Realization of Giant Negative Thermal Expansion via a Significantly Decreased Local Moment in a Kagome Ferromagnet with Spin Reorientation

Negative thermal expansion (NTE) materials are significant for a variety of modern advanced applications, yet discovering new NTE materials and understanding the underlying spin-lattice interactions remain challenging. Here, we report a pronounced volumetric NTE with a coefficient of thermal expansion, αV = -32.4 × 10-6 K-1 (100-200 K), in kagome (Sc,Ti)Fe2 with Fe-excess. Neutron diffraction, Mössbauer spectroscopy, and theoretical investigations indicate that the content of scandium affects the antiferromagnetic coupling of kagome layers hosting Fe in 6h lattice position and additional magnetic exchange interaction arising from the excess Fe stabilizes the ferromagnetic magnetic order. A sharp reduction of the magnetic moment of the Fe sublattice thus emerges on heating and yields the NTE. Intriguingly, the transition between ferromagnetic and antiferromagnetic stacking of the kagome layers is accompanied by a spin reorientation of Fe from the in-plane direction to the out-of-plane direction. This magnetic reorientation could be the result of a strong competition of the Fe single-site anisotropies on different sublattices and is thus regarded as a hallmark of the high-spin state/low-spin state transition on the frustrated Fe sublattice. This work provides further insight that different magnetic orders in the same material can host NTE and normal thermal expansion behavior.