Survival of magnetic correlations above the ordering temperature in the ferromagnetically ordered classical kagome magnet Li9Cr3(P2O7)3(PO4)2

Abstract

Motivated by the recent discovery of a semiclassical nematic spin liquid state in a Heisenberg kagom\'e antiferromagnet ${\mathrm{Li}}_{9}{\mathrm{Fe}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{7})}_{3}{({\mathrm{PO}}_{4})}_{2}$ (LFPO) with $S=5/2$ [Kermarrec $et\phantom{\rule{4pt}{0ex}}al$. Phys. Rev. Lett. 127, 157202 (2021)], we now investigate the impact of spin quantum number $S$ on the ground-state properties by studying the isostructural kagom\'e magnet ${\mathrm{Li}}_{9}{\mathrm{Cr}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{7})}_{3}{({\mathrm{PO}}_{4})}_{2}$ (LCPO) with active ${t}_{2g}$ orbitals and $S=3/2$. Thermodynamic measurements reveal that the ground-state properties of LCPO is dominated by the ferromagnetic interactions with a mean-field temperature $\mathrm{\ensuremath{\Theta}}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}3\phantom{\rule{0.28em}{0ex}}\text{K}(J<1\phantom{\rule{0.28em}{0ex}}\text{K})$ and the ordering temperature, ${T}_{c}$ \ensuremath{\sim}2.7 K, and the size of the ordered moment \ensuremath{\sim}$1.05\phantom{\rule{0.28em}{0ex}}\ifmmode\pm\else\textpm\fi{}0.25\phantom{\rule{0.28em}{0ex}}{\textmu{}}_{\mathrm{B}}$ is significantly reduced from that of a fully ordered moment. The ab initio electronic structure calculations nicely corroborate the thermodynamic results and suggest the presence of additional in- and out-of-plane further-neighbor antiferromagnetic couplings, although significantly weaker in comparison to the dominant first-nearest-neighbor ferromagnetic coupling. The spin-lattice relaxation rate measured with fields larger than the saturation field shows a magnetic field induced gap $(\mathrm{\ensuremath{\Delta}}\ensuremath{\propto}$ B) in the excitation spectrum, and in $B\ensuremath{\rightarrow}0$ limit the gap has a finite intercept \ensuremath{\sim}3 K, equivalent to the mean-field scale. We interpret the origin of this gap is associated with the magnetic interactions inherent to the material. With our experimental results, we establish the stabilization of a ferromagnetic like ground state and the persistence of magnetic correlations above the ordering temperature in LCPO.