Tuning the magnetic properties of Fe-CN-Co Mixed Valence System

RICARDO MARTÍNEZ GARCÍA; J. MARC BROTO

Encuentro; Third school of High Magnetic Fields (High Fields ALPHA Project); 2004

The MVS species, which become magnetic with illumination, is really a solid solution of Co(2+) and Co(3+) ferrocyanide with the appropriate amounts of an alkaline cation (Na, K) to balance the anion charge. The V(CN) stretching frequency in this material is a measure of the amount of Co(3+) in its composition. For ozonized cobaltous ferrocyanide the V(CN) absorption falls at 2125 cm-1, while for partially reduced cobaltous ferricyanide this absorption is observed in a wide frequency range from 2085 cm-1 (in a pure cobaltous ferrocyanide) up to 2120 cm-1 for the phase rich in Co(3+). When the MVS species is obtained from an equimolar solution of K3[Fe(CN)3] and K4[Fe(CN)6].3H2O, the IR spectra of the obtained precipitate shows two absorption bands at 2085 and 2120 cm-1 while its Mössbauer spectrum is composed of two quadrupole doublets of low spin Fe(II). Certain correlation is observed between the V(CN) value and the quadrupole splitting (D) derived from the Mössbauer spectra. These results suggest that the MVS species is a solid solution, where in same crystal structure there are clusters of Co(3+, S=0)-NC-Fe(II, S=0) and clusters of Co(2+, S=3/2)-NC-Fe(II, S=0). The reversibility of the photo-induced magnetization could be related with the existence of these last clusters, which retain the structural information of the whole material. As far as we know, the effect of the Co(2+):Co(3+) ratio in the MVS species on the properties of the photo-induced magnetic state has not been studied. Shed light on this subject is the aim of the present project.