We report magnetization and specific-heat measurements on polycrystalline samples of and show that this compound is a frustrated square lattice with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions. The coupling constants and are determined from a fitting of the susceptibility data and confirmed by an analysis of the saturation field , the specific heat, and the magnetic entropy. undergoes magnetic ordering at about 1 K, likely toward a columnar antiferromagnetic state. We find that with the frustration ratio is closer to a critical (quantum spin liquid) region of the frustrated square lattice than any of the previously reported compounds. Positive curvature of the magnetization curve is observed in agreement with recent theoretical predictions for high-field properties of the frustrated square lattice close to the critical regime.
- Received 17 March 2008
©2008 American Physical Society