[en] A series of the double-doping samples La_(2+4x)/3Sr_(1-4x)/3Mn_1-xCu_xO₃(0=< x=<0.2)with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 and the single-doping samples La_2/3Sr_1/3Mn_1-xCu_xO₃(0=< x=<0.2) have been investigated. For the double-doping samples, though the ratio Mn³⁺/Mn⁴⁺=2:1 has been generally recognized the optimum ratio, the Curie temperature T_C and metallic-insulator transition temperature T_p1 are more rapidly decreased by Cu substitution than that corresponding to single-doping samples. And the resistivity ρ value for the double doping is larger about two or three orders of magnitude than that corresponding to single doping. At the same time, two resistivity peaks and two magnetoresistance (MR) peaks appear. We suggest that for the double-doping samples the A-site cation size < r_A> and the A-site mismatch factor σ² decreases with increasing doping level, which leads to the system microstructural distortion. This microstructural distortion makes the Mn³⁺-O-Mn⁴⁺ cut off more cluster-spin except for the clusters induced by Cu. These cluster interfaces contribute to ρ, which exceeds far the contribution of e_g electron decreasing with doping increasing in the single doping. At the same time, such interface scattering also gives rise to the appearance of second peak for the double-doping samples. The experimental results shows that double doping could be also a potential way in tuning colossal MR (CMR), which can give a guide for the adequate selection of CMR materials

[en] La_0.7Sr_0.3Mn_1-xTi_xO₃ films (0 ≤ x ≤ 0.5) were fabricated using chemical solution deposition. It was found that with an increase in Ti content, the lattice constant was first increased due to the substitution of Mn⁴⁺ with Ti⁴⁺, and then it tended to decrease when x ≥ 0.3 due to the substitution of Mn³⁺ by Ti⁴⁺ with a larger ionic radius, as well as when excess oxygen was introduced. With the increase in x, the Curie temperature was decreased and broadened; however, it was found that all the films showed ferromagnetic properties at 300 K, which indicated that the excess oxygen played an important role. The transport properties showed that with an increase in Ti content the resistivity increased and the insulator-metal transition tended to a lower temperature due to the depression of the double exchange and the dilution of the Mn³⁺-O-Mn⁴⁺ network. Moreover, the temperature dependence of magnetoresistance (MR) results showed that with the Ti content increasing, the maximum MR increased and the transition was broadened suggesting that it is useful to tune the MR behaviour through Ti doping, and when x ≤ 0.1 the derived films showed no obvious MR at low temperature; however, when x = 0.3, the film did show obvious MR at low temperature

[en] The magnetization, resistivity ρ, thermoelectric power S, thermal conductivity κ and specific heat C_p in the layered cobalt oxide Nd_1-xSr_1+xCoO₄ (x = 0.25 and 0.33) have been investigated systematically. Based on the temperature dependence of susceptibility χ_m(T) and Seebeck coefficient S(T), a combination of the low-spin state for Co⁴⁺ and the intermediate-spin (IS) state for Co³⁺ can be suggested. The very large magnitude of S is suggested to originate from the configurational entropy of charge carriers enhanced by their spin and orbital degeneracy between Co³⁺ and Co⁴⁺ sites. In addition, based on the analysis of the temperature dependence of S(T) and ρ(T), the high temperature small polaron conduction and the low temperature variable range hopping conduction are suggested, respectively. As to thermal conduction κ(T), the decrease in κ with increasing Sr-doping is suggested to come from the combined effect of the scattering of the Co³⁺O₆ Jahn-Teller distortion with the IS state and the disorder to the phonon. Meanwhile, in the specific heat C_p(T) curves, the samples containing magnetic rare-earth Nd³⁺ ions reveal an obvious Schottky-type anomaly due to the crystal-field effects

[en] We have investigated the current conduction of NiO thin film embedded with Ni nanocrystals (nc-Ni) and the influence of charging in the nc-Ni/NiO thin film on the current transport. The hole trapping in the thin film under a negative charging voltage is found to greatly increase the current conduction measured at a positive voltage due to the enhancement of electric field at the interface between the thin film and Si substrate. Moreover, the current-voltage (I-V) characteristic follows a power-law relationship. In addition, the dc resistance of the thin film strongly depends on the magnitude of the charging voltage and charging time. These results could be used to realize a charging-controlled resistive memory effect.

