Influence of grain size on magnetic and transport properties of polycrystalline La0.8Sr0.2MnO3 manganites

In this study, we have investigated the effects of grain size on the magnetic and transport properties of polycrystalline La_0.8Sr_0.2MnO₃ samples. The samples were prepared by solid-state reaction method. In order to obtain a series of samples with different grain sizes, the bulk sample was subjected to high energy ball milling. The magnetization decreases by decreasing grain size. A decrease in magnetization by decreasing grain size is due to the increasing of magnetically disordered states at the surface of grains. By decreasing the grain size the resistivity increases and a second peak was observed at temperatures well bellow T_c. The intrinsic CMR decreases by decreasing grain size while the extrinsic intergranular MR is promoted to larger values. A model, proposed by Zhang et al. [N. Zhang, W. Ding, W. Zhong, D. Xing, Y. Du, Phys. Rev. B 56 (1997) p. 8138.], involving domain walls contribution and a spin polarized tunneling between neighboring grains of manganites below transition temperature may be responsible for this behavior.     

Structural, magnetic and magnetotransport properties of La0.8Sr0.2MnO3/xLaMnO3 composites

The effect of LMO doping on the structure, magnetic and magnetotransport properties of La_0.8Sr_0.2MnO₃ (LSMO)/xLaMnO₃ (LMO) has been investigated. Two types of LSMO/xLMO composites, named as SL_x (low temperature sintered samples) and SH_x (high temperature sintered samples) samples, were prepared by different sintering temperature and solid-state reaction method. The presence of LMO at the grain boundaries increases the disordered states at the surface of the grains and therefore the magnetization and transition temperature decrease by increasing the amount of LMO doping level. Results show that the rate of decreasing of transition temperature is much more for high temperaure sintered samples. Also the resistivity of samples increases by the increase of LMO doping level. Results also show that the LMO doping has an effect on a low field magnetoresistance (LFMR). The value of LFMR increases for low doping level of 0 ≤ x ≤ 15, for SL_x samples and 0 ≤ x ≤ 10 for SH_x samples. Also LFMR decreases at high doping level. The spin dependent tunneling and scattering at the interfaces of the grain boundaries are responsible for the increase of LFMR at low doping level, while reduction of LFMR at high doping level may result from the grain boundary becoming too thick for electron tunneling.     

Influence of sintering temperature and oxygen stoichiometry on electrical transport properties of La0.67Na0.33MnO3 manganite

With a view to investigate the influence of sintering temperature and oxygen stoichiometry on electrical and magnetic properties of sodium-doped lanthanum manganite sintered at different temperatures, a series of samples were prepared by the sol–gel route. The samples were characterized by the XRD studies and the data were analyzed using the Rietveld refinement technique and it has been observed that the materials are having rhombohedral structure with space group. The electrical resistivity and thermoelectric power studies were investigated both as a function of crystallite size and oxygen content. To understand the conduction mechanism, the electrical resistivity data have been analyzed and it has been concluded that the variation of electrical resistivity in the ferromagnetic region can be explained by electron–electron scattering process (T²) and two magnon scattering processes, while that in the paramagnetic region is explained by the small polaron hopping mechanism. Similarly, the variation of thermopower in the ferromagnetic region is explained on the basis of electron–magnon scattering.     

Colossal and constant magnetoresistance over a large temperature range between 230 and 4.2 K in La0.8Li0.2MnO3 prepared by a partial melting technique

La_0.8Li_0.2MnO₃ was prepared by a partial melting technique. The Li-doped LaMnO₃ compound has a rhombohedral perovskite structure and shows a wide semiconductor-to-metal transition between 230 and 175 K in resistance versus temperature. Nearly constant magnetoresistance, Δρ/ρ₀, above 20–60% was achieved with application of magnetic fields from 1 to 8 Tesla (T) over a very large temperature range from the ferromagnetic transition at 230–4.2 K. As a comparison, a partial melted La_0.7Ca_0.3MnO₃ shows a very narrow ferromagnetic transition around 230 K which is the same as that of single crystals and epitaxy thin films.     

