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 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.     

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.     

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.     

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.     

Synthesis of Nd0.7Sr0.3MnO3 ceramics by high-energy ball milling

Strontium doped perovskite-type manganite Nd_0.7Sr_0.3MnO₃ is synthesized by high-energy ball milling. Nd_0.7Sr_0.3MnO₃ single phase with perovskite structure is formed completely after milling 4 h and the perovskite phase decreases gradually and changes into amorphous state with increasing ball milling time up to 12 h. The interesting point in this work is that the Nd_0.7Sr_0.3MnO₃ perovskite can be formed again from the amorphous with subsequent 6 h ball milling. In addition, the re-obtained perovskite seems to be more stable than Nd_0.7Sr_0.3MnO₃ compound prepared using solid-state reaction method.     

Giant negative magnetoresistance in the vicinity of the ferromagnetic phase transition TC in Sm0.55Sr0.45MnO3

The temperature dependence of the resistance in Sm_0.55Sr_0.45MnO₃ at zero magnetic field and its first derivative served for the determination of T_C which turned out to be equal to 129.1 K. Then, the resistance measurements were carried out as functions of the stationary magnetic fields up to 14 T at the following temperatures: 118.6, 124.6, 133.6 and 139.7 K. Two of them are below and two of them are above T_C. It turned out that at all the four temperatures the resistance reveals a hysteresis. It turned also out that the closer the temperature to T_C the greater this hysteresis. At all the four temperatures the magnetoresistance is negative, its absolute value depends on the magnetic field and reaches 100% at 12 T. At lower fields the absolute value of the negative magnetoresistance increases monotonically with the decrease of temperature starting from 139.7 K. On the other hand, a maximum observed on all the primary and secondary curves above T_C moves with the decrease of temperature towards the lower magnetic fields and disappears on all the curves below T_C. On the secondary curves above T_C this maximum is flat.     

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.     

Magnetoelectric properties of Fe-Ga/BaTiO3 laminate composites

Magnetostrictive and piezoelectric laminate composites of Fe-Ga and BaTiO₃ have been studied. The magnetoelectric (ME) coefficients have been characterized for the transversely magnetized and transversely polarized transverse-transverse (TT) mode. At lower frequencies, the ME voltage coefficient of the laminate was 12.5 mV/Oe. Near the natural resonant frequency (∼95 kHz) of the laminate, the ME voltage coefficient was found to be dramatically increased to 28.5 mV/Oe. In addition, the induced ME voltages were near linear functions of AC magnetic field.     

La1-xKxMnO3的微波吸收特性

用溶胶-凝胶法制备La1-xKxMnO3粉晶,用X射线衍射仪和扫描电镜表征样品的晶体结构和微观形貌,用微波矢量网络分析仪测试了该样品在2~18 GHz微波频率范围的复介电常数和复磁导率,并计算损耗角正切及微波反射率,分析K掺杂量和样品厚度对体系微波吸收性能的影响及微波损耗机制。结果表明:晶体结构为钙钛矿型,颗粒形貌为不规则椭球状或短棒状;当样品厚度为2.40 mm、x=0.3时,吸收峰值为27.1 dB,10 dB以上有效吸收频带宽度达10.6 GHz。纳米La1-xKxMnO3兼具介电损耗和磁损耗,介电损耗相对较强。磁损耗因子和介电损耗因子随微波频率的变化相反,是基体中铁磁与反铁磁团簇在微波电磁场作用下相互转变引起。     

Elastic behavior of La0.67Ca0.33MnO3:ZrO2 composites

A series of Colossal Magneto Resistance materials, with compositional formula (1 − x) La_0.67Ca_0.33MnO₃ + xZrO₂ (where x = 0%, 10%, 20%, 40%, 60%, 80%) were prepared by sol–gel technique. When characterized structurally by X-ray diffraction they are found to have cubic structure. After measuring their bulk densities, the ultra sonic longitudinal (V_l) and shear velocities (V_s) were measured at room temperature using the pulse transmission technique. Using the ultrasonic data, the values of Young's and rigidity moduli along with Poisson's ratio and Debye temperatures have been calculated. As the materials are porous, zero porous elastic moduli have also been arrived at using a well-known model. The observed variation of elastic moduli with varying ZrO₂ concentration has been explained qualitatively.     

Magnetic properties of Pb2Sr2PrCu3O8

The magnetic properties of Pb₂Sr₂PrCu₃O₈ were determined using X-ray absorption, inelastic neutron spectroscopy and magnetic susceptibility measurements. X-ray absorption on the Pr L₃-edge strongly indicates a trivalent oxidation state. Inelastic neutron scattering results are modeled assuming a ³H₄ ground-state multiplet split by a crystalline electric field potential similar to PrBa₂Cu₃O_x. This potential correctly predicts the value of the Curie–Weiss moment, 2.72 μB, obtained from magnetic susceptibility measurements, and so explains the reduction from the free-ion value. The very broad magnetic response found in the inelastic neutron scattering experiments indicates a strong interaction between the Pr 4f electrons and the CuO₂ bands.     

