Effects of ferrites on magnetic transport properties in La2/3Sr1/3MnO3/ferrites composites

Effects of soft-magnetic MnZn ferrite (Mn0.5Zn0.5Fe2O4, MZF) and hard-magnetic Ba ferrite (BaO·6Fe2O3, BaM) on the structure and magnetic transport properties of [La2/3Sr1/3MnO3] (LSMO)/(x) [ferrites] (ferrites=MZF, BaM) composites have been investigated. It was found that the inclusion of MZF phase reduces magnetization and ferromagnetic-paramagnetic transition temperature (Tc) of the composites. With increasing the content of the dopants, the high-temperature magnetoresistance (MR) decreases, whereas low-...     

Preparation of submicron LaCaMnO submicron powder and LaCaMnO–alumina composites with CMR effects by a new method: Precursor sintering technique

We have successfully synthesized submicron LaCaMnO powder (with a nominal composition of La_0.67Ca_0.33MnO₃) and the LaCaMnO–Al₂O₃ composite powder by a newly invented precursor sintering technique. It is found that the precursor solution containing polyacrylamide and citric acid can facilitate the formation of LaCaMnO powders at a relatively lower sintering temperature because PAM and citric acid form a polymer network in the solution and the metal ions evenly distribute in the precursor solution. The critical sintering temperature was carefully studied based on X-ray diffraction patterns and scanning electron microscopy images. It turns out that a low-temperature sintering results in nano-sized powders with a particle size of 50–100 nm, but a high-temperature sintering leads to larger clusters of 1–3 μm. For the LaCaMnO–alumina composites system, secondary phases appear if the sintering temperature is relatively high (1200 °C). Magnetoresistance of the LaCaMnO wafer made from the obtained LaCaMnO powder has a maximum of 56.7% at 269 K. More appealing is that LaCaMnO–alumina composite powders have a very high MR, 82.5%, even in the same order of the MR of the epitaxial-grown LaCaMnO thin film. Besides, the advantages of the PST method include simple equipment, common chemical compounds and low-cost.     

应用于伺服液压缸的磁阻式位移传感器研究

介绍了基于磁阻式位移传感器的特点、工作原理及传感芯片的结构,并把该传感器应用到直驱式电液伺服液压缸上.研究结果表明,磁阻式位移传感器具有很强的抗干扰和环境适应能力.     

Effect of heat treatment condition on magnetic,electrical transport and magnetoresistance properties of La0.67Sr0.33MnO3 manganite coatings

Effects of three different heat treatment conditions (air, vacuum and nitrogen) on the structure, magnetic, electrical transport and low-field magnetoresistance behaviors of La_0.67Sr_0.33MnO₃ manganite coatings synthesized by sol-gel and screen printing methods have been systematically investigated. The detailed crystallographic analyses revealed that the diffraction peak positions of the samples subjected to annealing in vacuum and nitrogen ambients shifted to lower-angle regions, and lattice parameters and structure distortion increased as compared with those of the sample annealed in air, which were due to the introduction of oxygen deficiency under the annealing conditions of vacuum and nitrogen. The content of oxygen deficiency of the samples could be determined by the change of lattice parameters. Furthermore, the vacuum and nitrogen annealed samples exhibited a dramatic increase in the resistivity, accompanied by the reduction in ferromagnetism, the resistivity peak temperature and the ferromagnetism transition temperature. These phenomena were attributed to the decrease of average manganese oxidation state, as a result the mobile charge carriers were reduced, the ferromagnetic coupling and the double exchange effect between manganese ions decreased while the magnetic inhomogeneity increased. In the meanwhile, magnetoresistance ratios of the oxygen-deficient samples were enhanced around the percolation threshold.     

Structure,magnetic, electrical transport and magnetoresistance properties of La0.67Sr0.33Mn1−xFexO3 (x=0–0.15) doped manganite coatings

We have undertaken a systematic study of the effect of Fe³⁺ doping on the crystal structure, magnetic, electrical transport and magnetoresistance properties of La_0.67Sr_0.33Mn_1−xFe_xO₃ (0≤x≤0.15) polycrystalline coatings prepared by the sol-gel and screen printing method. The X-ray powder diffraction and Reitveld refinement results indicate that the partial substitution of Mn³⁺ ions by Fe³⁺ ions does not introduce noticeable lattice distortion and structural transition. Magnetic measurements show that Fe³⁺ ions significantly lowers the Curie temperature, and the magnetization at low temperatures first increases and then decreases with further Fe³⁺ doping. The Fe³⁺ doped manganites (x>0.05) are semiconductors with high resistivity, as the consequence of low number of available hopping sites of charge carriers. The most stimulating result obtained is that the magnetoresistance property is greatly enhanced around the percolation threshold (x=0.05) of Fe³⁺ ions in this system and the percolative phase separation is responsible for the anomalous behavior.     

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.     

Insight into the magnetism of a distorted Kagome lattice,Dy3Ru4Al12, based on polycrystalline studies

The layered compound with distorted Kagome nets, Dy₃Ru₄Al₁₂, was previously reported to undergo antiferromagnetic ordering below (T_N=) 7 K, based on investigations on single crystals. Here, we report the results of our investigation of AC and DC magnetic susceptibility (χ), isothermal remnant magnetization (M_IRM), heat-capacity, magnetocaloric effect and magnetoresistance measurements on polycrystals. The present results reveal that there is an additional magnetic anomaly around 20 K, as though the Néel order is preceded by the formation of ferromagnetic clusters. We attribute this feature to geometric frustration of magnetism. In view of the existence of this phase, the interpretation of the linear-term in the heat-capacity in terms of spin-fluctuations from the Ru 4d band needs to be revisited. Additionally, in the vicinity of T_N, AC χ shows a prominent frequency dependence and, below T_N, M_IRM exhibits a slow decay with time. This raises a question whether the antiferromagnetic structure in this compound is characterized by spin-glass-like dynamics. In contrast to what was reported earlier, there is a change in the sign of the magnetoresistance (MR) at the metamagnetic transition. A butter-fly-shaped (isothermal) MR loop (interestingly spanning over all the four quadrants) is observed at 2 K with distinct evidence for the magnetic phase co-existence phenomenon in zero field after travelling through metamagnetic transition field. The results on polycrystals thus provide additional information about the magnetism of this compound, revealing that the magnetism of this compound is more complex than what is believed, due to geometric frustration intrinsic to Kagome net.     

Spin injection and transport in organic spin-valves based on fullerene C60

We report the fabrication and magnetoresistance (MR) of the La_0.67Sr_0.33MnO₃/C₆₀/Co spin valves. The introduction of 1.5 nm AlO_x barrier between the C₆₀ layer and cobalt top electrode prevents effectively interfacial diffusion and Co penetration, and thus an appreciable positive MR (as large as 3.65%) at room temperature was exhibited for the devices in the thickness range (5–40 nm) of C₆₀ studied. Possible mechanisms on the MR polarity are proposed. Furthermore, based on the temperature- and thickness- dependent MR and I–V characteristics, we have obtained clear evidences that the MR of C₆₀-based spin-valves originates from the tunneling of spin-polarized electrons at the low thickness of C₆₀, however at the larger thickness (>20 nm) the electrons are injected into and subsequently hopping transport within the C₆₀ spacer.