Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The temperature dependence of the resistivity for composite samples of (1−x)La_0.67Ba_0.33MnO₃+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃+92 mol% ZrO₂). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (T_P) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.     

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.     

Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite

An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La_0.67Ca_0.33MnO₃/YSZ and La_0.67Ca_0.33MnO₃/Fe₃O₄. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe₃O₄ is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La_0.67Ca_0.33MnO₃ and the impurity doped La_0.67Ca_0.33MnO₃ composites. Compared with pure La_0.67Ca_0.33MnO₃, a possible interpretation is presented by considering the influences of YSZ and Fe₃O₄ on the structure of grain boundaries and/or surfaces of La_0.67Ca_0.33MnO₃grains.     

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.     

Hydrogen annealing effects on polycrystalline La0.67Ba0.33MnO3 compound

Upon annealing polycrystalline La_0.67Ba_0.33MnO₃ bulk samples in flowing 95%Ar:5%H₂ mixed gas at 700 °C for different time, the insulator–metal transition temperature, _TP$T_{\mathrm{P}}$ and the amplitude of AC magnetic susceptibility were decreased first, then increased, finally decreased again. While the resistivity was increased monotonically. This anomalous behavior was explained by the combinational effects of oxygen loss and Ba ion vacancies caused by the segregation of Ba ion related impurity phase.     

Fe3O4 Segregation and Transport Properties of La0.67Ca0.33MnO3

Effect of Fe3O4 segregation at grain boundaries on the electrical transport and magnetic properties of La0.67Ca0.33MnO3 is investigated. The experimental results show that the Fe3O4 segregation not only shifts the paramagnetic-ferromagnetic transition temperature of La0.6TCa0.33MnO3 to a lower temperature region but also induces a new transition in a lower temperature region. Meanwhile, the transition processes observed in both the resistivity and magnetization curves are obviously widened. Compared to pure La0.67Ca0.33MnO3, we assume that the Fe3O4 segregation level at the grain boundaries can modify the electrical transport and magnetic properties of La0.67Ca0.33MnO3.     

Revival of metastable behavior in phase separated La0.5Ca0.5MnO3+δ

The La_xCa_1−xMnO_3+δ compositions close to charge ordering (x∼0.5) show a gradual relaxation from a metallic/ferromagnetic state to an insulating/antiferromagnetic state with thermal cycling. Here, we report on the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We also show the changes in the magnetization and the thermoelectric power as the revived metastable state is cycled. We find that the changes in the thermoelectric power extend well into the region above the charge ordering temperatures. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well.     

Large enhancement of room temperature magnetoresistance in Ag-added La0.67(Ca0.65Ba0.35)0.33MnO3

The electrical and magnetoresistant properties of La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃/Ag_x (abbreviated by LCBMO/Ag_x) have been studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for x=0.27 sample in 10 kOe magnetic field is 41%, almost 20 times larger than that for x=0 sample. This enhancement is related to that the Curie temperature (T_c) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of experimental results for x=0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.     

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.     

Electric current-induced giant electroresistance in epitaxial La0.67Sr0.33MnO3 thin films

The electronic transport behavior of La_0.67Sr_0.33MnO₃ epitaxial thin films with different thicknesses has been investigated under various applied DC currents. The 20 and 70 nm thick films show a giant negative electroresistance (ER). In contrast, the films with 100 nm thickness show unusual giant positive ER, which can reach 30% with the current density of 1.8×10⁸ A/cm² at room temperature. It is interesting that the electric current can also change the magnetoresistance of the films. The results were explained by considering the spin polarized current induced increase of ferromagnetic metallic phase and current-induced lattice distortion via electron wind force under high current density.     

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.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

Effect of Al doping on the magnetic and electrical properties of layered perovskite La1.3Sr1.7Mn2−xAlxO7

The effect of Al substitution for Mn site in layered manganese oxides La_1.3Sr_1.7Mn_2−xAl_xO₇ on the magnetic and electrical properties has been investigated. It is interesting that all the samples undergo a similar and complex transition with lowing temperature; they transform from the two-dimensional short-range ferromagnetic order at T*, then enter the three-dimensional long-range ferromagnetic state at T_C, at last they display the canted antiferromagnetic state below T_N. T*, T_C and T_N are all reduced with Al content. Resistivity increases sharply with increasing Al concentration, and the metal-insulator transition disappears when x reaches 10%. Additionally, magnetoresistance (MR) effect is weakened. Al substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction, and further suppresses FM ordering and metallic conduction. Owing to the anisotropic interaction in the layered perovskite, the magnetic and electrical properties are more sensitive to Al doping level than those in ABO₃-type perovskite.     

