Magnetic structure and field-induced transitions in the triangular spin-1 antiferromagnet NiNb₂O₆: Observation of a triple point

A comprehensive experiment-theory study on the magnetic field-induced spin-flip transitions (H-C1 and H-C2) and the field-temperature (H-T) phase diagram of nickel niobate (NiNb2O6) polycrystals is reported. Magnetic susceptibility (chi) measurements reveal the presence of quasi-one-dimensional magnetism with spin-1 Ni2+ Heisenberg spin chains, instead of Ising chains, along the easy axis (c axis). The ferromagnetic intrachain exchange-interaction strength, J(0)/k(B) (=4.20 K), between Ni2+ ions in the c axis is estimated from the molecular-field theory approximation. The interchain exchange interactions J(1)/k(B)=-1.335 K and J(2)/k(B)=-1.298 K have been determined from the experimentally obtained magnitudes of H-C1=10.8 kOe and spin-saturated field H-S=38 kOe. These magnitudes are consistent with the total magnetic exchange-interaction strength, J/k(B)=7.00 K obtained from chi(-T) data fitted with the Heisenberg linear chain model and our density-functional theory (DFT+U) calculations. The overall antiferromagnetic behavior of the NiNb2O6 system, correctly captured within our DFT+U calculations, occurs below T-N=5.59 +/- 0.02 K as determined from the analysis of the specific heat (C-P-T) data. The C-P vs T data near T-N is fitted to power-law expression C-P=A|T-T-N|(-alpha) yielding the critical exponent alpha=0.349(2) [0.349(1)] for T>T-N (T更多