Publications (51)59.65 Total impact
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Dataset: On the Curie temperature dependency of the magnetocaloric effect
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ABSTRACT: We investigate the magnetocaloric effect dependency on the most important microscopic parameters of ferromagnetic materials, such as the Curie temperature (T C), the spin value (J), and the magnetic field change (DH). Second-and first-order phase transition systems are considered, using the Bean-Rodbell model [C. P. Bean and D. S. Rodbell, Phys. Rev. 126, 104 (1962)] of magnetovolume interactions on the Weiss mean-field model [P. Weiss, J. Phys. Theory Appl. 6, 661 (1907)]. The magnetocaloric effect simulations show a surprising T À2=3 C linear dependence of the maximum entropy change (DSm max), which is observed for all simulated systems. An approximate state equation establishing the dependence of DSm max on T C , DH, J, and the magnetic atoms density (N) is presented. The dependence of maximum magnetic entropy change on T À2=3 C is validated by a wide set of experimental results of second-and first-order phase transition materials that are promising for magnetic refrigeration applications at room temperature. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4726110] For the past fifteen years, since the discovery of the giant magnetocaloric effect (MCE) in the R 5 (Si,Ge) 4 system 1 near room temperature (RT), a very devoted effort has been made into the understanding of this phenomenon. The magnetic re-frigeration near RT, which makes use of the MCE, possesses several eco-friendly features, namely, its high efficiency, ab-sence of environmentally harmful gases, and low maintenance costs. 2–4 Nevertheless, the magnetic refrigeration near RT still has not been able to compete with the conventional refrigera-tion technology. One of the major achievements to be accom-plished is to find the most suitable magnetocaloric material in this temperature range. Namely, an intense debate on which kind of magnetic transitions (first or second order) are prefera-ble for the RT magnetic refrigeration devices is still occurring nowadays. 5–8 So far, the most promising compound families from the point of view of technological applications at RT include R 5 (Si,Ge) 4 (R is for Rare Earth), RM 2 (M is for Al, Co or Ni), MnFe(P 1Àx As x), La(Fe 13 À x , Si x), and the mangan-ites R 1Àx M x MnO 3 (M is for Ca, Sr and Ba). All these materi-als have been thoroughly characterized and their most important magnetic properties are known and are well reviewed in the following references. 2–4,9,10 The most conven-tional way to characterize the MCE of a given magnetic mate-rial is through the temperature dependence of its magnetic entropy change (DSm(T)) for a given applied field change (DH), namely, through its peak value (DSm max), Curie tem-perature (T C), which establishes the optimal operating temper-ature, and the refrigerant capacity (product of full width at half maximum, FWHM DSmðTÞ , and the DSm max), which esti-mates the range of operating temperature. A considerable the-oretical effort has been made on applying various models to the study of the MCE, from phenomenological theories 11–13 to microscopic models. 14 Nevertheless, there is still a notable lack of direct relations between the MCE and microscopic magnetic parameters, as recently evidenced in Ref. 15. Herein, we have performed theoretical simulations to study the dependence of the DSm(T) curves on the following parameters: Curie temperature (T C), spin value (J), the applied magnetic field change (DH) for both first-and second-order magnetic transitions. The simulated DSm(T) curves were obtained within the basis of the molecular mean field model, 16 by solving numerically the following state equation for spin J: -
Article: Perturbed angular correlations investigations on YMnO3 multiferroic manganite
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ABSTRACT: The Perturbed Angular Correlation (PAC) technique was applied to study the yttrium local environment in YMnO3 multiferroic manganite. The electric field gradients (EFG) at the Y site have been measured as function of temperature, covering both ferroelectric and magnetic transitions. The results were compared with point charge model (PCM) calculations. The experimental results show two different EFG distributions for all temperatures. Only one can be directly attributed to the yttrium crystalline site in the hexagonal structure. KeywordsPAC–Manganites–Phase separationHyperfine Interactions 04/2012; 197(1):83-88. · 0.21 Impact Factor -
Article: On the Curie temperature dependency of the magnetocaloric effect (vol 100, 242407, 2012)
Applied Physics Letters 01/2012; 101(7). · 3.84 Impact Factor -
Article: Dynamic off-centering of Cr3+ ions and short-range magneto-electric clusters in CdCr2S4
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ABSTRACT: The cubic spinel CdCr2S4 gained recently a vivid interest, given the relevance of relaxor-like dielectric behavior in its paramagnetic phase. By a singular combination of local probe techniques, namely, pair distribution function and perturbed angular correlation, we firmly establish that the Cr ion plays the central key role on this exotic phenomenon, namely, through a dynamic off-centering displacement of its coordination sphere. We further show that this off-centering of the magnetic Cr ion gives rise to a peculiar entanglement between the polar and magnetic degrees of freedom, stabilizing, in the paramagnetic phase, short-range magnetic clusters, clearly seen in ultralow-field susceptibility measurements. Moreover, the Landau theory is here used to demonstrate that a linear coupling between the magnetic and polar order parameters is sufficient to justify the appearance of magnetic cluster in the paramagnetic phase of this compound. These results open insights on the hotly debated magnetic and polar interaction, setting a step forward in the reinterpretation of the coupling of different physical degrees of freedom. DOI: 10.1103/PhysRevB.86.224418Physical Review B 01/2012; 86(22). · 3.69 Impact Factor -
