Simple Electron Spin Resonance Demonstration Published 6th February 2021

The aim of this project was to design and test a very simple setup that would demonstrate electron spin resonance. The system consists of: - a marginal oscillator (consisting of two npn transistors) - a 1kHz oscillator (LM555) - simple Helmholtz coils (diameter 14mm) - power supply (0-5V/100mA) - voltmeter (0-5V) - radio receiver (USB RTL-SDR dongle and SDRSharp software or Yaesu FT-817) Please find below collection of captioned pictures related to the project.

To demonstrate electron spin resonance, a good sample containing unpaired electrons is essential. Here two samples were tested with excellent results: - 300mg of solid state electrolyte extracted from a 220uF/10V solid state OS-CON Sanyo electrolytic capacitor. OS-CON (1995-2010) series capacitors used the charge transfer salt TTF-TCNQ (tetracyanoquinodimethane) as a solid organic electrolyte, which provides a source of unpaired electrons. The extraction of TCNQ from an OS-CON capacitor must be supervised by a scientist/engineer who is experienced with respect to this process, and who will assume full responsibility for the safety of those involved. TCNQ and TTF may be regarded as low-toxicity compounds [link], however solid organic electrolytes may contain other toxic compounds. - 300mg of TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl); this compound is available on eBay CuSO4 5H2O (copper sulfate pentahydrate) was also tested in attempt to register an electron paramagnetic resonance (epr) signal but unfortunately the signal was extremely weak.

An approximately uniform magnetic field is produced by two identical miniature electromagnets (diameter 14mm) in a Helmholtz configuration (Helmholtz coil). Each electromagnet consist of two windings: - 0-2mT winding: 200 turns of 0.1mm enameled copper wire (internal diameter 14mm) - 0.1mT 1kHz modulation coil of 50 turns of 0.1mm enameled copper wire (internal diameter 14mm) A 5ml single-use syringe was used as a former for the above coils. Next, the coils were removed from the former and secured with isolation tape. Finally, .4mm enameled copper wire was wound toroidally around the coils to produce an electric shield and heat sink.

The RF coil consists of 10.5 turns of 0.7mm enameled copper wire with internal diameter 5mm. A drill bit was used as a former for this coil.

All the coils were soldered to a small piece of prototyping PCB. The Helmholtz coil consists of two electromagnets with an internal diameter of 14mm and a separation of 7mm. The magnetic field created by the Helmholtz coils is perpendicular to the oscillating magnetic field created by RF coil. A 30pf capacitor is added parallel to the RF coil to produce an RF tank. The windings for 0-2mT (2x 200turns) are connected in series. Resistance = 50 Ohm 3 Volts produce 1.7mT The windings for 0.1mT 1kHz (modulation, 2 x 50 turns) are connected in parallel. Resistance = 12.5Ohm 200mV produce 0.1mT

NanoVNA - was used to measure the resonant frequency of the LC tank ( = 52MHz) The NanoVNA is very small handheld Vector Network Analyzer (VNA); designed by edy555, it is a portable, high-performance standalone device with an lcd display, touchscreen and battery. It was originally designed to work at 50kHz-300MHz.

A differential cross-coupled LC oscillator is used as marginal oscillator. Even though the number of circuit elements is small (2 x npn transistor, LC circuit and single resistor), the theory is complicated. A passive LC tank resonator is activated through a differential negative oscillator. The presented circuit is very unstable. The absorption of RF by the sample inserted to RF coil affects both amplitude and frequency (allowing both AM and FM detection). The circuit is also very sensitive for voltage fluctuation (can used as a voltage controlled oscillator)

A tinySA is used to measure the frequency and assess the sensitivity of the cross coupled oscillator. The sensitivity depends on the value of the resistor and the voltage of the power supply. The circuit is powered from a CR2032 coin battery and the resistance R is between 1 -10kOhm. If a fly (insect) passes near the RF coil (few cm) this will measurably change the frequency and amplitude of the oscillator due to RF absorption by the fly (obviously using some small object like a pencil or finger is more practical to measure sensitivity). Please note that the cross coupled oscillator's frequency is below the resonant frequency of the LC tank. For LC = 52MHZ, the oscillator generates between 25-50MHz depending of the value of resistor R and the voltage of the power supply. The tinySA is a small spectrum analyzer, primarily intended for 0.1MHz to 350MHz. Aternatively, an RTL-SDR can be used as spectrum analyzer.

