Tuning the flat bands of the kagome metal CoSn with Fe, In, or Ni doping

Abstract

CoSn is a Pauli paramagnet with relatively flat d-bands centered about 100 meV below the Fermi energy, EF. Single crystals of CoSn lightly doped with Fe, In or Ni are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, ac susceptibility, specific heat and resistivity measurements. Within the rigid band approximation, hole doping with a few percent of Fe or In should move the flat bands closer to EF, whereas electron doping with Ni should move the flat bands further away from EF. We provide evidence that this indeed occurs. Fe and In doping drive CoSn toward magnetism, while Ni doping suppresses CoSn's already weak magnetic response. The resulting ground state is different for Fe versus In doping. For Fe-doped crystals, Co1-xFexSn, with 0.02 < x <0.27, the magnetic and specific heat data are consistent with the formation of a spin glass, with a glass temperature, Tg, ranging from 1 K for x =0.02 to 10 K for x = 0.27. Powder and single crystal neutron diffraction found no evidence of long-range magnetic order below Tg for samples with x ≈ 0.17. For In-doped crystals, CoSn1-yIny, both the magnetic susceptibility and the Sommerfeld coefficient, g, increase substantially relative to pure CoSn, but with no clear indication of a magnetic transition for 0.05 < y < 0.2. CoSn crystals doped with Ni (Co0.93Ni0.07Sn) have a significantly smaller magnetic susceptibility and g than pure CoSn, consistent with flat bands further from EF.

Chemical Control of Magnetism in the Kagome Metal CoSn1 – xInx: Magnetic Order from Nonmagnetic Substitutions

“…As noted above, CoSn is a Pauli paramagnet, and In substitution up to 20% has been shown to increase the overall magnetic moment, 28 consistent with the electronic structure response just described. However, no new ground state was observed at that modest In content.…”

“…Previously, we found that adding Fe to CoSn did add holes and moved the flat bands closer to E F , but each Fe appeared to carry a local magnetic moment of about 2 μ B that dominated the magnetic response and obscured any subtle effects from the flat bands. 28 Indium doping was limited in that study to concentrations up to 20%. The results indicated an approach toward a flat band-related instability with increasing In content but revealed no associated phase transition.…”

“…The results indicated an approach toward a flat band-related instability with increasing In content but revealed no associated phase transition. 28 Here, we report results obtained by replacing up to 40% of the Sn in CoSn with In. This results in a movement of E F into the flat bands while avoiding the complication of local moment magnetism seen with Fe doping.…”

“…As a structural motif, the kagome net is common to many material families. , Electronic and topological behaviors are actively studied in many examples, including Fe 3 Sn 2 , , the CoSn and FeSn family, ,− Co 3 Sn 2 S 2 -related shandites, ,, and, very recently, Ni 3 In and AV 3 Sb 5 (A = K, Rb, or Cs) . Evidence for the kagome-related flat bands impacting the physical properties is not common since the material’s electron count must be tuned so that the chemical potential (or Fermi energy, E F ) is in the vicinity of the flat bands.…”

Chiral phonons in honeycomb sublattice of layered CoSn-like compounds

“…Two of the seven studied compounds have been previously reported to be magnetically ordered. The magnetic moments on Fe atoms are aligned ferromagnetically (FM) in FeGe [86,87], while in FeSn the antiferromagnetic (AFM) order is observed [88]. In AFM structure the spins are arranged ferromagnetically in each FeSn layer and antiferromagnetically ordered in neighboring layers.…”