Enhanced temperature coefficient of resistance and magnetoresistance of Co-doped La0.67Ca0.33MnO3 polycrystalline ceramics

Recently, La_0.67Ca_0.33MnO₃ has garnered significant research attention due to its peculiar physical properties, such as colossal magnetoresistance and metal insulator transitions. The practical applications of these materials are mainly determined by the temperature coefficient of resistance (TCR) and magnetoresistance (MR). As a mature synthesis route, the sol-gel method can prepare high-quality ceramic targets. Herein, using methanol as a solvent, La_0.67Ca_0.33Mn_1-xCo_xO₃ (0 ≤ x ≤ 0.06) polycrystalline ceramics are prepared using the sol-gel method and the influence of Co doping on electrical and magnetic properties is systematically studied. Co doping increases the grain size, and is helpful to improve TCR and MR. In addition, with increased Co doping, the double exchange is weakened, and the ferromagnetism is depressed, which leads to a decrease in T_MI. The results reveal that the TCR and MR can be optimized by tuning the Co content. For instance, we have achieved a TCR value of 44.2% · K^?1 at x = 0.02 and an MR value of 76.3% at x = 0.04, showing the promise of Co-doped La_0.67Ca_0.33MnO₃ ceramics in a wide array of applications, such as bolometers and magnetic sensors.     

Enhanced room temperature coefficient of resistivity (RT-TCR) and broad metal-insulator transition temperature (TMI) of La0.67Ca0.33-xAgxMnO3 polycrystalline ceramics

Polycrystalline La_0.67Ca_0.33-xAg_xMnO₃ (LCAMO, x = 0, 0.06, 0.15, 0.18, and 0.24) ceramics were fabricated by conventional sol-gel route at relatively low sintering temperature of 1100 °C for 12 h. Effects of silver content (x) on crystal structure, grain size, resistivity and magnetic properties of as-prepared LCAMO specimens were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), elemental mapping and energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and standard four-probe method (ρ-T). The data from XPS, XRD and EDS revealed that silver existed as Ag⁺ ions in the lattice matrix position of LCAMO ceramics. Broad metal-insulator transition temperature (T_MI) values ranging from 267.0 K (x = 0) to 302.6 K (x = 0.24) were obtained with LCAMO specimens prepared with variable Ag⁺ added contents. Peak temperature coefficient of resistivity (TCR) enhanced from 4.1% K⁻¹ at 263.2 K (for x = 0) to 10.9% K⁻¹ at 278.5 K (for x = 0.18), and reached 7.5% K⁻¹ at room-temperature (295.9 K) for x = 0.24. Meanwhile, magnetoresistance (MR) of materials reached 17.7% at room-temperature (299.2 K) for x = 0.24. Overall, these findings demonstrated that Ag doping was beneficial for improving electrical and magnetic properties of LCAMO materials. In summary, LCAMO ceramics achieved RT-TCR and MR at optimal Ag stoichiometric ratio, promising for applications in infrared bolometers or magnetic sensors.     

多晶La0.67Ca0.33MnO3材料的热敏和湿敏特性

通过电阻温度测试系统来测试由La0.67Ca0.33MnO3多晶材料制成的敏感电阻,发现La0.67Ca0.33MnO3敏感电阻的电阻值不仅随着温度呈线性变化,而且也随着湿度呈线性变化。将封装后的La0.67Ca0.33MnO3敏感电阻与标准Pt100热敏电阻进行测温比较,相对误差为0.2%。     

多晶La0.67Ca0.33MnO3材料的热敏性和湿敏性

通过电阻温度测试系统来测试由La0．67Ca0.33MnO3多晶材料制成的敏感电阻，我们发现La0．67Ca0.33MnO3敏感电阻的电阻值不仅随着温度呈线性变化，而且也随着湿度呈线性变化。将封装后的La0．67Ca0.33MnO3敏感电阻与标准Pt100热敏电阻进行测温比较，相对误差为0.2％。     

Effect of Gd doping on electrical transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

