Electrochemical synthesis of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles in alkaline aqueous solutions containing complexing agents

Ultrafine magnetite particles are prepared through an electrochemical process, at room temperature, from an iron-based electrode immersed in an alkaline aqueous medium containing complexing compounds. XRD and chemical analysis indicate that the product is pure magnetite, Fe₃O₄. The size and morphology of the particles are studied by SEM. The magnetite nanoparticles present a magnetoresistance of almost 3%, at 300 K, under a magnetic field of 1 T. A reactive mechanism for the electrochemical process is proposed.     

钙钛矿型稀土锰氧化物La0.7Sr0.3FexMn1—xO3的磁性和巨磁电阻效应

采用溶胶-凝胶工艺制备了La     

Crystal structure and a giant magnetoresistance effect in the new Zintl compound Eu3Ga2P4

Single-crystalline samples of a new Zintl compound, Eu(3)Ga(2)P(4), have been synthesized by a Ga-flux method. Eu(3)Ga(2)P(4) is found to crystallize in a monoclinic unit cell, space group C2/c, isostructural to Ca(3)Al(2)As(4). The structure is composed of a pair of edge-shared GaP(4) tetrahedra, which link by corner-sharing to form Ga(2)P(4) two-dimensional layers, separated by Eu(2+) ions. Magnetic susceptibility showed a Curie-Weiss behavior with an effective magnetic moment consistent with the value for Eu(2+) magnetic ions. Below 15 K, ferromagnetic ordering was observed and the saturation magnetic moment was 6.6 μ(B). Electrical resistivity measurements on a single crystal showed semiconducting behavior. Resistivity in the temperature range between 280 and 300 K was fit by an activation model with an energy gap of 0.552(2) eV. The temperature dependence of the resistivity is better described by the variable-range-hopping model for a three-dimensional conductivity, suggesting that Eu-P bonds are involved in the conductivity. A large magnetoresistance, up to -30%, is observed with a magnetic field H = 2 T at T = 100 K, suggesting strong coupling of carriers with the Eu(2+) magnetic moment.     

Superconductivity and magnetism in LuRh1.2Sn4.0

The electrical resistivity, magnetoresistance and upper critical field of single-crystal LuRh_1.2Sn_4.0 have been studied. A superconducting transition temperature T_c = 4.1K was observed and the critical-field slope near T_c was found to be 17kOeK⁻¹. The most remarkable observation was a logarithmic decrease in the resistivity with increasing temperature and a minimum at 210 K as in the case of dense Kondo materials. Our low temperature resistivity and magnetoresistance data suggested a weak itinerant antiferromagnetic ordering below 5.2 K.     

微观结构对溶胶-凝胶法合成的La0.67Sr0.33MnO3电磁性能影响

采用溶胶-凝胶法制备了不同烧结温度的钙钛矿类锰氧化物La0.67Sr0.33MnO3样品。实验结果表明,在1573 K以上烧结的样品,晶粒出现异常长大,晶界效应明显。随着烧结温度的提高,磁化强度逐渐增大,但样品的居里温度基本不变。此外,在1173和1573 K温度下烧结的样品,均出现了低于居里温度的金属-半导体导电行为转变。在合适的烧结条件下,可以观察到隧道磁电阻(TMR)和超大磁电阻(CMR)2种磁电阻效应。实验表明,自旋电子的输运,不仅与样品平均粒径的大小和密度有关,而且与晶界的微观结构有密切关系。     

d-Electron-induced negative magnetoresistance of a pi-d interaction system based on a brominated-TTF donor

A new pi-d interaction system (EDT-TTFBr2)2FeBr4 (EDT-TTFBr2 = 4,5-dibromo-4',5'-ethylenedithiotetrathiafulvalene) and its nonmagnetic anion analogue (EDT-TTFBr2)2GaBr4 based on a brominated TTF-type organic donor are investigated. The salts featured by quasi-1D pi-electronic systems are metallic with metal-insulator transitions taking place at about 20 and 70 K for the FeBr4- and GaBr4- salts, respectively, where the low-temperature insulating state is associated with charge ordering or a Mott insulator followed by an antiferromagnetic transition at lower temperatures. The FeBr4- salt is featured with an antiferromagnetic transition of the anion d spins at a Neel temperature (TN) = 11 K, which is significantly high despite its long anion-anion Br-Br contact, suggesting the importance of the pi-d interaction in the magnetism. The surprisingly strong pi-d interaction, ca. -22.3 K estimated from the magnetization curve, evidences the usefulness of the chemical modification of the donor molecule with bromine substitution to achieve strong intermolecular interaction. The antiferromagnetic state of the anion d spins affects the transport of the conducting pi electrons through the strong pi-d interaction, as evidenced by the presence of a resistivity anomaly of the FeBr4- salt at TN. Below TN, the FeBr4- salt shows negative magnetoresistance that reaches -23% at the highest magnetic field investigated (B=15 T), whereas only a small positive magnetoresistance is observed in the pi-electron-only GaBr4- salt. The mechanism of the negative magnetoresistance is explained by the stabilization of the insulating state of the pi electrons by the periodic magnetic potential of the anion d spins in the FeBr4- salt, which is modified by applying the external magnetic field.     

