Unveiling the degeneracy of bound magnon crystals from magnetic and thermodynamic features of the spin-1/2 Heisenberg octahedral chain

TL;DR

This study explores the magnetic and thermodynamic behaviors of a spin-1/2 Heisenberg octahedral chain, revealing complex bound magnon phases, magnetization plateaus, and potential for efficient magnetocaloric cooling.

Contribution

It introduces a generalized localized-magnon theory for frustrated regimes and characterizes distinct bound magnon phases in the Heisenberg octahedral chain.

Findings

Identification of two fragmented phases with distinct singlet formations.

Observation of magnetization plateaus at one-fifth and three-fifths of saturation.

Pronounced double-peak structure in specific heat near field-driven transitions.

Abstract

Magnetic, thermodynamic, and magnetocaloric properties of a spin-1/2 Heisenberg octahedral chain with three distinct exchange interactions are investigated in an external magnetic field using the variational method, extended localized-magnon approach, and exact diagonalization. Variational arguments rigorously establish two distinct fragmented phases in the frustrated regime. In the former phase all four spins of each square plaquette form a collective plaquette singlet, whereas in the latter phase two dimer singlets are formed along diagonals of each square plaquette. These bound two-magnon states, supplemented with three localized one-magnon states, enable us to elaborate a generalized localized-magnon theory that is applicable in a frustrated regime across the entire field range as confirmed by comparison with exact diagonalization data. The concept of localized magnons provides a consistent description of low-temperature magnetization curves featuring intermediate one-fifth and three-fifths plateaus, which intersect each other at temperature-independent crossing points determined by the relative degeneracies of competing bound-magnon phases. Field variations of the specific heat reveal a pronounced double-peak structure near each field-driven transition with peak heights depending on the relative degeneracies of the respective bound-magnon states. Our results demonstrate that the system supports highly efficient cooling via adiabatic demagnetization, making it a promising candidate for magnetocaloric refrigeration.