Theory of Fractionally-magnetized Quantum Ferromagnet

TL;DR

This paper develops a theory for fractional magnetization in quantum ferromagnets, revealing entangled states with emergent antiferromagnetic features and demonstrating the phenomenon through rigorous and numerical methods.

Contribution

It introduces a novel fractional magnetization phase in ferromagnetic spin chains, linking it to spin-liquefaction and entanglement, and generalizes the concept to higher dimensions.

Findings

Identification of a fractional magnetized phase with M=1-1/(2S)

Ground states exhibit quantum entanglement akin to antiferromagnetic states

The theory applies to various lattice dimensions and magnetic field conditions

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-$S$ $({\geqq} 1)$ bilinear-biquadratic spin chain based on (i) a rigorous eigenstate correspondence between the spin-$S$ model and spin-$\frac12$ 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-$\frac12$ 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.