Dynamic theory of ferromagnetic—to—spin-glass transition

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Abstract
We study the interaction between spin waves and magnetic two-level systems (TLS) in a ferromagnet with frustration. The coupling between the magnons and the TLS is due to the modulation of the angle between spins, by the rearrangement of the spins directions when the system tunnels from one metastable state to another. The magnon-TLS interaction causes an instability of the ferromagnetic phase with decreasing temperature due to the appearance of a soft mode. We associate this instability with the disappearance of long-range ferromagnetic order observed in reentrant ferromagnets.