Effects of quantum impurity spins on the magnetic properties of zigzag and linear spin chains

TL;DR

This study explores how quantum impurity spins affect the magnetic ground states and low-energy excitations in zigzag and linear spin chain compounds, revealing impurity-dependent gapped or gapless behaviors and impurity-induced magnetic order.

Contribution

It provides new insights into impurity effects on magnetic properties of spin chains, highlighting the role of spin state, symmetry, and frustration in determining magnetic phases.

Findings

Ni doping induces a spin gap in chains.

Zn and Co doping maintain gapless excitations.

Co doping enhances magnetic order and critical temperatures.

Abstract

We investigated the magnetic ground state and low-energy excitations of the spin chains compounds SrCuO$_{2}$ (zigzag chains) and Sr$_{2}$CuO$_{3}$ (linear chains) in the presence of quantum impurities induced by lightly doping ($\leq 1 \%$) with Zn$^{2+}$ ($S = 0$), Co$^{2+}$ ($S =1/2$) and Ni$^{2+}$ ($S = 1$) impurities at the Cu$^{2+}$ site. We show that the ground states and the nature of low-lying excitations (i.e., gapped or gapless) depend on the spin state and symmetry of the defects. For Ni doped chains a spin gap is observed but for Zn and Co doping the excitations remain gapless. Co-doped chains exhibit magnetic order with critical temperatures significantly enhanced compared to those of the pristine compounds. In the specific case of 1 \% Co impurities, the linear chains exhibit long-range order below 11 K, while the zigzag chain is characterized by a quasi-long range ordered phase below 6 K with correlation lengths of about 12\textit{a} and 40\textit{c} units along the crystal axes \textit{a} and \textit{c}, respectively. The different magnetic behaviours of these two compounds with comparable intra- and interchain couplings underpin the role of spin frustration in the zigzag chains.