# Unveiling Spin Transition at Single-Particle Level in Levitating Spin Crossover Nanoparticles

## TL;DR

Researchers demonstrated real-time control of spin transitions in levitating nanoparticles, enabling low-power optical switching and sensing.

## Contribution

A contact-free platform for observing and manipulating spin transitions in single SCO nanoparticles with optical and pressure control.

## Key findings

- Reversible optovolumetric changes of up to 10% were observed in levitating Fe(II)–triazole nanoparticles.
- Spin transitions were manipulated using laser heating without substrate interference.
- Pressure modulation confirmed mechanical control over the same bistable spin transition.

## Abstract

The ability to control and understand phase transitions
of individual
nanoscale building blocks is key to advancing the next generation
of low-power reconfigurable nanophotonic devices. To address this
critical challenge, molecular nanoparticles (NPs) exhibiting spin
crossover (SCO) phenomenon are trapped by coupling a quadrupole Paul
trap to a multispectral polarization-resolved scattering microscope.
This contact-free platform simultaneously confines, optically excites,
and monitors the spin transition in Fe­(II)–triazole NPs in
a pressure-tunable environment, eliminating substrate artifacts. Thus,
we demonstrate light-driven manipulation of the spin transition in
levitating NPs, enabled by laser heating and free of substrate-induced
effects. Using the robust spin bistability near room temperature of
our SCO system, we quantify reversible optovolumetric changes of up
to 10%, revealing precise switching thresholds at the single-particle
level. Independent pressure modulation produces a comparable volume
increase, confirming mechanical control over the same bistable transition.
These results constitute full real-time control and readout of spin
states in levitating SCO NPs, with operating conditions compatible
with ultralow-power optical switching, data storage, and nanoscale
sensing.