Recent neutron scattering measurements indicate that NaFe 1−x Cu x As forms an antiferromagnetic stripe phase near x ≈ 0.5 in a Mott insulating state. This copper concentration is well in excess of that required for superconductivity, x < 0.04. We have investigated the development of magnetism in this compound using 23 Na nuclear magnetic resonance (NMR) spectra and spinlattice relaxation measurements performed on single crystals (x = 0.13, 0.18, 0.24, and 0.39). We find multiple inequivalent Na sites, each of which is associated with a different number of nearest neighbor Fe sites occupied by a Cu dopant. We show that the distribution of Cu substituted for Fe is random in-plane for low concentrations (x = 0.13 and 0.18), but deviates from this with increasing Cu doping. As is characteristic of many pnictide compounds, there is a spin pseudo gap that increases in magnitude with dopant concentration. This is correlated with a corresponding increase in orbital NMR frequency shift indicating a change in valence from Cu 2+ to a Cu 1+ state as x exceeds 0.18, concomitant with the change of Fe 2+ to Fe 3+ resulting in the formation of magnetic clusters. However, for x ≤ 0.39 there is no evidence of long-range static magnetic order.

Neutron scattering measurements have demonstrated that the heavily Cu-doped NaFe1−xCuxAs compound behaves like a Mott insulator exhibiting both real space Fe-Cu stripes, as well as antiferromagnetism below a Néel temperature for x < ∼ 0.5. We have investigated evolution of structural and magnetic ordering using 23 Na and 75 As NMR for single crystals (x = 0.39 and 0.48), confirming antiferromagnetism in the form of magnetic stripes. We show that end-chain defects in these stripes are the principal source of magnetic disorder and are responsible for cluster spin-glass transitions in both compounds, in the latter case coexistent with antiferromagnetism. Aided by our numerical simulation of the 75 As spectra, we show that a staggered magnetization at the Fe sites is induced by non-magnetic Cu dopants.

Measurements of the17 O nuclear magnetic resonance (NMR) quadrupolar spectrum of apical oxygen in HgBa2CuO 4+δ were performed over a range of magnetic fields from 6.4 to 30 T in the superconducting state. Oxygen isotope exchanged single crystals were investigated with doping corresponding to superconducting transition temperatures from 74 K underdoped, to 78 K overdoped. The apical oxygen site was chosen since its NMR spectrum has narrow quadrupolar satellites that are well separated from any other resonance. Non-vortex contributions to the spectra can be deconvolved in the time domain to determine the local magnetic field distribution from the vortices. Numerical analysis using Brandt's Ginzburg-Landau theory was used to find structural parameters of the vortex lattice, penetration depth, and coherence length as a function of magnetic field in the vortex solid phase. From this analysis we report a vortex structural transition near 15 T from an oblique lattice with an opening angle of 73• at low magnetic fields to a triangular lattice with 60• stabilized at high field. The temperature for onset of vortex dynamics has been identified from spinspin relaxation. This is independent of the magnetic field at sufficiently high magnetic field similar to that reported for YBa2Cu3O7 and Bi2Sr2CaCu2O 8+δ and is correlated with mass anisotropy of the material. This behavior is accounted for theoretically only in the limit of very high anisotropy.

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