Emergence of Kondo-assisted Néel order in a Kondo necklace model

Abstract

The interplay between Kondo screening and magnetic order has long been a central issue in the physics of strongly correlated systems. While the Kondo effect has traditionally been understood to suppress magnetism through the formation of local singlets, recent studies suggest that Kondo interactions may enhance magnetic order under certain conditions. However, these scenarios often rely on complex electronic structures, including orbital and charge degrees of freedom, making the essential mechanisms difficult to isolate. Here we report the realization of a spin-(1/2,1) Kondo necklace model in a Ni-based complex-a minimal spin-only analog of the Kondo lattice that isolates quantum spin correlations by eliminating charge degrees of freedom. Thermodynamic measurements identify a magnetic phase transition and a field-induced quantum phase transition. Perturbative analysis reveals that the Kondo coupling mediates effective antiferromagnetic interactions between the spin-1 sites, stabilizing the Néel order across the entire chain. Our results establish a universal boundary in Kondo physics, where coupling to spin-1/2 moments yields singlets, but to spin-1 and higher stabilizes magnetic order.

Figures (4)

• Figure 1: Crystal structure and Kondo necklace model of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$.a, Molecular structure of Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$. b, Crystal structure forming the Kondo necklace model along the $a$ axis. Hydrogen atoms are excluded to enhance clarity. The green nodes represent the spin-1/2 of the radicals. The thick lines represent exchange interactions. c,Spin-(1/2,1) Kondo necklace model comprising intermolecular $J_{\rm{1}}$ and intramolecular $J_{\rm{2}}$. $\boldsymbol{s}$ and $\boldsymbol{S}$ denote the spins on the radical and Ni$^{2+}$, respectively.
• Figure 2: Magnetic behavior of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$.a,Temperature dependence of magnetic susceptibility ($\chi$ = $M/H$) of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$ at 0.1 T. The inset shows corrresponding $\chi T$ values. b, Magnetization curve of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$ at 1.4 K. The dashed lines represent the QMC results for the spin-(1/2,1) Kondo necklace model with $J_{1}/k_{\rm{B}}$ = 20.3 K and $J_{2}/k_{\rm{B}}$ = 9.9 K $J_{2}/J_{1}$ ($J_{2}/J_{1}$ = 0.49). For the magnetization curve, a radical purity of 95 ${\%}$ is considered for the calculation. c, Calculated magnetization curves at $T/J_{1}$ = 0.05 with the representative values of $J_{2}/J_{1}$. The magnetic moment and the magnetic field are normalized by the values at the saturation. For $J_{2}/J_{1} \geq 1.0$, a clear 1/3 plateau emerges, reflecting full polarization of the effective spin-1/2 within the $\boldsymbol{s}$-$\boldsymbol{S}$ dimer mediated by $J_{2}$. Conversely, for $J_{2}/J_{1} \ll 1.0$, the magnetization exhibits a steep rise toward 2/3, consistent with full polarization of the Ni spins caused by the effective decoupling of $J_{2}$.
• Figure 3: ESR of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$.a, Frequency dependence of ESR absorption spectra of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$ at 1.8 K. The arrows indicate the resonance fields. b, Frequency-field plot of the resonance fields. Solid lines indicate the calculated resonance modes of the spin-1 monomer along the principal axes, obtained using an on-site anisotropy of $D/k_{\rm{B}}$=$-$1.2 K and $g$-values of $g_x$=2.20, $g_y$=2.25, and $g_z$=2.34. c, Calculated energy branch of the the spin-1 monomer for $H$//$z$, $H$//$x$ and $H$//$y$. Those for $H$//$x$ and $H$//$y$ are qualitatively equivalent. Arrows indicate spin-allowed transitions from the ground state, which correspond to the resonance modes.
• Figure 4: Specific heat of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$.a, Temperature dependence of the specific heat $C_{\rm{p}}$ of $[$Ni($p$-Py-V-$p$-F)(H$_2$O)$_5$$]$$\cdot$2NO$_3$ at various magnetic fields for $H$//$b$, perpendicular to the chain direction. The lines are guides for the eye. b, Low-temperature region of $C_{\rm{p}}$. For clarity, the values for 1.0, 1.3, 1.5, 1.7, 1.8, 2.0, and 3.0 T have been shifted up by 43, 37, 32, 25, 19, 13, and 6 J/ mol K, respectively. The broken lines represent the Schottky-type specific heat of the spin-1 monomer for $H$//$y$, obtained using an on-site anisotropy of $D/k_{\rm{B}}$=$-$1.2 K and $g_y$=2.25. The experimental results approach the behavior of the spin-1 monomer with increasing fields, suggesting the critical field causing the decoupling of $\boldsymbol{S}$ is approximately 2 T. The higher temperature deviations are considered to arise from the spin-1/2 chain and lattice contributions.