Structural, magnetic and transport properties in the manganites La0.7Sr0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Sr_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show a rhombohedral structure with the space group . It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of Te content. The Curie temperature T_C decreases with increasing Te-doping level, in contrast, the magnetization magnitude of Te-doping samples at low temperatures increase with increasing x as x≤0.05 and then decrease with further increasing x to 0.15. The results are discussed in terms of the combined effects of the opening of the new double exchange (DE) channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ for Sr²⁺ and the reduction of the transfer integral b due to the decrease of the Mn-O-Mn bond angle.     

Structural, transport, and magnetic properties in the Ti-doped manganites LaMn1−xTixO3 (0≤x≤0.2)

The magnetism and transport properties of the samples LaMn_1−xTi_xO₃ (0≤x≤0.2) were investigated. All samples show a rhombohedral structure () at room temperature. The sample with x=0 undergoes the paramagnetic-ferromagnetic (PM-FM) transition accompanied by an insulator-metal (I-M) transition due to the oxygen excess. The doped samples show ferromagnetism and cluster behavior at low temperatures. Though no I-M transition associated with the PM-FM transition appears, the magnetoresistance (MR) effect was observed especially at low temperatures under the applied fields of 0.5 T. Due to the fact that the oxygen content in the Ti-doped samples is nearly stochiometry (3.01) and the Hall resistivity at room temperature is negative, the ferromagnetism in LaMn_1−xTi_xO₃ (0.05≤x≤0.2) is believed to be consistent with the Mn²⁺-O-Mn³⁺ double exchange (DE) mechanism. These results suggest that DE can be obtained by direct Mn-site doping.     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

Study of structural, transport and magneto-resistive properties of La0.7Ca0.3−xCexMnO3 (0≤x≤0.2)

We have synthesized a series of La_0.7(Ca_0.3−xCe_x)MnO₃ (0≤x≤0.2) by standard solid-state reaction method. X-ray diffraction (XRD) measurement was carried out for structural studies and Rietveld refinement was done for structural analysis. The transport properties were studied using four probe technique. The temperature dependence of the resistivity was measured in the temperature range of 20 K to room temperature. It is found that all samples show a systematic variation in metal to insulator transition at transition temperature (T_P) and resistivity (ρ) with the relative concentration of hole and electron doping in the system. The samples showed varying amounts of colossal magnetoresistance depending upon temperature and applied magnetic field. The magnetoresistance values as high as 72% were observed in x=0 sample.     

The effect of Gd-doping on the charge ordering state of Bi0.3−xGdxCa0.7MnO3 (0≤x≤0.30)

The effect of Gd-doping on the charge ordering (CO) state in perovskite-type manganates Bi_0.3−xGd_xCa_0.7MnO₃ with x=0, 0.02, 0.05, 0.1, 0.3 has been investigated by transport and magnetic property measurements. It is found that CO temperature (T_CO) and antiferromagnetic (AFM) ordering temperature T_N occurring below T_CO decrease obviously with increasing Gd-doping level. Accompanying the variation of T_CO, the increased magnetization and the decreased resistivity are observed. In addition, the increased magnetic inhomogeneity has been also observed in the samples based on the difference between the zero-field-cooling (ZFC) magnetization M_ZFC and field-cooling (FC) magnetization M_FC, which is ascribed to the competition between ferromagnetic (FM) phase induced by Gd-doping and CO AFM phase. The experimental results indicate that the Bi³⁺ lone pair electron with 6s² character plays a dominating role on the CO state of Bi_0.3Ca_0.7MnO₃.     

Magnetoresistance in La1−xCaxMnO3 (0≤x<0.4)

The magnetic and transport properties of La_1−xCa_xMnO₃ (0≤x<0.4) have been systematically studied. The magnetoresistance (MR) maximum appears at x=0.2-0.25 and the temperature dependence of MR for x>0.25 shows a much broader profile than that of samples for x=0.2-0.25. Based on a scenario in which there is a short-range charge ordering (CO) state coexisting in the ferromagnetic state matrix for x>0.25, and the least or even no short-range CO state exists in samples for x=0.2-0.25, the above observations can be understood.     

Magnetic study of phase separation and charge ordering in La1−xSrxMnO3 near x=0.5

We have investigated the magnetic phase diagram of polycrystalline and single-crystal La_1−xSr_xMnO₃ near 0.46≤x≤0.50. It turns out that for x<0.48, the polycrystalline material is ferromagnetic (FM), but for x≥0.48, incipient charge ordering takes place along with antiferromagnetism. At x=0.48, the ferromagnetic-antiferromagnetic phase transition in ceramics occurs at less than 85 kOe but requires significantly larger field for increasing x. These observations are in contrast to what is found in the single crystals, which are all FM.     

