Realization of a triangular spin necklace in a verdazyl-based Ni complex

TL;DR

This paper reports the synthesis and characterization of a verdazyl-based Ni complex forming a one-dimensional triangular spin necklace, demonstrating frustrated magnetic interactions and a field-tunable antiferromagnetic order in a molecular system.

Contribution

It introduces a novel molecular design of a frustrated quantum spin chain with both spin-1/2 and spin-1 units, providing a platform to study frustration-driven quantum phases.

Findings

Strong antiferromagnetic interactions between radical pairs

Observation of a phase transition to antiferromagnetic order

Field suppression of the magnetic phase transition

Abstract

We successfully synthesized a verdazyl-based complex, ($m$-Py-V)$_3$[Ni(NO$_3$)$_2$], in which Ni$^{2+}$ ions and verdazyl radicals form a one-dimensional, triangular spin necklace consisting of spin-1/2 and spin-1 units. Molecular orbital calculations reveal strong antiferromagnetic (AF) interactions between inversion-related radical pairs that form spin-1/2 singlet dimers. The remaining verdazyl and Ni$^{2+}$ spins form frustrated triangular units, creating a distinctive spin network. Magnetic susceptibility and specific heat measurements identify a phase transition to an AF order. The application of magnetic fields suppresses the phase transition signal, suggesting field-induced decoupling of the spin-1 moments. Electron spin resonance measurements are used to evaluate the easy-axis anisotropy of spin-1, which may promote the AF order. This work provides a rare example of a geometrically frustrated quantum spin chain realized via molecular design, thereby offering a platform for exploring frustration-driven quantum phases in low-dimensional materials.