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We report on the synthesis of polycrystalline thin films of PbZr0.3Ti0.7O3 (PZT) by the so-called chemical solution deposition technique. The thin films were deposited on Pt∕Ti∕SiO2∕Si substrates by the spin-coating method, and were heat treateded at 700°C in air and under several oxygen pressures (10

The manganite Nd0.5Ca0.5MnO3 has a CE-type antiferromagnetic-insulating (AFI) ground state at low temperatures. This compound undergoes a charge-ordering (CO) transition at TCO∼250 K followed by a paramagnetic to AFI (orbital-ordered) OO phase transition at TN∼170 K. The ferromagnetic (FM) double-exchange interaction between Mn ions is quenched by such a CO effect. Upon Cr doping, as in single crystals of Nd0.5Ca0.5Mn0.96Cr0.04O3, suppression of the quenched CO-OO state occurs at TC∼140 K and is accompanied by a FM-metal-insulator transition. These Cr-doped materials have a low temperature ground state comprised of a mixture of 20–30 nm domains of the FM phase embedded in the CO-OO matrix, as inferred from Lorentz microscopy. Our results show that these Cr-doped materials undergo a field-dependent magnetic transition in a large range of temperature T

The dynamic magnetic properties of Ni nanoparticles diluted in an amorphous SiO2 matrix prepared from a modified sol-gel method have been studied by the frequency f dependence of the ac magnetic susceptibility \c{hi}(T). For samples with similar average radii ~ 3-4 nm, an increase of the blocking temperature from TB ~ 20 to ~ 40 K was observed for Ni concentrations of ~ 1.5 and 5 wt.%, respectively, assigned to the effects of dipolar interactions. Both the in-phase \c{hi}'(T) and the out-of-phase \c{hi}"(T) maxima follow the predictions of the thermally activated N\'eel-Arrhenius model. The effective magnetic anisotropy constant Keff inferred from \c{hi}"(T) versus f data for the 1.5 wt.% Ni sample is close to the value of the magnetocrystalline anisotropy of bulk Ni, suggesting that surface effects are negligible in the present samples. In addition, the contribution from dipolar interactions to the total anisotropy energy Ea in specimens with 5 wt.% Ni was found to be comparable to the intrinsic magnetocrystalline anisotropy barrier., Comment: 12 pages, 2 figures

We have performed measurements of dc electrical resistivity ρ(T), magnetic susceptibility χ(T), and impedance spectroscopy Z(T, ω) in both single crystals and polycrystalline samples of Nd0.5Ca0.5Mn1−xCrxO3 (0.00⩽x⩽0.07). The pristine compound exhibits a transition to a charge ordered (CO) state at TCO∼250 K. However, no evidence of either metal-insulator (MI) or ferromagnetic (FM) transitions are observed at lower temperatures. The substitution of Mn by Cr partially suppresses the CO transition and induces a FM/MI ordering at lower temperatures TC=TMI∼140 K, which is essentially Cr independent. Both ρ(T) and χ(T) measurements indicate the development of a ferromagnetic (FM) component at low temperatures T

We have measured electrical resistance R(T), magnetization M(T), and for the first time impedance spectroscopy Z(T,ω) of polycrystalline samples of La0.3Pr0.4Ca0.3MnO3 compounds. The combined results suggest the coexistence of two metallic ferromagnetic phases below the Curie temperature of the system. The R(T) data exhibit two important features: the occurrence of a metal–insulator (MI) transition at temperatures close to TMI≈170 K and a large thermal hysteresis below TMI. The first feature was found to be associated with the development of ferromagnetism and takes place when the magnetization of the samples becomes about 15% of its saturated value at low temperatures. The second feature suggests two contributions to R(T) below the Curie temperature TC. The Z(T,ω) data taken from 77 to 300 K and frequency varying from 5 to 107 Hz are much more valuable. These results reveal two well defined bulk contributions to the transport properties of these manganites below ∼170 K: one occurring at high frequencies ...