Gd doped La_0.8Sr_0.2MnO₃ (La_0.8-xGd_xSr_0.2MnO₃, LGSMO) ceramics were prepared by a sol-gel method. X-ray diffraction (XRD) patterns showed that all samples exhibited distorted perovskite structures, R_3c. When the Gd³⁺ content x > 0.03, the crystal structure changed to orthorhombic, Pnma. Scanning electron microscopy (SEM) images showed that the ceramics characterize high density and grain boundary connectivity, and higher Gd³⁺ doping decreased the grain size from 26.72 μm to 7.42 μm. The temperature dependence of resistivity showed a transition from a low-temperature metal to a high-temperature insulator. The resistivity increased with Gd doping content, and the metal-insulator transition temperature, T_P, increased first and then decreased, while the temperature coefficient of resistance (TCR) of the samples first decreased and then increased with Gd³⁺, and the magnetoresistance (MR) increased first and then decreased. The peak TCR at x = 0.06 was 5.18%·K^?1, and MR at 0.04 was 34.57%. The electrical transport properties of the ceramics were explained based on the double exchange (DE) interaction mechanism. The obtained material may have application prospects in magnetic devices and infrared detectors.     

Extrinsic behavior in La0.67Ca0.33MnO3–BaTiO3 composites

A manganite matrix based nano-composite series, (1 ? x)La_0.67Ca_0.33MnO₃(LCMO)–(x)BaTiO₃(BTO), has been prepared by the pyrophoric method. Influence of BTO phase on structural and magneto-transport properties of LCP phase has been studied using structural and transport investigations. The series exhibits a conduction threshold at x_m ～ 0.30. Overall pattern of temperature dependence of resistivity for this series has been fitted with a percolation model. Almost 200% improvement has been observed by the formation of composite when compared to the parent sample.     

Robust temperature coefficient of resistance of polycrystalline La0.6Ca0.4MnO3 under magnetic fields at room temperature

In the presence of magnetic field, reducing the loss in temperature coefficient of resistance (TCR) and increasing metal−insulator transition point (T_MI) to room temperature are the most important concerns for the application of perovskite manganites in high−precision thermistors. Based on emerging evidence, relevant factor to address these problems lies in the interaction between Jahn−Teller (JT) distortion and magnetic field, which results in spin−orbital coupling (SOC) effect and significantly influences TCR and T_MI. In this work, we studied the magnetic field induced SOC effect in polycrystalline La_1−xCa_xMnO₃ (x = 0.225–0.45) materials synthesized via sol−gel technique. Compounds undergo the JT distortions with increasing Ca dopant content, and the most pronounced distortion of 0.0144 at x = 0.40 is correlated with a basal−plane distortion mode. All samples exhibit T_MI values between 262 K and 288 K, indicating the enhancement of doping−induced double−exchange interaction. In the magnetic field of 1 T, SOC effect sufficiently suppresses the deterioration of TCR caused by deficient magnetization in La_0.6Ca_0.4MnO₃ to a value of 2.9%·K⁻¹ at room temperature (287 K).     

La0.67Ca0.33CoO3的结构和磁电阻效应研究

采用溶胶凝胶法制备了La0.67Ca0.33CoO3样品,利用X射线衍射和磁电阻测量,对样品进行了结构和磁电阻效应的研究。结构研究表明样品呈现十分优异的单相性。电阻测量表明在H=0 T和H=0.3 T的磁场下,样品电阻值都随着温度的降低急剧增大。样品在H=0.3 T的磁场下在T<20 K时,呈现正的磁电阻效应,最高达21%,在20 K相似文献   

Electrical transport properties of Sm-doped La0.7Ca0.3MnO3 polycrystalline ceramics

Improving the magnetoresistance effect of perovskite ceramic materials under a low applied magnetic field to expand its application range is one of the main research directions of this type of material. In this study, La_0.7Ca_0.3MnO₃ was doped with different levels of Sm by the sol-gel method to yield a series of La_0.7-xSm_xCa_0.3MnO₃ (LSCMO) polycrystalline ceramics. X-ray diffraction (XRD) results revealed that LSCMO ceramics possessed standard perovskite structures. Scanning electron microscopy (SEM) showed grains closely connected without obvious holes. In addition, the grain size gradually decreased with the increase in Sm doping content. The resistivity temperature curves displayed a clear metal-insulator transition behavior of LSCMO accompanied by a steep change from ferromagnetic to paramagnetic behavior (FM-PM). The metal-insulator transition temperature (T_p) values of the as-obtained LSCMO gradually shifted toward lower temperatures with increase in Sm content. Moreover, resistivity temperature coefficient (TCR) and magnetoresistance (MR) values also gradually increased with Sm doping content. The transport properties in polycrystalline ceramics could be adequately explained by the double exchange model, which would be useful for interpreting the CMR effects when used in magnetic devices.     

