Survival of magnetic correlations above ordering temperature in a ferromagnetically ordered classical kagom\'e 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 Li9Fe3(P2O7)3(PO4)2 (LFPO) with S=5/2 [Kermarrec et 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 Li9Cr3(P2O7)3(PO4)2 (LCPO) with active t2g 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 $\theta \sim$ 3 K (J < 1 K) and the ordering temperature, Tc ~ 2.7 K, and the size of the ordered moment $\sim 1.05 \pm 0.25 $ ~${\mu}_B$ 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, though 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 ($\Delta \propto B$) in the excitation spectrum, and in $B \rightarrow 0$ limit the gap has a finite intercept ~ 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.