Polycrystalline samples of LnNi0.3Co0.7O3 (Ln=La, Pr) were prepared through sol-gel precursors. The samples were subjected to several heat treatments at temperatures ranging from 650 to 1000 °C and under oxygen pressure as high as 70 bar. Characterizations of x-ray powder diffraction XRD magnetization M(T), and ac magnetic susceptibility χac(T) were performed on these compounds. The results of XRD showed that samples of LaNi0.3Co0.7O3 crystallize in a rhombohedral distorted perovskite structure (R3c) and that PrNi0.3Co0.7O3 compounds displayed an orthorhombically distorted structure (Pbnm). The temperature dependence of M(T) revealed the presence of a sharp cusp occurring at temperatures TP∼60 and ∼ 30 K for Ln=La and Pr, respectively. Below TP, the M(T) curves taken after different thermal cycles, i.e., field cooled and zero-field cooled processes, are histories dependent on the magnitude of M(T) decreasing with increasing applied magnetic field. We have also observed that TP decreases with increasing a...

We have studied the structural, magnetic, and transport properties of polycrystalline (Nd1−xGdx)1.85Ce0.15CuO4−y; x=0.0, 0.05, 0.10, 0.25, and 0.50; produced under different atmospheres and cooling rates after the reduction process. From x-ray analysis the T′-structure is preserved for all samples studied. We also found that quenched samples in argon atmosphere exhibit superconductivity below ∼22 K. The temperature dependence of the electrical resistivity ρ(T) displays a striking double resistive superconducting transition with no evidence of a zero resistance state down to 1.5 K. The electrical resistivity has a ≈20% drop at the upper critical temperature Tci followed by a plateau down to a lower temperature Tcj. While Tci is both weakly dependent on the Gd content and external applied magnetic field, the behavior of ρ(T) at low temperatures is strongly sensitive to the Gd concentration, excitation current, and applied magnetic fields. Also, the behavior of ρ(T) below ≈3 K is almost field independent. Th...

Polycrystalline samples of Eu2−xCexCuO4−y (0.0≤x≤0.18) were prepared from a sol‐gel precursor and sintered in air at different temperatures. From the x‐ray diffraction results, the beginning of Eu2−xCexCuO4−y phase formation is observed by increasing the sintering temperature up to 500 °C, as indicated by the presence of very broad Bragg reflections belonging to the desired phase. At 700 °C, Eu2−xCexCuO4−y coexists with Eu2O3, CeO2, and CuO. Additional sintering at 950 °C in air for 20 h results in single phase materials. The effectiveness of Eu replacement by Ce in the Eu2CuO4 pristine phase has been confirmed by a decrease in the lattice parameter c with increasing x. Also, the solubility limit of Ce in these series was found to be higher than x=0.15. The results of thermal analysis reveal that the eutectic temperature Te≊1020 °C is Ce independent. On the other hand, the peritectic temperature Tp increases significantly with increasing Ce concentration. It is close to Tp≊1180 °C for Eu2CuO4 and Tp≊1195 ...

A systematic study of transport and magnetic properties of polycrystalline Sm2−xCexCuO4−y (0.15≤x≪0.18) specimens obtained from a sol‐gel precursor reveals a striking double resistive superconducting transition. One of the resistive transitions takes place at a temperature Tci and is almost insensitive to Ce concentration, applied magnetic field, and current density, while the other transition occurs at a lower temperature Tcj and is very sensitive to Ce content, applied magnetic field, and current density. Alternating‐current magnetic susceptibility χac measurements reveal two interesting features. There is only one contribution to the diamagnetic signal below ∼20 K, which is particularly evident in the resistive component χ‘ of χac where a peak at Tcj can be identified. The application of an applied magnetic field shifts the χ‘ peak to lower temperatures and reveals a spin‐glass‐like behavior. All the results can be satisfactorily explained in terms of weakly linked Josephson junctions between small sup...

Pre-reacted powders of (Bi-Pb)2Sr2Ca2Cu3O10+δ (Bi-2223) were consolidated by using the spark plasma sintering (SPS) technique under vacuum and at two different temperatures TD: 750 and 830 °C. The results indicate the occurrence of grains with core-shell morphology, where the shell is oxygen deficient. A post-annealing heat treatment (PAHT), performed in air, at 750 °C, and for a brief time interval, is responsible for a 10 to 25-fold increase in the transport superconducting current density at 77 K. The role of the oxygen-deficient shell, before and after the PAHT, was investigated by means of magnetic and transport measurements. We argue that the PAHT is two folds: (i) it is responsible for the decrease of the width of the oxygen-deficient shell, then increasing the oxygen content along the grain boundaries; (ii) it promotes the formation of conduction current paths along the grain boundaries of the SPS material.

