  We present a new mechanism for a spin blockade effect associated with a
change in the type of magnetic anisotropy over oxidation state in a single
molecule transistor, by taking an example of an individual
Eu$_{2}$(C$_{8}$H$_{8}$)$_{3}$ molecule weakly coupled to non-magnetic
electrodes without linker groups. The molecule switches its magnetization
direction from in-plane to out-of-plane when it is charged. In other words, the
magnetic anisotropy of the molecule changes from easy plane to easy axis when
the molecule is charged. By solving the master equation based on model
Hamiltonian, we find that current through the molecule is highly suppressed at
low bias independently of gate voltage due to the interplay between spin
selection rules and the change in the type of magnetic anisotropy. Transitions
between the lowest magnetic levels in successive charge states are forbidden
because the magnetic levels differ by $|\Delta M| > 1/2$ due to the change in
the type of magnetic anisotropy, although the total spins differ by $|\Delta
S|=1/2$. This current suppression can be lifted by significant B field, and the
threshold B field varies as a function of the field direction and the strength
of magnetic anisotropy. The spin blockade effect shed light on switching the
magnetization direction by non-spin-polarized current and on exploring effects
of this property coupled to other molecular degrees of freedom.