Synthesis, structural and transport properties of nanocrystalline La1−xBaxMnO3 (0.0≤x≤0.3) powders

Nanocrystalline La_1−xBa_xMnO₃ (0.0≤x≤0.3) manganites have been prepared by a simple and instantaneous solution combustion method, which is a low temperature initiated synthetic route to obtain fine-grained powders with relatively high surface area. The phase purity and crystal structure of the combustion products are carried out by powder X-ray diffraction. The as-made nanopowders are in cubic phase. On calcination to 900 °C, barium doped manganites retain cubic phase, whereas barium free manganite transformed to rhombohedral phase. The scanning electron microscope (SEM) results revealed that the combustion-derived compounds are agglomerated with fine primary particles. The doped manganites have surface area in the range 24-44 m²/g. The surface area of the manganites increases with barium content, whereas it decreases on calcination. Both undoped and doped lanthanum manganites show two active IR vibrational modes at 400 and 600 cm⁻¹. The low temperature resistivity measurements have been carried out by four-probe method down to 77 K. All the samples exhibit metal-insulator behaviour and metal-insulator transition temperature (T_M-I) in the range 184-228 K and it is interesting to note that, as the barium content increases the T_M-I shifts to lower temperature side. The maximum T_M-I of 228 K is observed for La_0.9Ba_0.1MnO₃ sample.     

Vibrational anisotropy of MnO6 octahedron and phase separation in La0.67−yPryCa0.33MnO3 manganites

The role of vibrational anisotropy of Mn³⁺O₆ octahedron in the phase separation behavior of La_0.67−yPr_yCa_0.33MnO₃ (x=0, 0.15, 0.25 and 0.30) has been investigated by means of magnetization M, internal friction Q⁻¹, Young's modulus E along with the X-ray powder diffraction measurements. For the samples with y=0 and 0.15, the Q⁻¹ exhibits three peaks in the ferromagnetic region, which are attributed to the intrinsic inhomogeneity of ferromagnetic phase, i.e. the electronic phase separation with the coexistence of insulating and conducting phases. However, both the samples with y=0.25 and 0.30 undergo a magnetic phase separation with the coexistence of the antiferromagnetic and ferromagnetic phases, and the Q⁻¹ peaks related to the electronic phase separation have not been observed. In addition, the Q⁻¹ exhibits a peak in the paramagnetic region for all samples, which may result from the formation of magnetic clusters. We observed that the evolution from electronic to magnetic phase separation is close related to the rapid increase in the ratio of two kinds of Jahn-Teller distortion modes Q₃ and Q₂, i.e. Q₃/Q₂. A schematic phase diagram is given in the text, and it is suggested that the enhancement of vibrational anisotropy of Mn³⁺O₆ octahedron plays a key role in the evolution from electronic to magnetic phase separation.     

Lanthanum deficiency effect in magnetic transition and transport property of La0.8−δCa0.2MnO3

The evolution of magnetic and electrical phases in La_0.8−δCa_0.2MnO₃ was investigated in terms of La deficiency. We found that the increase of the La deficiency tends to raise the Curie temperature (T_C) in La_0.8−δCa_0.2MnO₃. The FM clusters formed in compounds with large La deficiency provide percolation paths above T_C. With increasing the La defect, the transport property changes from insulating to metallic state, which is in association with the crossover from a second order to a first order magnetic phase transition in the vicinity of T_C.     

Low-field transport properties of (1−x)La0.7Ca0.2 Sr0.1MnO3+x(ZnO) composites

Composite samples (1−x)La_0.7Ca_0.2Sr_0.1MnO₃(LCSMO)+x(ZnO) with different ZnO doping levels x have been investigated systematically. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and ZnO grains, and that ZnO segregates mostly at the grain boundaries of LCSMO. The magnetic properties reveal that the ferromagnetic order of LCSMO is weakened by addition of ZnO. The results also show that ZnO has a direct effect on the resistance of LCSMO/ZnO composites, especially on the low-temperature resistance. With increase of the ZnO doping level, T_P shifts to a lower temperature and the resistance increases. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 3 kOe. The maximum MR appears at x=0.1. The low field magnetoresistance (LFMR) results from spin-polarized tunneling. However, around room temperature, the enhanced MR of the composites is caused by magnetic disorder.     

Large magnetoresistance in low temperature metallic region of manganite compounds (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05≤x≤0.15)

Magnetoresistance (MR) and magnetization (dc and ac) measurements have been carried out on the manganites, (La_0.7−2xEu_x)(Ca_0.3Sr_x)MnO₃ (0.05≤x≤0.15), in the temperature range of 5-320 K. At 5 K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4-5 T. This large high-field MR occurs in the metallic region, far below the insulator-metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained in the light of various possibilities such as phase segregation, cluster spin-glass behavior, etc.     