(La0.67Ca0.33MnO3)x/(ZrO2)1-x复合体系的磁性和传输特性

将用溶胶-凝胶法得到的La0.67Ca0.33MnO3(LCMO)微粉与ZrO2的颗粒进行复合,制备了(LCMO)x/(ZrO2)1-x渗流复合体系.当LCMO的体积分数为40%时,复合体系达到渗流阈值,此时材料在低温下的磁电阻得到显著增强.77 K时在10 mT的磁场下,(LCMO)0.4/(ZrO2)0.6的磁电阻比为7.8%,相对于LCMO增加了712%.低场磁电阻(LFMR)的增强是由于载流子在二者界面处发生的自旋相关隧穿效应.由于界面反应,不可避免地产生了Zr离子对Mn离子的B位替代,从而引起材料磁性M和居里温度Tc的下降.在x＜60%时,Tc保持在220 K附近基本不变,说明该B位替代是有限的.     

Ni-Fe催化剂乙醇部分氧化制氢的研究

研究了Ni Fe催化剂对乙醇部分氧化制氢反应,系统地考察了不同O2/C2H5OH摩尔比及反应温度下催化剂的性能.发现Ni Fe催化剂对乙醇部分氧化制氢具有较好的催化活性,其中组成为Ni50Fe50催化剂最好,最佳的反应条件是O2/C2H5OH=1.0,T=573 K.XRD谱图表明催化剂主要由尖晶石结构的铁酸盐和FeNi3合金相组成. XPS结果说明,催化剂体相以还原态FeNi3合金相为主,表面以氧化态的铁酸盐为主.稳定性考察的结果表明,催化剂经40 h反应后,对氢的选择性明显下降,此时对应的FeNi3物相衍射峰强度也明显降低,表明催化剂对H2选择性的下降与FeNi3物相的转变有关.     

La1-xCaxMnO3外延薄膜的制备、结构及磁电阻效应

采用溶胶-凝胶旋涂法在单晶LaAlO3 (100)基片上生长了La1-xCaxMnO3(x=0, 0.05, 0.1, 0.2)外延薄膜. 利用X射线衍射仪(XRD)、 原子力显微镜(AFM)、 聚焦离子束系统(FIB)、 X射线光电子能谱分析仪(XPS)、 振动样品磁强计(VSM)和磁性测量系统(MPMS)对样品的结构、 形貌、 价态、 居里温度和磁电阻效应进行了研究. 结果表明, 薄膜为立方钙钛矿结构, 具有明显的(100)外延生长取向和平整的表面. 在居里温度附近, 样品发生铁磁-顺磁转变. 随着Ca2+掺杂量的增加, 样品的居里温度升高, La0.8Ca0.2MnO3的居里温度为264 K. 随温度的变化, 样品发生了金属-绝缘体转变. 样品还具有较大的磁电阻效应, 在H=2.0 T, T=210 K时, La0.95Ca0.05MnO3的磁电阻达到80.9%.     

