Two-magnon bound states in quasi-low-dimensional easy-axis ferromagnets

We have found the critical value of “easy-axis” anisotropy, starting from which a bound state of two magnons with wave vector k = 0 first appears in quasi-one-dimensional ferromagnets with nearest-neighbor interaction. This value is shown to be much higher than the magnitude of the weak three-dimensional interchain (interlayer) exchange interaction.

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