A method for preparing a material with spin transition in the form of particles, characterized in that it consists of: - in a first step, preparing a dispersion of particles of a spin transition material in a polar organic solvent, the dielectric constant of which, measured at 25°C, is greater than 19, and then - in a second step, achieving contact of the thereby obtained dispersion of particles with supercritical CO 2 at a temperature T and at a pressure P fulfilling the three following conditions: 1) 40 ≤ T ≤ 150°C; 2) 150 ≤ P ≤ 250 bars and 3) P.T > 6,000, in that the spin transition material is selected from a material formed by at least one compound fitting the following formula: AX b Y c wherein: - A fits the formula Fe(R-Trz) 3; - R-Trz represents a 1,2,4-triazole ligand bearing a substituent R on the nitrogen atom in position 4; - R is a methyl group or a group R 1 R 2 N- wherein R 1 and R 2 each represent independently of each other H or a methyl radical; - X represents at least one monovalent or divalent anion; - Y represents at least one anion which is a chromophore group; - b and c are selected so that the electroneutrality of the compound is observed, and a material formed by nanometric particles essentially comprising a compound fitting the following formula: Fe ⁢ L 3 w ⁢ L 3 ⁢ ⌊ Xʹ 2 / x ⁢ 1 - z / xʹ Yʹ 2 ⁢ τ / xʹ ⌋ w wherein: - L represents a 1,2,4-triazole ligand bearing a substituent R on the nitrogen atom in position 4; - X' is an anion with valence x, 1 ≤ x ≤ 2; - Y' is an anion different from X with the valence x', 1 ≤ x', ≤ 2; - R is a methyl group or a group R 1 R 2 N- wherein R 1 and R 2 each represent independently of each other H or a methyl radical; - 0 ≤ z ≤ 2 and 3 ≤ w ≤ 1500. - in a first step, preparing a dispersion of particles of a spin transition material in a polar organic solvent, the dielectric constant of which, measured at 25°C, is greater than 19, and then - in a second step, achieving contact of the thereby obtained dispersion of particles with supercritical CO 2 at a temperature T and at a pressure P fulfilling the three following conditions: 1) 40 ≤ T ≤ 150°C; 2) 150 ≤ P ≤ 250 bars and 3) P.T > 6,000, in that the spin transition material is selected from a material formed by at least one compound fitting the following formula: AX b Y c 1) 40 ≤ T ≤ 150°C; 2) 150 ≤ P ≤ 250 bars and 3) P.T > 6,000, - A fits the formula Fe(R-Trz) 3; - R-Trz represents a 1,2,4-triazole ligand bearing a substituent R on the nitrogen atom in position 4; - R is a methyl group or a group R 1 R 2 N- wherein R 1 and R 2 each represent independently of each other H or a methyl radical; - X represents at least one monovalent or divalent anion; - Y represents at least one anion which is a chromophore group; - b and c are selected so that the electroneutrality of the compound is observed, and - L represents a 1,2,4-triazole ligand bearing a substituent R on the nitrogen atom in position 4; - X' is an anion with valence x, 1 ≤ x ≤ 2; - Y' is an anion different from X with the valence x', 1 ≤ x', ≤ 2; - R is a methyl group or a group R 1 R 2 N- wherein R 1 and R 2 each represent independently of each other H or a methyl radical; - 0 ≤ z ≤ 2 and 3 ≤ w ≤ 1500.