High-density sol-gel derived,cold-isostatically pressed La0.67Ca0.27Sr0.06MnO3 polycrystalline ceramics and their room-temperature TCR improvement

In this study, high-density La_0.67Ca_0.27Sr_0.06MnO₃ (LCSMO) ceramics were successfully obtained by sol-gel method followed by cold isostatic pressing (CIP) under 250 MPa. Effects of sintering temperature (T_S, 1250 °C ≤ T_S ≤ 1450 °C) on structure, surface chemical, electrical, and optical properties of as-obtained LCSMO ceramics were investigated by X-ray diffraction (XRD, BDX3200), X-ray photoemission spectroscopy (XPS, Thermo Fisher Scientific), a four-probe system (East changing ET9005), Fourier transform infrared spectrometry (FTIR, TENSOR 27), Raman microscopy (RENISHAW inVia) and UV–vis spectrophotometry (HITACHI U-4100), respectively. All prepared samples showed orthorhombic structure with Pnma space group. As T_S increased, grain size gradually rose from 2.71 to 7.30 μm. XRD refinement, Raman, and FTIR all depicted constant of bond angle and length of Mn–O. Hence, variation in T_S did not cause lattice distortion inside samples. The best electrical properties were obtained at 1450 °C, where resistivity (ρ) fell below 0.008 Ω cm and temperature coefficient of resistivity (TCR) reached 13.0% K^?1 at 289.2 K (room temperature).It is worth noting that electrical performance was indeed improved due to high densities of samples under processing of CIP. Overall, the change in T_S from 1250 to 1450 °C greatly influenced electrical behaviors but showed little effect on structural and optical properties of LCSMO ceramics.     

Co-optimization of Na and K doping for improved room-temperature TCR of La0.7(Na0.3-xKx)MnO3 polycrystalline ceramics

Polycrystalline La_0.7(Na_0.3-xK_x)MnO₃ (LNKMO, x = 0.10, 0.15, 0.20, 0.25, and 0.26) ceramics were successfully compounded by adopting conventional sol-gel technology. The physical properties of as-prepared specimens were closely related to their morphology and internal structure, which were characterized and analyzed via X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. Results confirmed that La⁺ ions located at A-sites in crystal lattice were partially substituted by doped Na⁺ and K⁺ ions, which resulted in rotation and distortion of MnO₆ octahedron. Lattice distortion was primary factor behind double exchange (DE) mechanism and Jahn-Teller (JT) effects. In addition, Na and K dopants altered relative amount of Mn³⁺ and Mn⁴⁺ ions, causing intensity variation in DE effect. These changes contributed to a decline in resistivity and an increase in peak resistance temperature (T_k) with increasing K doping level. Meanwhile, optimal temperature coefficient of resistance (TCR) value of LNKMO ceramics reached 8.48% K^?1 at 292.14 K when x = 0.25. This work reveals the mechanism of Na and K co-doping to optimize electrical transport properties of LNKMO manganese oxides and provides excellent material for the fabrication of uncooled infrared bolometers.     

