We present zero-field muon spin relaxation ͑SR͒ measurements of La 1.6Ϫx Nd 0.4 Sr x CuO 4 with xϭ0.125,0.15,0.2; La 1.475 Nd 0.4 Ba 0.125 CuO 4 , La 1.875 Ba 0.125 CuO 4 , and La 1.875 Ba 0.125Ϫy Sr y CuO 4 with y ϭ0.025,0.065. All of the samples with dopant concentrations xϩyр0.15 show similar static magnetic order with coherent precession of the muon spins below T N Ϸ30 K, with a T→0 ordered Cu moment Ϸ0.3 B . The samples with xϭ0.20 show no coherent precession but manifest two distinct relaxation regimes, typical of quasistatic magnetism. We then present transverse-field SR hysteresis measurements of the La 1.45 Nd 0.4 Sr 0.15 CuO 4 and La 1.4 Nd 0.4 Sr 0.2 CuO 4 systems that show a large superconducting response below approximately 7 K and 12 K, respectively. We argue that superconductivity and magnetic order coexist in the xϭ0.15 system.

Muon spin rotation and relaxation experiments in the pyrochlore iridate Eu 2 Ir 2 O 7 yield a welldefined muon spin precession frequency below the metal-insulator/antiferromagnetic transition temperature T M = 120 K, indicative of long-range commensurate magnetic order and thus ruling out quantum spin liquid and spin-glass-like ground states. The dynamic muon spin relaxation rate is temperature-independent between 2 K and ∼T M and yields an anomalously long Ir 4+ spin correlation time, suggesting a singular density of low-lying spin excitations. Similar behavior is found in other pyrochlores and geometrically frustrated systems, but also in the unfrustrated iridate BaIrO 3 . Eu 2 Ir 2 O 7 may be only weakly frustrated; if so, the singularity might be associated with the small-gap insulating state rather than frustration.

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We have performed 69,71 Ga nuclear magnetic resonance ͑NMR͒, nuclear quadrupole resonance ͑NQR͒, and muon spin rotation and resonance on the quasi-two-dimensional antiferromagnet NiGa 2 S 4 , in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in NiGa 2 S 4 , we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the c axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the spin freezing occurring at T f , at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-lattice relaxation rate 1 / T 1 and nuclear spin-spin relaxation rate 1 / T 2 measured by Ga-NQR as well as the muon spin relaxation rate . The main sharp NQR peaks exhibit a stronger tendency of divergence, compared with the weak broader spectral peaks, indicating that the spin freezing is intrinsic in NiGa 2 S 4 . The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards T f , and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multicomponent T 1 were observed below 2 K, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between 2 K and T f , where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below T f , but keep fluctuating down to 2 K with the MHz frequency range. Below 0.5 K, all components of 1 / T 1 follow a T 3 behavior. We also found that 1 / T 1 and 1 / T 2 show the same temperature dependence above T f but different temperature dependence below 0.8 K. These results suggest that the spin dynamics is isotropic above T f , which is characteristic of the Heisenberg spin system, and becomes anisotropic below 0.8 K.

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