非线性响应对自旋阀磁电阻的影响

从非线性Kubo公式出发,考虑电子自旋相关体散射, 研究了自旋阀结构磁电阻效应.发现非线性响应在不同程度上影响巨磁阻效应.在零温附近,温度参数对磁电阻影响较小,而外加偏压对磁电阻的影响相对较大.     

Towards an Ab Initio Description of the Exchange Bias in Spin-Valve Systems

By considering collinear and non-collinear magnetic configurations, the interlayer exchange energy can be viewed as a continuous energy parameter k that can directly be correlated with the magnetoresistance. It was shown [Weinberger, P. (2002). Exchange bias due to configurational magnetic rearrangements. Phys . Rev . B , 65 , 014430] for the spin-valve system Co(111)/Co 6 /(CoO) n /Co 6 /Cu 6 /Co 6 /Co(111), n = 6,12, that the exchange bias refers to that value of k below which the magnetoresistance remains zero. Above this value a gradual change in the magnetoresistance is observed when considering non-collinear magnetic configurations.     

Anisotropic magnetoresistance and spin-valve effect in all-metal mesoscopic spin-valve devices

We have fabricated all-metal lateral spin-valve devices consisting of two permalloy electrodes and an interconnecting aluminum strip. The micromagnetic behavior of the device has been imaged with a magnetic-force microscope in external magnetic fields at room temperature. During a single cooling cycle at temperatures between 2 and 120 K we have measured the anisotropic magnetoresistance of both electrodes and the magnetoresistance of the entire device. In the latter, we can clearly identify the contributions of the anisotropic magnetoresistance and the mesoscopic spin-valve effect.     

GMR enhancement in spin valves structures with nano-semiconducting layer

We report on the giant magnetoresistance enhancement in Co/Ru/Co-based spin valve structures with nano-semiconducting layer. The films were grown by ion beam sputtering on glass substrate at room temperature. The soft layer is composed of Fe/Co bilayers, while the hard layer is ensured by the Co/Ru/Co artificial antiferromagnetic subsystem (AAF) as follows: Fe_5nm/Co_0.5nm/Cu_3nm/Co_3nm/Ru_0.5nm/Co_3nm/Cu_2nm/Cr_2nm. This structure shows a giant magnetoresistance (GMR) signal of about 1.7%. To confine the electrons inside the spin valve structure, a 1.5 nm thick ZnSe semiconducting layer has been grown on the top of the AAF. This induces a strong GMR increase, up to 4%, which can be attributed to a dominant potential step at the Co/ZnSe interface.     

Electric field-induced magnetoresistance in spin-valve/piezoelectric multiferroic laminates for low-power spintronics

Electric field-induced magnetic anisotropy has been realized in the spin-valve-based {Ni₈₀Fe₂₀/Cu/Fe₅₀Co₅₀/IrMn}/piezoelectric multiferroic laminates. In this system, electric-field control of magnetization is accomplished by strain mediated magnetoelectric coupling. Practically, the magnetization in the magnetostrictive FeCo layer of the spin-valve structure rotates under an effective compressive stress caused by the inverse piezoelectric effect in external electrical fields. This phenomenon is evidenced by the magnetization and magnetoresistance changes under the electrical field applied across the piezoelectric layer. The result shows great potential for advanced low-power spintronic devices.     

The remagnetization process in spin-valve structures

The process of magnetization reversal in ultrathin magnetic trilayer is analyzed. It is shown that the shape of magnetization hysteresis loops and the giant magnetoresistance essentially depend on the relative magnitudes of magnetic parameters of the top and the bottom layers. New types of hysteresis loops are found for characteristic relative magnitudes of the parameters. Analysis of the dependence of the shape of hysteresis loops on the magnitude of interlayer exchange is performed. The phase diagram which determines the regions of existence of characteristic hysteresis loops for different relative magnitudes of the uniaxial anisotropy constant and exchange constant J₁ is constructed.     

