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Our study shows that metastable quintet state of spin crossover complex [Fe(dpp)2(NCS)2] · py (dpp=dipyrido[3,2-a:2′,3′-c] phenazine, py=pyridine) at low temperatures may be realized by a rapid cooling. The relaxation from quintet state to singlet spin at low temperatures depends on the both of time and previous history of the sample. The U-shaped dents of the magnetic effective moment depending on the temperature after initial rapid zero-field cooling indicate an obvious affection of magnetic field to the realization of singlet state in the transition range. The observations for this sample are indeed consistent with the fact that cooperative effects play a very important role in the spin transition. © 1999 American Institute of Physics.