Article: Unveiling the (De)coupling of magnetostructural transition nature in magnetocaloric R5Si2Ge2 (R = Tb, Gd) materials
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ABSTRACT: We present a detailed study on the magnetization under high magnetic fields of Tb5Si2Ge2 and Gd5Si2Ge2 compounds. From the Arrott plot construction (A. Arrott, Phys. Rev. 108, 1394 (1957)), we were able to estimate the TC of each structure (M and O(I)) experimentally and found that the TC of the O(I) phase can be directly obtained by extrapolating the TC curve of this phase in the respective phase diagram. Using a physical model based on free energy considerations, one explains the (de)coupling of the magnetic and structural transitions in R5(SixGe1−x)4 (R = Tb, Gd) compounds.Applied Physics Letters 09/2011; 99(13):132510-132510-3. · 3.84 Impact Factor -
Article: Local distortions in multiferroic AgCrO2 triangular spin lattice
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ABSTRACT: In this work an electric-field gradient and magnetic hyperfine field study of the multiferroic AgCrO2 triangular spin lattice is presented. Perturbed angular correlation measurements at different temperatures, revealed the coexistence of two electric-field gradients, i.e., two distinct local environments at temperatures below 100 K. The emerging second local environment appears as a distortion of the Cr surrounding resulting in a local symmetry lowering. This local distortion emerges much above TN and concomitantly with the onset of short-range magnetic correlations. We claim that, through a magnetoelastic instability, distinct Cr-Cr exchange interaction pathways appear providing a channel for magnetic frustration release.Physical Review B 07/2011; 84(1):014434. · 3.69 Impact Factor -
Article: Local distortions in multiferroic AgCrO2 triangular spin lattice
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ABSTRACT: In this work an electric-field gradient and magnetic hyperfine field study of the multiferroic AgCrO2 triangular spin lattice is presented. Perturbed angular correlation measurements at different temperatures, revealed the coexistence of two electric-field gradients, i.e., two distinct local environments at temperatures below 100 K. The emerging second local environment appears as a distortion of the Cr surrounding resulting in a local symmetry lowering. This local distortion emerges much above T-N and concomitantly with the onset of short-range magnetic correlations. We claim that, through a magnetoelastic instability, distinct Cr-Cr exchange interaction pathways appear providing a channel for magnetic frustration release.Physical Review B. 01/2011; 84(1). -
Article: Magnetic hyperfine field at Cr site in AgCrO2 given by Perturbed angular correlations
Hyperfine Interactions 01/2010; 197(1-3):123-128. · 0.21 Impact Factor -
Article: High refrigerant capacity of PrNi5−xCox magnetic compounds exploiting its spin reorientation and magnetic transition over a wide temperature zone
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ABSTRACT: The ferromagnetically coupled cobalt ion is observed to create a magnetocrystalline anisotropy in the PrNi5−xCox structure above a critical composition of x = 2. The competition of the anisotropy energies between Co and Pr sublattices gives rise to a spin reorientation (SR) phenomenon in PrNi5−xCox compounds at a low temperature (~150 K) which is then followed by a magnetic transition at a higher temperature. Co-doping has a strong influence on the Curie temperature, changing it from ~60 K (x = 1.95) to ~537 K (x = 3). The magnetic entropy change is associated with SR as well as a magnetic transition, and correspondingly a large full width at half maximum (δTFWHM) is obtained for this series of compounds. For example, the PrNi2.85Co2.15 compound presents δTFWHM = 166 K at a 1 T field. This series therefore has an appreciable relative cooling power, which makes this material a suitable magnetic refrigerant over a large temperature span.Journal of Physics D Applied Physics 02/2009; 42(5):055002. · 2.54 Impact Factor -
Article: Percolation processes and spin-reorientation of PrNi5-xCox
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ABSTRACT: In the present work we report on the structural and magnetic behaviors of the PrNi5-xCox intermetallic compounds. Due to the competition between the anisotropy energies of both Co and Pr sublattices, this series has a spin-reorientation phenomenon at low temperature (140 K). The Curie temperature, as a function of Co content, has a sudden increase above a critical concentration x(c)similar to 1.9 and this feature is assigned as a percolation of geometrically spaced Co clusters. This assumption is explained based on the critical exponent of percolation theory. The series presents therefore a rich magnetic phase diagram, which could be established over a full doping range, i.e., from x=0 to x=5. We have also studied these compounds on the magnetocaloric point of view and found a quite large full width at half maximum (delta T-FWHM) of the magnetic entropy change curves for some of the compositions, due to the merging of the Delta S peaks associated with the spin-reorientation process and the Curie temperature T-C. In addition, the series has an appreciable relative cooling power, which is therefore suitable to be used in a magnetic refrigerator operating in a large range of temperature.Physical Review B 01/2009; 79(1). · 3.69 Impact Factor -
Article: New phase transition in the Pr1-xCaxMnO3 system: evidence for electrical polarization in charge ordered manganites.