Magnetic phase evolution in Fe substituted GdBaCo2O5.5

Magnetization and magneto-resistance experiments have been carried out on well characterized samples of the GdBaCo_2−xFe_xO_5.5 series. Zero field cooled magnetization measurements in the low concentration Fe samples suggest, that the low temperature anti-ferromagnetic phase transforms sequentially to several ferromagnetic phases, before transforming to a paramagnetic state with increase in temperature. The anti-ferromagnetic to the first ferromagnetic phase transition is associated with a large negative magneto-resistance for Fe fractions upto x=0.075. Isothermal magnetization measurements in the ferromagnetic like region of the samples, suggests the presence of mixtures of two ferromagnetic phases. Similar measurements performed at low temperatures where anti-ferromagnetic-like phase is stabilised suggest the presence of a mixture of anti-ferromagnetic and ferromagnetic phases. Magnetization and magneto-resistance are seen to collapse for Fe fractions, x>0.1. Based on these studies a plausible scenario of the evolution of magnetism with Fe substitution in GdBaCo₂O_5.5, is suggested.     

Hard magnetic CoCr layer in ferromagnetic tunnel junctions

In magnetic tunnel junctions a highly spin-polarizing layer is usually exchange biased by an antiferromagnetic layer, an artificial antiferromagnetic layer system or a combination of both, while the magnetically soft layer is free to rotate. The use of a single layer of a hard magnetic material is rarely investigated up to now. In this paper, we present the electric and magnetic properties of tunnel junctions with a hard magnetic Co₈₃Cr₁₇ layer. The soft magnetic electrode consists of either a single Co layer or a Co/Ni₈₀Fe₂₀ bilayer. The magnetic anisotropy and coercive field H_C of the CoCr layer depend on its thickness and the kind of the bottom layer (Cu or Ta) and can vary from H_C=50–700 Oe. It is found that a thin Co cap layer also influences the hysteretic behavior. Furthermore, only small changes after annealing up to 450°C promise a high thermal stability for the application in magnetic tunnel junctions. Measurements of the tunnel magnetoresistance on large area junctions, however, show a strong magnetic coupling of the hard and soft electrodes.     

Magnetotransport spectroscopy of a quantum dot: effects of lead opening and phase coherence

We compare open quantum dot magnetoconductance spectra from experiment and theory in the presence of environmental coupling and attributed broadening. Estimates of the phase-breaking time in experiment, and effective broadening in simulation, are determined independently. In a larger, more open dot, with a significantly shorter phase-breaking time, the observed spectrum is broadened, most noticeably about B=0. The required broadening in simulation is characterized by effective temperatures higher than estimates from experiment; however, without accounting for disorder, which will further broaden the spectrum, the agreement is reasonable.     

Magnetism in FeNiW disordered alloys: Experiment and theory

We report here an experimental study of magnetization of FeNiW alloys at different compositions. We have studied variation of magnetization with temperature (at low external fields) and magnetic field (at low temperatures). The alloy shows para to ferromagnetic transitions across the composition range. We do not find any indication of the spin-glass phase. We have supplemented the experimental work with theoretical analysis using the first-principles tight-binding linear muffin-tin orbitals based augmented space recursion method. Our theoretical estimates of magnetic moment and Curie temperatures agree well with experiment. Our mean-field phase analysis also does not indicate the possibility of a spin-glass phase.     

Normal state magnetoresistance in stable magnetic fields up to 20 T in single crystalline La1.76Sr0.24CuO4

The in-plane normal state resistance R_n(T, H) in an overdoped La_2−xSr_xCuO₄ crystal (x = 0.24) has been measured in magnetic fields up to 20 T parallel to the c-axis. The R_n(T) curves in constant fields show a quadratic behavior in a wide range of temperature above T_c(H). Some characteristic features in R_n(H) are observed. In the low-field region R_n(H) increases with increasing H, reaches a maximum and then decreases with further increasing H. Possible origins for the observed unusual R_n(T, H) behavior are discussed.     

Microstructural and magnetic characteristics of IrMn exchange-biased tunnel junctions

Tunneling magnetoresistance values above 20% and 40% were obtained for as-deposited and annealed tunnel junctions, Ta/NiFe/Cu/NiFe/IrMn/CoFe/Al-oxide/CoFe/NiFe/Ta, respectively. Exchange biasing field increased from 270 to 550 Oe after annealing resulting from sharpening of the IrMn/CoFe interface. dV/dI vs. V curves showed asymmetric profiles, which were due to asymmetry of the CoFe/Al-oxide interfaces and difference in microstructure of the CoFe layers.     

