Magnetic properties of BaCdVO ( PO 4 ) 2 : A strongly frustrated spin - 1 2 square lattice close to the quantum critical regime

We report magnetization and specific-heat measurements on polycrystalline samples of BaCdVO PO4 2 and show that this compound is a S=1 /2 frustrated square lattice with ferromagnetic nearest-neighbor J1 and antiferromagnetic next-nearest-neighbor J2 interactions. The coupling constants J1 −3.6 K and J2 3.2 K are determined from a fitting of the susceptibility data and confirmed by an analysis of the saturation field 0Hs 4.2 T , the specific heat, and the magnetic entropy. BaCdVO PO4 2 undergoes magnetic ordering at about 1 K, likely toward a columnar antiferromagnetic state. We find that BaCdVO PO4 2 with the frustration ratio =J2 /J1 −0.9 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.