ESR study of the ordering transformation inκ−(BEDT−TTF)2Cu[N(CN)2]Br

Ordering transformation and its effect on the electronic structure of the $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}{\mathrm{Cu}[\mathrm{N}(\mathrm{CN})}_{2}]\mathrm{Br}$ organic superconductor was studied by measuring annealing induced changes in low temperature ESR spectra and spin susceptibility temperature dependence. The transformation is shown to proceed through coexistence of two phases. Besides resistivity decrease, ordering was shown to increase ESR spin susceptibility (below about 160 K) with a tendency for restoration of the temperature independent Pauli term. Gaplike spin susceptibility behavior below 60 K is shown to develop only at some stage of ordering. These effects point to a relationship between temperature dependent spin susceptibility in the salt and incomplete ordering.