Fluctuations and Instabilities of Ferromagnetic Domain Wall pairs in an External Magnetic Field

Details Fluctuations and Instabilities of Ferromagnetic Domain Wall pairs in an External Magnetic Field Fluctuations and Instabilities of Ferromagnetic Domain Wall pairs in an External Magnetic Field

1994-12-02

arxiv.org/abs/cond-mat/9412009v2

Physics

Condensed Matter

15 pages RevTex 3.0 (twocolumn), 9 figures available on request, to appear in Phys Rev B, Dec (1994)

Scientific paper

10.1103/PhysRevB.50.16485

Soliton excitations and their stability in anisotropic quasi-1D ferromagnets are analyzed analytically. In the presence of an external magnetic field, the lowest lying topological excitations are shown to be either soliton-soliton or soliton-antisoliton pairs. In ferromagnetic samples of macro- or mesoscopic size, these configurations correspond to twisted or untwisted pairs of Bloch walls. It is shown that the fluctuations around these configurations are governed by the same set of operators. The soliton-antisoliton pair has exactly one unstable mode and thus represents a critical nucleus for thermally activated magnetization reversal in effectively one-dimensional systems. The soliton-soliton pair is stable for small external fields but becomes unstable for large magnetic fields. From the detailed expression of this instability threshold and an analysis of nonlocal demagnetizing effects it is shown that the relative chirality of domain walls can be detected experimentally in thin ferromagnetic films. The static properties of the present model are equivalent to those of a nonlinear sigma-model with anisotropies. In the limit of large hard-axis anisotropy the model reduces to a double sine-Gordon model.

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