Papers by Keyword: Colossal Magnetoresistance

Abstract: We have investigated the structural, microstructure and electrical transport properties of nanosized Pr_0.85Na_0.15MnO₃ (PNMO) synthesized by sol-gel technique and sinter from 600°C to 1000°C. The grain size increases from 67 nm (S600) up to 284 nm (S1000) due to the grain growth during heat treatment. XRD showed that single phase orthorhombic crystal structure of PNMO is fully forms started at 600°C. The resistivity decreased with the increased of grain size and crystallite size due to the reduction of grain boundary effect (dead magnetic layer) which improved their grain conductivity.All samples showed semiconductor behavior where their metal insulator transition temperatures (T_MIT) were estimated to be lower than 80K.

Abstract: The influence of sintering temperature on structural, microstructure and transports properties of La_0.7Sr_0.3MnO₃ (LSMO) prepared via sol-gel techniques is introduced. Sintering temperature of LSMO varied from 650°C to 950°C with the interval of 100°C. All samples show single phase structure with hexagonal setting. Refinement was done on the XRD data obtained. Microstructure investigation displayed an increment in average grain size of LSMO due to the grain growth promotion as the sintering temperature increased. Magnetization of all samples was enhanced by the grain growth promotion. Resistivity experienced a reduction trend and T_p shifted to higher temperature as the sintering temperature increased. All sintered LSMO exhibit LFMR effect at low applied field and low temperature.

Abstract: Colossal magnetoresistance of La_0.7Sr_0.3MnO₃ compounds (x=0~0.18) were prepared by the sol-gel technique. Partial Mn atoms were replaced by Fe atoms and its influence on the structure, magnetic properties and colossal magnetoresistance were also studied. Our results show that the Curie temperature decreases fast, the ferromagnetic property is weakened, the electric conductivity is decreased and the colossal magnetoresistance is enhanced as an increase of replacing Mn by Fe atoms. It was convinced by the theory of molecular field that doping Fe atoms can decrease the ferromagnetic exchange coupling of La_0.7Sr_0.3MnO₃

Abstract: The polycrystalline La_0.62Sb_0.38MnO₃ sample has been prepared by the solid-state reaction method. Structural, magnetic and electrical transport properties have been researched. X-ray diffraction analysis confirms the hexagonal crystal symmetry. Magnetization measurements indicate La_0.62Sb_0.38MnO₃ experienced from paramagnetic to ferromagnetic state transition with decreasing temperature at about 225 K. Resistivity dependences on temperature exhibit metal-insulator transition (MIT), and the maximum magnetoresistance (MR) ratio is about 33 % at temperature of 189 K and magnetic field of 2 T.

Abstract: The transport properties of polycrystalline La_0.9Sb_0.1MnO₃ (LSMO) bulk prepared by the solid-state reaction were investigated. We find that transport behaviors heavily depend on the synthesis process. The resistivity of LSMO1 for less rubbing time shows one metal-insulator transition (MIT) peak at temperature of 201 K, while the resistivity of LSMO2 for more rubbing time shows a MIT and a shoulder at about 240 and 140 K, respectively. The magnetoresistance (MR) ratio of LSMO2 reaches 41% under magnetic field of 2 T. Moreover, the MR ratio keeps significant value within broad temperature range. The infrared (IR) absorption spectra of LSMO2 show that the stretch-mode peak split into two Gaussian peaks with the gap about 70 cm^-1. This large splitting indicates there are strong distortion and disorder in LSMO2 sample. The results are interpreted in terms of the disorder system and phase separation in perovskite manganites.

