Magnetic properties of λ-BETS2(FexGa1−x)Cl4exhibiting a superconductor-to-insulator transition(0.35<x<0.5)

The crystal of λ-${\mathrm{}\left(\mathrm{BETS}\right)\mathrm{}}_2{(\mathrm{F}\mathrm{e}}_x{\mathrm{Ga}}_{1-x}{)\mathrm{C}\mathrm{l}}_4$ with $x\approx 0.45$ undergoes successive superconducting and superconductor-to-insulator transitions around liquid helium temperature. The large diamagnetic susceptibility (≈-35 emu/mol at 4 K) observed in the superconducting phase of the system with $x\approx 0.47,$ which exhibits a superconducting transition at 4.3 K and superconductor-to-insulator transition at 3.7 K, can be regarded as evidence for the bulk nature of the superconductor-to-insulator transition. The anisotropy of the susceptibility and spin-flop behavior indicate that the system takes a π-d coupled antiferromagnetic insulating ground state, which is essentially the same as that of λ-${\mathrm{}\left(\mathrm{BETS}\right)\mathrm{}}_2{\mathrm{FeCl}}_4$ exhibiting a coupled metal-insulator and antiferromagnetic transition at 8.5 K. The easy axis is parallel to the c direction. Unlike the Ga-rich system $\mathrm{}(x<\mathrm{}0.35\mathrm{})\mathrm{}$ with superconducting ground state, the Fe-rich system such as λ-${\mathrm{}\left(\mathrm{BETS}\right)\mathrm{}}_2{(\mathrm{F}\mathrm{e}}_{0.7}{\mathrm{Ga}}_{0.3}{)\mathrm{C}\mathrm{l}}_4$ shows a metal-insulator transition at ambient pressure. At high pressure, however, the π and d electrons tend to be independent of each other and the system takes a superconducting ground state.