The RTL-SDR is an ultra cheap software-defined radio receiver based on DVB-T TV tuners with the RTL chip. The RTL-SDR is used in this project as a radio receiver for 40-50Mhz. SDR# (SDR Sharp) is a very popular Windows SDR program often used with the RTL-SDR. In this project an SDR# waterfall graph allows us to observe electron spin resonance. Additionally, an NFM detector allows us to listen to the 1kHz tone when resonance conditions are satisfied.

Block diagram of first experiment: 1. The magnetic field is slowly increased from 0 to 2mT by manually increasing the voltage on the power supply 2. The antenna connected to the RTL-SDR with SDR# software (radio receiver) is positioned a few centimeters from the cross-coupled oscillator and an SDR# frequency waterfall graph is observed. There is no "physical" connection between the cross-coupled oscillator and radio receiver - there is only a weak radio link with a range of about 10-20cm.

Cross coupled oscillator with a sample in the RF coil.

At resonance conditions (around 1.7mT for 48MHz) an clear dip is visible on the waterfall display.

The previous experiment can be significantly improved by adding a 1kHz magnetic field (audio tone, second modulation). A simple circuit is presented to the right, it can be replaced by any generator capable of producing a 0.1mT variable magnetic field.

Audio modulation tone with frequency around 800Hz and amplitude around 200mV. The shape of the audio tone is not important for a simple demonstration.

Block diagram of the second experiment: 1. The 1kHz 0.1mT magnetic field is switched on. 2. The magnetic field is slowly increased from 0 to 2mT by manually increasing the voltage on the power supply 3. The antenna connected to the RTL-SDR with SDR# software (radio receiver) is positioned a few centimeters from the cross-coupled oscillator and the SDR# frequency waterfall graph and audio output is observed. There is no "physical" connection between the cross-coupled oscillator and radio receiver - there is only a weak radio link with a range of about 10-20cm.

Setup of the second experiment: Cross coupled oscillator Helmholtz coils 1kHz 0.1mT generator RTL-SDR (receiver)

At resonance conditions (around 1.7mT for 48MHz) on a waterfall frequency display there is a clear frequency deviation together with amplitude modulation sidebands. Additionally, there is a clear audio tone (please watch the youtube video on the top of this page).

The tinySA can be used during experiments to monitor the frequency of the cross-coupled oscillator. Alternatively, a frequency meter or RTL-SDR with spectrum analysis software can be used.

A magnetometer can be used to monitor the magnetic field. Initially, an Arduino Nano with an SS49E linear Hall-effect sensor was used with very poor results. An HT-20 magnetometer (to the right) produced slightly better results but there is still poor reproducibility, a floating zero and low resolution (0.2-0.3mT). The TD8620 has much better specifications (resolution 0.01mT, auto zero) with a cost comparable to that of the HT-20.

The RTL-SDR (NFM modulation) works perfectly fine, however other much more expensive radio receivers were also tested with similar results. Specifically, the Yaesu FT-817 to the left (best results for NFM modulation) and Yupiteru MVT-7100 was tested, which interestingly for ESR demonstration work better in AM mode.

The setup for another experiment recorded on the youtube video (top of this page) consists of a cross-coupled oscillator, 1kHz generator, magnetometer, Yaesu FT-817 and oscilloscope (top blue channel - audio output from receiver, bottom yellow channel - audio modulation). When the oscilloscope is observed carefully (as in the youtube video on the top of the page) the audio signal is clearly visible, with maximal amplitude just before and after resonance, with zero amplitude at resonance. Additionally, a 180 degree change of phase of audio output signal is clearly visible.

And finally, when the radio audio output signal is connected to a slow scan, fast sampling digital oscilloscope (Hantek 6022BE), a characteristic radiospectroscopy butterfly is recorded. Any asymmetry of the butterfly may be due to a very low field, potential lack of field homogeneity or manual sweeping of the magnetic field.

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