C13 NMR study of the stabilization of the antiferromagnetic ground state and the emergence of unconventional magnetic state in a molecular π−d system

We have performed $^{13}\mathrm{C}$ NMR measurements on $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{STF})}_{2}{\mathrm{Fe}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Cl}}_{4}$ and evaluated Fe content dependence of the magnetic properties. We found that the angle dependence of the Knight shift of $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{STF})}_{2}{\mathrm{Fe}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Cl}}_{4}$ changes depending on the Fe content, which is explained by the exchange fields through the $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction and dipole fields. An antiferromagnetic ordering was observed in all Fe content samples except for $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{STF})}_{2}{\mathrm{GaCl}}_{4}$ ($x=0$). This indicates that the antiferromagnetic ground state is stabilized by the interaction between $\ensuremath{\pi}$ and $3d$ spins. From the linewidths of the NMR spectra measured at the lowest temperature, we confirmed that the size of the ordered $\ensuremath{\pi}$ spin moment is independent of the Fe content. Based on the results, we discuss the role of $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction for the stabilization of the antiferromagnetic ordering and the mechanism of the unconventional magnetism of $3d$ spins observed in $\ensuremath{\pi}\text{\ensuremath{-}}d$ systems.