Physique

Etude des promi&& cristallographiques et mae;n&iaues du cornnod s r &p aconducteur YBa&+O~, dop4 ou bien substitu6 avec quelaues Ql&nents de transitions. Cette ktude a &B men&e sur des Cchantillons dopCs (x% moles d'oxydes de Cr, de Mn ou de Ni avec (100-x)% moles d'YBaCu0 ) et sur des &antillons de formule yBa2 (GUI, Mx ) 3 0~~ avec M = Cr, Mn, Ni et x = 0.00; 0.02; 0.05 et 0.10. Notre but est de d&term&r la limite de solubilitk de ces 61Cments de transitions dans la matrice pdrovskite ainsi que leurs localisations et leur effets sur la temperature de transition supraconductrice. La diffraction des rayons X et la mesure de la susceptibilitd magnktique en champ alter. natif ont &tc effectukes.

A B S T R A C T Fe 0.5 Ge 0.5 nanocomposite films with different film thicknesses were fabricated using cosputtering. The films are composed of Ge, Fe and Fe 3 Ge 2 , and are ferromagnetic at room temperature. The saturation magnetization and magnetic interaction including dipolar interaction and exchange coupling increase with the increasing film thickness. The electrical conductance mechanism turns from metallic to semiconducting and the saturation Hall resistivity r xys increases with the decreasing film thickness. At 28 nm, r xys is $137 mV cm at 2 K, about 150 times larger than that of pure Fe film (0.9 mV cm) and four orders larger than that of bulk Fe. The r xy -H curves of all the films show the same linearity character in low-field range even though the temperature-independent slope is different at different film thicknesses. At high temperatures, the skew scattering mechanism is dominant. At low temperatures, side-jump effect should be dominant at large resistivity r xx regime for the thin films, and the skew scattering is dominant at small r xx regime for the thick films.

La complexité du magnétisme observé dans l'étude des matériaux Ln 2 M 12 P 7 a demandé un travail relativement considérable. Il nous fallait donc dans un premier temps limiter l'analyse magnétique aux mesures de la susceptibilité magnétique, de l'aimantation en fonction du champ à température constante et de l'aimantation en fonction de la température à champ constant. Les mesures de susceptibilités paramagnétiques ont été éffectuées entre la température ambiante et 1000 K. Les moments paramagnétiques effectifs déduits à partir des courbes 1/χ = f (T ) en supposant un moment nul sur les atomes du métal de transition sont en bon accord avec les valeurs théoriques calculées pour les ions libres Ln 3+ . La valeur du moment à saturation et la température de Curie paramagnétique indiquent un comportement antiferromagnétique pour Ce 2 Co 12 P 7 à basse température. Le moment magnétique de Sm 2 Co 12 P 7 mesuré à 4,2 et à 78 K sous un champ de 2,66 T indique une structure magnétique complexe à basse température. Cette étude du magnétisme confirmant l'intérêt de ces matériaux montre que les propriétés magnétiques de Ln 2 M 12 X 7 peuvent être interprétées à partir des études précédentes sur les phases terre rare -métal de transition Ln-M.

La 0.57 Nd 0.1 Sr 0.33 Mn 1−x Al x O 3 (LNSMAO) compounds were prepared for x = 0.0 to 0.3 by solid-state route. The X-ray powder diffraction has shown that all our synthesized samples are a single phase and have crystallized in the hexagonal symmetry with R3 c space group. It is also worth mentioning that the unit cell volume decreases along with an increasing aluminum (Al) content. The scanning electron microscopy (SEM) has shown smooth and densified structures, clean and pure images. The magnetization and electrical measurements vs. temperature proved that all our samples exhibit a ferromagnetic to paramagnetic transition and a metallic to semiconductor one when the temperature increases. The substitution of Mn by aluminum (Al) leads to a continuous decrease of both the Curie temperature T C (from 342 K for x = 0.0 to 238 K for x = 0.3) and the resistivity transition temperature T P (from 338 K for x = 0.0 to 208 K for x = 0.3). When analyzing the electrical resistivity data, it has been concluded that the metallic (ferromagnetic) part of the resistivity ( ) (below T P ) can be explained by the following equation = 0 + 2 T 2 + 4.5 T 4.5 , signifying the importance of the grain/domain boundary, electron-electron and two magnon scattering processes. However, at a higher temperature (T > T P ) the paramagnetic semiconducting regime, the adiabatic small polaron and Variable Range Hopping models are found to fit well.

