Spin Damping Research Articles

Wall Streaming in Ferromagnetic Thin Films

In nickel-iron films thicker than about 100 nm (1000 Å), fast-rising field pulses along the hard anisotropy axis lead to a new kind of wall displacement called ``wall streaming,'' which does not require any field component parallel to the wall necessary for all other wall motion processes. The walls are displaced with a very regular wall step width per pulse which strongly depends on the strength of the pulse field and on the pulse rise and fall times, but not on the pulse duration. The direction of wall motion reverses if the pulse polarity is reversed. During the application of a sequence of uniform pulses, the direction of wall motion may also reverse, beginning at one end of the wall, with a reversing line (interpreted as being a Néel line moving along the wall. The phenomenon is explained as a consequence of the gyromagnetic behavior of the magnetization in the Bloch walls. A detailed theory is presented, in which the intrinsic spin damping and the film inhomogeneity (characterizable by the coercivity) are to be taken into account. Wall streaming may contribute to a destruction of the information stored in magnetic films thicker than 100 nm (e.g., in plated-wire memories) as soon as fast-rising (tr ≲20 nsec) pulses are applied.

Theory of Dynamical Behaviors of Ferromagnetic Spins

The dynamical behavior of ferromagnetic spins is studied on the basis of the statistical mechanics of irreversible processes. A macroscopic equation determining the change in time of an inhomogeneous magnetization is derived with explicit expressions for the frequency spectrum and damping constant. With the use of the general expressions thus obtained, the following problems are discussed on the basis of the Heisenberg model of ferromagnetic spins: (1) the pair correlation of spins, (2) the magnetic scattering cross section of neutrons, (3) the frequency spectrum and the damping constant of spin waves, (4) the damping of the longitudinal spin component above and below the Curie point. In the low temperature limit, a straightforward reduction of our expressions leads to the spin wave frequency and damping equivalent to Dyson's theory of spin wave interactions. A general expression is given for van Hove's parameters describing the asymptotic behavior of the spin pair correlation at large distances. It is shown that the longitudinal spin damping in the low temperature limit exhibits a variety of k dependences, depending upon the relative magnitudes of the spin wave energy Dk2, the effective magnetic field H coming from other than the exchange interaction and the thermal energy kBT; in particular, for gµBH ≪Dk2 ≪KBT, the longitudinal damping is proportional to k2, namely, the change in time of the longitudinal spin density obeys the diffusion equation. In the vicinity of the Curie point and in the paramagnetic region, the longitudinal damping is shown to obey the diffusion equation. The diffusion constant thus obtained vanishes at the Curie point and is in good agreement with the values observed by Ericson and Jacrot for iron above the Curie point.