Magnetic and ESR studies in a Quasi-two-dimensional Triangular magnet Ba3MnNb2O9

We report the magnetic, specific heat and electron spin resonance (ESR) studies on Ba3MnNb2O9, as one of quasi-two-dimensional triangular–lattice magnet. The magnetic susceptibility and specific heat measurement show that it undergoes an magnetic transition at TN=3.2 K. The magnetic capacity (Cmag) follows T2 dependence, and magnetic entropy (Smag) reaching to 14.5 J K−1mol−1 at 32 K satisfactorily agrees with the theoretical estimations of Mn2+ ion in high-spin configuration with S=5/2. From the ESR spectra, the Landé factor (geff), the linewidth (ΔHpp) and the ESR intensity (I) are derived and analyzed as a function of temperature. As temperature decreases to T⁎~18 K, the g-factor shows a minima with geff=1.984(2) before shifts to larger value at lower temperatures, and the ESR intensity starts to be suppressed below this temperature. This anomalous phenomenon is explained in terms of the establishment of short-range AFM spin correlations at much higher temperatures above TN.