Theory of Doping Induced High-Spin in a Model of Polyene-Based Molecular Magnets

Control of intramolecular spin alignment is studied theoretically in a model of polyene-based molecular magnets in which delocalized $\pi$ electrons are coupled with localized radical spins. In a previous paper [Phys. Rev. Lett. 90 (2003) 207203], we have demonstrated that charge doping is an effective way to realize controllable high-spin in the $\pi$-conjugated molecular magnets. In this paper, we clarify the dependence of spin-alignment on the exchange interaction between the localized spin and $\pi$ electron and the electron-electron interactions. The antiferromagnetic exchange interaction plays a role different from the ferromagnetic counterpart in doped molecules. To understand complex interplay of charge and spin degrees of freedom in the doped systems, we carry out a systematic study on the phase diagram of spin alignment in the parameter space. The mechanism of the spin alignment is discussed based on the spin densities of $\pi$ electrons. The calculated results are consistent with experiments, providing a theoretical basis for the control of spin alignment.