Theory of Domain Wall Dynamics under Current

The present information technology is based on electron transport and magnetism. Magnetism has been most successful as high-density storages such as hard disks. In magnetic storages, read-out mechanism of the information had so far several significant developments. The oldest idea of detecting magnetic information would be to use Faraday’s induction law by scanning a read head (a coil) on the stored magnetic bits. More efficient read-out mechanism was developed by using anisotropic magnetoresistance (AMR) effect. AMR is a resistivity dependent on the angle between the magnetization and the electric current, which arises from the coupling of magnetization and electrons’ oribtal motion due to spin-orbit interaction.1 The resistivity change is only of order of a few %, but AMR is more efficient than using Faraday’s induction used in magnetic tape and hard disks in early days. Magnetic head with higher sensitivity was developed by use of GMR (giant magnetoresistance) effect in thin magnetic multilayers discovered in 1988.2,3 Strong magnetization dependence of the resistivity arises from the spin-dependent scattering of the electron at the interface between thin ferromagnetic layer and non-magnetic metallic layers. Quite recently GMR heads are being replaced by TMR (Tunneling MR) heads, where the non-magnetic layer is replaced by an insulating barrier.4 These rapid developments of read-out mechanism by use of solid state systems made possible so far the rapid increase of recording density.