Pablo Alvarez-Alonso | University of Oviedo / Universidad de Oviedo (original) (raw)

Books by Pablo Alvarez-Alonso

E n esta memoria de Tesis Doctoral se presentan los resultados del estudio del efecto magnetocaló... more E n esta memoria de Tesis Doctoral se presentan los resultados del estudio del efecto magnetocalórico y magnetovolúmico que se ha llevado a cabo en dos familias de compuestos ricos en Fe: aleaciones R 2 Fe 17 , sintetizadas en forma policristalina, y cintas amorfas de composición FeZrBCu. Estas aleaciones presentan transiciones magnéticas de segundo orden con temperaturas críticas en torno a temperatura ambiente.

Papers by Pablo Alvarez-Alonso

Journal of Alloys and Compounds, 2024

This work addressed the synthesis of melt-spun ribbons of the ternary intermetallic ferromagnetic... more This work addressed the synthesis of melt-spun ribbons of the ternary intermetallic ferromagnetic phases RFeSi with R = Tb and Dy, presenting a thorough characterization of their structural, magnetic, and magnetocaloric behavior. Both compounds have been referred to as suitable magnetic refrigerants in the natural gas liquefaction temperature range. The primary phase in ribbon samples of both compounds shows the CeFeSitype tetragonal crystal structure (space group P4/nmm), with Curie temperatures of 124 K and 89 K for TbFeSi and DyFeSi, respectively. Notably, a sizeable increase in the coercive field appeared below 20 K. However, for a magnetic field change of 2 T, the maximum values of the magnetic entropy change (|ΔSM| max) were 3.4 J kg-1 K-1 and 3.8 J kg-1 K-1 for TbFeSi and DyFeSi ribbon samples, respectively, reflecting a reduction of approximately 60 % compared to reported findings for bulk alloys. In contrast, the refrigerant capacity under a magnetic field change of 2 T measured 120 J kg-1 for TbFeSi and 109 J kg-1 for DyFeSi. The reduction in |ΔSM| max is mainly due to a broader magnetic transition compared to the previous results for bulk alloys. Based on the RFeSi ribbons obtained, a biphasic magnetocaloric composite with a broad table-like magnetic entropy change curve, as that required for an Ericsson-type refrigeration cycle, is designed.

Journal of Alloys and Compounds, 2024

Single-phase nanocrystalline R 2 Fe 17 (R = Pr and Nd) ribbons with rhombohedral Th 2 Zn 17-type ... more Single-phase nanocrystalline R 2 Fe 17 (R = Pr and Nd) ribbons with rhombohedral Th 2 Zn 17-type crystalline structure (space group Rm) have been fabricated by melt-spinning technique. The microstructure of the polycrystalline ribbons is composed of quasi-spherical grains with an average size below 100 nm. Transmission electron microscopy reveals that these grains are formed by agglomeration of smaller nanocrystalline entities around 15 nm in diameter, separated by disordered boundaries where the long-range crystalline order is lost. Two different ferro-to-paramagnetic phase transitions are observed, one of them coincides with that of the parent bulk alloy (290 and 326 K for R = Pr and Nd, respectively), and the other one can be ascribed to the disordered boundaries (323 and 350 K for R = Pr and Nd, respectively). For R= Pr, this fact gives rise to a significant broadening (c.a. 120 K under a magnetic field change of 2 T) of the full-width at the half-maximum of the magnetic entropy change curve, ∆S M (T) (that adopts a "table-like" shape), resulting in a considerable increase of the refrigerant capacity.

