Structural and magneto-transport properties of LCMO–STO composites

A manganite matrix-based composite series, (1 ? x)La_0.67Ca_0.33MnO₃(LCMO) ? (x)SrTiO₃ (STO), has been prepared by the solid state route. Influence of STO phase on structural and magneto-transport properties of LCMO phase has been investigated. By X-ray diffraction, scanning electron microscopy, and Fourier transform of infrared spectroscopy, we find that there is no interdiffusion between the LCMO and STO phases. Measurements of resistivity on these samples reveal that the parent sample shows a distinct metal–insulator (M–I) transition of intrinsic type at a temperature close to the Curie temperature, whereas composite samples show two possible transitions, intrinsic as well as extrinsic. The series exhibits a conduction threshold at x = x _m ～ 20%, up to which extrinsic M–I transition temperature decreases along with an increase in extrinsic magnetoresistance; whereas, above x _m these trends of variation are reversed.     

晶界对庞磁电阻颗粒薄膜的磁学和输运性能的影响

采用脉冲电子束沉积技术，在Si(100)单晶衬底上沉积庞磁电阻La_0.67Ca_0.33MnO₃颗粒薄膜，并对它的磁学性能和电学输运性能进行了表征.研究晶界对庞磁电阻薄膜的物理性能的影响，结果表明，晶界的存在使得晶粒之间的耦合变弱，在变温磁化过程中表现出团簇玻璃态行为，金属—绝缘体转变温度(T_p)远远低于铁磁—顺磁转变温度(T_c).低温下电子输运具有弱局域化行为.在低磁场下，晶界的存在掩盖了La_0.67Ca_0.33MnO₃的本征磁电阻行为. 关键词： 脉冲电子束沉积 晶界 磁学和电学输运性能 庞磁电阻     

二元掺杂镧锰氧化物La-Ca-Ba-Mn-O的庞磁电阻特性

用固相反应法制备了二元掺杂的镧锰氧化物La_0.67(Ca_0.6Ba_0.4)_0.33MnO_z(LCBMO)立方多晶体材料,研究了其磁特性和庞磁电阻特性,并与用类似方法制备的一元掺杂的La_0.67Ca_0.33MnO_z(LCMO)及La_0.67Ba_0.33MnO_z(LBMO)的庞磁电阻特性进行了比较.研究表明,LCBMO的居里温度T_C为312K,介于LCMO和LBMO的T_C（分别为280,362K）之间,其金属-半导体转变温度T_p和μ₀H=0.6T下的磁电阻（MR）峰值温度T_m分别为306,298K,接近于其T_C,也介于LCMO和LBMO的T_p和T_m之间.μ₀H=0.6T下,在各自的T_m处,LCMO,LCBMO和LBMO的庞磁电阻值分别达到41%,24.7%和8%,但在室温（300K）处,LCBMO的MR值仍达到20％,远大于LCMO和LBMO的值（分别为2.0％和2.4％）.研究还发现,在温度远低于T_m时,LCMO仍保持一定的磁电阻效应,而LCBMO和LBMO的磁电阻温度降低而增加,这些低温磁电阻特性与材料的结构特征（如晶界和致密度）有关. 关键词：     

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.     

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.     

Transport and Magnetic Properties of Eu and Sr Doped Manganite Compound La0.7Ca0.3MnO3

