Spin effects in ferromagnetic single-electron transistors

Electron tunneling in ferromagnetic single-electron transistors is considered theoretically in the sequential tunneling regime. A formalism is developed, which operates in a two-dimensional space of states, instead of one-dimensional 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 detail. 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.