Spin effects in ferromagnetic single-electron transistors

Electron tunneling in ferromagnetic single-electron transistors is consider ed theoretically in the sequential tunneling regime. A formalism is develop ed, which operates in a two-dimensional space of states, instead of one-dim ensional space used in the spinless case. It is shown that spin fluctuation s can be significantly larger than the charge fluctuations. The influence o f discrete energy spectrum of a small central electrode on tunneling curren t, charge and spin accumulation, charge and spin fluctuations, and on tunne l magnetoresistance is analyzed in detail. Two different scales are found i n the bias dependence of the basic transport characteristics; the shorter o ne originates from the discrete energy spectrum and the longer one from dis crete charging of the central electrode. The features due to discrete spect rum and discrete charging disappear at high temperatures.