The effect of simultaneous increase in carrier density and size-disorder on the transport and magnetic properties of La_0.7Ca_0.3MnO₃ has been investigated by studying the (La_{0.7 − 2x} Eu _x )(Ca_0.3Sr _x )MnO₃ (0.05 ≤ x ≤ 0.2) (LECSMO) compounds. These compounds have been compared with standard La_{1 − x} Ca _x MnO₃ (0.3 ≤ x ≤ 0.5) (LCMO) in which carrier density alone varies. In LECSMO, the insulator-metal transition temperature (T _p) decreases from 180 K for x = 0.05 to 80 K for x = 0.15 sample vis-à-vis ∼240 K for x = 0.35 to ∼225 K for x = 0.45 in LCMO system. Similarly, the Curie temperature (T _C) in LECSMO, decreases from 205 K for x = 0.05 to 75 K for x = 0.2 sample against 240 K for x = 0.35 to ∼220 K for x = 0.45 in LCMO system. Also, in LCMO the T _C and T _p are coincident whereas, in LECSMO system, with increasing x there is an increasing disparity between the two. At 5 K, in the metallic region, a large MR (>90%) is observed in x = 0.15 sample and is discussed in terms of phase segregation and inter-grain magnetoresistance.     

Preparation and study of silver added La0.67Ca0.33MnO3 film

La_0.67Ca_0.33MnO₃ (LCMO) and Ag admixed La_0.67Ca_0.33MnO₃ (Ag-LCMO) polycrystalline films have been prepared on SrTiO₃ single crystal (100) substrates by ultrasonic spray pyrolysis technique. These films are characterized using XRD, SEM, and temperature dependence of resistivity (ρ-T) and ac susceptibility (χ-T). The films are having cubic structure with lattice parameters as 3.890 and 3.885 Å for LCMO and Ag-LCMO films, respectively. The peek in ρ-T curve (T_p) and the ferromagnetic transition temperature (T_C) for the Ag-LCMO film is higher than that of LCMO film. The stability of both the films was tested by repeated measurements of its characteristics over a period of one week after several thermal cycling from room temperature to 77 K. In the LCMO film, the peak in the ρ-T curve (T_p) is found to shift towards lower value and conduction noise of the film increases in the subsequent measurements. In the case of Ag-LCMO the value of T_p, T_C and conduction noise of the film did not change even after several measurements. Silver segregating at the grain boundaries in Ag-LCMO polycrystalline film seems to be responsible for improving the characteristics of Ag-LCMO films.     

Search for high temperature coefficient of resistance La2/3Ca1/3MnO3 polycrystalline ceramics

La_2/3Ca_1/3MnO₃ polycrystalline ceramics were synthesized by sol–gel method. Sharp temperature coefficient of resistance (TCR) variation (with peak value up to 22 %) has been observed near the metal-insulator transition temperature T _MI (273 K) for the sample sintered at 1,450 °C. This TCR value is much higher than the previously reported values for the undoped and Ag-doped La_0.67Ca_0.33MnO₃ samples and is comparable to the optimized thin films. It was concluded that the improved physical properties of the La_0.67Ca_0.33MnO₃ material are due to its improved microstructure and homogeneity.     

LaCaMnO/LaNdCaMnO/LaCaMnO三层膜中巨磁电阻增强效应

通过直流磁控溅射方法在SrTiO₃(001)衬底上制备了一系列La_0.67Ca_0.33MnO₃/(La_0.35Nd_0.65)_2/3Ca_1/3MnO₃/La_0.67Ca_0.33MnO₃(以下简写为LCMO/LNCMO/LCMO)外延三层膜，其中间层 关键词：     

Enhancing low-field magnetoresistance of La0.67Ca0.33MnO3 films deposited on anodized aluminium-oxide membranes

In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide （AAO） membranes, The magnetic and electronic transport properties are investigated by using the Quantum Design physics properties measurement system （PPMS） and magnetic properties measurement system （MPMS）. The resistance peak temperature （Tp） is about 85 K and the Curie temperature （To） is about 250 K for the LCMO film on an AAO membrane with a pore diameter of 20nm. Large magnetoresistance ratio （MR） is observed near Tp. The MR is as high as 85% under 1 T magnetic field. The great enhancement of MR at low magnetic fields could be attributed to the lattice distortion and the grain boundary that are induced by the nanopores on the AAO membrane.     