高铁CaO-FeOx-SiO2三元体系氧化过程相变热力学分析

通过详细的热力学计算和推导, 对高铁CaO-FeOx-SiO2(CFS)体系(铁含量50%-60%, x=1-1.5)氧化过程中铁氧化物的价态和组分相赋存状态变化规律进行了分析. 结合SEM、EDX及XRD确定物相组成, 图像分析仪对相分布进行定量测量, 化学法分析铁组分变化情况. 研究了氧化过程中, 体系中相变的热力学规律以及磁铁矿相的析出特性, 对不同温度条件下磁铁矿相析出情况进行了详细讨论, 并简要推算了CaO的含量变化对体系相变的影响. 结果表明, 随熔体氧位的增加, 磁铁矿相逐渐形成并饱和以晶体析出, 铁组分会不断向磁铁矿相转移和富集. 体系冷却后主要由磁铁矿、铁橄榄石和钙铁硅酸盐固溶体组成, 氧化过程中, 铁橄榄石相减少, 磁铁矿相增加. 在1423 K以上温度, 控制氧平衡分压lg(pO2/p0)>-7.89时, 体系中的铁组分主要以磁铁矿形式存在, 并在冷却过程析出; 冷却过程中, 磁铁矿是初晶相, 体系中铁离子的摩尔比n(Fe3+)/n(Fe2+)为1/4 时, 磁铁矿初始析出的温度约为1640 K, 随n(Fe3+)/n(Fe2+)比值的增加, 磁铁矿析出温度升高, 在n(Fe3+)/n(Fe2+)为1.8/1 时, 磁铁矿初始析出的温度约为1720 K; 体系中氧化钙含量的增加, 可提高铁在磁铁矿相的富集程度.     

La1-xLixMnO3体系的输运特性及EPR研究

利用固相反应法合成了掺杂锂的La_1-xLi_xMnO₃(x=0,0.10,0.15,0.20,0.25,0.30)钙钛矿氧化物.XRD测试表明,所有样品均为菱方结构.除x=0,0.3外,其它样品均随温度的降低在液氮温区可观察到从绝缘态到金属态的转变,其中x=0.15样品的转变温度最高为167K.在H=1T的磁场下,出现了负磁阻现象.EPR谱上的g=2.00信号与Mn³⁺和Mn⁴⁺组成的复合团簇有关.     

Negative magnetoresistance in a magnetic semiconducting Zintl phase: Eu(3)In(2)P(4)

A new rare earth metal Zintl phase, Eu(3)In(2)P(4), was synthesized by utilizing a metal flux method. The compound crystallizes in the orthorhombic space group Pnnm with the cell parameters a = 16.097(3) A, b = 6.6992(13) A, c = 4.2712(9) A, and Z = 2 (T = 90(2) K, R1 = 0.0159, wR2 = 0.0418 for all data). It is isostructural to Sr(3)In(2)P(4). The structure consists of tetrahedral dimers, [In(2)P(2)P(4/2)](6-), that form a one-dimensional chain along the c axis. Three europium atoms interact via a Eu-Eu distance of 3.7401(6) A to form a straight line triplet. Single-crystal magnetic measurements show anisotropy at 30 K and a magnetic transition at 14.5 K. High-temperature data give a positive Weiss constant, which suggests ferromagnetism, while the shape of susceptibility curves (chi vs T) suggests antiferromagnetism. Heat capacity shows a magnetic transition at 14.5 K that is suppressed with field. This compound is a semiconductor according to the temperature-dependent resistivity measurements with a room-temperature resistivity of 0.005(1) Omega m and E(g) = 0.452(4) eV. It shows negative magnetoresistance below the magnetic ordering temperature. The maximum magnetoresistance (Deltarho/rho(H)) is 30% at 2 K with H = 5 T.     

Biocompatible and biodegradable polymer nanofibers displaying superparamagnetic properties.

Superparamagnetic polymer nanofibers intended for drug delivery and therapy are considered here. Magnetite (Fe3O4) nanoparticles in the diameter range of 5-10 nm were synthesized in aqueous solution. Polymer nanofibers containing magnetite nanoparticles were prepared from commercially available poly(hydroxyethyl methacrylate), PHEMA, and poly-L-lactide (PLLA) by the electrospinning technique. Nanofibers with diameters ranging from 50 to 300 nm were obtained. Nanofibers containing up to 35 wt % magnetite nanoparticles displayed superparamagnetism at room temperature. The blocking temperature was about 50 K for an applied field of 500 Oe, and the saturation magnetization was 3.5 emu g(-1) and 1.1 emu g(-1) for Fe3O4/PHEMA and Fe3O4/PLLA nanofibers, respectively, and depended on the amount of Fe3O4 nanoparticles in the nanocomposites. To test such magnetic nano-objects for applications as drug carriers and drug-release systems we incorporated a fluorescent albumin with dog fluorescein isothiocyanate (ADFI).     