Studies on La0.67Ca0.33MnO3-SrTiO3 composites using two-phase model

In this study we report the structural, electrical and magnetic properties of (1 − x)La_0.67Ca_0.33MnO₃ (LCMO)-(x)SrTiO₃(STO) composites. For this series we have observed a minute change in ferromagnetic (FM)-paramagnetic (PM) transition temperature with STO addition in LCMO matrix; however a reasonable change is observed in metal-insulator transition temperature, along with the occurrence of percolation threshold for x = 0.30 sample. Overall pattern for temperature dependence of resistivity for this series has been best-fitted using the formula 1/ρ = (1 − f)/ρ_PM + (f/ρ_FM), whereρ_PM and ρ_FM are the resistivities of the PM and FM contents in the sample and f is the volume fraction of FM phase in the sample. Investigations on magnetoresistance (MR) using magnetic field up to 3 T show enhancement of extrinsic MR in the composite samples which can be viewed in the light of spin polarized tunneling.     

Magnetic and electrical transport properties of nanostructured La0.67Ca0.33MnO3 networks

Contiguously nanostructured networks of La_0.67Ca_0.33MnO₃ (LCMO) are fabricated successfully by pulsed electron deposition (PED) onto the surface of porous anodic alumina oxide (AAO). The Curie temperature (T_C) is about 250 K. The metal–insulator transition temperature (T_p) is about 145 K without the magnetic field. The magnetoresistance can reach 84% near the peak temperature, which suggesting a strong magnetic correlated electronic transportation process. Zero field cooling (ZFC) and field cooling (FC) are split below the paramagnetic–ferromagnetic transition. The ZFC curves exhibit a typical blocking process. The observed spin glass and weak localization phenomena are due to the size effect, and it is found that the resistance dependent temperature curve above metal–insulator transition temperature (T_p) is more suitable to small polaron hopping (SPH) model.     

Size mismatch, grain boundary and bandwidth effects on structural, magnetic and electrical properties of Pr0.67Ba0.33MnO3 and Pr0.67Sr0.33MnO3 perovskites

We have studied the effect of size mismatch on the structural, magnetic and electrical properties of Pr_0.67Ba_0.33MnO₃ (PBMO) and Pr_0.67Sr_0.33MnO₃ (PSMO) perovskites with considerable difference in their variance σ² values. Samples were prepared by ceramic route at 1200 °C. Morphological study, microstructure and chemical composition were examined by scanning electron microscopy, EDX analyses and Rietveld structure refinement. Both compounds exhibit single orthorhombic Pnma crystalline phase and with strongly connected and larger grains for PSMO than for PBMO that gives a beginning of single crystal growth in PSMO case. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due to grain boundaries) transitions in the resistivity behaviour, and with higher transition temperatures than those usually reported in the literature. Extrinsic effects, however, are not observed in the lower σ² PSMO sample. Both compounds exhibit Curie temperature (T_C) values significantly higher than those reported in the literature, and with higher T_C for PSMO due to its larger bandwidth W. The experimental paramagnetic effective moment for PSMO is very close to the theoretical one, whereas there is a significant difference between these moments for PBMO sample, probably due to the considerable size mismatch effect between Pr and Ba, and to the ferromagnetic correlations in the paramagnetic state. Ferromagnetic-metallic regime in the two compounds seems to emanate from the electron-(phonon, magnon) scattering processes with a larger effect for PBMO than that for PSMO, due to the prominent role of the grain boundary in PBMO. Above paramagnetic-insulating transition temperature the data were well fitted by both variable range hopping (VRH) and small polaron hopping (SPH) models giving higher density state, and lower activation energy and Mott temperature T₀ for PSMO than those for PBMO, essentially due to their considerable difference in their variance values.     