[en] In this work, Ge nanocrystals (nc-Ge) embedded in SiO₂ thin films have been synthesized by ion implantation. Both the higher and the lower implantation dose/energy samples exhibit significant memory effect as a result of charge trapping in the nc-Ge. Under a negative gate voltage, either electron trapping or hole trapping dominates, depending on the magnitude of gate voltage and charging time as well as the distribution of nc-Ge. However, under a positive gate voltage, only electron trapping is observed, and the flat-band voltage shift is also affected by the nc-Ge distribution. These results demonstrate that the unconventional memory effect can be modulated by the distribution of nc-Ge.

[en] The structural, magnetic, electrical and thermal transport properties have been investigated in two-dimensional layered perovskite Sr_1.05Ln_0.95CoO₄ (Ln = La, Ce and Nd) compounds. The variations of Jahn-Teller distortion of the CoO₆ octahedron and the effective magnetic moment indicate that the induced holes are mostly accommodated in the t_2g orbital states keeping the intermediate-spin configuration. The detailed comparison of temperature dependence of magnetization, resistivity, thermoelectric power and thermal conductivity suggests that the tolerance factor t plays a very crucial role in the transport behaviours. With decreasing A-site rare earth ionic radius r_Ln³⁺, the tolerance factor t decreases and the distortion of CoO₆ octahedron enhances, which leads to the increased bending of the Co-O-Co bond, the narrowing of the bandwidth and the decrease in the mobility of e_g electrons. In addition, from the viewpoint of application, the huge Seebeck coefficients (>600 μV K^-1) indicate that the present layered cobaltites can be good candidates for thermoelectric materials.

[en] The structural, magnetic, electrical and thermal properties have been investigated in single-layered perovskite compounds of SrLnCoO₄ (Ln = La, Ce, Pr, Nd, Eu, Gd and Tb) with a K₂NiF₄-type structure. The variation of Jahn-Teller distortion of the CoO₆ octahedron reflected by two different Co-O bond lengths indicates that all the samples are mostly accommodated in the t_2g orbital states keeping intermediate spin configuration. The detailed comparison of the temperature dependence of magnetization, resistivity, thermoelectric power and thermal conductivity suggests that the tolerance factor t plays a very crucial role in transport behaviour. With decreasing A-site rare earth ionic radius r_Ln³⁺, the tolerance factor t decreases and the distortion of the CoO₆ octahedron enhances, which results in the increased bending of the Co-O-Co bond, the narrowing of the bandwidth and the decrease in the mobility of e_g electrons caused by the substitution of smaller Ln³⁺ ions for a larger Sr²⁺ ion

[en] In this letter, Ni-rich NiO thin film is deposited on p-type Si substrate by dc magnetron sputtering to form a metal-insulator-semiconductor structure. The charge trapping in the Ni nanocrystals (nc-Ni) embedded in NiO matrix induces a flatband voltage shift and capacitance modulation, which could be used for memory applications. The charging of nc-Ni depends on the voltage polarity, as well as the charging time and magnitude of gate voltage. The capacitance modulation can be described by an equivalent circuit model.

[en] The effect of K doping on structural, electronic transport, magnetic and specific heat properties in 4d perovskites Ba_1-xK_xMoO₃ (0 ≤ x ≤ 0.2) has been investigated. Partial substitution of K⁺ ions for Ba²⁺ ions does not change the space group, but decreases the lattice parameter a monotonously. All samples exhibit metallic behavior in the whole measured temperature range. The K doping does not change the Pauli-paramagnetic behavior in high temperature region, but a weak ferromagnetic (FM) transition is induced at about 50 K, which can be related to the deficiency of Mo. With K-doping content increasing, the resistivity increases, while the magnetic susceptibility and electronic specific heat coefficient decrease. The results are discussed according to the electron localization due to the disorder effect induced by dopants and the decrease in the density of state (DOS) at Fermi surface.

[en] Systematic studies of X-ray, resistivity (ρ), Hall effect, thermal conductivity (κ), and thermopower (S) have been performed on polycrystalline Sr_0.9Ce_0.1MnO₃ sample. While cooling, it undergoes a first-order metal-insulator transition at around 318K, which is associated with a structural transition due to Jahn-Teller ordering. Both Hall coefficient (R_H) and thermopower (S) show negative values, indicating that the carriers in the sample are electrons. In the whole measuring temperature range, dκ/dT>0, with rather low thermal conductivity value, is related to unusually large static and dynamic lattice distortions of the MnO₆ octahedra. Below 200K, both ρ and S follow the variable range hopping (VRH) model. In contrast, in the temperature range of 200-320K, a small polaron behavior can be invoked by the electron-phonon interaction