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Deformed spin-1/2 square lattice in antiferromagnetic NaZnVOPO4(HPO4)

Guchhait, S.; Ambika, D. V.; Ging, Q.-P.; Uhlarz, M.; Furukawa, Y.; Tsirlin, A. A.; Nath, R.

We report the structural and magnetic properties of a new spin-1/2 antiferromagnet NaZnVOPO4(HPO4) studied via x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, and 31P nuclear magnetic resonance (NMR) measurements, as well as density-functional band-structure calculations. While thermodynamic properties of this compound are well described by the J1-J2 square-lattice model, ab initio calculations suggest a significant deformation of the spin lattice. From fits to the magnetic susceptibility we determine the averaged nearest-neighbor and second-neighbor exchange couplings of J1 ≃ −1.3 K and J2 ≃ 5.6 K, respectively, resulting in the effective frustration ratio α = J2/J1 ≃ −4.3 that implies columnar antiferromagnetic order as the ground state. Experimental saturation field of 15.3 T is consistent with these estimates if 20 % spatial anisotropy in J1 is taken into account. Specific heat data signal the onset of a magnetic long-range order at TN ≃ 2.1 K, which is further supported by a sharp peak in the NMR spin-lattice relaxation rate. The NMR spectra mark the superposition of two P lines due to two nonequivalent P sites where the broad line with the strong hyperfine coupling and short T1 is identified as the P(1) site located within the magnetic planes, while the narrow line with the weak hyperfine coupling and long T1 is designated as the P(2) site located between the planes.

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Publ.-Id: 35476