Journal of Materials Chemistry C, 2023

Nowadays, Ni@C nanostructured materials are attracting a great deal of attention due to their mul... more Nowadays, Ni@C nanostructured materials are attracting a great deal of attention due to their multiple catalytic or magnetic functionalities. In this article we report on the investigation of the correlation between microstructure and magnetic properties of Ni nanoparticles embedded in a carbon matrix. The samples were obtained following a two-step procedure that ensures protection against nanoparticle oxidation, and was carried out in the following way: i) the synthesis of a nickelimidazole-based metal-organic framework (MOF) by a simple method in aqueous medium at moderate temperature (95 ºC); and ii) carbonization of the MOF at different temperatures between 400 and 600 ºC to obtain a carbon-supported hybrid material, containing Ni nanoparticles with "artichoke-like" morphology, where a Ni-FCC core is surrounded by "bracts" of Ni-HCP and Ni3C. The average size of the nanoparticle slightly changes from 7 to 10 nm as the carbonization temperature is increased, but the Ni-FCC core diameter ranges from 3 to around 6 nm. We show how the information obtained from the evolution of the magnetic behaviour with carbonization temperature, X-ray diffraction and electron microscopy, complements each other by providing consistent structural and magnetic characteristics of the investigated Ni@C nanoparticles. In fact, this joint analysis allows us to explain the formation and transformation of different Ni-based crystalline phases along the synthesis process, including Ni3C and Ni with both hexagonal and cubic crystalline structures. The amount of conventional Ni-FCC is below 10 wt.% for the sample treated at 400 ºC and can reach up to 50 wt.% for that of 600 ºC. Finally, based on our current findings we propose an explanation for understanding the magnetic properties of Ni@C, in which the Ni-FCC core spins mainly govern the magnetic coupling of the whole system.

Nanomaterials, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Journal of Materials Research and Technology, 2020

Melt spun ribbons NiFeGa Mechanical properties Elastocaloric effect a b s t r a c t In the presen... more Melt spun ribbons NiFeGa Mechanical properties Elastocaloric effect a b s t r a c t In the present work, the structural, mechanical and shape memory properties of Ni 55 Fe 16 Ga 29 (at.%) melt-spun ribbons have been studied with a prospective of application in the elastocaloric devises. Particularly, a special thermo-mechanical treatment, consisting in the thermal aging under constant external stress, was elaborated to control the residual internal stress generated due to the melt-spinning processing and improve thermome-chanical and superelastic responses of the ribbon. The stress-induced entropy change was evaluated for the ribbon with improved thermomechanical properties.

Journal or Rare Earths, 2020

ErNi 2 ribbons were produced by rapid solidification using the melt spinning technique. Their str... more ErNi 2 ribbons were produced by rapid solidification using the melt spinning technique. Their structural, magnetic and magnetocaloric properties in the as-solidified state were studied by X-ray diffraction, scanning electron microscopy, magnetization and specific heat measurements. Samples are single phase with the MgCu 2-type crystal structure, a Curie temperature T C of 6.8 K and a saturation magnetization at 2 K and 5 T of 124.0 A$m 2 /kg. For a magnetic field change m 0 DH of 5 T (2 T) ribbons show a maximum magnetic entropy change jDS M peak j of 24.1 (16.9) J/(kg$K), and an adiabatic temperature change DT ad max of 8.1 (4.4) K; this is similar to the previously reported literature for bulk alloys that were processed through conventional melting techniques followed by prolonged thermal annealing. In addition, the samples also show slightly wider DS M (T) curves with respect to bulk alloys leading to a larger refrigerant capacity.

Journal of Alloys and Compounds, 2020

We report the design and fabrication of a highly dense (>97%) two-phase magnetocaloric composite ... more We report the design and fabrication of a highly dense (>97%) two-phase magnetocaloric composite operating in the cryogenic temperature range based on the binary Laves phases HoAl2 and ErAl2 with a nearly table-like magnetic entropy change curve ΔSM(T) at 2 T. Such particular ΔSM(T) dependence was obtained, from the individual ΔSM(T) curves of the precursors, for the composition 28 wt% (ErAl2) + 72 wt% (HoAl2). This composition was selected to make a composite by spark plasma sintering from homogeneously mixed melt-spun ribbons of both intermetallic compounds. The calculated and experimental ΔSM(T) curves obtained for the as-sintered composite were found to be similar. Our results highlight the potential of this processing metallurgical technique for producing two-phase active magnetic regenerators with a designed entropy change curve for specific magnetic refrigeration applications.