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ABSTRACT: In this Letter a detailed study of the electric field gradient (EFG) across the Pr(1-x)Ca(x)MnO(3) phase diagram and its temperature dependence is given. Clearly, distinct EFG behavior for samples outside or inside the charge order (CO) region are observed. The EFG temperature dependence evidences a new phase transition occurring over the broad CO region of the phase diagram. This transition is discontinuous and occurs at temperatures between the charge ordering and the Néel temperatures. The prominent features observed in the EFG are associated with polar atomic vibrations which eventually lead to a spontaneous local electric polarization below CO transition.Physical Review Letters 04/2008; 100(15):155702. · 7.37 Impact Factor -
Article: New phase transition in $Pr_{1-x}Ca_xMnO_3$ system: evidence for electrical polarization in charge ordered manganites
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ABSTRACT: A detailed study of the electric field gradient (EFG) across the Pr1-xCaxMnO3 phase diagram and its temperature dependence is given. Clearly, distinct EFG behaviours for samples outside/inside the charge order (CO) region are observed. The EFG temperature dependence evidences a new phase transition occurring over the broad CO region of the phase diagram. This transition is discontinuous and occurs at temperatures between the charge ordering and the Neel temperatures. The features observed in the EFG are associated with polar atomic vibrations which eventually lead to a spontaneous local electric polarization below CO transition. Comment: 8 pages, 4 figures02/2008; -
Article: The effect of chemical distribution on the magnetocaloric effect: A case study in second-order phase transition manganites
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ABSTRACT: Ferromagnetic manganites of general formula ABMnO(3) (where A is a trivalent rare-earth ion and B is a divalent dopant) are candidates for magnetic cooling applications, since they can present either second-or first-order magnetic phase transitions, and the perovskite structure allows for substantial chemical substitution and tuning of the magnetocaloric properties. The consequent chemical distribution from substitution affects the magnetic and magnetocaloric properties of the compound. The change of relative cooling power with chemical disorder is discussed by the use of the molecular mean-field model. We present experimental results of the ferromagnetic, second-order phase transition La(0.70-x)(Er, Eu)(x)Sr(0.30)MnO(3) system. (c) 2008 Elsevier B.V. All rights reserved.Journal of Non-Crystalline Solids 01/2008; 354(47-51):5301-5303. · 1.54 Impact Factor -
Article: Specific heat of clustered low dimensional magnetic systems
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ABSTRACT: Homometallic ferrimagnetism is quite difficult to find in Nature, and this fact makes Na2Cu5Si4O14 an interesting material due to its ferrimagnetic arrangement of Cu ions in a zigzag chain with dimers and trimers. In view of this, we developed a theoretical model for the magnetic specific heat of a homometallic ferrimagnet and then compared it with experimental data for Na2Cu5Si4O14 compound. The successful comparison validates the model and further analysis makes it possible to predict anomalous behavior of this low dimensional magnetic system, due to the crossing of the energy levels.Journal of Physics Condensed Matter 10/2007; 19(44):446203. · 2.55 Impact Factor -
Article: Spin Wave Localization in Random Anisotropy Systems: Amorphous (DyxGd1-x)Ni
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ABSTRACT: Magnetization measurements performed in the random anisotropy (amorphous) system (DyxGd1-x)Ni are analysed in terms of spin wave excitations in the temperature range 1.5 to 20 K and magnetic fields up to 60 kOe. Besides the Zeeman term, the field-dependent spin wave gap contains a contribution coming from the random magnetic anisotropy. This contribution decreases with H, from a zero-field value proportional to the random anisotropy field, D2/J. This observation, in quantitative agreement with the theory, constitutes the first evidence for the existence of a random anisotropy gap in amorphous alloys. The field decreasing dependence of this gap induces a progressive delocalization of spin waves with the application of a magnetic field, opposing to the effect of the disorder of local anisotropy directions. For zero field the excitations are localized in space, in regions of the order of the ferromagnetic correlation length (Imry and Ma domains). Field-induced reorientations of the magnetization of these domains favour the propagation of spin waves to much more extended regions (spin wave delocalization).EPL (Europhysics Letters) 07/2007; 22(2):139. · 2.17 Impact Factor -
Article: Weak Localization in Amorphous Alloys in the Presence of Magnetic Impurities: Enhancement of the Spin-Splitting Contribution
