Interplay Between Domain Walls and Spin Waves in Magnetic Nanostructures

We theoretically study field-induced domain wall motion in an electrically insulating ferromagnet with hard- and easy-axis anisotropies. Domain walls can propagate along a dissipationless wire through spin wave emission locked into the known soliton velocity at low fields. In the presence of damping, the usual Walker rigid-body propagation mode can become unstable for a magnetic field smaller than the Walker breakdown field. We also numerically investigated the properties of spin waves emitted by the domain wall motion, such as frequency and wave number, and their relation with the domain wall motion. For a wire with a low transverse anisotropy and in a field above a critical value, a domain wall emits spin waves to both sides (bow and stern), while it oscillates and propagates at a low average speed. For a wire with a high transverse anisotropy and in a weak field, the domain wall emits mostly stern waves, while the domain wall distorts itself and the domain wall center propagates forward like a drill at a relative high speed.

 

The spin wave transportation through a transverse magnetic domain wall in a magnetic nanowire is studied. It is found that in a 1D nanowire, the spin wave passes through a domain wall without reflection. A magnon, the quantum of the spin wave, carries opposite spins on the two sides of the domain wall. As a result, there is a spin angular momentum transfer from the propagating magnons to the domain wall. This magnonic spin transfer torque can efficiently drive a domain wall to propagate in the opposite direction to that of the spin wave. Micromagnetic simulations show that generally, in nanostrips, spin waves (or magnons) interact with magnetic domain walls in a more complicated way that a domain wall can propagate either along or against magnon flow. However, thermally activated magnons always drive a domain to the hotter region of a nanowire of magnetic insulators under a temperature gradient. We theoretically illustrate why it is surely so by showing that domain wall entropy is always larger than that of a domain as long as material parameters do not depend on spin textures. Equivalently, the total free energy of the wire can be lowered when the domain wall moves to the hotter region. The larger domain wall entropy is related to the increase of magnon density of states at low energy originated from the gapless magnon bound states.