Single Crystal 31P NMR Studies of the Frustrated Square Lattice Compound Pb2(VO)(PO4)2

TL;DR

This study uses 31P NMR to investigate the magnetic properties and ordering in the frustrated square lattice compound Pb2(VO)(PO4)2, revealing antiferromagnetic order and lattice distortions.

Contribution

First detailed 31P NMR analysis of Pb2(VO)(PO4)2 elucidating magnetic interactions and lattice distortions in a frustrated square lattice compound.

Findings

Identified two inequivalent P sites with hyperfine coupling asymmetry.

Estimated exchange couplings: J1 = -5.4 K (ferromagnetic), J2 = 9.3 K (antiferromagnetic).

Observed columnar antiferromagnetic order below 3.65 K.

Abstract

The static and dynamic properties of V^{4+} spins (S = 1/2) in the frustrated square lattice compound Pb2(VO)(PO4)2 were investigated by means of magnetic susceptibility chi 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 \simeq 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 towards 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 to be J1/kB = (-5.4 \pm 0.5) K (ferromagnetic) and J2/kB = (9.3 \pm 0.6) K (antiferromagnetic), respectively. 1/(T1 T 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/(T1 T 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 \simeq -1.72 ratio. The critical exponent beta = 0.25 \pm 0.02 estimated from the temperature dependence of 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.