Magnetization-dependent shift in ferromagnet/superconductor/ferromagnet trilayers with a strong ferromagnet

We have measured the superconducting transition temperature Tc of Ni/Nb/Ni trilayers when the magnetizations of the two outer Ni layers are parallel (P) and antiparallel (AP). The largest difference in occurs when the Nb thickness is just above the critical thickness at which superconductivity disappears completely. We have observed a difference in Tc between the P and AP states as large as 41 mK--a significant increase over earlier results in samples with higher Tc and with a CuNi alloy in place of the Ni. Our result also demonstrates that strong elemental ferromagnets are promising candidates for future investigations of ferromagnet/superconductor heterostructures.     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

Magnetization-orientation dependence of the superconducting transition temperature in the ferromagnet-superconductor-ferromagnet system: CuNi/Nb/CuNi

The superconducting critical temperature (T(c)) of ferromagnet-superconductor-ferromagnet systems has been predicted to exhibit a dependence on the magnetization orientation of the ferromagnetic layers such that T(AP)(c)>T(P)(c) for parallel (P) and antiparallel (AP) configurations of the two ferromagnetic layers. We have grown CuNi/Nb/CuNi films via magnetron sputtering and confirmed the theoretical prediction by measuring the resistance of the system as a function of temperature and magnetic field. We find an approximately 25% resistance drop occurs near T(c) in Cu0.47Ni0.53(5 nm)/Nb(18)/CuNi(5) when the two CuNi layers change their magnetization directions from parallel to antiparallel, whereas there is no corresponding resistance change in the normal state.     

The effect of Pd doped in Ru layer on theinterlayer coupling in Co／RuPd multilayers

2.0nmCo/tnmRu层间反磁铁耦合 磁性录音 振动交换 添加效应2.0nmCo/tnmRu1-xPdx multilayers with x=0, 0.05, 0.08, 0.24, 0.39 and 0.48 were prepared by magnetron sput-tering. The spacer layer thickness of both Ru (before doping Pd) and RuPd (after doping Pd) varies from 0.2nm to 1.6nm. Two effects have been investigated: (1) the dependence of the interlayer coupling on the thickness of Ru1-xPdx as a function of x and (2) the dependence of the interlayer coupling on Pd doping density, x, as a function of thickness of Ru1-xPdx. Our results indicate that the interlayer coupling is strongly dependent on the doping density and the spacer layer thickness. The saturation field Hs increases when very low concentration of Pd doped in the Ru layers and a suitable spacer thickness are adopted.     

Reentrant superconductivity in superconductor-ferromagnetic-alloy bilayers

Oscillating behavior of superconductivity in ultrathin bilayers of niobium and ferromagnetic alloy Cu₄₁Ni₅₉ has been observed. This phenomenon was most pronounced at a Nb layer thickness of about 7.3 nm: the superconducting transition temperature T _c first sharply decreased with an increase in the ferromagnetic alloy thickness to complete suppression of superconductivity at the ferromagnetic alloy thickness d _CuNi ≈ 4 nm. With a further increase in the thickness d _CuNi, the superconductivity was restored at d _CuNi ≥ 13 nm. This strongly nonmonotonic and reentrant behavior of superconductivity in Nb/Cu₄₁Ni₅₉ bilayers is attributed to implementation of a state in the ferromagnetic alloy that is similar to the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov state.     

Room-temperature ferromagnetism induced by Cu vacancies in Cu_x(Cu_2O)_(1-x) granular films

Cux(Cu2O)1-x(0.09 x 1.00) granular films with thickness about 280 nm have been fabricated by direct current reactive magnetron sputtering. The atomic ratio x can be controlled by the oxygen flow rate during Cux(Cu2O)1-x deposition. Room-temperature ferromagnetism(FM) is found in all of the samples. The saturated magnetization increases at first and then decreases with the decrease of x. The photoluminescence spectra show that the magnetization is closely correlated with the Cu vacancies in the Cux(Cu2O)1-x granular films. Fundamentally, the FM could be understood by the Stoner model based on the charge transfer mechanism. These results may provide solid evidence and physical insights on the origin of FM in the Cu2O-based oxides diluted magnetic semiconductors, especially for systems without intentional magnetic atom doping.     

Spin dynamics in a superconductor-ferromagnet proximity system

The ferromagnetic resonance of thin sputtered Ni80Fe20 films grown on Nb is measured. By varying the temperature and the thickness of the Nb the role of the superconductivity on the whole ferromagnetic layer in these heterostructures is explored. The change in the spin transport properties below the superconducting transition of the Nb is found to manifest itself in the Ni80Fe20 layer by a sharpening in the resonance of the ferromagnet, or a decrease in the effective Gilbert damping coefficient, showing that the superconductivity affects the macrospin of the ferromagnetic layer. We interpret this in terms of the spin-pumping model.     