Magnetic properties and negative colossal magnetoresistance of the rare earth Zintl phase EuIn2As2

Large, high quality single crystals of a new Zintl phase, EuIn(2)As(2), have been synthesized from a reactive indium flux. EuIn(2)As(2) is isostructural to the recently reported phase EuIn(2)P(2), and it is only the second reported member of the group of compounds with formula AM(2)X(2) (A = alkali, alkaline earth, or rare earth cation; M = transition or post-transition metal; and X = Group 14 or 15 element) that crystallizes in the hexagonal space group P6(3)/mmc (a = 4.2067(3) A, c = 17.889(2) A and Z = 2). The structure type contains layers of A(2+) cations separated by [M(2)X(2)](2-) layers along the crystallographic c-axis. Crystals of the title compound were mounted for magnetic measurements, with the crystallographic c-axis oriented either parallel or perpendicular to the direction of the applied field. The collective magnetization versus temperature and field data indicate two magnetic exchange interactions near 16 K, one involving Eu(2+)...Eu(2+) intralayer coupling and the other involving Eu(2+)...Eu(2+) coupling between layers. EuIn(2)As(2) is metallic and magnetoresistive, as is the isostructural phosphide, and both compounds have coincident resistivity and magnetic ordering transitions, consistent with the observation of colossal magnetoresistance. Negative colossal magnetoresistance (MR = {[rho(H) - rho(0)]/rho(H)} x 100%) of up to -143% (at T = 17.5 K, H = 5 T) is observed for EuIn(2)As(2), approximately half of that observed for the more resistive phosphide, which has a higher magnetic ordering temperature and local moment coupling strength.     

Anisotropic magnetoresistance and piezoresistivity in structured Fe3O4-silver particles in PDMS elastomers at room temperature

Magnetorheological elastomers, MREs, based on elastic organic matrices displaying anisotropic magnetoresistance and piezoresistivity at room temperature were prepared and characterized. These materials are dispersions of superparamagnetic magnetite forming cores of aggregated nanoparticles inside silver microparticles that are dispersed in an elastomeric polymer (poly(dimethylsiloxane), PDMS), curing the polymer in the presence of a uniform magnetic field. In this way, the elastic material becomes structured as the application of the field induces the formation of filaments of silver-covered inorganic material agglomerates (needles) aligned in the direction of the field (parallel to the field). Because the magnetic particles are covered with silver, the MREs are not only magnetic but also electrical conductors. The structuration induces elastic, magnetic, and electrical anisotropic properties. For example, with a low concentration of particles in the elastic matrix (5% w/w) it is possible to obtain resistances of a few ohms when measured parallel to the needles or several megaohms in the perpendicular direction. Magnetite nanoparticles (Fe(3)O(4) NP) were synthesized by the coprecipitation method, and then agglomerations of these NPs were covered with Ag. The average size of the obtained magnetite NPs was about 13 nm, and the magnetite-silver particles, referred to as Fe(3)O(4)@Ag, form micrometric aggregates (1.3 μm). Nanoparticles, microparticles, and the MREs were characterized by XRD, TEM, SEM, EDS, diffuse reflectance, voltammetry, VSM, and SQUID. At room temperature, the synthesized magnetite and Fe(3)O(4)@Ag particles are in a superparamagnetic state (T(B) = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Young's modulus of the MREs were measured as a function of the orientation using a texture analysis device. The magnetic anisotropy in the MRE composite was investigated by FMR. The electrical conductivity of the MRE (σ) increases exponentially when a pressure, P, is applied, and the magnitude of the change strongly depends on what direction P is exerted (anisotropic piezoresistivity). In addition, at a fixed pressure, σ increases exponentially in the presence of an external magnetic field (H) only when the field H is applied in the collinear direction with respect to the electrical flux, J. Excellent fits of the experimental data σ versus H and P were achieved using a model that considers the intergrain electron transport where an H-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The H-dependent barrier decreases with the applied field, which is attributed to the increasing match of spin-polarization in the silver covers between grains. The effect is anisotropic (i.e., the sensitivity of the magnetoresistive effect is dependent on the relative orientation between H and the current flow J). In the case of Fe(3)O(4)@ Ag, when H and J are parallel to the needles in the PDMS matrix, we obtain changes in σ up to 50% for fields of 400 mT and with resistances on the order of 1-10 Ω. Magnetoresistive and magnetoelastic properties make these materials very interesting for applications in flexible electronics, electronic skins, anisotropic pressure, and magnetic field sensors.     