Structural characterization and magnetic properties of single-crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires

Herein, structural and magnetic properties of single-crystalline (La_0.6Pr_0.4)_0.67Ca_0.33MnO₃ nanowires synthesized via a hydrothermal process are reported. The nanowires are crystallized in an orthorhombic structure (Pnma space group). Their lattice parameters follow the relationship a ≈ c ≈ b/√2 These nanowires exhibited a clean and smooth surface with diameters of 60-120 nm and an average length of approximately 2.0 μm. High-resolution transmission electron microscopy images confirmed the single-crystalline nature of the nanowires growing along the [100] direction. The nanowires demonstrated magnetic hysteresis loops at low temperatures and a weak exchange bias (EB) effect. Paramagnetic (PM)–ferromagnetic (FM) phase transition occurred at a Curie temperature (T_C) of 224 K, and strong irreversibility between zero-field-cooled (ZFC) and field-cooled (FC) magnetization (M_ZFC and M_FC, respectively) curves was observed at 273 K. The M_ZFC curve exhibited a significantly broad peak with a maximum at a freezing temperature (T_f) of 134 K. Relative difference between M_FC and M_ZFC in the nanowires [(M_FC − M_ZFC)/M_FC] rapidly increased below T_f and reached approximately 50% below 35 K. The effective magnetic moment deduced from the Curie constant is larger than the theoretical value, indicating short-range FM interactions in the nanowires. A positive PM T_C (θ_p) implies dominant FM interactions in the nanowires, and θ_p > T_C observed herein indicates the existence of short-range ordered states above T_C.     

Synthesis and transport properties of La0.67Sr0.33MnO3 conformally-coated on carbon nanotubes

We report for the first time the fabrication of nanostructured ferromagnetic lanthanum strontium manganite La_0.67Sr_0.33MnO₃ (LSMO) conformally coated onto bamboo-like carbon nanotubes (BCNTs) by pulsed laser deposition. Scanning electron microscopy revealed that one-dimensional LSMO/BCNTs with diameters ranging from 100-160 nm and lengths over 10 μm were obtained. Line-scanned energy-dispersive X-ray spectroscopy profiles, selected area electron diffraction rings, and energy-filtered transmission electron microscopy maps provided further insight into the hybrid nanostructures. LSMO/BCNT and BCNT were also investigated via in situ electrical probing in a transmission electron microscope using a piezo-driven scanning tunneling microscopy holder. Modeling of the I–V characteristics of individual hybrid nanotubes yielded the resistivity, bandgap, and electron density of LSMO and BCNT. The results show that the transport properties of LSMO/BCNT are superior to those of BCNT. This research advances the integration of oxide materials and carbon nanotubes, bringing forth new avenues for miniaturization and fabrication of one-dimensional core-multishell materials with multifunctional properties that can be used as building blocks in nanodevices.     

La0．67Ca0．33CoO3的高场磁电阻效应研究

采用溶胶凝胶法制备了La0．67Ca0．33CoO3样品,利用物性测量系统（PPMS）对样品的磁电阻进行了测量,研究了样品在磁场下的磁电阻效应。电阻（R）测量表明在H=OT和H=9T的磁场下,在4K〈T〈300K的温度范围内,样品的电阻和磁电阻都随着温度的降低而升高,磁电阻最高达60％。从温度对eg电子的影响的角度进行了解释。     

Effect of manganese stoichiometry at B-site on magneto-transport and magnetic properties of La0.67Sr0.33MnO3manganites

To study the effect of manganese non-stoichiometry at B-site, a series of manganites with compositional formula La_0.67Sr_0.33Mn_1±xO₃ (where x = 0, 0.05 and 0.1) was synthesized by oxalate precursor method. X-ray diffraction data confirm the rhombohedral structure of La_0.67Sr_0.33Mn_1±xO₃ along with minor phases of Mn₃O₄. The average grain size is found to be 266 nm for x = 0 whereas its magnitude decreases with excess or deficiency in manganese concentration. An increase in the manganese non-stoichiometry leads to the coexistence of ferromagnetic and antiferromagnetic interactions. The effect of Mn_1±x on the magnetotransport properties could be understood on the basis of collective behaviour of magnetic spins, double exchange mechanism and ratio of Mn⁴⁺/Mn³⁺ ions. A crossover from negative to positive magnetoresistance behavior above metal-insulator transition temperature was observed for LSP-0.95 sample, whereas a positive magnetoresistance over the entire temperature region was noticed for LSP-1.10 sample.     

