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The Mn spin correlations were studied near the O'-O phase transition at T{JT} = 750 K, up to 950 K with 17O and 139La NMR in a stoichiometric LaMnO3 crystalline sample. The measured local hyperfine fields originate from the electron density transfered from the eg- and t2g-orbitals to the 2s(O) and 6s(La) orbits, respectively. By probing the oxygen nuclei, we show that the correlations of the Mn spins are ferromagnetic in the ab-plane and robust up to T{JT} whereas along the c-axis, they are antiferromagnetic and start to melt below T{JT}, at about 550 K. Above T{JT} the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room temperature orbital mixing angle, {\phi}nmr = 109+-1.5{\deg}, of the eg ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted MnO6 octahedra. For T > T{JT} LaMnO3 can be described in terms of non-polarized eg-orbitals since both eg-orbitals are equally occupied., Comment: 7 pages, 7 figures

We present the synthesis and the NMR characterization of a 17O enriched LaMnO3 crystalline sample. We checked that it is single phase and, more important, stochiometric in oxygen. Its 17O enrichment estimated by NMR is about 5.5%. These first 17O NMR results obtained at T=415K in a undoped parent LaMnO3 manganite demonstrate that the two oxygen sites of the structure probe very different Mn spin correlations in the paramagnetic orbital ordered phase. This work opens the way to study experimentally the interactions responsible for the orbital order., Comment: 8 pages,4 figures

The effect of atomic disorder induced by neutron irradiation on the crystal structure and electronic states near $E_F$ of the lightly overdoped LaFeAsO$_{0.85}$F$_{0.15}$ ($T_c=21K$) was studied by X-ray diffraction and $^{75}$As NMR. The irradiation of the polycrystalline sample by "moderate" neutron fluence of $\Phi=(0;0.5;1.6)\cdot10^{19}cm^{-2}$ at $T=50^{\circ}C$ leads to the suppression of superconductivity. It is shown that neutron irradiation produces an anisotropic expansion of the tetragonal lattice almost due to an increase of the Fe-As distance. A partial loss of the 2D character of the FeAs layer is accompanied with a suppression of the gap-like feature in temperature dependence of the spin susceptibility. In the most disordered state the $^{75}$As spin-lattice relaxation rate follows the Korringa law $^{75}T_1^{-1} \sim T$, the thermal behavior being typical for an isotropic motion of the conducting electrons.

We present a $^{209}$Bi and $^{17}$O NMR study of the Mn electron spin correlations developed in the charge ordered state of Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$. The unusually large local magnetic field $^{209}H_{loc}$ indicates the dominant $6s^{2}$ character of the lone electron pair of Bi$^{3+}$-ions in both compounds. The mechanism connecting the $s$ character of the lone pairs to the high temperature of charge ordering $T_{CO}$ is still not clarified. The observed difference in $^{209}H_{loc}$ for Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ to Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is probably due to a decrease in the canting of the staggered magnetic moments of Mn$^{3+}$-ions from. The modification of the $^{17}$O spectra below $T_{CO}$ demonstrates that the line due to the apical oxygens is a unique local tool to study the development of the Mn spin correlations. In the AF state the analysis of the $^{17}$O spectrum of Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ prompts us to try two different theoretical descriptions of the charge-ordered state, a site-centered model for the first manganite and a bond-centered model for the second one., Comment: 10 pages, 7 figures

Spin susceptibility of stabilized \delta phase in the Pu-Ga alloy is studied by measuring {69,71}^Ga NMR spectra and nuclear spin-lattice relaxation rate {69}T_{1}^{-1} in the temperature range 5 - 350 K. The shift ({69}^K) of the {69,71}^Ga NMR line and {69}^T_{1}^{-1} are controlled correspondingly by the static and the fluctuating in time parts of local magnetic field arisen at nonmagnetic gallium due to transferred hyperfine coupling with the nearest f electron environment of the more magnetic Pu. The nonmonotonic with a maximum around 150 K behavior of {69}^K(T) \chi_{s,5f}(T) is attributed to the peculiarities in temperature dependence of the f electron spin susceptibility \chi_{s,5f}(T) in \delta phase of plutonium. The temperature reversibility being observed in {69}^K(T) data provides strong evidence for an electronic instability developed with T in f electron bands near the Fermi energy and accompanied with a pseudogap-like decrease of \chi_{s,5f}(T) at T<150 K. The NMR data at high temperature are in favor of the mainly localized character of 5f electrons in \delta phase of the alloy with characteristic spin-fluctuation energy \Gamma(T) T^{0.35(5)}, which is close to $\Gamma(T) T^{0.5} predicted by Cox et al. [J. Appl. Phys. 57, 3166 (1985)] for 3D Kondo-system above T_Kondo}. The dynamic spin correlations of 5f electrons become essential to consider for {69}^T_{1}^{-1}(T) only at T<100 K. However, no NMR evidences favoring formation of the static magnetic order in \delta-Pu were revealed down to 5K ., Comment: 6 pages, 4 figures

