Exploring the impact of magnetic field and nanoparticle size on the magnetic properties of Fe(II)-triazole spin-crossover nanoparticles.

Three types of iron(II)-triazole spin-crossover nanoparticles were synthesized using a micellar exchange approach, each coated with a thin silica shell. Two were obtained under optimized conditions, yielding rod-shaped nanoparticles (1np, 2np). The synthesis of the third one involved a reduced co-surfactant amount, which led to oval-shaped nanoparticles (3np). The particle size of 1np (42 nm × 21 nm), 2np (113 nm × 53 nm), and 3np (234 nm × 137 nm), and their quality were confirmed using TEM microscopy with an energy-dispersive X-ray (EDX) microanalyzer and powder X-ray diffraction. The spin-crossover properties of these compounds were then investigated through DC magnetometry and DSC calorimetry, considering the influence of particle size and magnetic field strength. Only the thermal hysteresis width ΔT1/2 turns out to be field independent, whereas the spin transition temperature (T↑1/2 and T↓1/2) and high-spin (HS) remnant fraction in the low-spin (LS) state change with varying the applied field. The magnetic field exerted the most significant influence on the HS remnant fraction in the LS state; specifically, applying a 70 kOe field reduced the HS phase by up to 24% compared to low-field conditions. Quantitative analysis of the spin-crossover behaviour was performed using the Slichter and Drickamer model.