Spin-glass-like magnetic ground state of the geometrically frustrated pyrochlore niobateTb2Nb2O7

Abstract

dc $(\ensuremath{\chi})$ and ac $({\ensuremath{\chi}}_{\mathrm{ac}})$ magnetic susceptibilities, field dependence of isothermal magnetization and specific heat ${C}_{\mathrm{P}}$ measurements have been performed on a pyrochlore geometrically frustrated niobate ${\mathrm{Tb}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}.$ The $\ensuremath{\chi}$ characteristics is consistent with collective paramagnetism down to at least $T\ensuremath{\approx}2\mathrm{K}$ with effective nearest-neighbor antiferromagnetic interactions, ${J}_{\mathrm{eff}}=\ensuremath{-}1.5\mathrm{K},$ among magnetic ions. Crystal-field calculation shows that ${\mathrm{Tb}}^{3+}$ ground state is an isolated Ising-like $S=1/2$ doublet with anisotropic g tensor and ${\mathrm{Tb}}^{3+}$ spins pointing along local $〈111〉$ directions of tetrahedral unit of pyrochlore structure. The ground-state doublet is further modified by the short-range spin-spin correlations, giving rise to a rounded peak at 1.9 K in zero-field magnetic specific heat ${C}_{\mathrm{mag}}.$ In addition, the frequency-dependent peak in ${\ensuremath{\chi}}_{\mathrm{ac}}$ at about 0.8 K indicates spin-glass-like freezing of ${\mathrm{Tb}}^{3+}$ spins in ${\mathrm{Tb}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}.$ ${\mathrm{Tb}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ is compared with other geometrically frustrated Tb-based pyrochlores, e.g., a spin-glass ${\mathrm{Tb}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ and a collective paramagnet-spin liquid ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}.$