Perpendicular magnetization and exchange bias in epitaxial NiO/[Ni/Pt]2 multilayers

The realization of perpendicular magnetization and perpendicular exchange bias(PEB)in magnetic multilayers is important for the spintronic applications.NiO(t)/[Ni(4 nm)/Pt(1 nm)]₂multilayers with varying the NiO layer thickness t have been epitaxially deposited on SrTiO;(001)substrates.Perpendicular magnetization can be achieved when t<25 nm.Perpendicular magnetization originates from strong perpendicular magnetic anisotropy(PMA),mainly resulting from interfacial strain induced by the lattice mismatch between the Ni and Pt layers.The PMA energy constant decreases monotonically with increasing t,due to the weakening of Ni(001)orientation and a little degradation of the Ni–Pt interface.Furthermore,significant PEB can be observed though NiO layer has spin compensated(001)crystalline plane.The PEB field increases monotonically with increasing t,which is considered to result from the thickness dependent anisotropy of the NiO layer.     

Superconducting properties of ZnO-doped (Bi, Pb)-2223 thick film on Ni and NiO substrates prepared by spray deposition technique

The superconductivity of ZnO-doped (Bi, Pb)-2223 thick film on the Ni and NiO substrates, which was prepared by the spray deposition technique with cold forging, was investigated by characterizing the critical current density (J_c), the critical temperature (T_c), the orientation factor (f), and the microstructure of the film. The thickness of the thick film prepared by the spray deposition method was approximately 10 μm. The maximum J_c value of (Bi, Pb)-2223 film on NiO substrate was approximately 2200 A/cm² (I_c = 110 mA) when the film was sintered at 865 °C for 1 h with a cooling rate of 0.5 °C/min from 865 °C to 650 °C; in the case of Ni substrate, a maximum J_c value of approximately 2000 A/cm² (I_c = 100 mA) was obtained for the (Bi, Pb)-2223 thick film when a cooling rate was 3 °C/min. Such a difference in the J_c values of (Bi, Pb)-2223 thick film on Ni and NiO substrates is attributed to the presence of reaction layer at the (Bi, Pb)-2223 and substrate interface. In addition, the variations in the orientation factor of (Bi, Pb)-2223 thick film on NiO substrate related to those of J_c values. The J_c values of (Bi, Pb)-2223 film on NiO substrate with ZnO doping extremely depended on the amount of ZnO doping and the 0.5 wt% ZnO-doped (Bi, Pb)-2223 thick film deposited on NiO substrate, which was sintered at 835 °C for 1 h in air with a cooling rate of 1 °C/min, showed a J_c value of approximately 1200 A/cm² (I_c = 60 mA). Thus, it is considered that a small amount of ZnO doping was effective in lowering the sintering temperature of (Bi, Pb)-2223 thick film, resulting the improvement in the intragranular weak bonding or Josephson junction.     

High critical current density YBa2Cu3O7−δ tapes prepared by the surface-oxidation epitaxy method

YBa₂Cu₃O_7−δ (YBCO) films with high critical current density (J_c) were successfully fabricated on nickel tapes buffered with epitaxial NiO. NiO was prepared on the textured nickel tape by the surface-oxidation epitaxy (SOE) method. We have reported so far a critical temperature (T_c) of 87 K and J_c=4–6×10⁴ A/cm² (77 K, 0 T) for the YBCO films on NiO/Ni tapes. To enhance the superconducting properties of the YBCO films on the SOE-grown NiO, depositions of thin oxide cap layers such as YSZ, CeO₂, and MgO on NiO were investigated. These oxide cap layers were epitaxially grown on NiO and provided the template for the epitaxial growth of YBCO films. Substantially improved data of T_c=88 K and J_c=3×10⁵ A/cm² (77 K, 0 T) and 1×10⁴ A/cm² (77 K, Hc, 4 T) were obtained for YBCO film on NiO, by using a MgO cap layer with a thickness of 50 nm. The method described in this paper is a simple way to produce long YBCO tape conductors with high-J_c values.     

Asymmetric double-barrier S–I1–N–I2–S Josephson heterojunctions: experiment and theory

Double-barrier highly asymmetric Nb–Al oxide–Al–Nb oxide–Nb structures with reproducible characteristics were fabricated. The heterocontacts with the middle Al layer thickness ranging from 4 to 6 nm exhibited a well-defined d.c. Josephson supercurrent I_c at 4.2 K and characteristic voltages V_c=I_cR_N (R_N is the normal resistance, V_c defines the response time of the junction) from 0.3 to 0.4 mV. Two prominent features in the quasiparticle current–voltage curves have been observed: a so-called ‘knee' in the energy-gap region and an additional (to the linear voltage dependence) current at higher biases. They are discussed within a simple Landauer–Büttiker scattering approach to the phase-coherent quasiparticle transport in a quasiballistic S–I₁–N–I₂–S heterostructure with an extremely great difference between the barrier transparencies.     

