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Multi-modal nanoprobes reveal hidden magnetism

10 Jan 2020 Isabelle Dumé
Multi-messenger nanoprobes. (Courtesy: Ella Maru Studio)

A “multi-messenger” approach that was first employed to study astrophysical phenomena such as black hole mergers can also bring insights to the ultra-small realm of quantum physics. So say researchers at Columbia University in the US, who have investigated the electrical, magnetic and optical properties of a strained metal oxide thin film at the nanoscale using a combination of several imaging and measurement techniques. The strategy, which has already unearthed an unexpected magnetic phase in the film, represents a new way to explore these quantum materials and could make it easier to design new ones with tailor-made properties.

The multi-messenger approach in astronomy involves combining simultaneous measurements from different instruments, including infrared, optical, X-ray and gravitational-wave telescopes. In the past few years, researchers have begun using this strategy to build up a much more detailed picture of intergalactic phenomena than would be possible with individual techniques alone.

A team led by Dmitri Basov has now extended this revolutionary approach to the nanoscale. In their work, Basov and colleagues studied La2/3Ca1/3MnO3 (LCMO), which belongs to a family of colossal magnetoresistive manganites (AE1−xRExMnO3, where AE is alkali earth and RE, rare earth) that contain two distinct and competing phases: insulator and metal. This property makes it a promising material for making phase-programmable memories for next-generation computing applications.

A combination of measurement techniques

In a previous work, Basov and colleagues found that a magnetic metallic phase can be unexpectedly switched on in LCMO by simultaneously applying mechanical strain and laser light pulses. In their latest experiments, the researchers placed a thin film of LCMO (grown on a NdGaO3 substrate) under the tip of super-resolution scanning near-field optical microscope. They then applied strain in one direction to the material while also exciting it with 130-femtosecond-long light pulses from a 1.5 eV (visible light) laser.

By using a combination of measurement techniques – atomic force microscopy, scanning near-field optical microscopy, magnetic force microscopy and ultrafast laser excitation, all at low temperatures – the researchers were able to measure the photo-induced phase transition as the antiferromagnetic insulator material became a conducting ferromagnetic metal containing nanometre-scale domains. This unexpected switch is completely reversible, and could make for the basis of a phase-programmable memory,  the researchers say.

“The process is non-thermal, meaning that there is a large energy barrier between the two phases,” team member Jingdi Zhang tells Physics World. “Thermal fluctuations therefore do not destroy the stability of the ferromagnetic phase. The phase change is also ultra-fast.”

Team member Alex McLeod adds that while it is relatively common to study these materials with scanning probes, this is the first time that optical nano-imaging has been combined with magnetic nano-imaging, and all at the very low temperatures where quantum materials show their merits. Investigating quantum materials with multi-messenger nano-probes could accelerate worldwide efforts to engineer these materials with new properties, he says.

“Technological leap”

The researchers say that they would like to better understand the dynamics of the phase transition they observed. To that end, they are performing ultrafast X-ray scattering measurements (in conjunction with super-resolution near-field microscopy) that can probe lattice, charge, and magnetic degrees of freedom at the nanoscale, hoping to unravel the mechanism behind this ultra-stable and all-optical phase change.

The scientists, who report their work in Nature Materials, say they are also looking for “sibling” material systems to LCMO that undergo phase changes upon ultrafast optical excitation. The multimodal nanoscience approach to studying quantum physics phenomena is, they say, a “technological leap for how scientists can explore quantum materials to unearth new phenomena and guide future functional engineering of these materials for real-world applications.”

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