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  • 1
    Electronic Resource
    Electronic Resource
    Low temperature properties of soft magnetic materials: Magnetic viscosity and 1/f thermal noise : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5363-5365 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We study the magnetic properties of soft ferromagnetic cores in the low field linear regime at low temperature (T≤4.2 K) in view of their use in connection with superconducting quantum interference device magnetometers. We have tested several amorphous and polycrystalline alloys in the form of strip-wound toroidal cores. Both high and near zero magnetostrictive alloys are studied. All samples are found out to be the source of thermal magnetic noise of 1/f spectral density in agreement with the fluctuation dissipation theorem. This type of thermal noise comes from a frequency independent imaginary permeability. This feature is equivalently described in terms of the magnetic viscosity effect or by considering the constant (residual) term in the classical Legg's equation of magnetic losses at low fields. Measurements on a few Co-based amorphous alloys show that the magnetic viscosity is independent of temperature. Explanations in terms of thermal activation processes and quantum tunneling are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353732
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  • 2
    Electronic Resource
    Electronic Resource
    Magnetic viscosity, thermal relaxation, and thermal equilibrium noise in Co-based amorphous alloys at milliKelvin temperatures (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 4820-4825 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report measurements of the magnetic reversible complex permeability and of the related thermal noise for a few Co-based soft ferromagnetic amorphous alloys. The measurements were performed in the 16 mK–4.2 K temperature range and in the 0.1 Hz–1 kHz frequency range with a superconducting quantum interference device magnetometer. The data show the existence of a temperature independent magnetic viscosity together with a thermal relaxation of the spin subsystem to the heat bath. This last feature shows up as a partial rolloff of the real permeability from an isothermal low-frequency value to an adiabatic high-frequency one. The noise due to the sample is found in good agreement with the standard fluctuation-dissipation prediction for the thermal noise.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352045
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  • 3
    Electronic Resource
    Electronic Resource
    Random telegraph signals and low-frequency voltage noise in Y-Ba-Cu-O thin films (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 5440-5449 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Excess low-frequency noise extending to MHz frequencies was observed in dc current biased granular high-Tc thin films. At particular bias conditions random telegraph signal produced by a single, fast two-level fluctuator dominated the noise properties of the sample. Lifetimes of the low- and high-voltage states of the fluctuating system were found to be exponentially distributed. Power spectra of the excess noise signal could be well fitted with a single Lorentzian contribution. Duty cycle dependence of the random telegraph signal on bias conditions was used to get an insight into physical mechanism causing the fluctuations. Charge trapping events in the intergranular intrinsic Josephson junctions and trapped flux hopping were identified as possible alternative sources of the observed noise.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350202
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  • 4
    Electronic Resource
    Electronic Resource
    Spectral properties of rf emission from high Tc films (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 3029-3031 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Spectral properties of rf radiation from intrinsic Josephson junctions in high Tc Y-Ba-Cu-O thin film have been measured in the frequency range up to 1.5 GHz. Narrow emission lines with the 3 dB bandwidth of the order of 20 MHz were detected indicating that Josephson clusters radiate coherently. Synchronization conditions are determined by dc current and external magnetic field bias. Frequency locking of radiation to external resonant circuit was also observed. Spectral line narrowing due to resonant lock was distinguished from the coherence-induced narrowing by different tuning properties of the emission line. Noncoherent Josephson radiation manifests itself as a broadband background noise increase. A pronounced 1/f-like tail sensitive to dc bias and magnetic field was observed in a low frequency part of the spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.346392
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  • 5
    Electronic Resource
    Electronic Resource
    Thermal equilibrium noise with 1/f spectrum in a ferromagnetic alloy: Anomalous temperature dependence : The 6th joint magnetism and magnetic materials (MMM)−intermag conferenc (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 6332-6334 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on the magnetization noise in a soft ferromagnetic alloy for frequencies f〈10 Hz, temperatures T〈4.2 K, and excitation field amplitudes 〈150 μA/m. The spectra agree, without any adjustable parameter, with the fluctuation dissipation formula and the measured complex susceptibility of the material. The spectra show a 1/f shape coming from a frequency-independent imaginary susceptibility χ‘0. χ‘0 is found to be proportional to T above ≈2.5 K but tends to be independent of T below this value. After annealing the sample to relieve the internal stresses, χ‘0 is found nearly temperature independent above ≈2.5 K and to sharply increase by decreasing the temperature below that value. We discuss these findings in terms of the hopping of the magnetization by activation or tunneling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358257