Materials, 2020

Fe-Pd magnetic shape-memory alloys are of major importance for microsystem applications due to th... more Fe-Pd magnetic shape-memory alloys are of major importance for microsystem applications due to their magnetically driven large reversible strains under moderate stresses. In this context, we focus on the synthesis of nanostructured Fe 70 Pd 30 shape-memory alloy antidot array thin films with different layer thicknesses in the range from 20 nm to 80 nm, deposited onto nanostructured alumina membranes. A significant change in the magnetization process of nanostructured samples was detected by varying the layer thickness. The in-plane coercivity for the antidot array samples increased with decreasing layer thickness, whereas for non-patterned films the coercive field decreased. Anomalous coercivity dependence with temperature was detected for thinner antidot array samples, observing a critical temperature at which the in-plane coercivity behavior changed. A significant reduction in the Curie temperature for antidot samples with thinner layer thicknesses was observed. We attribute these effects to complex magnetization reversal processes and the three-dimensional magnetization profile induced by the nanoholes. These findings could be of major interest in the development of novel magnetic sensors and thermo-magnetic recording patterned media based on template-assisted deposition techniques.

Applied Physics Letters, 2016

Heusler-type Ni-Mn-based metamagnetic shape memory alloys (MetaMSMAs) are promising candidates fo... more Heusler-type Ni-Mn-based metamagnetic shape memory alloys (MetaMSMAs) are promising candidates for magnetic refrigeration. To increase heat exchange rate and efficiency of cooling, the material should have a high surface/volume ratio. In this work, the typical Ni50Mn35In15 MetaMSMA was selected to fabricate thin ribbons by melt-spinning. The characteristic transformations of the ribbons were determined by calorimetry, X-ray diffraction, scanning electron microscopy and thermomagnetization measurements. The inverse and conventional magnetocaloric effects (MCEs) associated to the martensitic transformation (MT) and the ferromagnetic transition of the austenite (TC A), respectively, were measured directly by the adiabatic method (∆Tad) and indirectly by estimation of the magnetic entropy change from magnetization measurements. It is found that the ribbons exhibit large values of ∆T ad =-1.1K at µ 0 ∆H = 1.9T, in the vicinity of the MT temperature of 300K for inverse MCE, and ∆T ad = 2.3K for conventional MCE at T C A = 309K. This result strongly motivates further development of different MetaMSMAs refrigerants shaped as ribbons. Magnetic refrigeration (MR), a solid state technology based on the magnetocaloric effect (MCE), has attracted great interest because of several environmental advantages (for instance, its larger energy efficiency) compared with traditional cooling systems. 1,2 Among the candidates as magnetic coolants for MR at room temperature, Ni-Mn-based Heusler alloys, a family of metamagnetic shape memory alloys (MetaMSMAs), are outstanding materials because (i) they can exhibit abrupt changes in their magnetic properties induced by a magnetostructural transition from the weakly magnetic martensite to a ferromagnetic austenite, which is usually associated with giant inverse MCE (i.e., with the opposite sign compared with conventional magnetocaloric), 3 and (ii) the MT temperatures can be easily tuned in a wide temperature range including the room temperature by slight changes in the composition, since they are highly dependent on the valence electron concentration per atom e/a. 4 Aside from investigating new materials with enlarged magnetocaloric responses, a growing research field consists of finding new performance optimization methods of MR systems; in