Electrical conductance and X-ray diffraction (XRD) measurements of lanthanum-deficiency La 1Àx & x MnO 3 (x ¼ 0.05, 0.10 and 0.20) polycrystalline samples were performed to examine the effect of the internal pressure at B-site on the conduction mechanism. The structural study reveals that all samples crystallize in the rhombohedral system. The electronic conduction appears to be thermally activated at high temperature, which indicates the presence of semiconductor behaviour. The increase of the x content converts 3x Mn 3+ to 3x Mn 4+ ions with smaller ionic radius, which reduces the internal pressure and leads to the increase of the one-electron bandwidth W. This increase causes the appearance of metallic behaviour at low temperature for x ¼ 0.10 and 0.20 content.

Binuclear complex Crystal structure Magnetism Electronic structure calculations a b s t r a c t Single crystals of a novel copper (II) complex with the amino acid L-arginine (L-arg) and cyclotetraphosphate, [Cu(L-arg) 2 (H 2 O)] 2 (P 4 O 12 ) Á 8H 2 O, were prepared in aqueous solution and were characterized by X-ray diffraction, spectroscopy (diffuse reflectance, UV-vis and IR) and thermal analysis. Magnetic measurements and electronic structure calculations were also performed. Crystal structure determination reveals that this compound has a nonsymmetrical atomic arrangement, containing mainly a binuclear [Cu(L-arg) 2 (H 2 O)] 2 4 þ cation and an uncoordinated cyclotetraphosphate counter-anion (P 4 O 4À 12 ). The tetravalent cation contains two independent Cu(II) ions in two different chemical environments. This compound exhibits an antiferromagnetic (AFM) to paramagnetic (PM) phase transition at a temperature (T N ) lower than 2 K. The values of paramagnetic Curie-Weiss temperature (y cw ) and the exchange parameter (J/K B ) emphasize the existence of an antiferromagnetic interaction between the neighboring copper ions.

An intense e4ort has recently been devoted to studying the interplay between structure, magnetism, and transport in manganese perovskite Ln 1؊x A x MnO 3 (Ln ‫؍‬ La, Pr, Nd, Sm; A ‫؍‬ Ca, Ba, Sr). As a function of temperature, applied magnetic 5eld, doping, A-site ionic radius 1r A 2, and A-site size disorder, this system displays a rich phase diagram for both magnetotransport and structural properties. We have investigated the structural, magnetic, and transport properties of (La 1؊x Nd x ) 0.7 Sr 0.3 MnO 3 . The crystal structure was examined by X-ray powder di4raction which indicated that all the samples were single phase and revealed a transition from rhombohedral to orthorhombic structure with increasing x. The magnetization and resistivity investigation shows that for all values of x, (La 1؊x Nd x ) 0.7 Sr 0.3 MnO 3 are ferromagnetic+metallic at low temperatures and paramag-netic+semiconductor above the Curie temperature T c .

In this paper, a new complex magnetic materials La 0.70 Ca 0.15 Sr 0.15 Mn 1−x Fe x O 3 (x = 0.000, 0.025, 0.050, 0.075 and 0.100), suitable for the Ericsson cycle, has been investigated. For these materials, the effect of Fe doping can be attributed to a combination of doping disorder, Fe-Mn super exchange interactions and a site-percolation mechanism, which suppress the metallic conduction and ferromagnetism. We have found that these compounds crystallize in the rhombohedral structure with the R3c space group. We have also verified that the substitution of the manganese with iron leads to an important decrease of the Curie temperature T C from 335 to 225 K for 0%-10% of Fe. The magnetocaloric study exposes a quite large value of the magnetic entropy change, which decreases when increasing Fe concentration. For an applied magnetic field of 2 T, the Relative Cooling Power (RCP) values are found to vary between 80 and 125 J/kg. As a result, the studied compounds could be considered as potential material for magnetic Ericsson refrigeration by using complex magnetocaloric compounds.

Bis (1,4-bis (3-aminopropylamine) piperazinium) chloride pentachlorocuprate (II) trihydrate (C 10 H 26 N 4 ) 2 Cl CuCl 5 .3H 2 O has been prepared and characterized by various physicochemical techniques including Raman spectroscopy and magnetic properties. A preliminary single crystal X-ray diffraction structural analysis reveals that the title compound belongs to the orthorhombic system with space group Pnma.

