S.A. Gudoshnikov

Hysteresis losses in a dense superparamagnetic nanoparticle assembly

The hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D = 25 nm are measured in the frequency range f = 10 – 200 kHz for magnetic field amplitudes up to H0 = 400 Oe. The low frequency hysteresis loops of the assembly are obtained by means of integration of the electro-motive force signal arising in a small pick-up coil wrapped around a sample which contains 1 – 5 mg of a magnetite powder. It is proved experimentally that the specific absorption rate diminishes approximately 4.5 times when the sample aspect ratio decreases from 11.4 to 1. Theoretical estimate shows that experimentally measured hysteresis loops can be approximately described only by taking into account appreciable contributions of magnetic nanoparticles of both very small, D 30 nm, diameters. Thus the wide particle size distribution has to be assumed.

Magnetic flux guide for high-resolution SQUID microscope

Magnetic properties of a permalloy needle designed for use in a high-spatial-resolution scanning SQUID magnetic microscope as a magnetic flux guide have been examined. The distributions of the magnetic field normal component over a needle were imaged at 77 K in various applied magnetic fields. Influences of the flux guide on the high temperature superconducting dc SQUID characteristics have been investigated.

Relaxation-oscillation-driven DC SQUIDs

Simple and reliable relaxation-oscillation (RO)-driven DC SQUIDs (superconducting quantum interference devices) have been fabricated with a flux-to-voltage factor H up to 1 mV/ Phi /sub 0/ and internal energy sensitivity E/sub v/ of (4+or-1) * 10/sup -31/ J/Hz at frequencies above 10 Hz and 1*10/sup -29/ (1 Hz/f) J/Hz at lower frequencies. Supplied with a 50- mu m-wire 20-turn spiral input coil and a 6-cm/sup 2/ pickup loop, such SQUIDs provided magnetic field sensitivity close to (6+or-3)*10/sup -15/ T/Hz/sup 1/2/ above 10 Hz. The dynamic range of the magnetometers was close to 1.5*10/sup 7/ Hz/sup 1/2/ above 10 Hz. The maximum slew rate was about 10 mu T/s at 0.1 kHz. Successful records of Earth-ionosphere magnetic resonances have been obtained with these magnetometers housed in a stainless steel transport dewar. A balanced version of the RO-driven DC SQUID is suggested with improved H and E/sub v/. >

Magnetic field maps of YBCO thin films obtained by scanning SQUID microscopy for HTSC microelectronics

Scanning SQUID microscopy has been extended to record current-induced magnetic field maps of superconductor topologies based on YBCO film structures. The technique described can be used as a powerful tool for the diagnostics of superconductor integrated circuits and highly sensitive bolometers. This kind of diagnostics predicts the degree of integration which can be achieved at the modern level of HTSC microelectronics technology. One would expect an overall yield of 100-element chips at a level of in the production of superconducting YBCO/MgO bolometers.

SQUID microscope for magnetic structure visualization in magnetoimpedance elements

An attempt to apply a SQUID microscope for visualization of magnetic structure in giant magnetic impedance (GMI) elements was undertaken. The measurements were carried out on the 1/spl times/10/sup -3/ mm thick permalloy strip with the in-plane sizes 6 mm/spl times/0.3 mm. The topography of the vertical component of magnetic field was imaged with a high spatial resolution close to 30/spl times/10/sup -3/ mm over the central part of a sample. The images taken in the applied magnetic field varied over the range /spl plusmn/4 kA/m reveal the domain structure with anisotropy perpendicular to longitudinal sample axis.

Direct-coupled electronics for high-temperature superconductor DC SQUID-based magnetometer

A low-volume, high-performance, direct-coupled electronic system for high-T/sub c/ dc SQUID magnetometers has been developed and tested. This system is based on a liquid nitrogen-cooled amplifier and a modified integrator. The amplifier has a white noise voltage level close to 0.25 nVHz/sup - 1/2 /, a current noise of the order of 3 pAHz/sup - 1/2 /, a gain equal to 3000, a 3 dB bandwidth of about 370 kHz, and a package volume less than 3 cm/sup 3/. The modified integrator has an input resistance of the order to 10 k/spl Omega/ and a considerably high output drive current of about 30 mA. This unique combination results in the typical electronic 3 dB bandwidth of about 1 MHz, a dynamic range of /spl plusmn/5/spl times/10/sup 6/ Hz/sup - 1/2 / for frequencies up to 1.6 kHz, and a slew rate of 1/spl times/10/sub 6/ flux quanta/s in a wide signal frequency range for a middle-quality high-T/sub c/ dc SQUID sensor.

A planar picoamperemeter based on a superconducting quantum interferometer

An optimized picoamperemeter based on a superconducting quantum interferometer device (SQUID) for a metal cold-electron bolometer is fabricated and experimentally studied. The intrinsic SQUID current noise caused by the input coil is estimated to be less than 1 pA/Hz1/2. Owing to the application of modulation electronics, current sensitivity in the input coil reaches 5 pA/Hz1/2 in the frequency band from 10 Hz to 10 kHz.

HTS SQUID microscopy for measuring the magnetization relaxation of magnetic nanoparticles

HTS SQUID microscopy is applied for measuring the magnetization relaxation of the ensembles of noninteracting Fe/sub 3/O/sub 4/ nanoparticles dispersed in rigid polymer matrix preventing nanoparticles from agglomeration. Transmission electron microscopy (TEM) is used to determine the size distribution of magnetic nanoparticles and to control the homogeneity of their spatial distribution in the matrix. High sensitivity of SQUID microscope allows us to study samples at low contents of the magnetic component (0.1-0.5 vol%) magnetized in low magnetic field (/spl sim/10/sup -4/ T) produced by a low-inductance coil with short switching time (20 /spl mu/s). A low content of the magnetic component provides the absence of interparticle dipolar interactions, thus simplifying significantly the theoretical description of the magnetization relaxation process. The detection of magnetization relaxation starts after the external magnetizing field is switched off and proceeds for several minutes. A series of samples with a mean size of nanoparticles of about 7 nm possessing relaxation times less than 5 min was measured at a temperature of 77 K. The calculation formula based on the Stoner-Wohlfarth model for single-domain particles with a size distribution as determined from the TEM images will enable to obtain reliable data on the anisotropy constant and the saturation magnetization of Fe/sub 3/O/sub 4/ nanoparticles studied.

A direct readout high-T/sub c/ dc SQUID electronics with ac bias and a liquid-nitrogen-cooled preamplifier

Direct readout dc SQUID electronics based on a liquid-nitrogen-cooled preamplifier and alternating current bias was developed. The preamplifier with an intrinsic voltage noise of 0.18 nV/Hz/sup 1/2/ was used as the first stage of this electronics. A mathematical description of the noise suppression by this electronics was done. Low-frequency noise was reduced by a factor of 6 at 1 Hz by this technique.

A dc SQUID-based magnetic microscope study of the magnetic properties of the Ni thin films

We have applied a developed HTS dc SQUID based magnetic microscope for study of the magnetic properties of ultrathin Ni films. A magnetic moment sensitivity close to 10/sup -15/ A.m/sup 2//Hz/sup 1/2/ has been obtained at 77 K. The space resolution was determined at level of 100 /spl mu/m by the separation between the SQUID and the studied film surface. One-domain structure, in-plane orientation of magnetic moment and ferromagnetic-like ordering in 26 /spl Aring/ thick film were found. An absolute value of the film magnetic moment has been estimated.