Hysteresis and vertical anisotropy of magnetoresistance in La0.67A0.33MnOz (A=Ca,Sr) polycrystalline films deposited on amorphous quartz substrates

Polycrystalline La_0.67A_0.33MnO_z (A=Ca, Sr) thin films with [202] preferred orientation were synthesized on amorphous quartz substrates by means of metal organic deposition (MOD) technology. The Curie temperature (T_C) and metal?insulator transition temperature (T_p) are 172 K, 247 K and 227 K, 335 K for La_0.67Ca_0.33MnO_z (LCMO) film and La_0.67Sr_0.33MnO_z (LSMO) film, respectively. The hysteresis and vertical anisotropy behaviors of MR are exhibited by the films below T_C, which strongly relies on the magnetization process. The magnitude of these magnetotransport properties would decline with increasing temperature. The in-plane hysteric MR behaviors could be explained by multiple-domain model. For vertical anisotropy of MR, demagnetization effect plays an important role. Especially, compared to LCMO film, LSMO film still presents the above hysteretic MR effect and vertical anisotropic MR at RT due to its higher T_C.     

Synthesis and magnetic properties of complex oxides La0.67Sr0.33MnO3 nanowire arrays

As a new approach of melt-injection-decomposition method, it has been successfully adopted for the synthesis of the complex oxides La_0.67Sr_0.33MnO₃ nanowire arrays. X-ray diffraction studies confirmed the formation of perovskite manganite phase of the sample. Transmission electron microscope and scanning electron microscope characterizations showed a large quantity of one-dimensional nanowires is obtained and the nanowires are continuous, concrete, oriented and rather uniform with an average diameter of 170 nm and length of several tens of micrometers. Magnetic measurements exhibited good ferromagnetic properties at the temperature of 10 K and 300 K respectively. The transition temperature of the complex oxides La_0.67Sr_0.33MnO₃ nanowire arrays is about 350 K, which will endow their great potential applications in magnetoresistance, spintronics or sensor fields at room temperature.     

Improvement of electrical and magnetic properties in La0.67Ca0.33Mn0.97Co0.03O3 ceramic by Ag doping

For the perovskite manganite La_1-xCa_xMnO₃, achieving high temperature coefficient of resistance (TCR) and magnetoresistance (MR) is the key to realize its potential applications. In this study, high-quality La_0.67Ca_0.33Mn_0.97Co_0.03O₃:Ag_x polycrystalline ceramics were prepared by the sol-gel method. The results show that Ag doping has important impact on metal-insulator and ferromagnetic-paramagnetic transitions. A Ag doping amount increases, the grain size of the samples increases at x = 0.05 and then decreases. The doping of Ag can improve the crystalline quality of the samples and enhance the connectivity between grains, thereby improving the metallicity of the system. Additionally, with Ag doping amount increases, the resistivity of the samples gradually decreased, while the Curie temperature and the metal-insulator transition temperature gradually increased. Especially after Ag doping, both the TCR peak (TCR_peak) and the MR peak (MR_peak) values are significantly improved. The TCR_peak reaches 65.2%·K^?1 at x = 0.1, while the MR_peak is as high as 82.6% at x = 0.05 under 1 T magnetic field. Doping perovskite manganite ceramics with Co and Ag can greatly optimize their TCR and MR, favoring the potential applications of these materials.     

Effect of Y doping on transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

In this study, La_0.8-xY_xSr_0.2MnO₃ (LYSMO) polycrystalline ceramics were prepared by means of sol-gel technique using methanol as solvent. X-ray diffraction (XRD) showed all samples to possess standard perovskite structure. Scanning electron microscopy (SEM) revealed samples with high compactness and grain size from 27.80 to 29.73 μm. Resistivity–temperature tests indicated sharp metal-insulator transition behavior of all samples accompanied by rapid transformation from ferromagnetism to paramagnetism (FM-PM). As Y³⁺ doping amounts rose, radius of A-site ions decreased, metal-insulator transition temperature (T_p) of polycrystalline samples shifted to lower temperatures, and resistivity increased. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by introduction of Y³⁺. At x = 0.06, peak TCR and peak MR reached 4.85% K⁻¹ and 52.34%, respectively. Using double exchange (DE) interaction mechanism, electric transport performances of as-prepared ceramics were explained. These findings look promising for future applications of LYSMO materials in magnetic devices and infrared detectors.