Magnetic properties of the one-dimensional S=3/2 Heisenberg antiferromagnetic spin-chain compound Na2Mn3O

We have performed magnetic susceptibility (chi), heat capacity (C-p), and nuclear magnetic resonance (NMR) measurements on the layered compound Na2Mn3O7 in which the magnetic Mn4+ ions constitute a maple-leaf lattice. At high temperature, chi follows the Curie-Weiss (CW) law with CW temperature as high as 152 K and Mn4+ ions are found to be in the high-spin state (S = 3/2). With decreasing temperature, chi passes through a broad maximum at 115 K and decreases rapidly at low temperature. A similar broad peak appears at around 62 K in the magnetic contribution to heat capacity (C-m). The temperature dependence of chi and C-m are the characteristics of an one-dimensional Heisenberg antiferromagnetic spin chain with a nonmagnetic ground state. The obtained values of spin-gap energy (Delta(s)) and nearest-neighbor antiferromagnetic exchange coupling (J) are 28 and 27 K, respectively. There is no evidence of a long-range magnetic ordering down to 2 K in heat capacity data, in agreement with spin-singlet ground state. These observations are further corroborated by Na-23 NMR studies through measurements of both the NMR shift and the spin-lattice relaxation rate.