CuO nanograins (NGs) and CuO nanowires (NWs) on the top of nanograins samples were produced by the electrical resistive heating method. The NGs sample shows absence of long range magnetic order and strong field-induced ferromagnetic behavior. In the sample comprised of NWs on the top of NGs, a long-ranged antiferromagnetic ordering is induced by a magnetic field of 5 kOe and coexists with an enhanced ferromagnetic-like contribution. The ferromagnetic behavior is observed below and above the temperature-induced TN suggesting that this behavior is not dependent on the original magnetic state of the system, whether it is paramagnetic (PM) or antiferromagnetic.

The spark plasma sintering (SPS) technique, by using a compacting pressure of 50 MPa, was used to consolidate pre-reacted powders of Bi1.65Pb0.35Sr2Ca2Cu3O10+δ (Bi-2223). The influence of the consolidation temperature, TD, on the structural and electrical properties has been investigated and compared with those of a reference sample synthesized by the traditional solid-state reaction method and subjected to the same compacting pressure. From the X-ray diffraction patterns, performed in both powder and pellet samples, we have found that the dominant phase is the Bi-2223 in all samples but traces of the Bi2Sr2CaCu2O8+x (Bi-2212) were identified. Their relative density were ∼85 % of the theoretical density and the temperature dependence of the electrical resistivity, ρ(T), indicated that increasing TD results in samples with low oxygen content because the SPS is performed in vacuum. Features of the ρ(T) data, as the occurrence of normal-state semiconductor-like behavior of ρ(T) and the double resistive super...

The effect of weak dipolar interactions (DIs) between Ni nanoparticles (NPs) in samples with different Ni concentrations was investigated by performing a detailed characterization of their structural and magnetic properties. From the determination of several physical parameters of Ni NP assemblies, it was found that the ac and dc magnetic susceptibility measurements are valuable for identifying the DIs between NPs while hysteresis loops measurements showed to be very insensitive, provided that the strength of the DI field is much smaller than the maximum coercive field. Therefore, the sensitivity of the observed static and dynamical magnetic properties to the effect of weak DI depends on the measurement protocols used.

We have performed a systematic study of the magnetic properties of a series of ferrimagnetic nanoparticles of MgxFe3−xO4 (0.8≤x≤1.5) prepared by the combustion reaction method. The magnetization data can be well fitted by Bloch’s law with T3/2. Bloch’s constant B determined from the fitting procedure was found to increase with Mg content x from ∼3.09×10−5 K−3/2 for x=0.8 to 6.27×10−5 K−3/2 for x=1.5. The exchange integral JAB and the spin-wave stiffness constant D of MgxFe3−xO4 nanoparticles were also determined as ∼0.842 and 0.574 meV and 296 and 202 meV A2 for specimens with x=0.8 and 1.5, respectively. These results are discussed in terms of cation redistribution among A and B sites on these nanostructured spinel ferrites.

Polycrystalline La5∕8−yPryCa3∕8MnO3, 0⩽y⩽0.625, samples were synthesized by the solid-state reaction method and studied using x-ray powder diffraction, magnetic susceptibility [χ(T)], and magnetoresistivity [ρ(T,H)] measurements. Some features such as an appreciable thermal hysteresis observed in both ρ(T) and χ(T) curves indicated a competing scenario due to the coexistence of different phases. We have also found that there is a critical region in the phase diagram, for the Pr concentration ranging from ∼0.30 to 0.40, where the magnitude of the insulator to metal transition temperature (TMI), the Curie temperature (TC), the magnetoresistance, and the residual resistivity (ρ0) are characterized by abrupt changes with little increase in y. Our data also indicate that the physical properties of these manganites in this critical region are dominated by a strong competition between coexisting ferromagnetic metallic and charge-ordered insulating phases.

Electrical conductive textured LaNiO3/SrTiO3 (100) thin films were successfully produced by the polymeric precursor method. A comparison between features of these films of LaNiO3 (LNO) when heat treated in a conventional furnace (CF) and in a domestic microwave (MW) oven is presented. The x-ray diffraction data indicated good crystallinity and a structural orientation along the (h00) direction for both films. The surface images obtained by atomic force microscopy revealed similar roughness values, whereas films LNO-MW present slightly smaller average grain size (∼80 nm) than those observed for LNO-CF (60–150 nm). These grain size values were in good agreement with those evaluated from the x-ray data. The transport properties have been studied by temperature dependence of the electrical resistivity ρ(T) which revealed for both films a metallic behavior in the entire temperature range studied. The behavior of ρ(T) was investigated, allowing to a discussion of the transport mechanisms in these films.

