  Single-molecule memory device based on a single-molecule magnet (SMM) is one
of the ultimate goals of semiconductor nanofabrication technologies. Here, we
study how to manipulate and readout the SMM's spin states of stored information
that characterized by the maximum and minimum average value of the
$Z$-component of the SMM's spin, which are recognized as the information bits
"1" and "0". We demonstrate that the switching time depends on both the
sequential tunneling gap $\varepsilon_{se}$ and the spin-selection-role allowed
transition-energy $\varepsilon_{trans}$, which can be tuned by the gate
voltage. In particular, when the external bias voltage is turned off, in the
cases of the empty-occupied and doubly-occupied ground eigenstates, the time
derivative of the transport current and that of the average value of the SMM's
spin states can be used to read out the SMM's spin states of stored
information. Moreover, the tunneling strength of and the asymmetry of
SMM-electrode coupling have a strong influence on the switching time, but that
have a slight influence on the readout time that being on the order of
nanoseconds. Our results suggest a SMM-based memory device, and provide
fundamental insight into the electrical controllable manipulation and readout
of the SMM's spin-state of stored information.
