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Single-crystal P31 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, and D. C. Johnston
Phys. Rev. B 80, 214430 – Published 31 December 2009

Abstract

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 χ and P31 nuclear magnetic resonance (NMR) shift (K) and P31 nuclear spin-lattice relaxation rate 1/T1 measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below TN3.65K. 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/(T1Tχ) is almost T independent at high temperatures due to random fluctuation of spin moments. Below 20 K, the compound shows an enhancement of 1/(T1Tχ) 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/J11.72 ratio. The critical exponent β=0.25±0.02 estimated from the temperature dependence of the sublattice magnetization as measured by P31 NMR at 11.13 MHz is close to the value (0.231) predicted for the two-dimensional XY model.

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  • Received 18 September 2009

DOI:https://doi.org/10.1103/PhysRevB.80.214430

©2009 American Physical Society

Authors & Affiliations

R. Nath1,2,*, Y. Furukawa1, F. Borsa1,3, E. E. Kaul2, M. Baenitz2, C. Geibel2, and D. C. Johnston1

  • 1Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
  • 2Max Planck Institut für Chemische Physik Fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany
  • 3Dipartimento di Fisica “A. Volta,” Università di Pavia, I-27100 Pavia, Italy

  • *Present address: School of Physics, Indian Institute of Science Education and Research, Thiruvananthapuram 695016, India.

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Vol. 80, Iss. 21 — 1 December 2009

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