La2/3Ca1/3MnO3∶Ag0.4与La2/3Ba1/3MnO3∶Ag0.4的制备及其性能研究

采用化学共沉淀法合成制备了Ag掺杂摩尔百分比为0.4的La2/3Ca1/3MnO3∶Ag0.4 (LCMO∶Ag0.4)、La2/3Ba1/3MnO3∶Ag0.4(LBMO∶Ag0.4)多晶复合材料,通过XRD和R-T分别对两种材料的结构和性能进行测试分析.实验结果表明:所得样品均为正交晶系钙钛矿结构;LCMO∶Ag0.4的金属-绝缘转变温度(Tp=280.2 K)比LBMO∶Ag0.4要高(Tp=256.8 K);LCMO:Ag0.4样品的电阻值比LBMO∶Ag0.4要小两个数量级;LCMO∶Ag0.4的TCR最大值为28％,要远远高于LBMO∶Ag0.4的TCR最大值(0.5％).     

Effect of load resistance on magnetoelectric properties in FeGa/BaTiO3/FeGa laminate composites

The electrical resistance load effects on an ME laminated composite of FeGa/BaTiO₃/FeGa are investigated. The results show that (i) The ME voltage coefficient increases and tends to be saturated with the increase in electrical resistance load, (ii) the cutoff frequency and the resonance frequency both shift to the lower frequency side and approach to the values under open-circuit condition with the increase in electrical resistance load, and (iii) the ME output power can be adjustable by changing the attached electrical resistance load to achieve the best active status. The present study provides the basis for the design of ME sensors and their electronic circuits.     

Effect of potassium doping on the structural, magnetic and magnetocaloric properties of La0.7Sr0.3−xKxMnO3 perovskite manganites

We investigate the effect of potassium doping on the structural, magnetic and magnetocaloric properties of La_0.7Sr_0.3−xK_xMnO₃ (x = 0.05, 0.1, 0.15 and 0.2) powder samples. Our polycrystalline compounds were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted rhombohedral system with space group. With increasing potassium content, the unit cell volume exhibits a broad maximum around x = 0.15. Magnetization measurements versus temperature showed that all our samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to decrease from 365 K for x = 0 to 328 K for x = 0.2 as well as the saturated magnetization M_sp which shifts from 3.68 μ_B/Mn for x = 0 to 3.05 μ_B/Mn for x = 0.2. The critical exponent γ defined as M_sp (T) = M_sp(0)[1−(T/T_C)]^γ is found to remain almost constant and equal to 0.33 for all our samples. The maximum of magnetic entropy changes |ΔS_max| of La_0.7Sr_0.3−xK_xMnO₃ for x = 0.05 and 0.15 is found to be respectively, 1.37 and 1.2 J kg⁻¹ K⁻¹ under a magnetic field change of 1 T.     

Mechanochemical effects on microstructure and transport properties of nanocrystalline La0.8Na0.2MnO3 ceramics

Mechanochemical effects on microstructure and transport properties of the sol–gel prepared nanocrystalline La_0.8Na_0.2MnO₃ ceramics have been studied. It was found that with the prolonging of the milling time the crystalline size as well as the particle size obviously decreased while the resistivity increased. The strain of the milled samples was calculated using the Williamson–Hall plot, which increased with the increase of the milling time. The surface layer with large defects and the strain led to the decrease of the insulator-metal transition temperature, but the magnetoresistance value within the whole measured temperature range of 80–300 K was remarkably improved, which indicated that it was an effective route to enhance the magnetoresistance value both at low temperature and at high temperature by mechanochemical processing.     

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.     

On the synthesis and characterization of a new manganite type CMR material: La0.7Hg0.3MnO3

A new manganite type CMR material, La_0.7Hg_0.3MnO₃ has been successfully synthesized and has been found to exhibit magnetoresistance (≈9%) at low fields (≈1.5 kG). The synthesis has been carried out through a solid state reaction route consisting of the formation of La_0.7MnO₃ followed by diffusion of Hg leading to La_0.7Hg_0.3MnO₃. The as grown samples are polycrystalline and correspond to an orthorhombic unit cell with the lattice parameters; a=5.5183 Å, b=5.6383 Å and c=7.5368 Å. The typical grain size as revealed by scanning electron microscopy is in the range of 0.5–2 μm. The ρ–T behaviour shows a peak at T_IM=227 K. The ρ–T behaviour above this temperature corresponds to that of an insulator and below this to that of a metal. The ρ–T behaviour remains unaltered when a magnetic field (H_dc=1.5 kG) is applied. However, with this magnetic field a drop in the resistivity is observed up to 77 K. At room temperature the magnetoresistance ratio (MRR) is too small but it steadily increases as the temperature is decreased. Thus, MRRs at 227.13 and 77 K are 3.41 and 9.05%, respectively, in an applied field of H_dc=1.5 kG. At a given temperature the variation in MRR with field H_dc is rapid at lower field values (H_dc<1.2 kG) and scales linearly for higher field values (H_dc>1.2 kG). It may be mentioned that the present work on the synthesis and magnetoresistance behaviour of La_0.7Hg_0.3MnO₃, is the first of its type.     

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...