Magnetic impurities in parts per trillion can be revealed when a crystal is rung like a bell

A new spectroscopy technique uses microwaves to ‘ring’ a crystal, revealing the concentration of impurities based on their unique vibrational signatures.

Scheelite, or calcium tungstate (CaWO₄), is a material of great interest for its potential uses in quantum and electronic technologies, and advanced dark matter physics experiments.

A 2024 EQUS study of the crystal sought to quantify the tell-tale markers of impurities, which can be used to modify the material’s electronic properties towards certain sensing applications.

The team used advanced techniques in microwave spectroscopy to detect and characterise tiny impurities with exceptional sensitivity (parts per trillion), based on the effects of their quantum spin on the material’s magnetic resonance.

The work supports the growing role of novel, designer materials in both technological innovation and fundamental physics.

Ringing a finely tuned bell

Microwave spectroscopy analysis of a crystal is like hitting a finely tuned bell.

When the microwaves striking this ‘bell’ align perfectly with the crystal’s intrinsic properties, resonance occurs, and the crystal ‘rings’ at its natural, resonant microwave frequency.

In this case, it is the magnetic moments of the crystal’s quantum electron spins that determine the intrinsic resonant frequency, with an applied external magnetic field allowing ‘tuning’ of the tone.

When impurities such as gadolinium and iron ions are present, they cause a unique and identifiable change in that resonant frequency, with analysis of the intensity of the change in ‘tone’ revealing the concentration of impurities present.

“The gadolinium tones originate from the spin transitions of unpaired electrons,” explains lead author EQUS PhD candidate Elrina Hartman (UWA).

“And the spin transition energy can be tuned by an applied magnetic field, so when the spin’s tone matches the intrinsic crystal’s tone, we see a modulation of the crystal’s tone.”

Because the ‘loudness’ of the gadolinium signal is dependent on the strength of the resonant coupling between microwaves and the electron spins, it is proportional to the number of electron spins.

This proportionality of resonant coupling and electron spins amplifies the response and allowed the research team to detect even the faintest impurities.

Remarkable levels of sensitivity

In the 2024 study, the team’s application of microwave electron spin resonance (ESR) spectroscopy to undoped single-crystal calcium tungstate revealed trace amounts of two types of paramagnetic impurities: gadolinium ions (Gd3+) and iron ions (Fe3+) in concentrations as low as parts per billion.

These impurities did not compromise the crystal’s performance as a high-fidelity resonator.

“In fact, doping a crystal with such impurities can be used to shift its electronic properties, opening doors to new applications,” explains Elrina.

The demonstration of exceptional sensitivity in detecting and characterising such minute levels of magnetic impurities opens the way to potential new uses for the material, using finely controlled doping to ‘tune’ its functional properties, for example in future sensing technologies.

The researchers further determined that the remarkable sensitivity of calcium tungstate as a resonator meant the ESR technique would be capable of detecting impurities at even lower concentrations, as small as parts per trillion.

This level of sensitivity would unlock a number of new applications in materials science and quantum physics.

Future directions and applications

Calcium tungstate’s performance as a high-quality resonator positions it as a key material for quantum computing, with embedded gadolinium ions potentially serving as spin qubits – the units of information that store and process quantum data.

The UWA study also bolsters efforts in precision metrology, where accurate measurements are essential for applications ranging from timekeeping to advanced sensing technologies.

The team also quantified the highly temperature-dependent dielectric properties of the material, with this offering potential use as a radiation sensor.

For example such sensing potential could be applied to dark matter detection, where interactions with weakly interacting massive particles (WIMPs) could be detected through temperature-dependent changes in the crystal’s properties.

“By investigation of the material’s low-temperature behaviour, we identified several promising research avenues for the future,” says Elrina.

“It was important to establish the dielectric properties of the crystal and impact of impurities at low temperatures, as these effects have not been explored before.” Ongoing experiments are already testing the material’s temperature response between millikelvin and 4 Kelvin.

While the study has laid a strong foundation, it also highlights several challenges that need to be addressed. One major area of focus is the interaction between spins in the crystal and photons, for example, which could unlock new capabilities in quantum sensing and information processing.

Additionally, understanding the nonlinear and thermal dynamics of the system under varying conditions will be critical for both clock development and dark matter research.

EQUS Chief Investigator, Maxim Goryachev, the corresponding author, emphasized the broader significance of this work: “Spins in solids like calcium tungstate offer long coherence times, which are essential for stability and precision in future hybrid quantum clocks.” As such, the work aligns with the research goals of the EQUS Clock flagship, which focuses on identifying spin systems with long coherence times to improve the stability and phase noise of quantum clocks.

This story was first published in the 2024 EQUS annual report.