  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.