Stress and defect induced enhanced low field magnetoresistance and dielectric constant in La0.7Sr0.3MnO3 thin films

Colossal magnetoresistive manganite La_0.7Sr_0.3MnO₃ (LSMO) films were prepared by pulsed laser deposition on three different single crystal substrates using different deposition parameters. Characterizations of their surface morphologies, structural, magnetic and magneto-transport properties show that films on MgO single crystal substrates contain higher amount of structural defects compared to those on SrTiO₃ (STO) and NdGaO₃ (NGO) substrates. Low deposition rate and thicker films give rise to polycrystallinity and grain boundaries. The films on MgO substrate showed a broad paramagnetic (PM) to ferromagnetic (FM) transition accompanied with metal-insulator transition (MIT) much below their Curie temperature (T_C) indicating growth of strained structures due to large lattice mismatch (9%) between the substrate and the film. The deposited films on STO and NGO show least effect of substrate induced strain exhibiting sharper PM-FM transition and metallic behavior below T_C. The magnetoresistance (MR) measured with 300 mT field clearly shows two contributions, one due to grain boundary tunneling and the other due to colossal MR effect. The highest low field MR effect of 17% was achieved for the film on MgO with the highest thickness and surface roughness indicating the presence of grain boundary related defects. Also a high dielectric constant was observed for the same film at room temperature up to 100 kHz frequency. Coexistence of defect induced large low-field MR and abnormally high dielectric constant can give rise to different exciting applications.     

Electric-field control of topological spin textures in BiFeO3/La0.67Sr0.33MnO3 heterostructure at room temperature

<正>Electric-field control of topological magnetic states in thinfilm heterostructures is promising for applications in nextgeneration memory or logic devices with ultrahigh density and low-power consumption.Multiferroic materials,where spin and polar degrees of freedom coexist,provide a versatile playground to manipulate magnetism (converse magnetoelectric effect).Here,we report that the topological spin textures can be controlled by electric field in rhombohedral BiFeO₃/monocli...     

Fabrication and performance of La0.8Sr0.2MnO3/YSZ graded composite cathodes for SOFC

The performance of multi-layer (1 − x) La_0.8Sr_0.2MnO₃/x YSZ graded composite cathodes was studied as electrode materials for intermediate solid oxide fuel cells (SOFC). The thermal expansion coefficient, electrical conductivity, and electrochemical performance of multi-layer composite cathodes were investigated. The thermal expansion coefficient and electrical conductivity decreased with the increase in YSZ content. The (1 -x)La_0.8Sr_0.2MnO₃/x YSZ composite cathode greatly increased the length of the active triple phase boundary line (TPBL) among electrode, electrolyte, and gas phase, leading to a decrease in polarization resistance and an increase in polarization current density. The polarization current density of the triple-layer graded composite cathode (0.77 A/cm²) was the highest and that of the monolayer cathode (0.13 A/cm²) was the lowest. The polarization resistance (R_p) of the triple-layer graded composite cathode was only 0.182 ω·cm² and that of the monolayer composite cathode was 0.323 ω·cm². The power density of the triple-layer graded composite cathode was the highest and that of the monolayer composite cathode was the lowest. The triple-layer graded composite cathode had superior performance.     

Transport and ultrasonic properties in La0.67Sr0.33Mn0.87Fe0.13O3 perovskite

The longitudinal and transverse ultrasonic sound velocity and attenuation, as well as electric resistance have been carefully measured in single-phase polycrystalline giant magnetoresistance perovskite La_0.67Sr_0.3Mn_0.87Fe_0.13O₃ at a frequency of 10 MHz, from 20 to 300 K. A big electric resistance peak was observed at 85 K (T_C). At the temperature above T_C, the resistivity can be fitted well by Mott’s law ρ=exp (T₀/T)^1/4 and both the longitudinal and transverse sound velocities show a lattice softening, which was accompanied by an attenuation peak. This simultaneous occurrence of electron and lattice softening implies electron–phonon coupling, known to exist for the octahedrally coordinated d⁴ ion, originating in the Jahn–Teller distortion. Below 55 K, pronounced sound-velocity softening for both longitudinal and transverse waves was observed; this may correspond to the formation of a spin-glass state.     