退火气压对于P3HT∶PCBM微观形貌及光伏性能的影响

通过精确设定不同的退火环境气压, 实现对P3HT(Poly(3-hexylthiophene -2,5-diyl)与PCBM(［6,6］-Phenyl C61 butyric acid methyl ester)体系中聚噻吩结晶度以及共混相分离程度的控制, 并在此基础上制备了结构为ITO/PEDOT∶PSS/P3HT∶PCBM/Al的正型光伏器件。在允许的压强设定范围内, 器件各项性能参数均随退火环境压强的增大表现出先升高后下降的变化规律, 并统一于气压设定为1 500 mTorr时获得最大值。从活性层的紫外-可见(UV-Vis)吸收光谱中发现P3HT在510 nm吸收峰以及550和600 nm肩峰附近的吸收强度随退火气压升高而增大, 在气压为1 500 mTorr时达到最高, 吸收强度的提升源于聚合物分子π—π堆叠的增加。原子力显微镜(AFM)进一步分析结果表明, 高气压环境(>1 000 mTorr)能够促进P3HT∶PCBM共混组分在退火过程中形成较大程度的相分离, 而当环境压强合适时(1 500 mTorr)适度的相分离利于聚合物形成良好有序结晶, 从而能够提升活性层内部载流子传输能力, 保证较高的短路电流与填充因子, 制备的器件也因此表现出良好的光伏性能, 光电转化效率达到3.56%。     

退火气压对于P3HT∶PCBM微观形貌及光伏性能的影响

通过精确设定不同的退火环境气压,实现对P3HT(Poly(3-hexylthiophene -2,5-diyl)与PCBM([6,6]-Phenyl C₆₁ butyric acid methyl ester)体系中聚噻吩结晶度以及共混相分离程度的控制,并在此基础上制备了结构为ITO/PEDOT∶PSS/P3HT∶PCBM/Al的正型光伏器件。在允许的压强设定范围内,器件各项性能参数均随退火环境压强的增大表现出先升高后下降的变化规律,并统一于气压设定为1 500 mTorr时获得最大值。从活性层的紫外-可见(UV-Vis)吸收光谱中发现P3HT在510 nm吸收峰以及550和600 nm肩峰附近的吸收强度随退火气压升高而增大,在气压为1 500 mTorr时达到最高,吸收强度的提升源于聚合物分子π—π堆叠的增加。原子力显微镜(AFM)进一步分析结果表明,高气压环境(>1 000 mTorr)能够促进P3HT∶PCBM共混组分在退火过程中形成较大程度的相分离,而当环境压强合适时(1 500 mTorr)适度的相分离利于聚合物形成良好有序结晶,从而能够提升活性层内部载流子传输能力,保证较高的短路电流与填充因子,制备的器件也因此表现出良好的光伏性能,光电转化效率达到3.56%。     

Improvement of refrigerant capacity of La0.7Ca0.3MnO3 material with a few percent Co doping

The influence of first and second order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity or relative cooling power (RCP) of La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials has been investigated. Large low-field-induced magnetic entropy changes are observed in La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials. The La_0.7Ca_0.3MnO₃ material experiences a large entropy change with a first-order magnetic phase transition at the Curie temperature, T_C. On the other hand, La_0.7Ca_0.3Mn_0.95Co_0.05O₃ displays a smaller entropy change with a second order phase transition. While a first-order magnetic transition material induces a larger MCE (7.528 J/kg K at 5 T) at T_C, this is limited to a narrow temperature range, resulting in a relatively small RCP (218 J/kg), while the Co-doped second-order magnetic transition material induces a smaller MCE (7.14 J/kg K for 5 T), but it is spread over a broader temperature range, resulting in a larger RCP (308 J/kg). The maximum magnetoresistance (MR, defined as ρ(0)/ρ(H)-1) under a field of 5 T is about 206% and 333% for La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃, respectively. The refrigeration capacity (RCP) is enhanced in La_0.7Ca_0.3Mn_0.95Co_0.05O₃ (by about 41%) due to small changes from Co doping. The magnetocaloric features of these materials at lower magnetic fields (MCE=3.163 for La_0.7Ca_0.3Mn_0.95Co_0.05O₃ and 4.63 J/kg K for La_0.7Ca_0.3MnO₃ at 1 T), and the high RCP and MR can provide some ideas for exploring novel magnetic refrigerants that can operate with permanent magnets rather than superconducting ones as the magnetic field source.     

