A Mossbauer study of57Fe-doped CsCoBr3and RbCoCl3

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Abstract
57Fe Mossbauer spectra of CsCo0.99757Fe0.003Br3 and RbCo0.9957Fe0.001Cl3 between 5 K and room temperature are reported. In both compounds, the quadrupole slitting is found to be negative, and it is deduced that the Fe2+ ion has a singlet ground state with nearly zero orbital and spin angular momentum for temperatures above the three-dimensional magnetic ordering temperature TN1. The Mossbauer spectra of CsCo0.99757Fe0.003Br3 and RbCo0.9957Fe0.01Cl3 are consistent with a transition to a partially disordered antiferromagnetic phase at TNl, and a transition to a fully ordered ferrimagnetic phase at TN2. The transition temperatures TN1 are not significantly lower than those in the pure compounds, i.e. 28.3 K in CsCoBr3 and 28 K in RbCoCl3. TN2 is found to be 12 K in CsCo0.99757Fe0.003Br3, and 4-14 K in RbCo0.9957Fe0.01Cl3. Two types of relaxation process are found in the Mossbauer spectra of these two compounds at temperatures below TNl. One is attributed to electronic relaxation between the lowest states of the Fe2+ ion; the other, which is present only at TN2 and above, to solitons in the chains of magnetic ions that are excluded from the three-dimensional order between TN1 and TN2. The temperature variation of the linewidths in the Mossbauer spectra of RbCo0.9957Fe0.01Cl3 above TN1 is consistent with solitons being present in this material.

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