Effect of cold work on anisotropic magnetoresistance of Co25Ni75

Various states of the polycrystalline Co₂₅Ni₇₅ sample have been achieved by cold rolling and heat treatments after cold rolling. At each metallurgical stage, the electrical resistivity

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, measured in the saturated condition with a longitudinal field, and the resistivity

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, obtained under a similar condition with a transverse field, were studied at 77K. We used the quantity

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, defined as

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, where

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is the resistivity in zero field (or a demagnetized state), to check the magnetic anisotropy in the sample. It is found that cold work can induce an easy axis parallel to the roll direction. This phenomenon is associated with the short-range order and/or the directional Co–Co pair order formed during a cold-rolling process. Recovery and recrystallization were observed, if the rolled samples were annealed at various temperatures T_Q. Both stress relief and change of texturing lead to an enhancement of magnetic anisotropy of the longitudinal type. The anisotropic magnetoresistance

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of the cold-rolled sample (CR-0) is the lowest. When T_Q is increased,

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becomes recovered. However,

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of a plainly salt water quenched sample is always larger than that of a cold-rolled one.     

Size effects and interactions in La0.7Ca0.3MnO3 nanoparticles

La_0.7Ca_0.3MnO₃ (LCMO) nanoparticles with average diameter of 16-73 nm were prepared by reactive milling and thermal processing methods. Interaction and size effects on the magnetic properties of the LCMO nanoparticle samples were investigated. Phenomena related to the interparticle interaction, such as an un-overlapping of the M(H_ext,T)/M_S vs. H_ext/T scaling plots and a Curie-Weiss rather than Curie law behavior of the dc susceptibility at high temperatures were analyzed. The magnetization curves of interacting nanoparticles were well described by using the mean-field approximation. The dependence of the blocking temperature T_B on the strength of the interactions, magnetic anisotropy, as well as the thermal dependence of magnetization deviates from the expected Bloch law was also estimated.     

Coexistence of superconductivity and ferromagnetism in La1.85Sr0.15CuO4-La2/3Sr1/3MnO3 matrix composites

In order to clarify the interaction between superconductivity and magnetism, a series of (La_1.85Sr_0.15CuO₄)_1−x (La_2/3Sr_1/3MnO₃)_x matrix composites (x = 0-0.2, mole fraction) was successfully prepared by solid-state reaction method. Based on the electrical transport measurements, it is found that the superconductivity is gradually suppressed as increasing the content of La_2/3Sr_1/3MnO₃ (LSMO) manganites and that the superconductivity still exists in the composites even though plenty of LSMO is introduced into La_1.85Sr_0.15CuO₄ (LSCO) superconducting cuprate. At the same time, the results of the magnetic measurements also demonstrate the coexistence between superconductivity and ferromagnetism when the CuO₂ planes are intact as follows from the results of X-ray diffraction (XRD). In the whole, the present experiments show that the ferromagnetism in the microscale does not destroy superconductivity for LSCO cuprate in this kind of the matrix composites, and the intercalation of LSMO may lead to an electronic phase separation in LSCO with the hole rich and/or hole poor regions.     

