Abstract
The static and dynamic properties of spins in the frustrated square-lattice compound were investigated by means of magnetic susceptibility and nuclear magnetic resonance (NMR) shift and nuclear spin-lattice relaxation rate measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below . NMR spectra above show two distinct lines corresponding to two inequivalent P sites present in the crystal structure. The observed asymmetry in hyperfine coupling constant for the in-plane (P1) P site directly points toward a distortion in the square lattice at the microscopic level, consistent with the monoclinic crystal structure. The nearest- and next-nearest-neighbor exchange couplings were estimated by fitting versus temperature by a high-temperature series expansion for the spin susceptibility of the frustrated square lattice to be (ferromagnetic) and (antiferromagnetic), respectively. is almost independent at high temperatures due to random fluctuation of spin moments. Below 20 K, the compound shows an enhancement of which arises from a growth of antiferromagnetic spin correlations above . Below and for the field applied along the axis, the NMR spectrum for the P1 site splits into two satellites and the spacing between them increases monotonically with decreasing which is a direct evidence of a columnar antiferromagnetic ordering with spins lying in the plane. This type of magnetic ordering is consistent with expectation from the ratio. The critical exponent estimated from the temperature dependence of the sublattice magnetization as measured by NMR at 11.13 MHz is close to the value (0.231) predicted for the two-dimensional XY model.
2 More- Received 18 September 2009
DOI:https://doi.org/10.1103/PhysRevB.80.214430
©2009 American Physical Society