Anomalous spin-lattice coupling in the quasi-one-dimensional spin-1 corrugated skew-chain antiferromagnet: Ni2V2O7

We investigated spin-lattice coupling through detailed structural, magnetic, and lattice-dynamics studies of the S=1 quasi-one-dimensional lattice Ni2V2O7 compound by using x-ray diffraction, magnetic susceptibility [χ (T)], heat capacity (CP), x-ray absorption spectroscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. Further, the detailed study of lattice dynamics by first-principles calculation on Ni2V2O7 is presented. Our magnetic susceptibility χ (T), agreeing well with heat-capacity (CP) measurements, conclusively shows that the antiferromagnetic transitions occur at TN1∼6.7K and TN2∼5.8K. However, a broad peak centered at around Tmax∼100K in the CP/T data along with the divergence of the susceptibility from Curie-Weiss behavior indicates the formation of short-range magnetic correlations ∼100K, which is well above TN1. The temperature-dependent x-ray diffraction analysis suggests Ni-Ni bond shortening and NiO6 octahedral distortion significantly below ∼100K. Further, from detailed lattice dynamics, two Raman modes showed anomalous lattice softening below ∼100K which indicates the formation of the Ni atom dimerization as supported by ordering of the J1 exchange interaction. All the other Raman modes soften below ∼30K. This suggests that the different exchange interactions order at different temperatures. This again supports the presence of short-range magnetic correlations well above TN1 which is in good agreement with magnetization and heat-capacity data. Importantly, the spin-lattice coupling has been established in Ni2V2O7 below ∼100K and the spin-lattice coupling constant (λsp) for various Raman modes has been deduced which shows multiferroic behavior below TN1. The isostructural change observed in x-ray diffraction and Raman spectroscopy below 100 K provides structural origin for the multiferroic behavior shown by this compound.