Superconductivity and ferromagnetism in EuFe?(As(1-x)P(x))?

Superconductivity and ferromagnetism are two antagonistic cooperative phenomena, which makes it difficult for them to coexist. Here we demonstrate experimentally that they do coexist in EuFe?(As(1-x)P(x))? with 0.2 ≤ x ≤ 0.4, in which superconductivity is associated with Fe 3d electrons and ferromagnetism comes from the long-range ordering of Eu 4f moments via Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The coexistence features large saturated ferromagnetic moments, high and comparable superconducting and magnetic transition temperatures, and broad coexistence ranges in temperature and field. We ascribe this unusual phenomenon to the robustness of superconductivity as well as the multi-orbital character of iron pnictides.     

Magnetic proximity effects in antiferromagnet/ferromagnet bilayers: the impact on the Néel temperature

We present a study of the ordering temperature of an ultrathin antiferromagnetic film in the proximity of a ferromagnetic layer. The Néel temperature of a single-crystalline antiferromagnetic FexMn1-x film on Cu(001) in contact with a ferromagnetic Ni layer was monitored by the discontinuity in the coercivity as a function of temperature by magneto-optical Kerr effect measurements. It decreases by up to 60 K if the magnetization axis of the ferromagnet is switched from out of plane to in plane by deposition of a Co overlayer. These results give clear evidence for a magnetic proximity effect in which the ferromagnetic layer substantially influences the ordering temperature of the antiferromagnetic layer.     

EuS/Ta异质结的极大磁电阻效应

在铁磁/超导异质结中,铁磁体的交换场通过近邻效应将导致超导体准粒子态密度的塞曼劈裂.基于该效应,在外磁场不强的情况下,通过外加磁场可以有效地调节铁磁/超导界面处的交换作用,从而实现超导体在正常态和超导态之间转换,产生极大磁电阻.本文利用脉冲激光沉积方法制备了EuS/Ta异质结并研究了其电磁特性.Ta在3.6 K以下为超导态,EuS在20 K以下为铁磁态.在2 K时,EuS/Ta异质结中可观测蝴蝶型磁滞回线,证明在低磁场下(<±0.18 T)异质结中EuS铁磁态和Ta超导态共存.磁输运测试表明,通过施加外磁场可以有效调节EuS的交换场,随着交换场的增大,同时也加强了界面处的交换作用,从而抑制Ta的超导态,实现了Ta在超导态和正常态之间的转变,在EuS/Ta异质结中观测到了高达144000%的磁电阻.本文制备的EuS/Ta异质结具有极大磁电阻效应,在自旋电子学器件中有潜在的应用前景.     

Nonmonotonic d(x(2)-y(2)) superconducting order parameter in Nd2-xCexCuO4

Low energy polarized electronic Raman scattering of the electron-doped superconductor Nd2-x Ce x CuO4 ( x = 0.15, T(c) = 22 K) has revealed a nonmonotonic d(x(2)-y(2)) superconducting order parameter. It has a maximum gap of 4.4k(B)T(c) at Fermi surface intersections with an antiferromagnetic Brillouin zone (the "hot spots") and a smaller gap of 3.3k(B)T(c) at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.     

一种非连续纳米氧化层自旋阀的显微结构研究

利用高分辨电子显微学方法(HREM)研究了纳米氧化层镜面反射自旋阀多层结构Ta(35nm)Ni80Fe20(2nm)Ir17Mn83(6nm)Co90Fe10(15nm)NOL1Co90Fe10(2nm)Cu(22nm)Co90Fe10(15nm)NOL2Ta(3nm).该自旋阀的巨磁电阻(GMR)效应高达15%,较无此镜面反射纳米氧化层(NOL)的自旋阀提高近1倍,同时交换偏置场亦有所增强.高分辨显微结构分析表明,介于钉扎层与被钉扎层之间的氧化层(NOL1)并未完全氧化,即除氧化过程生成的CoFe氧化物 关键词： 自旋阀 纳米氧化层 高分辨电子显微学 巨磁电阻效应     

Synthesis and magnetic properties of multilayer Ni/Cu and NiFe/Cu nanowires

Highly ordered composite nanowires with multilayer Ni/Cu and NiFe/Cu have been fabricated by pulsed electrodeposition into nanoporous alumina membrane. The diameter of wires can be easily varied by pore size of alumina, ranging from 30 to 100 nm. The applied potential and the duration of each potential square pulse determine the thickness of the metal layers. The nanowires have been characterized by transmission electron microscopy (TEM), magnetic force microscopy (MFM), and vibrating sample magnetometer (VSM) measurements. The MFM images indicate that every ferromagnetic layer separated by Cu layer was present as single isolated domain-like magnet. This technique has potential use in the measurement and application of magnetic nanodevices.     