La0.67Sr0.33FexMn1-xO3制备和奇特磁电阻效应

The Ln0.67Sr0.33Fe.Mn1-xO3(LSFMO)(0.05< x <0.33) systems have been synthesized by co-precipitation. The X-ray diffraction patterns indicates that these compounds are of single phase with rhombohedral structure. The resistivity of samples was measured in zero field and applied field (H=4000Oe) by a standard four-probe method. Large magnetoresistance (MR) was found in all samples. There is a resistivity transition peak in LSFMO (x ≦0.13) in which MR negative, the temperature (TP) at which the resistivity peak occurs decreases with the increasing of x. LSFMO (x=0.23) to which the large negative MR is still found is a semiconductor, It is unexpected that a peculiar alternative positive negative MR was found in LSFMO(x=0.33) which also is a semicondutor.     

Synthesis and Magnetic Properties of Manganese-Doped GaP Nanowires

We characterized the structure and magnetic properties of Mn-incorporated GaP nanowires synthesized by thermal evaporation of GaP/Mn powders. The nanowires consist of twin-crystalline zinc blende GaP grown with the [111] direction and doped with about 1 at. % Mn. They are often sheathed with the bumpy amorphous outerlayers containing high concentrations of Mn and O. The ferromagnetic hysteresis curves at 5 and 300 K and temperature-dependent magnetization provide evidence for the ferromagnetism with the Curie temperature higher than room temperature. Magnetic properties of individual nanowires have been measured, showing a large negative magnetoresistance equal to about -5% at 5 K. We suggest that the Mn doping of GaP nanowires would form a dilute magnetic semiconductor.     

Spin blockade in the conduction of colloidal CdSe nanocrystal films

The conduction of thin films of n-type CdSe colloidal quantum dots is studied at low temperature and under magnetic field. At medium and high magnetic fields (10 T), the films exhibit positive magnetoresistance consistent with the variable range hopping model. At low magnetic field(<0.3 T) but in the strong electric field regime, there is a narrower magnetoresistance of order 10%-15%. The magnetoresistance shows a strong bias dependence, small and positive at low bias, increasing but still positive at higher bias, and turning negative at the highest bias. A similar behavior has been reported recently for thin film organics. Weak localization effects are ruled out. The explanation for the observations is based on spin blockade relaxed by the hyperfine interaction. The weak magnetoresistance at low bias is attributed to the diffusing paths taken by the hopping electrons. At higher bias, the more directed motion of electrons leads to increasingly positive magnetoresistance due to the more effective spin blockade. At the highest bias, the magnetoresistance becomes negative, which is attributed to the increased exchange interaction associated with the shorter tunneling distance.     

La_(0.67)Sr_(0.33)Fe_xMn_(1-x)O_3制备和奇特磁电阻效应

系列陶瓷氧化物La0.67Sr0.33FexMn1-xO3（0.055x0.33）通过共沉淀法工艺合成获得.经X射线衍射分析表明，所得氧化物均为单相，属三方结构.在零场和外加磁场下的四极法电阻测试结果表明：所有样品都具有较大的磁电阻效应，当x<0.13时，样品的电阻率-温度曲线中均出现电阻率峰（电阻率出现从半导体性向金属性的转变），该电阻率峰处的温度（Tp）随x的增大而向低温区移动，磁电阻效应是负值;x＝0.23时，样品已呈半导体性，磁电阻效应仍是负值;然而x＝0.33时，样品虽也表现为半导体性，但其中的磁电阻效应出现奇特的正负交替转换.     

Electrodeposited magnetite with large magnetoresistive response at room temperature and low magnetic fields

Iron oxide layers were obtained by electrodeposition method on Cu substrates. The electrolyte containing Fe₂(SO₄)₃, NaOH, and Triethanolamine was stirred at a temperature of 65 °C and the depositions were performed potentiostatically in a conventional three electrode cell at a potential of −1.1 V vs SCE. Parallel magnetoresistance values of −6.2% at 3 kOe were obtained for measurements at room temperature. The magnetoresistive curves showed reduced hysteresis loop, temporal stability, and no saturation for the maximum applied magnetic field. Our results show that for the case of the granular magnetite that we grow, the AMR has opposite sign of that of 3d magnetic alloys. Allied to high values of resistivity, these properties are potentially adequate for the development of magnetic devices such as field sensors. Additional characterization was obtained by using scanning electron microscopy and vibrating sample magnetometry.