) spin-crossover molecules using diamond quantum sensing microscopy*

Spin crossover (SCO) molecules are one of the promising candidates for molecular electronics owing to its thermal, magnetic, and optical switching phenomena. Fe(Htrz)₂(trz)](BF₄)] SCO polymers show thermal switching between high spin (HS) and low spin (LS) states which are applicable in the thermal sensors or switches [1]. While the bulk magnetic properties of Fe(Htrz)₂(trz)](BF₄)] molecules are widely studied by magnetometry techniques their properties at the individual level are missing [2]. Here we use nitrogen vacancy (NV) based magnetometry to image individual Fe(II) triazole SCO nano-rods of size varying from 200 to 1000 nm [3]. Scanning electron microscopy (SEM) and Raman spectroscopy are performed to find out the size of the Fe(II) triazole nano-rods and to confirm the spin state of the SCO molecule respectively. The stray magnetic fields produced by individual nano-rods are imaged by NV microscopy as a function of temperature (up to 150 ⁰C) and applied magnetic field (up to 3500 G) and correlated with SEM and Raman. We found that most of LS sates are paramagnetic in contrary to prediction of a diamagnetic behavior [4]. Further, we discuss temperature and size effects on Fe (II) triazole properties. [1] A. A. Blanco, et al., Wiley, Molecules. 27, 1213 (2022). [2] A. A. Blanco et al., Molecules 2022, 27(4), 1213 (2022). [3] I. Fescenko, A. Laraoui, et al., Phy. Rev. App. 11(3), 034029 (2019). [4] S. Lamichhane et al., under preparation.