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ABSTRACT: The effect of magnetic impurities on weak localization in amorphous metals is investigated with magnetoresistance in the series (DyxY1-x)Ni up to x = 0.12. In these samples we find that the weak-localization contribution to magnetoresistance is increased with respect to the nonmagnetic sample, YNi, due to an enhancement of the Zeeman spin splitting of the conduction bands by the exchange interaction with the magnetic ions. Over the whole series, the spin-splitting dependence on temperature and composition scales with the paramagnetic susceptibility of the samples. This shows its direct relation with the magnetic-exchange interaction of conduction electrons with Dy moments, which polarize the conduction band.EPL (Europhysics Letters) 07/2007; 21(1):61. · 2.17 Impact Factor -
Article: Physical meaning and measurement of the entropic parameter $q$ in an inhomogeneous magnetic systems
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ABSTRACT: In this paper we present a thorough analysis of two systems magnetically inhomogeneous: the manganite La$_{0.7}$Sr$_{0.3}$MnO$_3$/MgO and the amorphous alloy Cu$_{90}$Co$_{10}$. In both cases, the non-extensive statistics yield a faithful description of the magnetic behavior of the systems. In the model proposed here, the inhomogeneous magnetic system is composed by many Maxwell-Boltzmann homogeneous bits and the entropic parameter $q$ is related to the moments of the distribution of the inhomogeneous quantity. From the analysis of Scanning Tunnelling Spectroscopy (STS) images, the $q$ parameter can be directly measured.02/2006; -
Article: Preparation and properties of new superconductor material MgB2
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ABSTRACT: Bulk MgB2 samples were synthesized by hot isostatic pressing under pressures up to 200MPa at 950 degrees C. In these conditions, full densification of samples was obtained (similar to 98% of theoretical density). SEM, EDS and XRD analysis on final dense bodies were used to evaluate samples, and show increasingly better control over the amounts of secondary MgO (down to similar to 10%) and complete prevention of formation of MgB4 by using simple glass encapsulation techniques and addition of Mg(s) to the capsule. The samples display superconducting properties, including a narrow critical transition in electrical properties (Tc similar to 36-38K). Magnetic studies were performed, allowing the determination of the superconducting fraction and critical current density J(c) of the materials. Contrary to the Tc, the J(c) is quite sensitive to the processing and microstructure and values from 0.3 to 0.6x10(6) A/cm(2) are obtained at 10K. The reduction of J(c) with the applied magnetic field requires further improvements to reduce weak links.Advanced Materials Forum Iii, Pts 1 and 2. 01/2006; 514-516:333-337. -
Article: Experimental determination of the non-extensive entropic parameter $q$
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ABSTRACT: We show how to extract the $q$ parameter from experimental data, considering an inhomogeneous magnetic system composed by many Maxwell-Boltzmann homogeneous parts, which after integration over the whole system recover the Tsallis non-extensivity. Analyzing the cluster distribution of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ manganite, obtained through scanning tunnelling spectroscopy, we measure the $q$ parameter and predict the bulk magnetization with good accuracy. The connection between the Griffiths phase and non-extensivity is also considered. We conclude that the entropic parameter embodies information about the dynamics, the key role to describe complex systems. Comment: Submitted to Phys. Rev. Lett12/2005; -
Article: Field-tuned magnetocaloric effect in metamagnetic manganite system
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ABSTRACT: We have investigated the origin of the huge magnetocaloric effect in a manganite system with ferro–antiferromagnetic phase mixture at low temperatures. We carried out magnetic measurements in fields up to 100 kOe in order to show that both a high hysteretic behavior and a metamagnetic transition in the antiferromagnetic state are responsible for the large magnetic entropy change. The temperature where the maximum of the magnetic entropy change occurs can be tuned by varying the maximum value of the applied magnetic field to the system. This last procedure can open a new perspective for charge-ordered manganite applications to magnetic refrigeration at low temperatures.Applied Physics Letters 11/2004; 85(21):4974-4976. · 3.84 Impact Factor
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2004–2012
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University of Aveiro
Aveiro, Aveiro, Portugal
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1997–2007
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University of Porto
Porto, Distrito do Porto, Portugal
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