HC is far less than that for [(C5 i Pr5)Dy(μ-I)3Dy(C5 i Pr5)], which has HC > 140 kOe below 60 K, arising from strong intramolecular exchange coupling from its mixed-valent electronic structure . However, 191 Oe at 77 K, dropping to HC = 66 Oe at 80 K, i.e. closing at a rate of 42 Oe K -1 ) , HC for 1-Dy drops slowly (rate 2 Oe K -1 ) and remains non-zero until 100 K (Figure ). As SMMs are superparamagnets, their hysteresis behaviour derives from the underlying spin relaxation rates .