Abstract: The magnetic and electrical properties of the La_0.67Ca_0.25Sr_0.08MnO₃/xAg composite system are systematically investigated as a function of Ag-added content. With increasing the Ag dopant amount, the magnetization decreases a little while Curie temperature (T_C) is almost independent of Ag content around 312 K. The resistivity reduces rapidly with Ag addition ( x < 0.25 ) due to the decomposed metal Ag, and then it increases slightly which is probably induced by the less Ag content related to the volatilization of Ag during calcinations. At low temperature, the ρ – T curves fit well by the expression of ρ =ρ₀ +ρ₂T² +ρ_4.5T^4.5 while all data for the above T_C can be fitted by using the adiabatic small-polaron-hopping model ρ=ρ₀T exp (E/k_BT ). The enhancement of MR effect ( 41% ) at room temperature is mainly related to the coexistence of intrinsic MR properties and the spin dependent scattering of conduction electrons at the interfaces. These results indicate that combining the doping effect with the composite method is an effective selection in enhancement of MR at room temperature, which is very meaningful for the application research.

Abstract: The magnetic and electrical properties of La0.67Sr0.33MnO3 ( LSMO ) are influenced very much by the Nb dopant. However, this doping effect is restricted by the limited Nb solution into LSMO due to the low calcined temperature. As a result, a second phase LaNbO4 appears in our samples. Enhancements of the low-field magnetoresistance (LFMR) were observed both at 77 K and room temperature in the manganite system prepared by doping Nb2O5 into LSMO powders. The doping amount x of Nb ions ranges from 0-10 % molar ratio. The MR ratios at 77 K with H = 1 T and H = 0.1 T are 33.8 % and 24 % for the x = 0.07 doped sample, respectively. A MR effect up to 9 % was also found for the sample with x = 0.05 at room temperature, which is 2.2 times as large as that for LSMO (4.1%). The spin dependent tunneling and scattering at the interfaces of the grain boundaries are responsible for the LFMR while the high field magnetoresistance (HFMR) originates from the spin dependent transport related to noncollinear spin structure at the interfaces.

Abstract: A series of La0.67Sr0.33MnO3/Ag composites were synthesized using a sol-gel method followed by a conventional solid-state reaction route. The temperature dependence of the magnetic properties and electrical resistivity has been investigated between 80 and 450 K. Except for the sample with x = 0.01, Ag addition has increased the conductivity of this system dramatically. Curie temperature (TC) is almost independent of Ag content and is 352 K for Ag-dopant samples, while the metal–insulator transition temperature TP increases with Ag addition, which is mainly due to the improvement of grain boundaries and opening a new metal conductivity channel caused by the segregation of Ag on the grain surfaces. At low temperature, the ρ - T curves are fitted well by the expression of ρ= ρ0 + ρ2T2 + ρ4.5T4.5 while all data for the above TC can be fitted by using the adiabatic small-polaron-hopping model ρ = ρ0T exp(E/kBT ).

Abstract: The temperature dependences of the transverse and longitudinal hypersound velocities at the frequencies (0.5-0.7) GHz were measured in the La1-xSrx MnO3 (x = 0.125; 0.15; 0.175) compositions. Structural phase transitions the position of which is confirmed by the electric resistance and magnetic measurements data for the same samples are observed. These results were analyzed within the model of the competing Jahn-Teller distortions and magnetic ordering. Anomalies in the behavior of the longitudinal hypersound velocities were related to the local Jahn-Teller distortions the suppression of which upon magnetic ordering is considered as a possible origin of a colossal magnetoresistance.

Abstract: The transport and magnetic properties of the Eu1-xCaxB6 (0x0.4) single crystals were studied in the wide ranges of temperatures (1.8-300 K) and magnetic fields (up to 8 T). The experimental data allow to identify a metal-insulator transition (MIT) at the critical Ca concentration xMIT≈0.3, which agrees well with the predictions of double exchange model (V.M. Pereira et al. Phys. Rev. Lett., 93 (2004) 147202). A significant enhancement of magnetoresistive effect is observed below 100K for Eu1-xCaxB6 compounds corresponding to the metallic side of the MIT (xxMIT). The drastic decrease of paramagnetic Curie temperature evaluated from the magnetic and magnetotransport data is discussed in terms of quantum MIT scenario recently proposed for this low carrier density system.