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 K and 87 K for two samples of different origin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.     

The origin of spin current in YIG/nonmagnetic metal multilayers at ferromagnetic resonance

Spin pumping in yttrium-iron-garnet(YIG)/nonmagnetic-metal(NM) layer systems under ferromagnetic resonance(FMR) conditions is a popular method of generating spin current in the NM layer.A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications.It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer.Here,by combining microwave absorption and DC-voltage measurements on thin YIG/Pt and YIG/NM_1/NM_2(NM_1 =Cu or Al,NM_2 =Pt or Ta),we unambiguously showed that spin current in NM,instead of from the precessing YIG magnetization,came from the magnetized NM surface(in contact with thin YIG),either due to the magnetic proximity effect(MPE) or from the inevitable diffused Fe ions from YIG to NM.This conclusion is reached through analyzing the FMR microwave absorption peaks with the DC-voltage peak from the inverse spin Hall effect(ISHE).The voltage signal is attributed to the magnetized NM surface,hardly observed in the conventional FMR experiments,and was greatly amplified when the electrical detection circuit was switched on.     

Influence of temperature on thermal relaxation of exchange bias field in CoFe/Cu/CoFe/IrMn spin valve

A multilayered spin valve film with a structure of Ta(5 nm)/Co_(75)Fe_(25)(5 nm)/Cu(2.5 nm)/Co_(75)Fe_(25)(5 nm)/Ir_(20)Mn_(80)(12 nm)/Ta(8 nm) is prepared by the high-vacuum direct current(DC) magnetron sputtering. The effect of temperature on the spin valve structure and the magnetic properties are studied by x-ray diffraction(XRD), atomic force microscopy(AFM), and vibrating sample magnetometry. The effect of temperature on the exchange bias field thermomagnetic properties of multilayered spin valve is studied by the residence time of samples in a reverse saturation field. The results show that as the temperature increases, the Ir Mn(111) texture weakens, surface/interface roughness increases, and the exchange bias field decreases. Below 200℃, the exchange bias field decreases with the residence time increasing, and at the beginning of the negative saturation field, the exchange bias field Hex decreases first quickly and then slowly gradually. When the temperature is greater than 200℃, the exchange bias field is unchanged with the residence time increasing.     

Effect of unidirectional solidification rate on Jc for current lead

The dependence of transport J_c value on the traveling rate and the nominal composition was investigated by taking different nominal compositions of YBa₂Cu₃O_6+x (Y123) + n mol% Y₂BaCuO₅ (Y211) (n = 10, 20, 30, 40 and 50) with addition of 0.5 wt% of Pt on samples processed by different unidirectional solidification rates, namely 1, 3 and 5 mm/h. The highest J_c was found in the sample with 30% Y211 addition by the higher traveling rate adjusted to prevent the formation of polycrystals. In this φ 1.56 × 60 mm sample the values of transport I_c and J_c were 1370 A and 71 700 A/cm², respectively, which were obtained by the conventional DC four-probe method with criterion of 1 μV/cm at 77 K and self-field.     

Magnetic hysteresis of the magnetoresistance and the critical current density in polycrystalline YBa2Cu3O7−δ–Ag superconductors

A systematic study of the magnetic hysteresis in transport properties of polycrystalline YBa₂Cu₃O_7−δ–Ag compounds has been made based on two kinds of measurements at 77 K and under applied magnetic fields up to 30 mT: critical current density J_c(B_a) and magnetoresistance R(B_a). The R(B_a) curves show a minimum in their decreasing branch occurring at B=B_min which was found to be both the excitation current I_ex and the maximum applied magnetic field B_am dependent. In addition, for a certain value of B_am>5 mT, we have observed that B_min increases with increasing I_ex and reaches a saturation value. The J_c(B_a) curves show a maximum in decreasing applied magnetic fields occurring at B=B_max. We have also found that B_max increases with increasing B_am and reaches a saturation value. The minimum in the R(B_a) and the maximum in J_c(B_a) curves were found to be related to the trapped flux within the grains. All the experimental results are discussed within the context of the flux dynamics and transport mechanisms in these high-T_c materials.     

