Type: Article
Publication Date: 2012-08-08
Citations: 7
DOI: https://doi.org/10.1103/physrevb.86.085109
We study the electronic structure and magnetic interactions in methylamine-intercalated orthorhombic alkali-doped fullerene $({\mathrm{CH}}_{3}{\mathrm{NH}}_{2}){\mathrm{K}}_{3}{\mathrm{C}}_{60}$ within the density functional theory. As in the simpler ammonia intercalated compound $({\mathrm{NH}}_{3}){\mathrm{K}}_{3}{\mathrm{C}}_{60}$, the orthorhombic crystal-field anisotropy $\ensuremath{\Delta}$ lifts the ${t}_{1}\mathrm{u}$ triple degeneracy at the $\ensuremath{\Gamma}$ point and drives the system deep into the Mott-insulating phase. However, the computed $\ensuremath{\Delta}$ and conduction electron bandwidth $W$ cannot alone account for the abnormally low experimental N\'eel temperature, ${T}_{\mathrm{N}}=11$ K, of the methylamine compound, compared to the much higher value ${T}_{\mathrm{N}}=40$ K of the ammonia one. Significant interactions between ${\mathrm{CH}}_{3}{\mathrm{NH}}_{2}$ and ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ are responsible for the stabilization of particular fullerene-cage distortions and the ensuing low-spin $S=1/2$ state. These interactions also seem to affect the magnetic properties, as interfullerene exchange interactions depend on the relative orientation of deformations of neighboring ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ molecules. For the ferro-orientational order of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{2}$-K${}^{+}$ groups we find an apparent reduced dimensionality in magnetic exchange interactions, which may explain the suppressed N\'eel temperature. The disorder in exchange interactions caused by orientational disorder of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{2}$-K${}^{+}$ groups could further contribute to this suppression.