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Zusammenfassung:
We report static and dynamic properties of the antiferromagnetic compound Zn2(VO)(PO4)2, and the consequences of nonmagnetic Ti4+ doping at the V4+ site. 31P nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rate (1/T1) consistently show the formation of the long-range antiferromagnetic order below TN=3.8–3.9 K. The critical exponent β=0.33±0.02 estimated from the temperature dependence of the sublattice magnetization measured by P31 NMR at 9.4 MHz is consistent with universality classes of three-dimensional spin models. The isotropic and axial hyperfine couplings between the P31 nuclei and V4+ spins are Aisohf=(9221±100) Oe/μB and Aaxhf=(1010±50) Oe/μB, respectively. Magnetic susceptibility data above 6.5 K and heat capacity data above 4.5 K are well described by quantum Monte Carlo simulations for the Heisenberg model on the square lattice with J≃7.7 K. This value of J is consistent with the values obtained from the NMR shift, 1/T1, and electron spin resonance intensity analysis. Doping Zn2VO(PO4)2 with nonmagnetic Ti4+ leads to a marginal increase in the J value and the overall dilution of the spin lattice. In contrast to the recent ab initio results, we find neither evidence for the monoclinic structural distortion nor signatures of the magnetic one-dimensionality for doped samples with up to 15% of Ti4+. The Néel temperature TN decreases linearly with increasing the amount of the nonmagnetic dopant.
