircon phases and hence the magnetocaloric effect is expected to be larger for the scheelite polymorphs in comparison with the zircon ones. In this work a preliminary study has been done in order to determine the magnetocaloric parameters of the two polymorphic phases of TbCrO4 and HoCrO4. In this sense the isothermal entropy change, ΔST, as a function of temperature has been calculated for magnetic field variations from the measurements of magnetization versus field at different temperatures. Both magnetocaloric parameters, ΔST and the adiabatic temperature change, ΔTad, have also been evaluated from the entropy functions at different fields obtained from the heat capacity data.Peer Reviewe

A versatile magnetic refrigeration demonstrator

Trabajo presentado a la "6th International Conference on Magnetic Refrigeration at Room Temperature" celebrada en Victoria (Canadá) del 7 al 10 de septiembre de 2014.A versatile room temperature reciprocating magnetic refrigeration demonstrator has been designed, built and tested in order to check suitable magnetocaloric materials for magnetic refrigeration. Test experiments have been done with 31 g of Gd spheres of 0.2 – 0.4 mm diameter as refrigerant material, because it is a well-known benchmark material for magnetic refrigeration. The magnetic field is provided by a Halbach Nd2Fe14B permanent magnet with a slot of 10 mm width and a maximum field of 1.4 T. At optimized values of frequency (f = 0.7 Hz) and utilization factor (U = 0.19), the demonstrator achieves a maximum no load temperature span of 19.3 K. A maximum cooling power of 6 W at zero temperature span was obtained at optimized values f = 0.31 Hz and U = 1.1. Different thermodynamic cycles have been studied looking for the optimized parameters.Peer Reviewe

Peculiarities of the magnetocaloric properties in Ni-Mn-Sn ferromagnetic shape memory alloys

6 páginas, 6 figuras.-- PACS number(s): 75.30.Sg, 75.50.Cc.-- et al.Magnetocaloric properties of a Ni50Mn36Co1Sn13 ferromagnetic shape memory alloy have been studied experimentally in the vicinity of a first-order magnetostructural phase-transition low-temperature paramagnetic martensite↔high-temperature ferromagnetic austenite. The magnetic entropy change ΔSm calculated from the magnetization M(T) data measured upon cooling is higher than that estimated from M(T) measured upon heating. Contrary to ΔSm, the adiabatic temperature change ΔTad measured upon cooling is significantly smaller than that measured upon heating. The apparent discrepancy between ΔSm and ΔTad (larger ΔSm, smaller ΔTad upon cooling, and smaller ΔSm, larger ΔTad upon heating) is caused by the hysteretical behavior of this magnetostructural transition, a feature common for all the alloys in the family of Ni50Mn25+xZ25−x (Z=In,Sn,Sb) ferromagnetic shape memory Heusler compounds. The hysteresis causes the magnetocaloric parameters to depend strongly on the temperature and field history of the experimental processes.This work was partially supported by RFBR (Grants No. 07-02-13629, No. 08-02-91317, and No. 09-02-01274), FANI (Grant No. 02.513.123097), the EU Seventh Framework Programme (Contract No. 214864), MICINN and FEDER funding of Projects No. MAT2007-61621, No. MAT2008-1077, and No. CSD2007-00010.Peer reviewe

Analysis of the magnetocaloric effect in Heusler alloys: study of Ni50CoMn36Sn13 by calorimetric techniques

This is an open access article distributed under the Creative Commons Attribution License.Direct determinations of the isothermal entropy increment, −ΔST, in the Heusler alloy Ni50CoMn36Sn13 on demagnetization gave positive values, corresponding to a normal magnetocaloric effect. These values contradict the results derived from heat-capacity measurements and also previous results obtained from magnetization measurements, which indicated an inverse magnetocaloric effect, but showing different values depending on the technique employed. The puzzle is solved, and the apparent incompatibilities are quantitatively explained considering the hysteresis, the width of the martensitic transition and the detailed protocol followed to obtain each datum. The results show that these factors should be analyzed in detail when dealing with Heusler alloys.Financial support from Projects MAT2011-23791, MAT2013-44063-R and MAT2014-53921-R from the Spanish MEC, DGA Consolidated Groups E100 and E34, RFBR 12-07-00676-a, RF President MD-770.2014.2, RSF 14-12-00570 and from the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of MISiS are acknowledged.We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).Peer Reviewe

The ferrimagnetic compounds CoM[M'(EDTA)]2.4H2O (M,M'=Co,Ni): Magnetic characterization of CoCo[Ni(EDTA)]2.4H2O

Under the terms of the Creative Commons Attribution (CC BY) license to their work.We report on the magnetic properties of the ordered bimetallic compound CoCo(NiEDTA)2⋅4H2O (abbreviated as [CoCoNi]). The structure consists of ordered bimetallic layers formed by alternating octahedral sites of Co and Ni(II); tetrahedral Co sites connect different Co‐Ni layers. We discuss the low‐dimensional ferrimagnetic behavior of this compound in terms of a model that assumes three spin sublattices exchange coupled by an Ising interaction.This work was supported by the Comision Interministerial en Ciencia y Tecnologia (Grant No. PB 85-OI06-C02-02), the Iustitucio Valenciana d'Estudis i Investigació, and the National Science Foundation (Grant No. DMR-8515224). F. S. andP. G. R. thank the Spanish Ministerio de Educacion y Ciencia for a predoctoral and postdoctoral fellowship, respectively.Peer Reviewe