Nanotechnology, 2020

Development of perpendicular magnetic anisotropy (PMA) thin films is a requisite for many applica... more Development of perpendicular magnetic anisotropy (PMA) thin films is a requisite for many applications. In this work, we have illustrated the enhancement of the PMA of Hard (Co)/ Soft (Permalloy, Py) ferromagnetic bilayers by depositing them onto nanoporous anodic alumina membranes with different hole diameters varying in the range between 30 nm and 95 nm. A dramatic change in the hysteresis loops behaviour with hole size, D, and magnetic surface cover ratio parameters has been observed: (1) for samples with small antidot hole diameters, the in-plane (INP) hysteresis loops show single-step magnetic behaviour; (2) for D = 75 nm, the hysteresis loops of Co/Py and Py samples exhibit a multistep magnetic behaviour; (3) a decreasing coercivity in the INP hysteresis loops for antidot arrays samples with D > 75 nm has been detected as a consequence of the reduction of the in-plane magnetic anisotropy and the rising of the out-of-plane component. A crossover of magnetic anisotropy from the in-plane to out-of-plane for bilayer antidot samples has been observed for Co/Py ferromagnetic bilayers, favoured by the interfacial exchange coupling between the two ferromagnetic materials. These findings can be of high interest for the development of novel magnetic sensors and for perpendicular-magnetic recording patterned media based on template-assisted deposition techniques.

Journal of Alloys and Compounds, 2020

This work investigates the magnetic and the intrinsic exchange bias (EB) properties of the sol-ge... more This work investigates the magnetic and the intrinsic exchange bias (EB) properties of the sol-gel synthesised LaMnO3 and LaFeO3 perovskite compounds. The x-ray diffraction (XRD) has proved the high homogeneity of both compounds, which are crystallised in the orthorhombic Pnma structure (as proved by Rietveld refinement). The field cooling-zero field cooling magnetisation dependent temperature (M(T)) indicates the antiferromagnetic (AFM) nature of these compounds. Nevertheless, an anomalous ferromagnetic (FM) behaviour is observed, which is more likely, arises from the spin canting effect in the Mn3+ and Fe3+ ions. The thermal variation of the magnetisation reciprocal (M−1) has confirmed the presence of this FM component below 110K in LaMnO3 and the Curie-Weiss behaviour above 160K. Also, the magnetic hysteresis loops below 110K are corresponding to the FM-like behaviour. The spin of the FM component couples with the AFM phase spins, leading to the EB effect in both compounds that show maximum values of −1124Oe and 2343 Oe for the LaMnO3 and LaFeO3 compounds, respectively. It is observed that the EB effect is suppressed with the partial substitution of La3+ by Ba2+ due to the dominance of the FM phase and the absence of the FM/AFM coupling. Also, the influence of the FM-AFM phases co-existence on the magnetocaloric(MCE) properties of the LaMnO3 compound were studied. Where the LaMnO3 compound shows a magnetic entropy change (ΔS) of 0.42 J/kg.K with an adiabatic temperature change (ΔTad) of 0.1k that is improved in the La0·8Ba0·2MnO3 compound to 1.3 J/kg.K and 0.4K, respectively.

Journal of Alloys and Compounds, 2017

Applied Physics Letters, 2018

We have studied the correlation between the elastocaloric effect and the crystallographic directi... more We have studied the correlation between the elastocaloric effect and the crystallographic direction where a uniaxial stress is applied in a textured polycrystalline Ni-Mn-In-Cr ferromagnetic shape memory alloy; this alloy displays martensitic transformation around room temperature and presents an L21 cubic structure in the austenite phase. The texture in the material was induced by simple arc melting synthesis; using inverse pole figures, a favored grain growth was shown in the direction [001] perpendicular to the cooled surface. The elastocaloric effect was determined by direct measurements of the adiabatic temperature change (ΔTadme), while compressive stress was applied and released; hereby, it has been shown that it is possible to exploit the columnar growth texture in order to obtain a large and reversible elastocaloric effect. The reversible elastocaloric response was measured between 280 and 310 K by applying moderate stresses of 50, 75, and 100 MPa in the [001], [111], and [011] directions. A strong interrelation was found in the cyclic ΔTadme values of −3.9, −2.0, and −1.3 K after unloading a compressive stress of 100 MPa applied mainly in the [001], [111], and [011] directions, respectively.

