Magnetic and electronic structure studies on S = 1/2 Heisenberg antiferromagnetic alternating spin chain system KCuGa(PO4)2

The correlation between low dimensionality and spin interactions can foster the emergence of exotic singlet states. Herein, we report a thorough investigation of the $S=1/2$ spin chain system KCuGa(PO₄)₂, with alternating magnetic couplings $J_{min}$ and $J_{max}$. A broad peak appears around 12 K, followed by an exponential decay in $\chi \left(T\right)$, indicating the existence of a spin gap in the ground state. The magnetic susceptibility data are modeled with the Heisenberg antiferromagnetic alternating spin chain, which gives $J_{min}\approx -6.47$ K, $J_{max}\approx -16.18$ K, alternation parameter ($\alpha$) = $J_{min}/J_{max}\approx 0.40$, and a spin gap ($\Delta$) of 12 K. The $\chi \left(T\right)$ data were simulated using spin-dynamics based on the Landau–Lifshitz–Gilbert (LLG) method, showing excellent agreement with the experimental data. Magnetic heat capacity measurements further confirm the value of $\Delta$ as 12 K. Field-induced magnetic behavior is also observed. First-principles electronic structure calculations were performed to complement the experimental findings and investigate the electronic structure and exchange interactions. The calculated $\alpha \approx 0.40$ from DFT + U matches closely with the experimental estimate, confirming the validity of the HAFM alternating spin chain model.