We have measured magnetization M(T,H), electrical resistivity ro(T,H), thermal conductivity k(T,H), and thermopower S(T,H) of polycrystalline samples of Nd1-xEuxNiO3; 0, 7 pages, 3 figures, and 1 table

Carbon–nickel nanocomposite thin films were prepared by ion beam cosputtering a graphite target having a nickel chip attached to its surface. The amount of Ni incorporated in the films was controlled by the size of the Ni chip (from 5to22wt%), and measured by in situ x-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy images indicated that the films have a granular structure with nanosize Ni particles embedded in an amorphous carbon matrix. The Ni particles have an average radius of ∼2nm, with a rather narrow size distribution. Magnetization measurements revealed that films are superparamagnetic and present blocking temperatures TB

We have studied the general physical properties of polycrystalline samples of La0.7−xYxCa0.3Mn0.9957Fe0.01O3; x=0 and 0.1; manganites prepared through sol–gel precursors. Characterization of these samples through electrical resistivity, ρ(T), and magnetization, M(T), measurements indicated a transition to a ferromagnetic state at TC∼240 and 100 K for x=0 and 0.1, respectively. Mossbauer spectra taken from room temperature down to 4.2 K revealed the presence of two paramagnetic doublets from 300 to TC having temperature independent hyperfine parameters and, that below TC, the appearance of two ordered components that coexist with one paramagnetic phase. The coexistence between these phases is preserved from TC down to ∼20 K. The highest magnetoresistance effect was found in the temperature range in which the change of volumetric fraction of these phases is maximum.

We have performed measurements of acmagnetic susceptibility χac as a function of temperature and magnetic field on polycrystalline specimens of Sm2−x Ce x CuO4−y (0.15≤x≤0.18) prepared from a sol‐gel precursor. All the samples studied show a striking double resistive superconducting transition. One of the resistive transitions occurs at a higher temperatureT ci and the other one at a lower Josephson‐coupling temperatureT cj . Magnetic measurements reveal the appearance of superconductivity below T cj which is evident from the resistive component χ″ which peaks only at T cj . Measurements made on pellets and crushed samples with average grain size ∼1 μm show essentially the same features. This suggests that Josephson coupling occurs inside physical grains with dimensions close to 5 μm. We have also utilized the behavior of the peak in the χ″ component as a function of applied magnetic fields to build a H vs Tphase diagram for these compounds. Some interesting features were observed in this H vs Tphase diagram: (1) a H α, α=2/3, dependence at low applied magnetic fields (H≤2 kOe) and high temperatures, and (2) an abrupt increase in the magnitude of the exponent at high fields and low temperatures. The behavior at low fields and high temperatures suggests that the magnetic response of the system is mainly controlled by a disordered Josephson network. Possible connections between these two different behaviors are discussed.

Nd2CuO4 and Gd2CuO4 present antiferromagnetic ordering associated with the rare‐earth ions at about 2 and 6.8 K, respectively. We report measurements of the ac susceptibility in polycrystalline samples of Gd2CuO4 and (Nd1−xGdx)2CuO4. The data in Gd2CuO4 in fields up to 30 kG show evidence of the existence of a spin‐flop transiton around 2 kG at low temperatures. The samples of the (Nd1−xGdx)2CuO4 showed Neel temperature associated with the antiferromagnetic ordering of the rare‐earth ions between 6.8

A method to calculate the real and imaginary parts of the magnetic permeability and susceptibility of nonmagnetic metals by using Kelvin functions is presented. The exact treatment is shown for the massive cylindrical geometry. An expression for a hollow circular cylinder is discussed and expanded to the thin‐shell limit.

The eddy current decay method for determination of metallic samples resistivities is presented for the case of a hollow circular cylindrical geometry. The theory is developed for two kinds of experimental arrangements: the primary coil is driven by a voltage source or by a current source. The measurements made for several metals (Al, Cu, Nb, brass, and bronze) at room temperature and at the temperature of liquid nitrogen analyzed by the theory give accurate results compared with the four terminal method.