Thickness dependence of electroresistance in La0.67Ca0.33MnO3 epitaxial films

The electroresistance (ER) of La_0.67Ca_0.33MnO₃ (LCMO) epitaxial thin films with different thicknesses was studied. For the 110 nm thick LCMO film, its ER shows a maximum at T_p, where the resistance shows a peak, and decreases to zero at lower temperatures. While for the 30 nm thick LCMO film, its ER is remarkable in a wide temperature range. Another interesting observation in this work is that the electric current can tune the magnetoresistance of the ultrathin LCMO thin film. The results were discussed by considering the coexistence of ferromagnetic metallic phase with the charge ordered phase, and the variation of the phase separation with film thickness and electric current. This work also demonstrates that electric current can tune the magnetoresistance of the manganites, which is helpful for their applications.     

Structural and magnetic phase diagram and room temperature CMR effect of La1−xAgxMnO3

The study of the structural and magnetic phase diagram of the manganites La_1−xAg_xMnO₃ shows similarity with the La_1−x′Sr_x′MnO₃ series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn³⁺:Mn⁴⁺=2) with T_max=300 K (maximum of the ρ(T) curve in zero field).     

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.     

Synthesis and enhanced low-field magnetoresistance in La0.67Ca0.33MnO3 /CuO composites

The (1−x)La_0.67Ca_0.33MnO₃+xCuO composites have been synthesized by a new liquid phase method. The XRD and SEM measurements reveal that little CuO is soluble in the structure of La_0.67Ca_0.33MnO₃ and is mainly distributed at the grain boundary of La_0.67Ca_0.33MnO₃. As CuO content x increases, the magnetization M values increase until x=0.05 and M values decrease when x further increases at low temperature. For x=0.10, 0.20 and 0.30 composites, double metal-insulator transitions accompanying a single ferromagnetic transition are observed. Large low-field magnetoresistance is achieved for the composites and the largest magnetoresistance appeared when x=0.20.     

Effects of doping Na on the structure and physical properties of La2/3Ca1/3MnO3

Effects of doping Na on the structure, electrical and magnetic properties of La_2/3Ca_1/3MnO₃ are investigated. A structural phase transition from orthorhombic to rhombohedral structure takes place at y=0.375. All samples show metal-insulator (M-I) transition at the transition temperature and undergo the transition from paramagnetism to ferromagnetism at the Curie temperature T_C. and T_C increase monotonically with increasing Na content. However the Na-doped samples have a shoulder in their electrical transport curves found below and shows a widened magnetic transition process. On the other hand, intrinsic colossal magnetoresistance (CMR) peaks are observed in all the samples, but samples with y around 0.25 show two MR peaks which can be attributed to magnetic inhomogeneity induced by the doped Na⁺ ions. Here we propose a method to broaden the CMR peak of perovskite manganite, which is beneficial for practical applications.     

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.     

Magnetotransport properties of La0.67Ca0.33MnO3//La0.67Sr0.33MnO3 bilayers

The perovskite bilayers La0.67Ca0.33MnO3 （LCMO） （100 nm） / La0.67Sr0.33MnO3（LSMO） （100 nm） and LSMO （100 nm） / LCMO （100 nm） are fabricated by a facing-target sputtering technique. Their transport and magnetic properties are investigated. It is found that the transport properties between them are different obviously due to distinguishable structures, and the different lattice strains in both films result in the difference of metal-to-insulator transition. Only single-step magnetization loop appears in our bilayers from 5K to 320K, and the coercive force of LSMO/LCMO varies irregularly with a minimum ～ 2387A/m which is lower than that of LCMO and LSMO single layer films. The behaviour is explained by some magnetic coupling.