Magnetic properties of the one-dimensional S=3/2 Heisenberg antiferromagnetic spin-chain compound Na₂Mn₃O₇

Abstract

We have performed magnetic susceptibility (χ), heat capacity (Cp), 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, χ 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=32). With decreasing temperature, χ 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 (Cm). The temperature dependence ofvχ and Cm are the characteristics of an one-dimensional Heisenberg antiferromagnetic spin chain with a nonmagnetic ground state. The obtained values of spin-gap energy (Δs) 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 23Na NMR studies through measurements of both the NMR shift and the spin-lattice relaxation rate.