We have investigated the influence of chromium (Cr) doping on the magneto-electrical properties of polycrystalline samples La 0.75 Sr 0.25 Mn 1−x Cr x O 3 (0.15 ≤ x ≤ 0.25), prepared by the sol-gel method. Comparison of experimental data with the theoretical models shows that in the metal-ferromagnetic region, the electrical behavior of the three samples is quite well described by a theory based on electron-electron, electron-phonon and electron-magnon scattering and Kondo-like spin dependent scattering. For the high temperature paramagnetic insulating regime, the adiabatic small polaron hopping (SPH) model is found to fit well the experimental curves.

This paper presents a theoretical work on magnetocaloric properties of citric-gel La0.6Ca0.4MnO3 and La0.6Sr0.4MnO3 and ceramic La0.6Ca0.35Sr0.05MnO3 obtained by the mixture of the two first samples. The value of the magnetocaloric effect has been determined from theoretical work on magnetization as a function of temperature at several magnetic fields. Besides, the magnetic entropy change and specific heat were obtained, and the values of maximum magnetic entropy change, full-width at halfmaximum and relative cooling power at several magnetic field variations were calculated. The magnetocaloric effects of these materials are large and tunable, suggesting their possible application in moderate magnetic fields near room temperature.

In this work, structural and magnetic studies have been investigated to explore the influence of the oxygen-deficiency (ı) on the physical properties of Ca 2 MnO 4−ı (ı = 0 to 0.5) compounds. The samples cationic stoichiometry has been studied using energy dispersive X-ray analysis (EDAX). EDAX spectra, taken from the surface of the synthesized powders, have showed a nominal composition near the desired one. The structural study, by X-ray diffraction analysis, has confirmed the single-phase composition of all samples. The Rietveld refinement technique of the X-ray patterns, has revealed that samples crystallize in the tetragonal system with I41/acd space group for ı = 0 and in the orthorhombic one with Bbcm space group for ı > 0. The applied magnetic field (0 H) and the temperature (T) dependences of the magnetization (M) have been studied using a vibrating sample magnetometer (VSM). From magnetization measurements versus temperature under the field-cooled (FC) and zero-field-cooled (ZFC) modes, we have deduced the presence of a spin-glass behavior characterized by a distinctive separation of the FC and ZFC curves. This behavior was found to become more important, as increasing the oxygen-deficiency ı, which leads to the appearance of magnetic frustration phenomenon.

An intense e4ort has recently been devoted to studying the interplay between structure, magnetism, and transport in manganese perovskite Ln 1؊x A x MnO 3 (Ln ‫؍‬ La, Pr, Nd, Sm; A ‫؍‬ Ca, Ba, Sr). As a function of temperature, applied magnetic 5eld, doping, A-site ionic radius 1r A 2, and A-site size disorder, this system displays a rich phase diagram for both magnetotransport and structural properties. We have investigated the structural, magnetic, and transport properties of (La 1؊x Nd x ) 0.7 Sr 0.3 MnO 3 . The crystal structure was examined by X-ray powder di4raction which indicated that all the samples were single phase and revealed a transition from rhombohedral to orthorhombic structure with increasing x. The magnetization and resistivity investigation shows that for all values of x, (La 1؊x Nd x ) 0.7 Sr 0.3 MnO 3 are ferromagnetic+metallic at low temperatures and paramag-netic+semiconductor above the Curie temperature T c .

The La 0.75 Sr 0.1 Ca 0.15 MnO 3 sample have been prepared by sol-gel method. The crystallographic study shows that the sample crystallize in the rhombohedral structure with R 3c space group. The field dependence of magnetocaloric properties of manganite La 0.75 Sr 0.1 Ca 0.15 MnO 3 with second order phase transition material is investigated using a phenomenological model. The model parameters were determined from the magnetization data adjustment and were used to give better fits to magnetic transition and to calculate the magnetocaloric thermodynamic quantities. The magnetic entropy change reaches a peak of about 5.8 J/kg K and the relative cooling power (RCP) value of 195 J/kg at 305 K upon 5 T applied magnetic field. The critical exponents associated with ferromagnetic transition have been determined from magnetocaloric effect methods.