  Electron tunneling in ferromagnetic single-electron transistors is considered
theoretically in the sequential tunneling regime. A new formalism is developed,
which operates in a two-dimensional space of states, instead of one-dimensiona
space used in the spinless case. It is shown that spin fluctuations can be
significantly larger than the charge fluctuations. The influence of discrete
energy spectrum of a small central electrode on tunneling current, charge and
spin accumulation, charge and spin fluctuations, and on tunnel
magnetoresistance is analyzed in details. Two different scales are found in the
bias dependence of the basic transport characteristics; the shorter one
originates from the discrete energy spectrum and the longer one from discrete
charging of the central electrode. The features due to discrete spectrum and
discrete charging disappear at high temperatures.
