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14 Aug 2000

Volume 77, Issue 7, pp. 921-1064

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Low harmonic distortion in a Josephson arbitrary waveform synthesizer

S. P. Benz, C. J. Burroughs, and P. D. Dresselhaus

Appl. Phys. Lett. 77, 1014 (2000); http://dx.doi.org/10.1063/1.1288597 (3 pages) | Cited 18 times

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The use of broadband integrated filters has enabled practical operating margins for ac waveforms synthesized from the perfectly quantized voltage pulses of Josephson junction arrays. This improvement enabled the digital synthesis of arbitrary waveforms with low harmonic distortion; the second and higher harmonics are all at least 94 dB below the fundamental [−94 dBc (carrier)]. This is a 47 dB lower distortion compared to that of the same sine wave synthesized by the semiconductor-based code generator which drives the array. We present operating margins for synthesized dc voltages and demonstrate synthesized waveforms with multiple tones at kilohertz and megahertz frequencies with harmonic distortion and intermodulation products below −80 dBc.
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84.30.Ng Oscillators, pulse generators, and function generators
85.25.Hv Superconducting logic elements and memory devices; microelectronic circuits
85.25.Cp Josephson devices
06.20.F- Units and standards
84.37.+q Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)

Individual Shapiro steps observed in resistively shunted intrinsic Josephson junctions on Bi2Sr2CaCu2O8+x single crystals

H. B. Wang, Y. Aruga, J. Chen, K. Nakajima, T. Yamashita, and P. H. Wu

Appl. Phys. Lett. 77, 1017 (2000); http://dx.doi.org/10.1063/1.1288601 (3 pages) | Cited 17 times

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With a thin gold layer, we fabricated mesa-like shunted intrinsic Josephson junction stacks on Bi2Sr2CaCu2O8+x single crystals. Instead of the multibranch structure and large voltage jumps often observed in conventional intrinsic junctions, current–voltage characteristics typical of resistively shunted junctions were obtained. Individual Shapiro steps were clearly visible with irradiation at frequencies from a few to 20 GHz. The experiments demonstrated not only the ac Josephson effects directly but also the possibility of employing intrinsic Josephson junctions in high-frequency applications. © 2000 American Institute of Physics.
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74.50.+r Tunneling phenomena; Josephson effects
85.25.Cp Josephson devices
74.72.-h Cuprate superconductors
74.25.N- Response to electromagnetic fields
74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions

Magnetoresistance enhancement in specular, bottom-pinned, Mn83Ir17 spin valves with nano-oxide layers

A. Veloso, P. P. Freitas, P. Wei, N. P. Barradas, J. C. Soares, B. Almeida, and J. B. Sousa

Appl. Phys. Lett. 77, 1020 (2000); http://dx.doi.org/10.1063/1.1288672 (3 pages) | Cited 44 times

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Bottom-pinned Mn83Ir17 spin valves with enhanced specular scattering were fabricated, showing magnetoresistance (MR) values up to 13.6%, lower sheet resistance R□ and higher ΔR□. Two nano-oxide layers (NOL) are grown on both sides of the CoFe/Cu/CoFe spin valve structure by natural oxidation or remote plasma oxidation of the starting CoFe layer. Maximum MR enhancement is obtained after just 1 min plasma oxidation. Rutherford backscattering analysis shows that a 15±2 Å oxide layer grows at the expense of the initial (prior to oxidation) CoFe layer, with ∼12% reduction of the initial 40 Å CoFe thickness. X-ray reflectometry indicates that Kiessig fringes become better defined after NOL growth, indicating smoother inner interfaces, in agreement with the observed decrease of the spin valve ferromagnetic Néel coupling. © 2000 American Institute of Physics.
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75.47.De Giant magnetoresistance
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
72.15.Gd Galvanomagnetic and other magnetotransport effects
81.65.Mq Oxidation
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis

Spectacular decrease of the melting magnetic field in the charge-ordered state of Pr0.5Ca0.5MnO3 films under tensile strain

W. Prellier, A. M. Haghiri-Gosnet, B. Mercey, Ph. Lecoeur, M. Hervieu, Ch. Simon, and B. Raveau

Appl. Phys. Lett. 77, 1023 (2000); http://dx.doi.org/10.1063/1.1288673 (3 pages) | Cited 73 times

