Ferrimagnets, in which half the spins are disordered and half are ordered—the so-called half-fire, half-ice state—have been advanced theoretically as a route towards realizing exotic magnetic ground states. Here we demonstrate novel single-spin resonance in $\mathrm{SmM}{\mathrm{n}}_2\mathrm{G}{\mathrm{e}}_2$, a 1/3-fire, 2/3-ice material. Using ac susceptibility in the frequency range 0.1 Hz–1 kHz we find a Debye-like resonance with Orbach relaxation temperature dependence, yielding an energy barrier $E_B\approx 840k_B$ and attempt frequencies in the THz range. These quantities strongly suggest single-spin relaxation among paramagnetic Sm spins where the flipping field is provided by the ferromagnetically-ordered Mn spins at domain boundaries, a direct outcome of the 1/3-fire, 2/3-ice state.

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• Antiferromagnetism
• Ferromagnetism
• Magnetic domains
• Magnetism
• Spin relaxation
• Noncollinear magnets
• Rare-earth magnetic materials
• AC susceptibility measurements