Imaging the Thermal Hysteresis of Single Spin-Crossover Nanoparticles.

The magnetic hysteresis property during the spin transition of spin-crossover (SCO) materials holds great promise for their applications in spin electronics, information storage, thermochromic, and nanophotonic devices. Existing studies often measured the averaged property of a bulk sample consisting of lots of individuals. When considering the significant heterogeneity among different individuals and the inevitable interparticle interactions, ensemble measurement not only blurred the structure-property relationship but also compromised the intrinsic hysteresis property and cyclability. Herein, we employed a recently developed surface plasmon resonance microscopy (SPRM) method to measure the thermal hysteresis curve of single isolated SCO nanoparticles. The thermal-induced spin transition was found to alter the optical contrast of single SCO nanoparticles, which was optically readout using SPRM in a quantitative, nonintrusive, and high-throughput manner. Single nanoparticle measurements revealed an intrinsic transition temperature that was independent of the temperature scan rate and superior stability after over 11 000 cycles of single SCO nanoparticles. Correlations between the hysteresis and the size and morphology of the same individuals further uncovered the significant nanoparticle-to-nanoparticle heterogeneity with implications for the size-property relationship and rational design of SCO materials with improved performance.