Aplicaciones de las células madre en el estudio y tratamiento de enfermedades priónicas: una revisión bibliográfica

Las enfermedades priónicas son un grupo de trastornos neurodegenerativos raros causados por la acumulación de una isoforma mal plegada (PrPSc) de la proteína prion celular (PrPC). Tanto el mecanismo de patogenicidad como las funciones de la propia proteína prion celular siguen siendo desconocidos, y debido a que estas enfermedades son de progreso rápido y mortales, existe una necesidad urgente y no satisfecha de desarrollar terapias que permitan frenar la propagación del prion. Se han propuesto muchos fármacos para combatir esta enfermedad, no obstante, no han resultado efectivos para paliar el daño cerebral. Por ello, una técnica que se presenta como buena candidata terapéutica es el empleo de las células madre, células no especializadas con capacidad de autorrenovarse y dar lugar a distintos tipos de células. Se sabe que estas células migran hacia sitios de daño cerebral y pueden diferenciarse en tipos de células neuronales específicos. Además del uso en la medicina regenerativa, las células madre se emplean también como modelo in vitro de las enfermedades priónicas y para el estudio de nuevas terapias farmacológicas.<br /

Effect of Gd polarization on the large magnetocaloric effect of GdCrO4 in a broad temperature range

The ferromagnetic zircon-type phase of GdCrO4 presents high values for the magnetocaloric (MC) parameters. This compound has large isothermal entropy changes ΔST under the magnetic field action in a wide temperature range, from 5 to 35 K, reaching a maximum |ΔST|=29.0±0.1J/kgK at 22 K, for a field increment ΔB=9 T. It orders ferromagnetically at TC=21.3K via the Cr-Cr exchange interaction and shows a second transition at 4.8 K due to the ordering of the Gd sublattice. The large MC effect is enhanced by the polarization of the Gd3+ ions by the Cr5+ ones via a weaker Gd-Cr interaction. This effect is an interesting feature to be considered in the search for new compounds with a high MC effect in the range of liquid hydrogen or natural gas, regarding the liquefaction of gases by magnetization-demagnetization cycles. This paper contains experimental measurements of magnetization, heat capacity, and direct determinations of the MC effect. The magnetic contribution to the heat capacity Cm has been obtained after subtracting the lattice component. Approximate values for the exchange constants J1 (Cr-Cr) and J3 (Gd-Cr) have been deduced from Cm.This work has been funded by the Spanish MINECO through Projects No. MAT2013-44063-R and No. MAT2013-44964-R, DGA Consolidated Group E100, and Comunidad de Madrid Project No. S2009/PPQ-1626.Peer Reviewe

Thermophysical properties of the lanthanide sesquisulfides. IV. Schottky contributions, magnetic, and electronic properties of ϵ‐phase Yb2S3 and Lu2S3

The heat capacities of ϵ‐phase Yb2S3 and Lu2S3 have been determined from 6 to 350 K and their thermodynamic properties evaluated. The resolution of the Schottky and magnetic properties by evaluation of the lattice heat capacity is shown to be in accord with spectroscopically determined energy levels. The lattice heat capacity of Yb2S3 was determined by means of the Komada–Westrum phonon distribution model. Excess heat‐capacity contributions were thus evaluated and analyzed as Schottky and magnetic heat capacities. A phase transition associated with magnetic ordering was detected in the heat capacity of Yb2S3 near 7 K with an entropy content of 0.68R. The entropies at 298.15 K are 22.77R and 19.74R for Yb2S3 and for Lu2S3.  Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70016/2/JCPSA6-98-2-1458-1.pd

Thermophysical properties of the lanthanide sesquisulfides. II. Schottky contributions and magnetic and electronic properties of γ‐phase Pr2S3, Tb2S3, and Dy2S3

Heat‐capacity measurements by adiabatic equilibrium calorimetry are reported for γ‐phase Pr2S3, Tb2S3, and Dy2S3 between 5 and 350 K. Highly purified samples were prepared and their composition verified by chemical analysis. Precision lattice parameters were determined for each compound and are compared with literature values. The total heat capacity has been resolved into lattice, magnetic, and Schottky components by a volumetric approach. The experimental Schottky contributions accord with the calculated curves based on the crystal‐field splitting of the 2S+1LJ ground state of the lanthanide ions occupying sites of S4 symmetry in the Th3P4 lattice. The individual crystal‐field electronic energy levels have been obtained in part from an analysis of the hot‐band data observed in the absorption spectra of Pr2S3, Tb2S3, and Dy2S3, and from a calculated splitting in which the crystal‐field parameters Bkm, were determined from a lattice‐sum calculation. Molar thermodynamic properties are reported for all three compounds. The entropy at 298.15 K {S0−S0 (7 K)}, is 22.78R, 22.93R, and 23.36R, for γ‐phase Pr2S3, Tb2S3, and Dy2S3, respectively.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70592/2/JCPSA6-95-3-1964-1.pd