Fabrication and characteristics of all YBCO sandwich-type junctions using highly crystalline DC sputtered superconducting a-axis oriented Y1Ba2Cu3O7-x films

We report on the fabrication and characteristics of sandwich-type tunnel junctions with highly crystalline sputtered a-axis oriented thin film of Y₁Ba₂Cu₃O_7-x (YBC) as the base and the counter electrode. The junctions have been fabricated on SrTiO₃ (100) and MgO (100) substrates. A non-superconducting phase of YBC corresponding to a lattice constant of 4.08 Å is used as the barrier layer making this an all YBC sandwich junction. For all temperatures below T_ce (R=0) of the device, a zero voltage current was observed. The critical current density (J_c) of the device was found to be dependent on the thickness of the barrier layer and the crystallinity of the a-axis oriented YBC electrodes. At 40 K, such a junction fabricated on a SrTiO₃ (100) substrate was found to have a J_c of 1.8 X 10⁴ A/cm² and an I_cR_n product of 0.2 mV.     

Low frequency impedance of a round superconducting wire

We calculated the electric field E on the surface of a straight superconducting wire with circular cross-section carrying AC transport current I=I_acosωt. Performing the Fourier analysis of E, we found that both components of the first harmonic have the same form: the critical current I_c in prefactor and the rest depending on the ratio F=I_a/I_c. The in-phase component leads to the classical result of loss calculation, while the out-of-phase component was derived for the first time. Thus the wire can be symbolized by a complex self-inductance L₁(I)=L₁′(I)−jL₁″(I) where L₁′ represents the reactive power while L₁″ the losses. When the lock-in amplifier, used to sort out the components of the first harmonic, is utilized in the wide-band mode, it allows one to determine the magnetic flux penetrated in the wire volume at two significant moments of the AC cycle: at zero current (remanent flux) and at the amplitude value of current.     

Estimation of Jc(B) dependence from self-field alternating current (AC) losses measured on Bi-2223/Ag tapes

Transport AC losses measured in self-field conditions on multifilamentary Bi-2223 tapes are often found to be lower than those calculated within the framework of the critical state model for a bulk wire with elliptical cross section, though generally higher than predicted for a strip. This effect is sometimes ascribed to the non-ideal geometry of the tapes, which does not exactly reproduce either shape. Here we propose an alternative explanation assuming that the critical current density of superconducting material depends on magnetic field. In practice, we analyzed the AC loss curve and deduced different I_c values for the individual data points, using the standard Norris equation for elliptical conductor. This gives the relation between ‘calculated' I_c and the self-field associated to AC transport current, which can be regarded as an alternative way to qualify the dependence of J_c on magnetic field. Important is that this procedure covers the range of fields below the self-field at I_c where the measurement in background DC field can not be used to determine J_c(B).     

Nb/Au/(1 1 0)YBa2Cu3O7−δ Josephson junctions: evidence against atomic scaled “corner junctions”

We report on I–V characteristics for in situ formed Nb/Au/(1 1 0)YBa₂Cu₃O_7−δ (YBCO) Josephson junction, where the homoepitaxial (1 1 0)YBCO film shows ultra-smooth surface morphology. The field dependence of critical supercurrent I_c shows anisotropic large junction behavior with normal Fraunhofer patterns expected from BCS model of d_x²−y² wave superconductors. This strongly suggests that the Nb/Au/(1 1 0)YBCO junctions cannot be regarded as atomic scaled corner junctions, in contrast with (0 0 1)/(1 1 0)YBCO grain boundary junctions to show “π-junction” with a pronounced dip near zero fields in field modulation of I_c.     

NMR study of local hole distribution, spin fluctuation and superconductivity in Tl2Ba2Ca2Cu3O10

⁶³Cu, ¹⁷O and ²⁰⁵Tl NMR have been performed in the high-T_c superconductor Tl₂Ba₂Ca₂Cu₃O₁₀ whose T_c(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO₂ plane have been extracted from the nuclear quadrupole frequency ν_Q. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T₁ and T_2G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa₂Cu₃O₇, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher T_c is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, T_c would go up to 132 K which is quite close to the record T_c realized in the Hg based compound.     

Exchange coupling effects in NiO/Co and NiO/permalloy bilayers

NiO/Co and NiO/Ni₈₀Fe₂₀ bilayers were prepared at 293 onto SiO₂(1 0 1)/Si(1 1 1) and glass substrates using UHV (5×10⁻¹⁰ mbar) RF/DC magnetron sputtering. Results on magnetic measurements showed that the exchange biasing and coercive fields are inversely proportional to the Co and Ni₈₀Fe₂₀ (Py) layer thickness down to 2 nm. A maximal RT coupling energy for the NiO–Co and NiO–Py interface was estimated as 0.04 and 0.03 mJ/m² for the samples prepared onto SiO₂(1 0 1)/Si(1 1 1) substrates.     

