No abstract

Nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation rate T,' of "B have been measured in superconducting polycrystalline Mg13 2 with T°"s = 39.5 K. It is shown that (T,T) -' and the Knight shift K, are independent of temperature and nearly isotropic above 1. Both of these quantities are decreased gradually in going to the superconducting state. According to NMR data the density of states near the Fermi level is flat at the scale of about 500 K. Some conclusions on the orbital content of the density of states at the Fermi level were drawn and compared with the results of the band structure calculations.

NMR line shift and nuclear spin-lattice relaxation rate T −1 1 of 11 B and 25 Mg were measured in superconducting MgB2 (T ons c =38K) structurally disordered by nuclear reactor neutrons up to the fluence of thermal neutrons Φ = 1 • 10 19 cm −2 . The temperature of superconducting transition was shifted down to T ons c,irrad =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 11 B and 25 Mg are determined by hyperfine magnetic interactions with carriers. It was revealed that electronic states near EF of Mg are influenced negligibly by irradiation whereas partial DOS of the B 2px,y states reduces greatly in irradiated MgB2. Acording to the Mc Millan formula, NMR data show that critical temperature decreases in irradiated MgB2 mainly due to reduction in the partial DOS of p states of boron.

The Mn spin correlations were studied near the O -O phase transition at TJT = 750 K, up to 950 K with 17 O and 139 La NMR in a stoichiometric LaMnO3 crystalline sample. The measured local hyperfine fields originate from the electron density transfered from the e g -and t 2g -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 TJT whereas along the c-axis, they are antiferromagnetic and start to melt below TJT , at about 550 K. Above TJT the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room temperature orbital mixing angle, ϕnmr = 109 ± 1.5• , of the e g ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted MnO6 octahedra. For T > TJT LaMnO3 can be described in terms of non-polarized eg-orbitals since both eg-orbitals are equally occupied.