Journal of Alloys and Compounds, 2018

Ni50Mn35In15 compound has become an archetype for investigating the functional properties of meta... more Ni50Mn35In15 compound has become an archetype for investigating the functional properties of metamagnetic shape memory alloys. We have fabricated Ni50Mn35In15 melt spun ribbons to study the crystal structure, microstructure, martensitic transformation, magnetic properties and magnetocaloric effect as a function of the ribbon solidification rate controlled by the wheel speed. We have found that an increase of the cooling rate refines the alloy grain size, which, in turn, influences the chemical order of austenite phase and functional properties: ribbons produced at low wheel speed (10, 20 and 30 m/s) present majorly L21 structure associated with higher magnetic entropy change, ΔSM (up to 18.6 J/kgK for a magnetic field change of μ0ΔH = 5 T) and Curie temperatures of austenite, TCA, and martensite, TCM (TCA = 309 K and TCM = 199 K) compared with the B2-ordered single phase ribbons (ΔSM = 11.3 J/kgK for μ0ΔH = 5 T; TCA = 293 K; TCM = 178 K) obtained at higher cooling rates (40 and 50 m/s). Besides, we have also observed a correlation between the grain size reduction and a shift of the martensitic transformation to lower temperatures. Direct measurements of the adiabatic temperature change have been performed during both the first- and second-order phase transitions. The results disclose the correlation between structural and magnetic properties of the ribbon and the wheel speed, which opens an innovative tool to adjust the transformation characteristics and magnetocaloric properties through the solidification rate control.

Journal of Alloys and Compounds, 2018

Journal of Alloys and Compounds, 2024

Journal of Materials Chemistry C, 2023

Nanomaterials, 2022

Journal of Materials Research and Technology, 2020

Journal or Rare Earths, 2020

Journal of Alloys and Compounds, 2020

Materials, 2020

Applied Physics Letters, 2016

Nanotechnology, 2020

Journal of Alloys and Compounds, 2020

Journal of Alloys and Compounds, 2017

Applied Physics Letters, 2018

Journal of Alloys and Compounds, 2018

The inspection of simplified fabrication and/or processing routes in order to produce materials w... more The inspection of simplified fabrication and/or processing routes in order to produce materials with attractive magnetocaloric properties is of paramount importance for the development of environmentally friendly magnetic cooling technology. In this work, we have made use of the melt-spinning technique to obtain directly single-phase Y 2 Fe 17 polycrystalline ribbons avoiding any high-temperature annealing for phase consolidation and homogenization. The melt-spun ribbons, with hexagonal Th 2 Ni 17-type crystal structure, exhibit a moderate maximum value of the magnetic entropy change, | DS M peak | ¼ 2.4(4.4) J kg À1 K À1 under an applied magnetic field change of 2(5) T. Although these values are similar to those for the bulk alloy, the DS M (T) curves are manifestly broader, thus giving rise to an expansion of the working temperature range and the enhancement of about 15% in the refrigerant capacity. We also show that the magnetic field dependence of |DS M peak | at T ¼ T C follows a H 2/3 power-law.

Mn-rich Ni-Mn-Sn metamagnetic shape memory alloys exhibiting magnetostructural transformation are... more Mn-rich Ni-Mn-Sn metamagnetic shape memory alloys exhibiting magnetostructural transformation are of a great potential as the base materials for solid-state refrigeration. With the aim of fine tuning of the transformation characteristics and improving functional properties, in the present work we have fabricated polycrystalline Ni 50-x Fe x Mn 40 Sn 10 (x = 0, 2, 4, 6, 8 at.%) melt-spun ribbons, starting from the base alloy with x = 0, which is weakly magnetic in both austenitic and martensitic phases. By exploring martensitic transformation (MT) and magnetic behaviors as a function of Fe doping and magnetic field, we have found that Fe and/or magnetic field reduce the MT temperature and Curie temperature of austenite phase, becoming closer to each other as the Fe-content increases, accompanied by an increase of the magnetic moment of austenite, magnetization jump at MT, transformation volume, and magnetic contribution, ∆S M , to the total entropy change at MT. The ribbons present moderate values of ∆S M equal to 11 J kg-1 K-1 at 5 T for x = 8, moderate thermal hysteresis (10-14 K) nearly independent of Fe doping or magnetic field, and adjustable structural and magnetic transition temperatures close to room temperature.

