Nitrogen-vacancy (NV) centers are a rising star in the field of high-resolution magnetic imaging. Stable enough to capture condensed-matter phenomena at room temperature, these versatile, atom-sized defects are now soaring to new heights in the magnetism community—and descending to new depths. Research groups led by Ania Jayich from the University of California, Santa Barbara, and Patrick Maletinsky from the University of Basel in Switzerland, have independently developed NV-based sensors that can resolve nano-sized magnetic features at temperatures as low as a few degrees above zero Kelvin. With these sensors, condensed-matter physicists now have access to a wider range of magnetic imaging temperatures.
“There’s been a big push toward low-temperature operations because there is a lot of very exciting electronic systems that exist at low temperatures,” says Maletinsky, assistant professor of experimental physics. “Graphene, quantum Hall effects, spin Hall effects—there’s a big variety.” For Maletinsky, who has been pushing the limits of NV-based imaging since his days as a postdoctoral researcher at Harvard University, his group’s most recent advance, published in Nature Nanotechnology (doi:10.1038/NNANO.2016.63), represents an important technological breakthrough.
NV centers are naturally occurring defects in diamond that consist of a substitutional nitrogen atom and a neighboring lattice vacancy. In the laboratory, NV centers are created by ion implanting nitrogen atoms in a piece of diamond, knocking carbon atoms out of place in the lattice. Each implanted nitrogen atom and associated vacancy together behave as a single quantum spin. This allows scientists to track the energetic signature of an individual NV center (indicated optically by fluorescence) in response to a magnetic field across a sample. The atomic size and quantum behavior of NV centers have allowed scientists like Maletinsky to measure magnetic fields with nanoscale resolution. And because this sensitivity is preserved under ambient conditions, NV centers can be used to detect tiny magnetic fields in materials ranging from high-temperature superconductors to living cells—materials closed off to cold-temperature technologies such as superconducting quantum interference devices (or SQUIDs) and magnetic resonance force microscopes.
Nitrogen-vacancy (NV) magnetometry image of the bits of a hard disk at T = 6 K. Dark features correspond to the 5.3 G magnetic field contours (2892 MHz RF field). Scale bar, 100 nm. Credit: Nature Nanotechnology.
At cryogenic temperatures, however, the tables are turned. NV-based sensors have never been shown to operate under such cold conditions.
Maletinsky’s team addressed this problem by immersing their room-temperature NV sensor in a liquid-helium cryostat, allowing them to plunge to an operating temperature of about 4 K. The low tendency of helium to boil, Maletinsky says, allows their system to be very “quiet,” a key priority when the sensor is a single quantum spin nestled in a diamond nanopillar welded to the tip of an atomic force microscope.
With this system, the research team could quantitatively image the stray field emanating from magnetic vortices across a sample of YBa2Cu3O7–δ, a high-temperature superconductor known to form these quantum defects when cooled below its transition temperature (about 89 K) under an applied magnetic field. More importantly, the research team was able to extract a notoriously elusive measure known as the London penetration depth, which describes how far a magnetic field penetrates into a superconductor.
The key to making such highly sensitive, high-resolution measurements, Maletinsky says, is how close the delicate NV sensor can be brought to a sample surface. “Tip-to-sample distance is the crucial figure of merit in these experiments.”
Ania Jayich would agree.
An assistant professor in the Physics Department, Jayich leads a group who published their own work on cryogenic NV-based sensing concurrently with Maletinsky’s team in Nature Nanotechnology (doi:10.1038/NNANO.2016.68).
“High spatial resolution is possible because our sensor is so small,” Jayich says. “But the sensor is ultimately limited by the distance to the sample, not its size.”
And herein lies a significant challenge for researchers like Jayich and Maletinsky. A shorter tip-to-sample distance should mean higher sensitivity. But because one quantum phenomenon is being used to detect another, at short distances, sample surface effects can compromise the “quantumness” of a NV center, as Jayich puts it. “It’s a problem that plagues almost all quantum technologies,” she says. “Quantum behavior is very delicate and very sensitive to its environment.”
Within these limits, however, Jayich’s group resolved magnetic domains smaller than 100 nm in a hard disk using their NV-sensing system, which is unlike that developed by Maletinsky’s team.
Instead of a single diamond pillar, Jayich’s group glues an entire array of pillars to their scanning tip, producing a micro-hairbrush structure. This arrangement makes it difficult to precisely control how close a NV center on a single bristle reaches a sample surface. However, it allows the researchers to produce several NV centers at once and choose the brightest and most stable one for imaging. The structure also holds promise for conducting wide-field experiments in which multiple NV centers can be optically addressed to quickly scan across large areas. But this and other improvements to NV-based imaging, some argue, may not come until much later.
Eli Zeldov, a professor at the Weizmann Institute of Science in Rehovot, Israel, acknowledges that NV-based sensors have made significant progress within the last few years, with a performance ceiling on par with that of the nanoSQUID imaging technology he has helped pioneer. Nevertheless, he argues that NV-based sensing currently remains a relatively slow and complicated technique, both in fabrication and in operation.
“It requires microwaves, it requires optics, it requires very delicate equipment,” Zeldov says. “It’s tricky. This can improve with time, but there’s still quite a lot of room for improvement.”
But for NV researchers like Jayich and Maletinsky , there may be no time like the present.
“We have established this system that already has really good sensitivity, excellent spatial resolution, and is quantitative,” Maletinsky says. “I think now we’re in a position where we can demonstrate very meaningful applications with the performance we already have.”