Electrical transport behavior of La0.67Ca0.33MnO3/Fe3O4 composites

The influence of Fe₃O₄ contents on the electrical transport properties (resistivity and ac susceptibility) of a series of composite samples of La_0.67Ca_0.33MnO₃/Fe₃O₄ is studied. Results show that the Fe₃O₄ phase not only shifts the intrinsic insulator-metal (I-M) transition temperature T_P1 to a lower temperature, but also causes a new I-M transition at a lower temperature T_P2 (T_P2<T_P1). On the basis of an analysis by scanning electron microscopy and X-ray diffraction, we suggest that the decrease of the I-M transition temperature and the formation of the new I-M transition are caused by the segregation of a new phases related to the Fe₃O₄ at grain boundaries or surfaces of the La_0.67Ca_0.33MnO₃ grains.     

La0.7Ce0.3MnO3 epitaxial films fabricated by a pulsed laser deposition method

La_0.7Ce_0.3MnO₃ epitaxial films were successfully fabricated via a pulsed laser deposition method by controlling the experimental conditions. A series of experiments with varying the oxygen pressure and the substrate temperature demonstrated that the use of appropriate conditions is crucial for fabricating the epitaxial thin films. The existence of such suitable conditions was thermodynamically interpreted in terms of the stability of Mn²⁺ ion. Both XRD and EPMA measurements indicated that La_0.7Ce_0.3MnO₃ thin films fabricated herein form single phases, although it was difficult to present the direct experimental evidence to prove that Ce ion can really exist within the perovskite structure. The resultant films with oxygen annealing showed a metal-insulator transition and ferromagnetic property with Curie temperature of 275 K.     

Double-peak resistivity transport properties of La0.67Ca0.33MnO3 ceramics

Polycrystalline samples of La_0.67Ca_0.33MnO₃ were prepared by solid-state reactions by varying the pelletization force and the sintering temperature. Lowering the sintering temperature gave rise to smaller grains and increased the overall resistivity of the ceramic. Partial melting was observed in the ceramics sintered at higher temperatures (1400-1500 °C). Additionally, these ceramics showed two distinct resistivity peaks. The resistivity peak near the magnetic transition (T_C) was sharp, whereas the second peak was a broad one observed below T_C.     

Investigation of FIB irradiation damage in La0.7Sr0.3MnO3 thin films

In this work we analyse systematically how morphological and magnetotransport properties of manganite thin films are affected by the damage induced by focused ion beam (FIB) irradiation. We irradiate different areas of the same sample with doses ranging from 5×10¹² to 3×10¹⁷ ions/cm² and we find that the film becomes swollen for doses up to 10¹⁶ ions/cm² and is eventually eroded by ion milling for further irradiation. On the other hand, transport properties are much more sensitive to FIB irradiation: the metal–insulator transition temperature is found to decrease monotonically with increasing doses up to 1.8×10¹³ ions/cm². At doses higher than 5.6×10¹³ ions/cm² the metallic state is completely suppressed and likely, also ferromagnetism.     

Electrical transport and magnetic properties of La2/3Ca1/3MnO3/SiO2 composites

The influence of SiO₂ on the electrical transport properties of LCMO/SiO₂ composites with different SiO₂ contents x is investigated, where LCMO represents La_2/3Ca_1/3MnO₃. Results show that the SiO₂ phase not only shifts the metal–insulator transition temperature (T_p) to a high temperature range, but also has an effect on the magnetoresistance (MR) of the composites. The temperature dependence of resistivity indicates that the T_p of the composites is obviously higher than that of pure LCMO, and that the peak resistivity ρ_max of the composites is lower than that of pure LCMO. In the SiO₂ content x∼0.02, the T_P is the highest and ρ_max becomes the lowest. The experimental observation is discussed on the basis of the analysis of scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns. Compared with pure LCMO, a possible interpretation is presented by considering the influence of SiO₂ on the coupling between ferromagnetic (FM) domains of LCMO.     

Reply to the comment on: ‘Magnetoresistance in La1−xCaxMnO3(0≤x<0.4)’

The author of the comment objects to the characterization and the interpretation of magnetoresistance (MR) effects observed by us in La_1−xCa_xMnO₃ (0≤x<0.4) samples. In this reply, arguments are used to show that the samples' characterization and explanation of the MR by considering the role of the short-charge ordering (CO) regions and magnetic domains are reasonable and acceptable.     

The current-induced effect on the Jahn-Teller distortion in the La0.5Ca0.5MnO3 manganite

The influence of dc current on the resistivity ρ and the Young's modulus E of La_0.5Ca_0.5MnO₃ compound has been investigated by means of an in situ measuring method. At low temperatures, both the resistivity ρ and the relative modulus ΔE increase with the current. A relaxation behavior of ρ to the higher resistive state is observed at a fixed temperature and a constant current. After storing the sample for a few days, ρ decreases with the current, accompanying a slight drop of ΔE at low temperatures. Current-induced effects on ρ and ΔE are interpreted according to the current-induced interwinning of Mn³⁺O₆ octahedral distortion modes between Q₂- and Q₃-types, which is suggested to contribute to the variation of the resistivity.