Single-crystal 31P NMR studies of the frustrated square-lattice compound Pb2(VO)(PO4)2

R. NATH; Y. FURUKAWA; F. BORSA; E. E. KAUL; M. BAENITZ; C. GEIBEL; D. C. JOHNSTON

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

American Physical Society

Año: 2009 vol. 80 p. 2144301 - 2144310

0163-1829

The static and dynamic properties of V4+ spins (S=1/2) in the frustrated square-lattice compound Pb2(VO)(PO4)2 were investigated by means of magnetic susceptibility ? and 31P nuclear magnetic resonance (NMR) shift (K) and 31P nuclear spin-lattice relaxation rate 1/T1 measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below TN~3.65 K. NMR spectra above TN 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 K versus temperature T by a high-temperature series expansion for the spin susceptibility of the frustrated square lattice to be J1/kB=(-5.4±0.5) K (ferromagnetic) and J2/kB=(9.3±0.6) K (antiferromagnetic), respectively. 1/ (T1T_chi) is almost T independent at high temperatures due to random fluctuation of spin moments. Below 20 K, the compound shows an enhancement of 1/(T1T_chi ) which arises from a growth of antiferromagnetic spin correlations above TN. Below TN and for the field applied along the c axis, the NMR spectrum for the P1 site splits into two satellites and the spacing between them increases monotonically with decreasing T which is a direct evidence of a columnar antiferromagnetic ordering with spins lying in the ab plane. This type of magnetic ordering is consistent with expectation from the J2 /J1~-1.72 ratio. The critical exponent beta = 0.25±0.02 estimated from the temperature dependence of the sublattice magnetization as measured by 31P NMR at 11.13 MHz is close to the value (0.231) predicted for the two-dimensional XY model.