BaV3O8: A possible Majumdar-Ghosh system withS=12

Abstract

BaV${}_{\text{3}}$O${}_{\text{8}}$ contains magnetic V${}^{\text{4+}}$($S=1/2$) ions and also nonmagnetic V${}^{\text{5+}}$($S=0$) ions. The V${}^{4+}$ ions are arranged in a coupled Majumdar-Ghosh chainlike network. A Curie-Weiss fit of our magnetic susceptibility $\ensuremath{\chi}(T)$ data in the temperature region of $80--300$ K yields a Curie constant $C=0.39$ cm${}^{\text{3}}$K/mole V${}^{\text{4+}}$ and an antiferromagnetic Weiss temperature $\ensuremath{\theta}=\ensuremath{-}26\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The $\ensuremath{\chi}$(T) curve shows a broad maximum at $T\ensuremath{\simeq}25$ K indicative of short-range order (SRO) and an anomaly corresponding to long-range order (LRO) at ${T}_{\mathrm{N}}$$\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}6$ K. The value of the ``frustration parameter'' ($f=|\ensuremath{\theta}/{T}_{\text{N}}|\ensuremath{\sim}5$) suggests that the system is moderately frustrated. Above the LRO temperature, the experimental magnetic susceptibility data match well with the coupled Majumdar-Ghosh (or ${J}_{\mathrm{nn}}\ensuremath{-}{J}_{\mathrm{nnn}}$ Heisenberg) chain model with the ratio of the nnn (next-nearest neighbor) to nn (nearest neighbor) magnetic coupling $\ensuremath{\alpha}=2$ and ${J}_{\mathrm{nnn}}/{k}_{B}=40$ K. In a mean-field approach when considering the interchain interactions, we obtain the total interchain coupling to be about 16 K. The LRO anomaly at ${T}_{\mathrm{N}}$ is also observed in the specific heat ${C}_{\mathrm{P}}(T)$ data and is not sensitive to an applied magnetic field up to 90 kOe. A ${}^{\text{51}}$V NMR signal corresponding to the nonmagnetic vanadium was observed. Anomalies at 6 K were observed in the variation with temperature of the ${}^{51}$V NMR linewidth and the spin-lattice relaxation rate $1/{T}_{1}$ indicating that they are sensitive to the LRO onset and fluctuations at the magnetic V sites. The existence of two components (one short and another long) is observed in the spin-spin relaxation rate $1/{T}_{2}$ data in the vicinity of ${T}_{\mathrm{N}}$. The shorter component seems to be intimately connected with the magnetically ordered state. We suggest that both magnetically ordered and nonlong-range-ordered (non-LRO) regions coexist in this compound below the long-range-ordering temperature.