Fabrication and magnetic property of binary Co-Ni nanowire array by alternating current electrodeposition

Ordered binary Co-Ni nanowire arrays with different components have successfully been fabricated by ac electrodeposition. The as-obtained nanowires exhibit a diameter of about 49.2 nm and aspect ratio of more than 30. A highly preferential orientation of the Co-Ni nanowires has been obtained by XRD. The magnetic properties of Co-Ni nanowire arrays determined by VSM are as the function of the Co-Ni components. The maximum value of coercivities perpendicular to the array is 2073 Oe. However, the magnetic properties of such nanowire arrays exhibited a bad thermal stability at the medium temperature of 200 °C.     

Anisotropic magnetotransport near the quantum Hall state

We report experimental results on an anisotropic magnetotransport of a bilayer electron system at the Landau level filling factor under tilted magnetic fields. We find that the magnetoresistance changes when the direction of applied in-plane field B is changed with respect to the direction of the current while keeping the strength of B fixed. This anisotropy only appears at the ground state where interlayer electron tunneling is permitted. Therefore, this phenomenon indicates that bilayer systems at with interlayer tunneling inherently have a dissipation mechanism related to the direction of the in-plane field.     

Thermal stability of spin valve sensors using artificial Co/Ir based ferrimagnets

Hard-soft spin valve structures have been grown by molecular beam epitaxy on MgO(0 0 1) substrates. The hard magnetic layer consists of an artificial Co/Ir/Co ferrimagnet system (AFi), while a Fe/Co bilayer from the buffer has been used as a soft detection layer. The Fe layer has been grown at 600°C giving rise to a monocrystalline layer with a BCC structure. Consequently, this layer presents a hard and a easy magnetization axis, respectively, along the BCC [1 1 0] and the [1 0 0] directions. The AFi system presents dramatic differences after successive annealing steps up to 350°C. An increase of the GMR from 3% to 3.5% is observed after annealing at 250°C while the coercive field of the AFi and the GMR plateau are strongly reduced. After further annealing at higher temperature the GMR drops down accompanied with a strong decrease in the antiparallel alignment amount in the AFi system. Rutherford back scattering measurements have been performed to investigate the changes in the interface morphology and to correlate it to the magneto-transport properties.     

激发压缩真空态的相位概率分布

应用Pegg和Barnett提出的相位算符和相位态理论,研究了激发压缩真空态的相位概率分布特性.数值计算结果表明,激发压缩真空态的相位概率分布主要受到相位参量和压缩参量的调节,相位参量使相位概率分布呈峰值结构,压缩参量的变化将影响相位概率分布的峰值强度,此外激发光子数对相位概率分布也有较大的影响.     

Ionic size effect on magnetic and electronic properties of La0.70A0.05Sr0.25CoO3 (A=La,Nd, Gd,Ho, Y)

The structural, magnetic and electronic transport properties of La_0.70A_0.05Sr_0.25CoO₃ (A=La, Nd, Gd, Ho, Y) have been investigated by X-ray diffraction and the magnetization and resistivity measurements. All the samples crystallize into hexagonal lattice with the space group R-3C (A167) at room temperature. Small doped ions shrink the lattices almost linearly and isotropically, and bend the Co–O–Co bond angle. Although the Co atoms in all compounds are stabilized in intermediate-spin electronic configuration, the (residual) resistivity (T→0$T\to 0$) increase significantly with doping of the small ions. At low temperatures, the compounds change from metallic state of La_0.70A_0.05Sr_0.25CoO₃ (A=La, Nd, Gd) to insulated state of La_0.70A_0.05Sr_0.25CoO₃ (A=Ho, Y). The 5–7% magnetoresistance (MR) near Curie temperature can be interpreted in terms of the external field suppression of the spin fluctuation, but the relative large MR in the insulator state of La_0.70A_0.05Sr_0.25CoO₃ (A=Ho, Y) (∼20% up to 50-kOe field at 5 K) probably results from the field-induced transfer integral and the tunneling effect.