Influence of microstructures on exchange bias behaviors of La0.7Sr0.3MnO3/La0.33Ca0.67MnO3 bilayers

Ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) and antiferromagnetic La_0.33Ca_0.67MnO₃ (LCMO) layers were grown on SrTiO₃ (STO) substrates by the pulsed laser deposition technique. LSMO films had rougher surfaces and larger grain sizes than LCMO films. Fully strained bilayers, in which each layer was as thin as 10 nm, were prepared by changing their stacking sequences, i.e. LSMO/LCMO/STO and LCMO/LSMO/STO. The former had higher T_C (350 K) than the latter (300 K), and exchange bias effects were only observed in the former bilayers. This revealed that microstructures could play an important role in the transport and magnetic properties of manganese oxide thin films.     

Interfacial spin interactions of ferromagnetic and antiferromagnetic manganite bilayers

A pulsed laser deposition technique was used to grow ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) films on antiferromagnetic La_0.33Ca_0.67MnO₃ (LCMO) and Pr_0.7Ca_0.3MnO₃ (PCMO) films in bilayer forms. The LSMO film on the PCMO layer had a more elongated out-of-plane lattice than that on the LCMO layer. The former had a lower ferromagnetic transition temperature (320 K) than the latter (350 K). The enhanced low-temperature magnetoresistance of the LSMO/PCMO bilayer suggests that the spin frustration is stronger at this bilayer than in the LSMO/LCMO bilayer. These differences indicate that strain state and defect concentration play important roles in governing interfacial spin interactions.     

Pressure effects in ferromagnetic manganites and carrier scattering by disordered magnetic rare earths

The influence of hydrostatic pressure on the transport properties of (La_1-xR_x)_0.67Ca_0.33MnO₃ (,Tb) ferromagnetic manganites is investigated. The enhancement of the Curie temperature T_C under pressure agrees with previous data. In the paramagnetic range, the resistivity can be represented by a Mott localisation law, with a characteristic temperature T₀ decreasing with pressure. The variation of T_C with pressure is compared to the effect induced by replacing La by a magnetic rare earth in (La_1-xR_x)_0.67Sr_0.33MnO₃ manganites (, ..., Tm). The main effect is not related to the decrease of the mean radius of the cation, but to an additional scattering by the magnetic moment of the rare earth. Received: 15 May 1998 / Revised: 6 July 1998 / Accepted: 16 July 1998     

Electrical conductivity and magnetic properties of La1 ? xCaxMn1 ? yFeyO3 ceramic samples (x = 0.67, y = 0, 0.05)

The temperature dependences of the magnetic susceptibility χ(T) and the electrical resistivity ρ(T) of ceramic samples of La_{1 − x} Ca _x MnO₃ with x = 0.67 (LCMO) and La_{1 − x} Ca _x Mn_{1 − y} Fe _y O₃ with x = 0.67 and y = 0.05 (LCMFO) are investigated in magnetic fields B = 50–10⁵ G and the temperature range T = 4.2–400 K. Both samples undergo a transition from the paramagnetic state to a state with charge (orbital) ordering (CO) at temperatures T _CO ≈ 272 K for LCMO and T _CO ≈ 222 K for LCMFO. The behavior of the paramagnetic phase in the temperature range 320–400 K for LCMO and 260–400 K for LCMFO is described by the Curie-Weiss law with effective Bohr magneton numbers p _eff = 4.83 μ_B (LCMO) and 4.77 μ_B (LCMFO), respectively. The disagreement between the observed positive Weiss temperatures (θ ≈ 175 K (LCMO) and θ ≈ 134 K (LCMFO)) and negative Weiss temperatures required for the antiferromagnetic ground state can be explained by the phase separation and transition to the charge-ordered state. The magnetic irreversibility for T < T _CO is accounted for by the existence of a mixture of the ferromagnetic and antiferromagnetic phases, as well as the cluster glass phase. At low temperatures, doping with iron enhances the frustration of the system, which manifests itself in a more regular behavior of the decay rate of the remanent magnetization with time. The temperature dependence of the electrical resistivity in the range of the charge-ordered phase conforms to the variable-range hopping model. The behavior of the electrical resistivity is governed by the complex structure of the density of localized states near the Fermi level, which includes a soft Coulomb gap Δ = 0.464 eV for LCMO and 0.446 eV for LCMFO. It is established that the ratio between the localization radii of charge carriers a for LCMFO and a _und for LCMO is a/a _und = 0.88. Original Russian Text ? V.S. Zakhvalinskiĭ, R. Laiho, T.S. Orlova, A.V. Khokhulin, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 1, pp. 61–68.     

