Structural and magnetic properties of a quasi-one-dimensional spin-1/2 compound NaVOPO4 are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and 31 P nuclear magnetic resonance measurements, as well as complementary ab initio calculations. Whereas magnetic susceptibility of NaVOPO4 may be compatible with the gapless uniform spin chain model, detailed examination of the crystal structure reveals a weak alternation of the exchange couplings with the alternation ratio α 0.98 and the ensuing zero-field spin gap ∆0/kB 2.4 K directly probed by field-dependent magnetization measurements. No longrange order is observed down to 50 mK in zero field. However, applied fields above the critical field Hc1 1.6 T give rise to a magnetic ordering transition with the phase boundary TN ∝ (H − Hc1)

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NaVOAsO4 by means of x-ray diffraction, magnetization, electron spin resonance (ESR), and 75 As nuclear magnetic resonance (NMR) measurements as well as density-functional band structure calculations. Temperature-dependent magnetic susceptibility, ESR intensity, and NMR line shift could be described well using an alternating spin-1/2 chain model with the exchange coupling J/kB 52 K and an alternation parameter α 0.65. From the high-field magnetic isotherm measured at T = 1.5 K, the critical field of the gap closing is found to be Hc 16 T, which corresponds to the zero-field spin gap of ∆0/kB 21.4 K. Both NMR shift and spin-lattice relaxation rate show an activated behavior at low temperatures, further confirming the singlet ground state. The spin chains do not coincide with the structural chains, whereas the couplings between the spin chains are frustrated. Because of a relatively small spin gap, NaVOAsO4 is a promising compound for further experimental studies under high magnetic fields.PACS numbers: 75.50. Ee, 75.10.Pq, 75.30.Et, 71.20.Ps, 61.66.Fn J' J a b c Jc Ja J' J FIG. 1. Left panel: crystal structure of NaVOAsO4. The black dashed lines separate the spin chains. The VO6 and AsO4 polyhedra are shown in red and green colors, respectively. Right panel: spin model showing the alternating (J, J ) crossing chains and the frustrated inter-chain couplings in a different orientation.

The intermetallic semiconductor FeGa₃ acquires itinerant ferromagnetism upon electron doping by a partial replacement of Ga with Ge. We studied the electron spin resonance (ESR) of high-quality single crystals of FeGa_3-xGe_x for x from 0 up to 0.162 where ferromagnetic order is observed. For x = 0 we observed a well-defined ESR signal, indicating the presence of pre-formed magnetic moments in the semiconducting phase. Upon Ge doping the occurrence of itinerant magnetism clearly affects the ESR properties below ≈ 40 K whereas at higher temperatures an ESR signal as seen in FeGa₃ prevails independent on the Ge-content. The present results show that the ESR of FeGa_3-xGe_x is an appropriate and direct tool to investigate the evolution of 3d-based itinerant magnetism.

We report synthesis and magnetic properties of quasi-one-dimensional spin-12 Heisenberg antiferromagnetic chain compound BaNa2Cu(VO4)2. This orthovanadate has a centrosymmetric crystal structure, C2/c, where the magnetic Cu2+ ions form spin chains. These chains are arranged in layers, with the chain direction changing by 62∘ between the two successive layers. Alternatively, the spin lattice can be viewed as anisotropic triangular layers upon taking the interchain interactions into consideration. Despite this potential structural complexity, temperature-dependent magnetic susceptibility, heat capacity, electron spin resonance intensity, and nuclear magnetic resonance (NMR) shift agree well with the uniform spin-1/2 Heisenberg chain model with an intrachain coupling of J/kB≃5.6 K. The saturation field obtained from the magnetic isotherm measurement consistently reproduces the value of J/kB. Further, the 51V NMR spin-lattice relaxation rate mimics the one-dimensional character in the intermediate temperature range, whereas magnetic long-range order sets in below TN≃0.25 K. The effective interchain coupling is estimated to be J⊥/kB≃0.1 K. The theoretical estimation of exchange couplings using bandstructure calculations reciprocate our experimental findings and unambiguously establish the onedimensional character of the compound. Finally, the spin lattice of BaNa2Cu(VO4)2 is compared with the chemically similar but not isostructural compound BaAg2Cu(VO4)2.

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