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

Abstract Magnetic, thermodynamic, and magnetocaloric properties of a spin- <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mfrac> <m:mrow> <m:mn>1</m:mn> </m:mrow> <m:mrow> <m:mn>2</m:mn> </m:mrow> </m:mfrac> </m:math> $\frac{1}{2}$ 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.