The “LC resonator” method was greatly improved for high resolution measurements of magnetic-field penetration depth, λ, of radio frequency into thin films or plate-like high-Tc superconductors by replacing a solenoid testing coil by an open-flat coil (OFC) driven by a tunnel diode oscillator of a low power and a highly stable frequency. The assembled OFC magnetometer showed new technical advantages such as simplicity, convenience, and high resolution of Δλ∼1–3 Å (Δλ/λ∼10−6), ability of reliable operation in wide ranges of temperature (4.2–300 K) and magnetic field (up to 12 T), etc. This enables us to use it for studying peculiarities of the vortex dynamics with weak signal in small sample volume high-Tc cuprates. As a first test of the assembled 23 MHz measuring setup, the temperature dependence of the upper critical field, HC2(T), is determined from the magnetic transition curves at different temperatures for Y1Ba2Cu3O7−δ thin-film ring. The HC2(T) data obey the formula: [1−(T/Tc)2]β with β=1.22±0.03 at temperatures close to Tc. The methods to increase the resolution of the OFC magnetometer by several orders of magnitude are suggested. It is believed to be feasible to reach the level of detection of the suppression of superconductivity in samples of size of a few micrometers by penetration of a few flux quanta by decreasing the size of the open-flat coil.

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The simple method of modeling of tunnel diode (TD) oscillators’ work is described, and may be applied also to the other types of negative-differential conductance oscillators. In particular, the calculation of the harmonic oscillation amplitude is performed by the energy balance method, based on TD measured the current–voltage characteristic’ (I–V curve), using only one fitting parameter—the tunnel diode’s measured bias at which oscillations are quenched by means of which the oscillator’s effective total losses are determined also. The formula for calculation of the TD generator’s frequency is derived; it agrees with experiment and permits determination of its dependence on the radiotechnical scheme’s elements and TD I–V curve’s shape. The results of the calculation in comparison with the experiment are reported for the oscillators of various constructions operating at different conditions. For this, more attention is given to the constructive solutions applied in the low temperature experiments. Examples of the TD oscillators’ use in cryogenic techniques as well as in the superconducting detectors of particles and radiation are presented. The oscillators described generate, under the certain conditions, low-amplitude (<40 mV) sinusoidal signals with a highly stable frequency (not worse than ΔF/F=10−9/min at liquid-helium temperatures) and low intensity of higher harmonics in the soft mode. The power released is less than 10 μW and the measuring rf magnetic field’s amplitude at a sample, placed inside or near the oscillator coil, is less than 1 mOe. Using designed facilities we have already detected normal-to-superconducting phase transitions of tin grains with diameters of about 2 μm and phase transitions in micrometer thick and 20 μm2 high-Tc superconducting films.

The improved `LC-resonator' technique (the open-flat coil magnetometer), based on a low-power stable-frequency tunnel diode oscillator with a single-layer open-faced coil, is very sensitive to changes of position. About 1 Å absolute and better than 10-6 relative resolution can be reached in distance change measurements. It is an excellent MHz-range scientific instrument with which one may study weakly expressed fine peculiarities of the superconductive state in small-volume thin plate-like high-Tc superconductors (HTS). It enables changes to be detected of about 1 pH of the HTS film's magnetic inductance (changes of ~1-3 Å of the magnetic penetration depth, λ) with very high (~10-6) relative resolution. Owing to the pick-up coil's flat design, relatively low operation frequency and high relative resolution, the present method has advantages over others. It is essential for non-destructive studies of thin flat HTS materials (with a small signal) especially near Tc (at the beginning of the formation of `Cooper' pairs). Mainly as the result of these advantages a new `paramagnetic' peculiarity of the superconductive transition has been detected in Y-Ba-Cu-O film, which precedes the well known `diamagnetic' ejection. A possible explanation of the observed effect is given, and the influence of some external physical parameters on the effect is revealed and discussed.

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