ESR study of the Prussian blue analogue Rb_0.8Mn_1.1[Fe(CN)₆]·D ₂O

We performed electron spin resonance (ESR) studies of the mixed valence Prussian blue analogue,Rb_0.8Mn_1.1 [Fe(CN)₆]·D₂O in a wide frequency (9.5-319 GHz) and temperature (3.5-300 K) interval. Evidence for the proposed high-temperature Mn²⁺ (S = frac(5, 2))-NC-Fe³⁺ (S = frac(1, 2)) to low-temperature Mn³⁺ (S=2)-NC-Fe²⁺ (S=0) internal charge transfer has been detected: a characteristic reduction of χ_ESRT on cooling from room temperature to 100 K, a particular frequency dependence of the room temperature ESR linewidth, and the temperature dependence of the ESR linewidth. Short-range magnetic correlations, reflected in a large broadening and dramatic shift of the ESR signal develop just above T_C=10 K. Below T_C, both the linewidth and the shift in position mimic the temperature dependence of the order parameter. In a quench experiment T_C was reduced by 3 K. The observed ferromagnetic ordering arises from Mn³⁺ (S=2) moments in the low-temperature electronic configuration.