  First- and second-order phase transitions, Fulde-Ferrel (FF) inhomogeneous
superconducting (SC) state and quantum criticality in
ferromagnet/superconductor/ferromagnet double tunnel junctions are
investigated. For the antiparallel alignment of magnetizations, it is shown
that a first-order phase transition from the homogeneous BCS state to the
inhomogeneous FF state occurs at a certain bias voltage $V^{\ast}$; while the
transitions from the BCS state and the FF state to the normal state at $%
V_{c}$ are of the second-order. A phase diagram for the central superconductor
is presented. In addition, a quantum critical point (QCP), $% V_{QCP}$, is
identified. It is uncovered that near the QCP, the SC gap, the chemical
potential shift induced by the spin accumulation, and the difference of free
energies between the SC and normal states vanish as $% |V-V_{QCP}|^{z\nu}$ with
the quantum critical exponents $z\nu =1/2$, 1 and 2, respectively. The tunnel
conductance and magnetoresistance are also discussed.