Spin-pumping-enhanced magnetic damping in ultrathin Cu(0 0 1)/Co/Cu and Cu(0 0 1)/Ni/Cu films

The influence of the Cu capping layer thickness on the spin pumping effect in ultrathin epitaxial Co and Ni films on Cu(0 0 1) was investigated by in situ ultrahigh vacuum ferromagnetic resonance. A pronounced increase in the linewidth is observed at the onset of spin pumping for capping layer thicknesses d_Cu larger than 5 ML, saturating at d_Cu = 20 ML for both systems. The spin mixing conductance for Co/Cu and Ni/Cu interfaces was evaluated.     

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.     

Single-gap s-Wave superconductivity near the charge-density-wave quantum critical point in CuxTiSe2

The in-plane thermal conductivity kappa of the layered superconductor CuxTiSe2 was measured down to temperatures as low as Tc/40, at x=0.06 near where the charge-density-wave order vanishes. The absence of a residual linear term at T-->0 is strong evidence for conventional s-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material NbSe2, where the superconducting gap is 3 times larger on the Nb 4d band than on the Se 4p band, suggests that in Cu0.06TiSe2 the Se 4p band is below the Fermi level and Cu doping into the Ti 3d band is responsible for the superconductivity.     

Phase-sensitive order parameter symmetry test experiments utilizing Nd(2-x)Ce(x)CuO(4-y)/Nb zigzag junctions

Phase-sensitive order parameter symmetry test experiments are presented on the electron-doped high-T(c) cuprate Nd(2-x)Ce(x)CuO(4-y). These experiments have been conducted using zigzag-shaped thin film Josephson structures, in which the Nd(2-x)Ce(x)CuO(4-y) is connected to the low-T(c) superconductor Nb via an Au barrier layer. For the optimally doped as well as for the overdoped Nd(2-x)Ce(x)CuO(4-y), a clear predominant d(x2-y2)-wave behavior is observed at T=4.2 K. Both compounds were also investigated at T=1.6 K, presenting no indications for a change to a predominant s-wave symmetry with decreasing temperature.     

Mean-field pairing theory for the charge-stripe phase of high-temperature cuprate superconductors

Striped high-T(c) superconductors such as La(2-y-x)Nd(y)Sr(x)CuO(4) and La(2-x)Ba(x)CuO(4) near x = 1/8 show a fascinating competition between spin and charge order and superconductivity. A theory for these systems therefore has to capture both the spin correlations of an antiferromagnet and the pair correlations of a superconductor. For this purpose we present here an effective Hartree-Fock theory incorporating both electron pairing with finite center-of-mass momentum and antiferromagnetism. We show that this theory reproduces the key experimental features such as the formation of the antiferromagnetic stripe patterns at 7/8 band filling or the quasi-one-dimensional electronic structure observed by photoemission spectroscopy.     

基于多尺度模拟研究Ni/Cu薄膜压痕塑性的原子机制

基于准连续介质多尺度模拟方法研究了Ni/Cu双层薄膜初始压痕塑性的原子机制,结果主要包括:(1)当Ni晶体层厚度小于10nm时,随着Ni晶体层厚度的减少,薄膜弹性极限所对应的临界接触力逐渐降低,即Ni/Cu薄膜随Ni晶体层厚度减小而变软;(2)压头下方晶格Shockley分位错的开动、界面位错的分解、以及界面位错与晶格位错的相互作用是Ni/Cu薄膜初始塑性的微观原子机制,(3)根据模拟结果观察和位错弹性理论计算,承载初始塑性的界面位错数目变少是Ni/Cu薄膜软化的主要原子机制.本文研究结果能够为异质界面力学行为研究提供有益参考.     

Andreev reflections on Y(1-x)Ca(x)Ba(2)Cu(3)O(7-delta): evidence for an unusual proximity effect

We have measured Andreev reflections between an Au tip and Y(1-x)Ca(x)Ba(2)Cu(3)O(7-delta) thin films in the in-plane orientation. The conductance spectra are best fitted with a pair potential having the d(x2-y2)+is symmetry. We find that the amplitude of the is component is enhanced as the contact transparency is increased. This is an indication for an unusual proximity effect that modifies the pair potential in the superconductor near the surface with the normal metal.