Theory of Fractionally-magnetized Quantum Ferromagnet
Abstract
We present a theory to realize entangled quantum spin states with fractional magnetization. The origin of magnetization reduction is partly emergent antiferromagnetism, that is, spin-liquefaction of ferromagnetism. We study a ferromagnetic bilinear coupling region of the spin-S (≥q 1) bilinear-biquadratic spin chain based on (i) a rigorous eigenstate correspondence between the spin-S model and spin-12 model and (ii) a numerical exact-diagonalization calculation up to S=3. As a result, we obtain a fractional magnetized M=1-1/(2S) phase, where ground states have quantum entanglement-reflecting corresponding spin-12 antiferromagnetic ground states in a ferromagnetic background. This spin-liquefaction theory of ferromagnets can be generalized to any-dimensional lattices even under a magnetic field. This fractional ferromagnetism opens the new research field of quantum ferromagnets.
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