A new twist on measuring spin: NIST Develops Microresonator to Study Electron Spin in Small Volumes

Researchers at the National Institute of Standards and Technology (NIST) have developed a device to detect a quantum property of electrons, known as spin, in extremely small volumes of material and among samples that contain only trace amounts of molecules or ions of interest. The device may enable scientists to accurately measure this fundamental property, which is related to magnetism, in single crystals of biological materials, ultrathin films such as graphene, and other tiny specimens.

Electrons and other subatomic particles behave as if spinning around a central axis, which generates a tiny magnetic field. The spin of an individual electron can only have two values; the electron spin is said to be pointing either ‘up’ or ‘down’ analogous to the north or south poles of bar magnets. In most atoms and molecules, neighboring electrons pair off in such a way that their spins point in opposite directions, canceling each other out and producing a net magnetic field of zero. Atoms and molecules in which some electrons remain unpaired are known as paramagnetic.

Spin measurements of electrons in paramagnetic materials can reveal essential properties of the materials in which they reside, including their molecular and magnetic structure. They can age-date rocks and examine the concentration and reactivity of free radicals and other highly reactive oxygen species—byproducts of aerobic respiration that cause damage to biological tissues, leading to cancer, heart attacks, strokes, and neurodegeneration. The measurements can also reveal the nature of dopants—trace impurities deliberately added to a material to radically alter such properties as conductivity, or to probe microstructure. For instance, adding a dopant to a semiconductor may increase its conductivity.

The standard method to measure electron spin, called electron spin resonance (ESR) spectroscopy, requires relatively large samples of at least 10 microliters (millionths of a liter). However, this excludes many materials of interest, including those that cannot be grown into single crystals larger than a few micrometers (millionths of a meter), as well as substances that can only be produced in micrograms (millionths of a gram) or smaller quantities.

For example, thin films made of nanomaterials such as graphene have too small a volume, as do single crystals of proteins and other biological macromolecules that play an important role in biomedical research and drug development. Minerals known as perovskites, which have widespread application as sensors, transducers, actuators and solar cells, can be grown to larger sizes. However, it is only by studying the tiny, single-crystal form of perovskites that researchers can glean essential, unambiguous information about the paramagnetic structure of these materials.

To make an ESR measurement, a sample is placed in a fixed, external magnetic field. The tiny, spin-generated magnetic field of each electron interacts with the external field, causing the magnetic energy levels of the electrons to separate according to their spin. Then the sample is irradiated with microwaves. That frequency range is chosen because its energy quanta match the differences of the magnetic energy levels of electrons. The distribution of magnetic energy levels of the paramagnetic material depends sensitively on the material’s molecular structure. By characterizing these energy level distributions, ESR spectroscopy provides detailed information on the composition and electronic and magnetic structure of the material.

Like all electromagnetic waves, including visible light, microwaves consist of oscillating electric and magnetic fields that are oriented perpendicular to each other. In ESR spectroscopy, the oscillating magnetic field of the microwaves interacts with the magnetic moment generated by the spin of unpaired electrons in the paramagnetic sample. This interaction causes electrons to jump from one magnetic energy level to another. Amplification of the microwave magnetic field causes more electrons to change magnetic energy levels, increasing the sensitivity of the ESR method. To amplify the microwave magnetic field, the sample is placed in a microwave resonator, a device that enables microwaves of a particular frequency to reinforce each other, just as a resonating bell amplifies a sound wave of a particular frequency.

Although many samples researchers would like to study are smaller than one nanoliter (billionth of a liter), the size of the microwave resonator limits just how tiny a sample can be. (A single drop of water contains 50,000 nanoliters.) Smaller resonators allow smaller samples by concentrating the microwave fields in a small volume. However, they cannot do so efficiently and much of the microwave energy is lost through radiation, compromising their sensitivity.

To build a more efficient and sensitive microresonator, Veronika Szalai of NIST, Nandita Abhyankar of NIST and the University of Maryland, and their colleagues relied on a type of metamaterial—a composite material typically consisting of an array of a repeating unit geometry, resulting in properties not ordinarily found in nature. In this case, the researchers adapted the design of a metamaterial unit so that it retains much more of the microwave energy in a miniscule volume--less than a nanoliter. The researchers reported their findings in the Oct. 28 issue of Science Advances.

This ability to concentrate and efficiently retain the microwave energy within the microresonator results in greater sensitivity for measuring spin. Importantly, the geometry of these devices allows them to be easily modified to resonate at higher frequencies, which has been a major challenge in microresonator design and may enable scientists to conduct detailed studies to identify the origin of spectral features for small samples.

Using the new microresonator, the team measured the spins of small groups of electrons at room temperature in volumes smaller than 0.1 nanoliters and at low concentrations of 0.05 mol % (5 paramagnetic ions per 10000 non-paramagnetic ions). The new design may enable the first room-temperature measurements of electron spin at small concentrations in single crystals with dimensions smaller than 10 micrometers.

Paper: Nandita Abhyankar, Amit Agrawal, Pragya Shrestha, Russell Maier, Robert D. McMichael, Jason Campbell and Veronika Szalai. Scalable microresonators for room-temperature detection of electron spin resonance from dilute, sub-nanoliter volume solids. Science Advances, 28 Oct 2020: Vol. 6, no. 44, eabb0620. DOI: 10.1126/sciadv.abb0620