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  • 6
    Electronic Resource
    Electronic Resource
    Thermal equilibrium noise with 1/f spectrum and temperature-dependent magnetic viscosity in the amorphous alloy DyNi : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 5634-5636 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on the magnetization noise and the imaginary susceptibility for the random anisotropy amorphous alloy DyNi1.32 for frequencies f〈10 Hz, temperatures 1.2〈T〈4.2 K, and excitation field amplitudes 〈150 μA/m (1.9 μOe). The noise spectra agree, without any adjustable parameter, with the fluctuation dissipation formula and the measured imaginary susceptibility of the material. The spectra show a 1/f shape coming from a frequency-independent imaginary susceptibility that it is found to be temperature-dependent extrapolating to zero at zero temperature. We discuss the connection between this approach and the usual relaxation experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355617
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  • 7
    Electronic Resource
    Electronic Resource
    A high inductance kHz resonator with a quality factor larger than 106 (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 2916-2919 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Electrical LC resonators with superconducting coils of ≈3.5 H inductance, operating in the frequency range 250–1500 Hz with quality factors up to Q≈1.6×106 are presented here. The coil has a reduced, 〈100 pF, stray capacitance and is housed in a superconducting case. Measurements are made with a low coupling SQUID readout. Some possible applications of the device are briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144639
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  • 8
    Electronic Resource
    Electronic Resource
    Planar microwave biased radio frequency SQUIDs with a cryogenic preamplifier (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 61 (1992), S. 1231-1233 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on the design and performance of a planar microwave rf-SQUID readout by a cryogenic preamplifier. The SQUID-sensor consists of a planar half-wavelength microstrip resonator, into which the SQUID-loop is integrated. The pump frequency is 1.7 GHz. The cryogenic preamplifier employs a microwave high electron mobility transistor (HEMT), located close to the SQUID sensor. Measurements of the flux noise and the fractional step rise parameter were carried out in the temperature range between 4.2 and 1.5 K. In an open loop white flux noise values of 2×10−6 Φ0/(square root of)Hz were obtained at 4.2 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.107604
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  • 9
    Electronic Resource
    Electronic Resource
    Ultrahigh frequency thin film rf-SQUID magnetometer with a cryogenic preamplifier employing a high-electron-mobility transistor (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 5403-5407 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Ultrahigh frequency rf-SQUID magnetometer employing a cryogenic preamplifier and a commercially available thin film rf-SQUID sensor have been designed, manufactured, and tested. The system has been operated at 316 MHz, which is the maximum pump frequency accepted by the thin film sensor. The cryogenic preamplifier has been built around a microwave high-electron-mobility transistor which was made unconditionally stable at ultrahigh frequency band by means of an appropriate circuit design, namely, a cascode configuration. The total flux noise of the magnetometer has been found to be 8.4 × 10−6Φ0/(square root of)Hz at 4.2 K and 5.8× 10−6Φ0/(square root of)Hz at 1.4 K. The temperature dependence of the fractional step rise α of the current-voltage rf characteristic has been measured to be proportional to the 2/3 power of the SQUID sensor temperature. The noise performance of the system at 4.2 K has been limited by the intrinsic SQUID noise only, in good agreement with theoretical predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143409
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  • 10
    Electronic Resource
    Electronic Resource
    Thermal magnetic noise due to eddy currents in a strip wound ferromagnetic core at 4.2 K (abstract) : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6996-6996 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Many applications involving high impedance signal sources and use of the low input impedance SQUID magnetometer would take advantage of the use of low noise ferromagnetic cores in matching transformers. Noise in ferromagnetic cores at liquid helium temperatures has been shown to be limited by thermal fluctuation of the magnetization for the two following cases: for bulk cores, where the skin effect sets a low frequency (〈100 Hz) roll-off of the effective permeability, and the spectral density is white up to the roll-off, and for strip wound cores at frequencies lower than ≈100 Hz, where magnetic viscosity gives origin to a frequency independent imaginary permeability and correspondingly to a 1/ω shaped spectral density. Thanks to a high sensitivity dc SQUID magnetometer we have now been able to measure thermal noise due to the skin effect in a strip wound 15-μm-thick Ultraperm core where the roll-off frequency is ≈1 MHz. Data for the noise are in quite quantitative agreement with the prediction of the fluctuation dissipation formula and the measured values of μ(ω). At frequencies lower than a corner ≈1 kHz the noise is again dominated by the 1/ω tail due to magnetic viscosity. We discuss the relevance for applications.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356753
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