NMR spectra of 73 Ge (nuclear spin I = 9/2) in germanium single crystals with different isotopic compositions have been measured at the frequency of 17.4 MHz at room temperature. Due to the small concentration (~0.1%) of the magnetic (73 Ge) isotope, the magnetic dipole-dipole interaction is negligible in the samples studied, and the observed specific features of the resonance lineshapes (a narrow central peak and a wide plateau) are determined mainly by the quadrupole interaction of magnetic nuclei with the random electric field gradient (EFG) induced by the isotopic disorder. The second and fourth moments of the distribution function of the EFG are calculated taking into account local lattice deformations due to mass defects in the close neighborhood of the magnetic nuclei, as well as charge density redistributions and lattice strains induced by distant impurity isotopes. The simulated lineshapes, represented by a superposition of Gaussians corresponding to individual transitions between nuclear Zeeman sublevels, agree reasonably well with the measured spectra., Comment: 36 pages, 4 figures

An influence of the 16O-18O isotope substitutions on magnetic state of perovskite-type manganite (La0.25Pr0.75)0.7Ca0.3MnO3 is studied by 55Mn NMR. Successive cycling with an isochronal exposure at different magnetic fields up to H=8T is used to study the field-induced transition from antiferromagnetic insulating (AFI) state to the ferromagnetic metal (FMM) one in the 18O-enriched sample. After exposure at H>H_{cr} \sim 5.3T the NMR spectrum of the 18O-sample evidences for magnetic phase separation (PS) resulted in the coexisting AFI and FMM domains. Further increase of exposing field leads to a progressive growth of the FMM phase at the expense of AFI domains. Its relative fraction can be controlled by external magnetic field and the resulting magnetic structure in the PS region is discussed. Anomalous T-dependence of the 55Mn nuclear spin-lattice relaxation rate is revealed in the FMM state of both 16O-and 18O-enriched samples. The manifestation of the Pr magnetic ordering at T \sim 40K is considered.

The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for the first time by ^{17}O NMR. This local probe is sensitive to spin, charge and orbital correlations. Two transitions exist in this system: the charge and orbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition at T_N = 170 K. Both are clearly seen in the NMR spectra measured in a magnetic field of 7T. Above T_{CO} there exists only one NMR line with a large isotropic shift, whose temperature dependence is in accordance with the presence of ferromagnetic (FM) correlations. This line splits into two parts below T_{CO}, which are attributed to different types of oxygen in the charge/orbital ordered state. The interplay of FM and AF spin correlations of Mn ions in the charge ordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the hole hopping motion that is slowed down with decreasing temperature. The developing fine structure of the spectra evidences, that there still exist charge-disordered regions at T_{CO} > T > T_N and that the static (t > 10^{-6}s) orbital order is established only on approaching T_N. The CE-type magnetic correlations develop gradually below T_{CO}, so that at first the AF correlations between checkerboard ab-layers appear, and only at lower temperature - CE correlations within the ab-planes.

NMR line shift and nuclear spin-lattice relaxation rate T_1^{-1} of 11B and 25Mg were measured in superconducting MgB_2 (T_c^{ons}=38K) structurally disordered by nuclear reactor neutrons up to the fluence of thermal neutrons $\Phi=1 10^{19} cm^{-2}. The temperature of superconducting transition was shifted down to T_{c,irrad}^{ons}=7K under irradiation. The change due irradiation in the partial electron density of states (DOS) at the Fermi energy of boron and magnesium were traced by taking into account that T_1^{-1} of 11B and 25Mg are determined by hyperfine magnetic interactions with carriers. It was revealed that electronic states near E_F of Mg are influenced negligibly by irradiation whereas partial DOS of the B 2p_{x,y} states reduces greatly in irradiated MgB_2. The NMR estimates for DOS using the McMillan formula show that critical temperature decreases in irradiated MgB_2 mainly due to reduction in the partial DOS of p states of boron., Comment: 5 pages, 4 figures

The 17O and 207Pb NMR spectra were measured in ceramic samples in the metallic phase of BaPb_{1-x}Bi_{x}O_3 oxides (0. The decrease of <17Ks> with temperature revealed for x=0.33 evidences for an opening of the energy gap near E_F near the metal-semiconductor transition (x=0.35)., Comment: submitted to Phys. Rev. B

NMR spectra and nuclear spin-lattice relaxation rate $T_1^{-1}$ of $^{11}$B have been measured in superconducting polycrystalline MgB$_{2}$ with $T_C^{ons}\cong 39.5K$. It is shown that $(T_{1}T)^{-1}$ and the Knight shift $K_{s}$ are independent of temperature and nearly isotropic above $T_C$. Both of these quantities are decreased gradually in going to superconducting state. According NMR data the density of states (DOS) near the Fermi level is flat at the scale $\approx$ 500K. Some conclusions on the orbital content of the DOS at the Fermi level was done and compared with the results of the band structure calculations., Comment: 2 pages, 4 figures