We investigate the behaviour of photo-excitable, bistable systems, under permanent light irradiation, in presence of relaxation towards the non-excited state. Cooperativity causes bistability of the steady state, leading to light-induced thermal and optical hysteresis (LITH and LIOH). The light-induced instability is expected to induce demixtion, i.e. the coexistence of domains of the two stable steady states. Such effects are evidenced by magnetic and reflectivity measurements on the spin-crossover solid solution: [Fex Co1−x (btr)2(NCS)2] · H2O, with x = 0.3, 0.5, 0.85. Experimental data are in quantitative agreement with a simple macroscopic model which includes a non-linear relaxation term in the master equation. PACS. 64.60.My Metastable phases -70.60.Es Relaxation effects -75.90.+w Other topics in magnetic properties and materials -78.20.Nv Thermooptical and photothermal effects

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