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An insulator-to-metal transition below 240 K is induced by applying a 7 T magnetic field in Pr0.5Ca0.5MnO3 thin films grown by the pulsed laser deposition technique on [100]-SrTiO3 substrates. This value of the melting magnetic field, much lower that the one required in bulk ( ∼ 20 T), is assumed to be an effect of the tensile stress. These results confirm the importance of the bandwidth in the control of the physical properties of this compound and open the route to get colossal magnetoresistive properties by using strain effects. © 2000 American Institute of Physics.
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75.47.Gk Colossal magnetoresistance
71.30.+h Metal-insulator transitions and other electronic transitions
72.60.+g Mixed conductivity and conductivity transitions
75.50.Dd Nonmetallic ferromagnetic materials
71.45.-d Collective effects

Magnetocaloric effect in La1−xSrxMnO3 for x = 0.13 and 0.16

A. Szewczyk, H. Szymczak, A. Wiśniewski, K. Piotrowski, R. Kartaszyński, B. Dabrowski, S. Koleśnik, and Z. Bukowski

Appl. Phys. Lett. 77, 1026 (2000); http://dx.doi.org/10.1063/1.1288671 (3 pages) | Cited 63 times

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Magnetization of polycrystalline La1−xSrxMnO3 samples with x = 0.13 and x = 0.16 has been measured as a function of temperature and external magnetic field. Using the gathered data and the thermodynamic Maxwell’s relation, changes of the magnetic contribution to entropy under influence of the magnetic field, allowing to estimate the value of magnetocaloric effect, have been determined. The changes reach ∼7 J/(kg K) in the field of 7 T, which makes the studied samples good candidates for application for magnetic refrigeration. Expressions describing entropy and its change under influence of magnetic field have been derived in the molecular field approximation. Satisfactory agreement between the theoretical curves and the experimental dependencies has been found. © 2000 American Institute of Physics.
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75.30.Sg Magnetocaloric effect, magnetic cooling
07.20.Mc Cryogenics; refrigerators, low-temperature detectors, and other low-temperature equipment
75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
65.20.-w Thermal properties of liquids
65.40.gd Entropy

Superparamagnetic magnetization equation in two dimensions

D. C. Jiles, S. J. Lee, J. Kenkel, and K. L. Metlov

Appl. Phys. Lett. 77, 1029 (2000); http://dx.doi.org/10.1063/1.1288677 (3 pages) | Cited 9 times

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An equation for the dependence of magnetization on magnetic field in the case of two-dimensional (base plane) anisotropy has been derived. The resulting equation is expressed as an infinite series of modified Bessel functions, unlike the elementary function expressions that are applicable to the one-dimensional (axially anisotropic) and three-dimensional (isotropic) cases. Nevertheless, in the low-field limit, the series can be effectively truncated to give an approximate solution, while, in the high-field limit, an alternative expression has been derived which represents the limiting function as the field strength tends to infinity. The resulting expressions can be used to describe the superparamagnetic magnetization and susceptibility as a function of magnetic field in situations where the magnetic moments are constrained to lie in a plane, with no preferred direction within the plane. This can therefore be applied to two-dimensional structures, such as magnetic thin films, where magnetostatic energy confines the moments to the plane of the film, or to three-dimensional structures with planar magnetocrystalline anisotropy. © 2000 American Institute of Physics.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Gw Magnetic anisotropy
02.30.Gp Special functions
75.50.Tt Fine-particle systems; nanocrystalline materials

Exchange coupling in NiFe/NiMn films studied by pseudo-Hall effect

Guohong Li, Tao Yang, Qiang Hu, and Wuyan Lai

Appl. Phys. Lett. 77, 1032 (2000); http://dx.doi.org/10.1063/1.1289036 (3 pages) | Cited 3 times

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Pseudo-Hall effect in exchange biased NiFe/NiMn films was measured in magnetic fields rotated in the film plane. Besides accurate determination of the exchange biasing direction, coherent rotation and incoherent reversal of the magnetization of the NiFe layers can be distinguished from these measurements. Detailed magnetic anisotropy analysis shows that there is no additional uniaxial anisotropy induced by the interfacial coupling in this system, while the instability of antiferromagnetic grains may play an important role that is responsible for the difference between exchange biasing fields obtained by reversible and irreversible measurements. © 2000 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
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