Investigation of structure and magnetization reversal in mechanically alloyed SmCo6.8Zr0.2 magnets

Nanocrystalline SmCo_6.8Zr_0.2 permanent magnets are prepared by intensive milling and subsequent annealing. XRD patterns of the samples annealed at 600 °C and 700 °C show a single SmCo₇ phase with TbCu₇ structure, while the SmCo₇ phase decomposes into a mixture of Sm₂Co₁₇ and Co₂₃Zr₆ phase at higher annealing temperatures. The highest coercivity and energy product of about 2.06 T and 9.5MGOe are obtained in the sample annealed at 700 °C for 10 min. Further analysis reveals that a very strong inter-grain exchange coupling is observed in samples annealed at 700 °C for 3 (SZ-3) and 10 (SZ-10) min, indicating by the high remanence ratio and positive δm in henkel-plot. It was supposed that the magnetization reversal process is controlled by domain wall pinning.     

In situ dynamical control of the strain and magnetoresistance of La0.7Ca0.15Sr0.15MnO3 thin films using the magnetostriction of Terfenol-D alloy

We fabricated a magnetoelectric laminate structure consisting of a magnetostrictive Tb_0.3Dy_0.7Fe_1.92 (Terfenol-D) plate bonded to a La_0.7Ca_0.15Sr_0.15MnO₃ (LCSMO)/0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) structure where a LCSMO film was epitaxially grown on a PMN-PT single crystal substrate. When a dc magnetic field is applied perpendicular to the film plane, the magnetoresistance of the LCSMO film in the paramagnetic (ferromagnetic) state for the LCSMO/PMN-PT/Terfenol-D structure is larger (smaller) than that for the LCSMO/PMN-PT structure without Terfenol-D. These effects are caused by the magnetostriction-induced in-plane compressive strain in the Terfenol-D, which are transferred to the PMN-PT substrate, leading to a reduction in the in-plane tensile strain of the epitaxial LCSMO film and thereby modifying the magnetoresistance of the film.     

Magnetic and neutron diffraction studies on potassium uranate KUO3

The magnetic properties of KUO₃ with the cubic perovskite structure have been studied. It shows an antiferromagnetic-type of transition at ca. 17 K in the magnetic susceptibility vs. temperature curve. Magnetic hysteresis is observed at 4.5 K, but not observed at 20 K. Neutron diffraction measurements have been performed on the powdered KUO₃ below and above the transition temperature. The neutron diffraction pattern measured at 10 K shows no appreciable difference from that measured at 50 K. A small ordered magnetic moment of U⁵⁺ ion will make it very difficult to observe magnetic diffractions, even if a magnetic ordering occurs. If the magnetic spin alignment occurs in a small domain, no magnetic diffraction will be found in the neutron experiments. On the other hand, the magnetic phenomena of KUO₃ have been analyzed on the basis of an octahedral crystal field model.     

Lattice effect on the magnetic and magneto-transport properties of (La1/3Sm2/3)0.67Ba0.33−xSrxMnO3 (x = 0.0, 0.1, 0.2 and 0.33) compounds

The effect of substituting Sr for Ba on the magneto-transport and magnetic properties of (La_1/3Sm_2/3)_0.67Ba_0.33MnO₃ system, has been investigated. The samples, (La_1/3Sm_2/3)_0.67Ba_0.33−xSr_xMnO₃ (x = 0.0, 0.1, 0.2 and 0.33), synthesized by citrate gel route, crystallize in an orthorhombic structure (space group Pnma, no. 62). The unit cell volume decreases while the metal-insulator transition temperature (T_MI) increases with increasing Sr content. The localization of charge carriers occurs at low temperatures and becomes more pronounced with decreasing Sr content which leads to an enhancement of resistivity. This could be understood by the variation of MnOMn bond-distance and angle. Reappearance of semiconducting behavior (dρ/dT < 0) is observed only in samples with x = 0 and x = 0.1 below certain temperature (T < T_MI). These samples exhibit thermal irreversibility behavior for a field-cooled (FC) and zero-field-cooled (ZFC) magnetization data in a magnetic field of 100 Oe. This is ascribed to the competition between the superexchange and double exchange interactions. The change in physical properties has been correlated to chemical parameters such as ionic radii, tolerance factor, electronegativity and variation in MnOMn angle.