Epitaxial growth La0.67Ca0.33MnO3 thin film by radio frequency sputtering method

Perfect epitaxial growth of La_0.67Ca_0.33MnO₃ (LCMO) thin film has been achieved on (1 0 0) LaAlO₃ (LAO) single crystal substrate by radio frequency sputtering method. X-ray diffraction (XRD) and electron diffraction analysis indicates that La_0.67Ca_0.33MnO₃ film grows epitaxially on LaAlO₃ along [1 0 0] direction of the substrate. The resistivity variation with temperature of the film shows a sharp metal to semiconductor transition peak around 253 K, which is close to that of the target. The magnetoresistance (MR) also reveals high quality epitaxy film characteristic at low temperatures and near the metal to semiconductor transition temperature.     

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.     

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.     

Structure and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films grown coherently on (001)NdGaO<Subscript>3</Subscript>

40-to 120-nm-thick (001)La_0.67Ca_0.33MnO₃ films grown through laser evaporation on (001)NdGaO₃ were studied. The lattice parameters of the La_0.67Ca_0.33MnO₃ films measured in the substrate plane (a_∥=3.851 Å) and along the normal to its surface (a_⊥=3.850 Å) practically coincided with that of the pseudocubic neodymium gallate. The unit-cell volume of the La_0.67Ca_0.33MnO₃ film was slightly smaller than that of stoichiometric bulk samples. The position of the maximum in the temperature dependence of electrical resistivity did not depend on the thickness of the La_0.67Ca_0.33MnO₃ film. The negative magnetoresistance (MR≈?0.25, H=0.4 T) of La_0.67Ca_0.33MnO₃ films reached a maximum at 239–244 K.     

非化学计量配比La0.67Sr0.33-x□xMnO3的结构和输运性质的研究

利用固相反应法制备了非化学计量配比的类钙钛矿锰氧化物La_0.67Sr_0.33-x□_xMnO₃(0<x≤0.33),研究了A位空位对材料的晶体结构和输运性质的影响.对粉末X射线衍射谱的Rietveld全谱拟合表明样品均为单相,在x=0到x=0.33空位浓度范围内晶体对称性没有发生变化,均具有三方对称性,空间群为R3c< 关键词： 非化学计量配比锰氧化物 Rietveld全谱拟合 结构分析 输运性质     

Enhancement of magnetoresistances at room temperature in YSZ doping La0.67Sr0.33MnO3 system

The temperature dependence of the resistance of composite samples (1−x)La_0.67Sr_0.33MnO₃+xYSZ with different YSZ doping level x was investigated at magnetic fields 0-3 T, where YSZ represents yttria-stabilized zirconia. Results show that the YSZ dopant does not only adjust the metal-insulator transition temperature, but also increases the magnetoresistance effect. With increase of YSZ doping level for the range of x<2%, the metal-insulator transition temperature values T_P of the composites decrease, but T_P increases with increase of x further for the range of x>2%. Meanwhile, in the YSZ-doped composites, a broad metal-insulator transition temperature region was found at zero and low magnetic field, which results in an obvious enhanced magnetoresistance in the temperature range 10-350 K. Specially, a larger magnetoresistance value was observed at room temperature at 3 T, which is encouraging with regard to the potential application of magnetoresistance materials.