Timeline of tunnel diode oscillator based experimental methods

region, any small decrease in voltage will result in a corresponding increase in current. Connected to the oscillating voltage drop of an LC tank, the negative resistance can be used to compensate for the

resistance losses of the LC circuit, the necessary condition for stable oscillations, provided that certain conditions are met, which we will detail later. The resulting circuit is called a tunnel diode oscillator (TDO) and is capable of resonating at frequencies from kilohertz to well into the microwave band. Since its frequency is always at resonance, very small value changes of its LC components will result in significant changes in frequency. This characteristic provides the foundation of using the TDO circuit as an experimental tool in investigating the physical properties of matter. The most common application consists in probing the magnetic susceptibility of materials. A magnetically active sample brought in close proximity of the inductor of a TDO circuit, will result in a change in the value of its inductance which, in turn, will generate a measurable corresponding resonant frequency shift. An analogous frequency shift can be induced by changes in the capacitance value due to a change in the electric permittivity however we will just mention this case without giving it any further consideration.

The first reported use of a TDO circuit operating at 15 MHz, occurred in 1969 when R. Meservey et al. [117] published their work detailing temperature measurements of kinetic inductance of superconducting structures. They used the TDO technique to determine the carrier concentration from penetration depth in films and wires of superconducting samples placed inside an LC tank at liquid helium temperature (with the tunnel diode and the rest of the components at room temperature).

R. B. Clover and W. P. Wolf [118] used a similar circuit a year later and reported on the

successful use of the TDO method for paramagnetic susceptibility measurements at frequencies from 3 to 55 MHz, at temperatures from 1.2 to 77 K, and at magnetic fields up to 18 kG. They also proposed a semi-empirical formula for describing the frequency of TDO operation.

In 1971 Y. J. Kingma and V. Dvorak [119] proposed to use of a tunnel diode oscillator as a proximity switch based on a study of two mutually coupled resonant circuits. They found that, depending on the coupling strength of the two coils, the TDO can switch oscillation modes.

J. Aslam and W. Weyhmann published a paper in 1973 [120] presenting a tunnel diode oscillator used for NMR studies in ferromagnetic materials at VHF an UHF. Their circuit design included an electronic tuning of operating frequencies in a relatively broad range.

In 1975 C. T. Van Degrift [121] reported on the construction and the results of a systematic study of the design considerations of a tunnel diode oscillator for 0.001 ppm measurements at low temperatures Comprehensive calculations regarding the measured frequency, noise and dependence on bias voltage, magnetic field, and temperature of the TDO circuit are also presented in the paper. It also suggested that the TDO method can be used to detect extremely small changes in a number of material properties such as thermal expansion, surface impedance, and electric and magnetic permeability. Later, a number of

publications reported the use of a TDO technique to study the temperature and magnetic field dependence of the rf susceptibility in insulators [122], organic compounds [123] and superconducting and magnetic thin films and surfaces [124] with the latter reporting that the device is capable to detect a change in susceptibility equal to that of a change in Fe thin film thickness of 0.03 atomic layers.

A paper published in 1986 by J. G. Brisson and I. F. Silvera [125] discusses the use of and theory behind a transmission-line tunnel diode oscillator with quick response times and immunity to stray reactance of the reentrant cavity.

G. J. Athas et al. [126] in 1993 reported on the first application of a tunnel diodecircuit to investigate the de Haas–van Alphen effect and superconducting critical field values in small single crystal organic conductors.

The investigation of vortex dynamics and penetration depth in high Tc superconductors using a tunnel diode oscillator technique was reported by S. Patnaik et al. in 1999 [127].

In 1999 a paper by H. Srikanth et al. [128] describes the use of a TDO for precise measurements of relative magneto-impedance changes in materials directly from the measured shift in TDO resonance frequency.

In 2000 T. Coffey et al. [129] present the details of an apparatus that extended the tunnel diode techniques to measure the properties of materials in pulsed magnetic fields in a their paper. The sample is placed in the inductor of a small radio frequency (rf) tank circuit powered by a tunnel diode where the conductivity, magnetization, or penetration depth can be measured depending on the sample and configuration of the radio frequency field. A major innovation is reported regarding the stabilization of the tunnel diode oscillator during a magnet pulse by using compensated coils in the tank circuit.

In a series of papers spanning from 1997 to 2004, S. G. Gevorgyan et al. [130-138] make use of a flat coil based tunnel diode oscillator to increase the filling factor of thin films or plate like samples. They discuss the theory and modeling of tunnel diode oscillators and the use of their open flat coil

magnetometer to study the superconductive properties of HTC materials with temperature and magnetic fields.

In 2000 L. Spinu et al. [139] suggested and implemented the use of the resonant TDO technique to prove the field response of dynamic transverse susceptibility in magnetic nanoparticle systems resulting in a precise mapping of fundamental parameters such as magnetic anisotropy and switching fields. The tunnel diode oscillator has proved to be a great tool in probing the transverse magnetic susceptibility of novel materials and structures such as magnetic nanostructures [140, 141], nanoparticle systems [139, 141, 142] and arrays [142, 143], magnetic multilayered structures [144, 145], and synthetic antiferromagnets [146, 147].

R. Prozorov et al. [101] in 2000 showed that the variations in London penetration depth of disk or rectangular slab shaped superconductors is directly proportional to the resonant frequency shift of the tunnel diode oscillator. The linear relation together with the great sensitivity of the TDO technique makes it an unparalleled tool in probing the temperature dependence of the magnetic penetration depth and consequently in investigating the pairing symmetry of unconventional superconductors.

Over the last decade a large amount of publications reported the use of the TDO method for the low temperature study of London penetration depth in novel superconductors. A more detailed overview of the TDO use in probing the low temperature penetration depth behavior of superconductors will be presented in Chapter V, however, some of the most active groups in the experimental field are the group from Ames Laboratory Iowa State University (R. Prozorov and A. Tanatar), University of Illinois at Urbana (E. M. Chia and M. B. Salamon) and the H. H. Wills Physics Laboratory at University of Bristol (A. Carrington).