One of the challenges of modern societies consists in to increase the equipment energy efficiency... more One of the challenges of modern societies consists in to increase the equipment energy efficiency, whereby reducing the energy consumption. In this sense, the magnetic solid-state refrigeration technology based on the magnetocaloric effect (MCE), attracts an enormous interest because of its potential to substitute the conventional liquid-gas refrigerant systems due to, among other advantages, its superior efficiency (up to 60% of Carnot's cycle) [1,2]. However, to be commercially competitive, this technology still needs cheap materials with enhanced refrigerant properties. Among the potential materials, metamagnetic shape memory alloys (mainly, Heusler-type Ni-Mn-based alloys) occupy a unique place because, alongside the shape memory effect and superelasticity, they exhibit large magnetocaloric effect due to the sharp change of the magnetization associated to the magnetostructural martensitic transformation (MT) [4].
We will present our recent studies of both the magnetocaloric effect and the influence of magnetic field on MT in metamagnetic Ni-Mn-In alloys doped by Cu and Cr. This doping mode allows a fine tuning of both the MT temperature around the room temperature (278-315 K) and magnetization drop at MT. The adiabatic MCE measurements have been performed using in-house made set-up [3]. An application of 1.9 T magnetic field results in a maximum inverse adiabatic temperature change of ~ -2 K caused by magnetic field-induced MT. Besides, the austenite phase undergoes a ferro-to-paramagnetic transition to which a direct adiabatic temperature change of almost the same amplitude as for inverse effect is associated. Furthermore, MT moves to lower temperatures (around 40 K for Cu-doped alloy) in magnetic fields up to 10 T accompanied by a decrease of the transformation entropy change.
References:
1. M.-H. Phan and S.-C. Yu, J. Magn. Magn. Mater. 308, 325 (2007).
2. V. Franco, J.S. Blázquez, B. Ingale, and A. Conde, Annu. Rev. Mater. Res. 42, 305 (2012).
3. V.A. Chernenko et al., J. Magn. Magn. Mater. 324, 3519 (2012).
4. P. Álvarez-Alonso et al., Key Eng. Mater. 644, 215–218 (2015).

MCE and magnetovolume effects in Fe-based alloys 04/07/11 1 / 48 P. Alvarez (University of Oviedo... more MCE and magnetovolume effects in Fe-based alloys 04/07/11 1 / 48 P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 2 / 48 P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 2 / 48 P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 2 / 48 4 Conclusions P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 2 / 48 P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys MCE and magnetovolume effects in Fe-based alloys 04/07/11 6 / 48 P.Gorria et al., J. Phys D: Appl. Phys., 41 (2008) 192003 (5pp) P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 7 / 48 P.Gorria et al., J. Phys D: Appl. Phys., 41 (2008) 192003 (5pp) P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 7 / 48 4 Conclusions P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 8 / 48 Magnetovolume Anomalies Lattice Parameters a and c, and the cell volume V vs T for Sm 2 Fe 14 B P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 9 / 48 P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 9 / 48 P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 9 / 48 Electronic Microscopies (University of Oviedo) Scanning (JEOL JSM-6100) Transmission (JEOL 2000 EX-II) Diffraction X-ray (University of Oviedo) Synchrotron (ESRF: ID27) Neutron (ILL: D1A, D1B, D2B, D4) P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys MCE and magnetovolume effects in Fe-based alloys 04/07/11 19 / 48 P. Alvarez (University of Oviedo) MCE and magnetovolume effects in Fe-based alloys 04/07/11 35 / 48