Magnetocaloric and magneto-transport properties of Gd10Co20Si70 alloy

We report the results of magnetic, thermodynamic, transport and magnetocaloric effect (MCE) studies of newly synthesized Gd_(10)Co_(20)Si_(70) alloy. These measurements confirm an antiferromagnetic transition at T_N=9 K. Both MCE and magnetoresistance (MR) show quadratic dependence on the applied magnetic field, indicating the presence of spin fluctuations in the alloy. The maximum values of the magnetic entropy change determined from the isothermal magnetization data for magnetic field change of 7 and9 T are found to be 10.5 and 15.6 J/kg·K, respectively. As a consequence of the spin fluctuations effect, the MCE peaks are pulled towards high temperature side as asymmetrically broadened peak. The MR attains a large positive value of 73%at 2 K in 8 T. The large MR and reversible MCE make this alloy an attractive multifunctional magnetic material.     

Thermal characterisation of a tungsten magnetoresistive heat switch

A magnetoresistive heat switch has been developed to improve the performance of our flight-worthy cryogen-free ADR. We have characterised the switch’s thermal conductivity in the temperature range 0.3-4 K under an applied magnetic field of 1.8 T for two tungsten samples of different purity. The results are discussed relating to the key aspects of semi-classical magnetoresistance theory. We show that crystal purity has a strong effect on switch performance and magnetoresistive effect. Our findings are verified by comparison to results obtained by other authors. The measured switching ratio for our best sample is 1.75 × 10⁴ at 1.5 K and 1.51 × 10⁴ at 4.26 K. The lattice conductivity remains dominated by the electronic conductivity in the investigated range of temperatures under an applied magnetic field of 1.8 T. In order for the lattice conductivity to dominate a purity of >99.999% would be required.     

Graphene Physics in Graphite

Single layers of carbon dubbed “graphene”, from which graphite is built, have attracted broad interest in the scientific community because of recent exciting experimental results. Graphene is interesting from a fundamental research perspective, as well as for potential technological applications. Here, we provide a brief overview of recent developments in this field, focusing especially on the electronic properties of graphite. Experimental evidence indicates that high‐quality graphite is a multi‐layer system with nearly decoupled 2D graphene planes. Based on experimental observations, we anticipate that thin graphite samples and not single layers will be the most promising candidates for graphene‐based electronics.     

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.     

炭材料中的组织

炭材料的基本结构单元是各向异性碳六角层面。基于择优取向的图型，炭材料中的纳米大小的组织（纳米组织）可发为沿面、轴向点的取向以及任意取向。从这种纳米组织的分类可以容易地了解到为什么炭材料能有如此多的形态，层片、纤维、球和不规则的粒子，以及为什么有如此宽广的性质。文中还简要地介绍了表征这些纳米组织的主要技术。     

Magnetic tunnel junctions with doubly-plasma oxidized AlOx insulation layer

Magnetic tunnel junctions (MTJ), with the tunnel barrier plasma oxidized in two steps, were fabricated in order to obtain structurally uniform AlO_x insulator. The doubly oxidized junctions exhibited the magnetoresistance (MR) ratio of 27-31% without showing any noticeable drop in the MR ratio even after oxidation time was extended well beyond the optimal oxidation time for the normal junctions. Transmission electron microscopy of the junctions confirmed that the AlO_x thickness was thinner for the doubly oxidized junctions compared to the singly oxidized MTJ. X-ray photoelectron spectroscopy of the doubly oxidized junction also strongly suggested that the initial oxide layer prevents the over-oxidation of the bottom electrode. The AlO_x tunnel barrier oxidized in two steps improved the junction performance and widened the processing window.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).     

Correlation among hydrogenation,magnetoelastic coupling,magnetic anisotropy,and magnetoresistance in magnetostrictive,hydrogen-absorbing palladium-cobalt alloy films for hydrogen sensing

Hydrogen-absorbing magnetic alloy films, such as palladium-cobalt (PdCo) alloy films, are expected to play a significant role in the next generation of hydrogen sensors. However, effects of hydrogenation on such films are very complex, since these alloys show strong spin-orbit interaction, i.e., strong magnetoelastic coupling. Accordingly, we conducted integrated research on the hydrogenation, magnetoelastic coupling, magnetism, and galvanomagnetic effect of PdCo alloy films having different magnetic anisotropies of longitudinal and perpendicular magnetic anisotropies. As a result, it was revealed that the stress in the film determines its magnetic anisotropy. The magnetoresistance curves of films, consisting of ordinal and anisotropic magnetoresistance effects, correspond well to the magnetization-magnetic field curves. Hydrogenation results in the compressive stress and decreased magnetostriction, which both have a negative influence on the perpendicular magnetic anisotropy energy of the films. Moreover, the influence is observed also in ordinal and anisotropic magnetoresistances. In addition, the increases in coercivity and electronic resistivity due to the incorporated hydrogen atoms (and related defects) are detected. The results are summarized in a correlation diagram, which shows that hydrogen-absorbing magnetic alloy films are very suitable for use in hydrogen sensors—the films can detect hydrogen via various methods such as magnetic anisotropy, galvanomagnetic effect, coercivity, and resistivity.     

Physical properties of microwave and solid state synthesized La0.7Na0.3MnO3

We report the structural, magnetic, electrical and broadband microwave absorption in La_0.7Na_0.3MnO₃ sample synthesized by microwave (MW) irradiation (Na_0.3LMO_MW) and compare them to the sample synthesized by solid-state (SS) reaction method (Na_0.3LMO_SS). Single phase Na_0.3LMO_MW was synthesized at 800 °C in 30 min, whereas, Na_0.3LMO_SS sample was obtained by sintering at 1200 °C for 48 h. Although both these samples show ferromagnetic transition at T_C ～324.8 K, the MW-synthesized sample shows distinct physical properties: broad ferromagnetic transition, smaller saturation magnetization, a large difference between the magnetic ordering and metal-insulator transition temperatures, a large high-field magnetoresistance, a table top-like magnetocaloric effect, and a large low-field microwave absorption compared to the solid state synthesized sample. These differences are suggested to arise from magnetic heterogeneity induced by smaller grain size and surface spin disorder in the MW synthesized La_0.7Na_0.3MnO₃.     

薄膜厚度对NiO/NiFe/Cu/NiFe自旋阀多层膜的磁阻效应的影响

用射频磁控溅射方法制备多层膜,研究了双层膜NiO／NiFe的矫顽力Hc和交换耦合场Hex与反铁磁层NiO、铁磁层NiFe厚度的关系。结果表明：NiO厚度为70nm时,Hex最大;Hc随NiO厚度增大而增大。当NiFe厚度增加时,Hex近似线性减小;而矫顽力则随NiFe厚度增大开始有缓慢增加,然后才减小。对于NiO（70nm）／NiFe（t1nm）／Cu（2．2nm）／NiFe（t2nm）自旋阀多层膜材料,研究了NiFe膜厚度对磁阻效应的影响。结果表明：被钉扎层NiFe的厚度为3nm时,自由层NiFe的厚度为5nm时,MR值分别最大,约为1．6％。