Performance at 10–50 K of Bi-2212/Ag multilayer tape fabricated by using pre-annealing and intermediate rolling process

High-transport critical current density (J_c-oxide)>500 kA/cm² at 4.2 K, 10 T can be obtained for Bi-2212/Ag tapes fabricated by using pre-annealing and intermediate rolling (PAIR) and melt-solidification process. In this paper, we report high-temperature properties of PAIR-processed Bi-2212/Ag multilayer tape in order to show their potential for practical applications operated at cryocooling temperatures. Magnetic field dependence and angular dependence of critical current (I_c) are investigated at temperatures ranging 10–50 K by using helium gas cooling and liquid neon. Field-temperature curves for I_c=0.2 and 2.0 A are also determined in order to show the approximation of the irreversible field. High-temperature performance of the tape is attractive to consider future applications. For example, the best sample carries I_c=267 A (engineering-J_c=303 A/mm², J_c-oxide=151 kA/cm²) and 92 A (104 A/mm², 52 kA/cm²) at 27.1 K (in liquid neon), in magnetic fields (parallel to the tape surface) of 2 and 10 T, respectively. Engineering-J_c of 100 A/mm² is obtained even in the perpendicular field of 0.5 T at 27.1 K.     

NiFe/Pt薄膜中角度相关的逆自旋霍尔效应

NiFe/Pt双层薄膜样品在铁磁共振时, NiFe磁矩进动所产生的自旋流注入到Pt层中, 由于逆自旋霍尔效应产生直流电压V_ISHE, 此电压会叠加到NiFe薄膜由于自旋整流效应而产生的电压V_SRE 上, 实验测量所得电压为V_ISHE和V_SRE的叠加. 为了区分这两种不同机理对电压的贡献, 本文采取旋转外加静磁场的方法, 通过分析所测电压随磁场角度的变化从而分离出V_ISHE 的大小. 研究结果表明, 相比于单层NiFe(20 nm)薄膜样品, NiFe(20 nm)/Pt(10 nm)双层膜样品中由于NiFe自旋注入到Pt 中导致铁磁共振线宽增加. 与逆自旋霍尔效应产生的电压相比, 自旋整流效应的贡献较小, 但不可忽略. 本文工作有助于认清铁磁/非磁性金属材料中的自旋相关效应, 并提供了一种准确的分析逆自旋霍尔效应的方法.     

Microscopic measurements in La-NQR of the ternary carbide superconductor LaNiC2

¹³⁹La-NQR measurements have been carried out in the ternary carbide superconductor LaNiC₂. The nuclear quadrupole frequency and the asymmetry parameter of ¹³⁹La in LaNiC₂ were estimated to be about 1.9 MHz and 0, respectively. In the normal state, the nuclear spin relaxation rate (1/T₁) in the ¹³⁹La NQR signal was proportional to temperature (T) in zero external field above the superconducting transition temperature (T_c) or in an external field larger than the superconducting critical field, which means the system is in the Fermi-liquid state. In the superconducting state, on the other hand, 1/T₁ decreases no more linearly with T, but decreases rapidly exponentially as exp (−Δ/k_BT) at low T with an appreciable enhancement just below T_c. The value of the superconducting energy gap, 2Δ, was estimated to be 3.34k_BT_c, compared with 3.52k_BT_c of the BCS-value. This result strongly suggests that the superconductivity in LaNiC₂ is of a conventional BCS type.     

Influence of DC external magnetic field on AC transport current loss of HTS tape

We measured the AC transport current loss of Bi 2223 multifilament Ag-sheathed tape under DC external magnetic field of 0–0.2 T. There were discrepancies between the measured data and Norris' formula for elliptical model in the range of low value of I_p/I_c (I_p and I_c are peak of the AC transport current and critical current of the tape respectively), while without DC background field, the loss of the tape was close to Norris' formula. Theoretically speaking, even with the DC background field and decreased critical current the AC transport current loss of the tape follows Norris' formula which is derived from the Bean model. When DC background field is applied to the HTS tape, n value of the power law E–J characteristics decreases together with the decrease of J_c. Dependence of the AC transport current loss on the n value was analyzed by numerical calculation. The results show that the loss depends on the n value and that decrease of the n value is one of the causes of the discrepancies between the measured data and Norris' formula.     

Theory of optical conductivity and photoemission intensity for Cu---O plane superconductors

The quasiparticle self-energy and the dynamic spin and charge susceptibilities are calculated self-consistently in RPA for the two-dimensional Hubbard model with additional electron-phonon interaction. Vertex corrections lead to an enhancement of charge fluctuations and a suppression of spin fluctuations, thus increasing T_c. The resulting optical reflectivity in the normal and superconducting state is found to be in qualitative agreement with the experiment data on YBa₂Cu₃O₇ for intermediate values of λ_ph and U/t. We calculate also the photoemission intensity in the normal and superconducting state.