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A. Seffar

Bio: A. Seffar is an academic researcher from Autonomous University of Barcelona. The author has contributed to research in topics: Superconductivity & Type-II superconductor. The author has an hindex of 3, co-authored 4 publications receiving 419 citations.

Papers
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Journal ArticleDOI
TL;DR: The amplitude $\ensuremath{\rho}(T,H)$ and temperature ${T}_{M}$ and CMR are found to be controlled by the radius of the lanthanide $({L}^{3+})$ which modifies the bending of the Mn-O-Mn bond.
Abstract: The amplitude $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$ and temperature ${T}_{M}$, where the colossal magnetoresistance (CMR) response of ${L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ manganites are maximum, are found to be controlled by the radius of the lanthanide $({L}^{3+})$ which modifies the bending of the Mn-O-Mn bond. Increasing the bond distortion lowers ${T}_{M}$ and enhances $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$. Enhanced CMR arises from (1) a shift to lower temperatures of ${T}_{M}$, (2) a reduced mobility of the doping holes, and (3) an increase of the coupling between itinerant and localized electrons. The resistivity $\ensuremath{\rho}\left(H\right)$ follows an $\ensuremath{\approx}{\mathrm{BM}}^{2}\left(H\right)$ law and the parameter $B$ is also tuned by the Mn-O-Mn bond angle. The narrowing of the electronic bandwidth is the fundamental parameter controlling the observed CMR.

423 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the role of cationic stoichiometry on the superconducting properties of Nd2−xCexCuO4 oxides and found that the typical synthesis conditions lead to some Cu deficiency in the samples.
Abstract: The solid-state synthesis of Nd2−xCexCuO4 superconducting oxides is studied. In this paper we investigate the importance of: (1) the duration of the reduction time, (2) the possibility to induce superconductivity by fast quenching from high temperature and the role of cationic stoichiometry on the superconducting properties. We have found that the typical synthesis conditions lead to some Cu deficiency in the samples. This result, together with the fact that Cu non-stoichiometry suppresses superconductivity allows one to understand the role of CuO excess on the synthesis of superconducting materials.

9 citations

Journal ArticleDOI
TL;DR: In this article, a systematic study of the stoichiometry effects on some of the normal state and superconducting properties of Nd 2−x Ce x CuO 4−δ ceramics prepared under various conditions (temperature, oxygen pressure, processing time and cooling rate).
Abstract: We present a systematic study of the stoichiometry effects on some of the normal state and superconducting properties of Nd 2−x Ce x CuO 4−δ ceramics prepared under various conditions (temperature, oxygen pressure, processing time and cooling rate). We will show that superconductivity can be induced in this system without reduction at low oxygen pressure. We will also present conclusive evidence that Nd 2−x Ce x CuO 4−δ is not a thermodynamically stable phase under the typical synthesis conditions.

3 citations

Journal ArticleDOI
TL;DR: In this article, the field dependence of the Hall resistivity (ϱ xy ) of L 2−x Ce x CuO 4 single crystals has been analyzed, and it has been shown that the polarity of the anomalous Hall anomaly can have either the same sign or opposite to the Hall effect in the normal state.
Abstract: One of the most intringuing observations in type II superconductors is the anomalous behavior of the Hall effect To some extent the vortex dynamics is controlled by the ratio between the coherence length (ξ) and the mean free path (1) of the charge carries, so that, a change of one of these parameters may alter the dynamic. Most of the flux dynamics studies, particularly the Hall effect in the mixed state, in the HTSC superconducting cuprates have been performed on hole-type materials as YBaCuO and BiSCCO where ξ is rather short (typically ξ 2-x Ce x CuO 4 family are specially suited for this purpose because of its longer (about one order of magnitude) coherence length ξ and very short 1 values. In this work we present the field dependence of the Hall resistivity (ϱ xy ) of L 2−x Ce x CuO 4 single crystals. We have found that, the polarity of the Hall anomaly can have either the same sign or opposite to the Hall effect in the normal state. However, the crystals with higher resistivities do not show the anomalous ϱ xy (H) peak.

1 citations


Cited by
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Journal ArticleDOI
01 Sep 1997
TL;DR: In this paper, the authors review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoreduction associated with a ferromagnetic-toparamagnetic phase transition.
Abstract: We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

1,336 citations

Journal ArticleDOI
TL;DR: The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+−4+) of the Mn ions.
Abstract: The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

775 citations

Journal ArticleDOI
TL;DR: In this article, experimental data and their theoretical interpretation are presented for the colossal magnetoresistance (CMR) materials to which conventional ferromagnetic semiconductors and manganites belong.

449 citations

Journal ArticleDOI
Michael Ziese1
TL;DR: In this article, a review of extrinsic magnetotransport effects in ferromagnetic oxides is presented, focusing on grain-boundary, tunnelling and domain-wall magnetoresistance.
Abstract: Magnetic oxides show a variety of extrinsic magnetotransport phenomena: grain-boundary, tunnelling and domain-wall magnetoresistance. In view of these phenomena the role of some magnetic oxides is outstanding: these are believed to be half-metallic having only one spin-subband at the Fermi level. These fully spin-polarized oxides have great potential for applications in spin-electronic devices and have, accordingly, attracted intense research activity in recent years. This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surface and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.

441 citations

Journal ArticleDOI
TL;DR: Experimental, phenomenological, and theoretical analyses are given of the dependence on strain of the ferromagnetic Tc of the colossal magnetoresistance (CMR) rare earth manganese perovskites as discussed by the authors.
Abstract: Experimental, phenomenological, and theoretical analyses are given of the dependence on strain of the ferromagnetic Tc of the colossal magnetoresistance (CMR) rare earth manganese perovskites. It is found that Tc is extremely sensitive to biaxial strain; by implication other physical properties are also. The results indicate that biaxial strain is an important variable which must be considered in the design of devices based on thin films and provide evidence in favor of the relevance of the Jahn–Teller electron-phonon coupling to the CMR phenomenon.

369 citations