The electric quadrupole interaction of 188 Ir ͑I p 1 2 ; T 1͞2 41.5 h͒ in a Fe single crystal was measured for magnetization parallel to the crystallographic [100], [110], and [111] axes. Contrary to all previous experiments, a strong dependence of the electric field gradient on the direction of the magnetization with respect to the crystallographic axes was observed for the first time.[ S0031-9007(97) PACS numbers: 76.60. Jx, 75.50.Bb, 76.60.Gv, 76.80.+ y The electric field gradient (EFG) at the site of impurity nuclei in cubic ferromagnetic hosts such as Fe, Ni, and fcc-Co has been the subject of many investigations, both experimental [1-9] and theoretical [5,10,11]. It was first observed by nuclear magnetic resonance (NMR) of where eq V zz (henceforth denoted as EFG) is the principal component of the traceless EFG tensor and eQ is the nuclear spectroscopic quadrupole moment. If the EQI is collinear with the magnetic hyperfine interaction, the magnetic resonance frequency is split into a set of 2I equidistant subresonances, where I is the nuclear spin. The subresonance separation is given byThe satellite resonances were considerably broader than the central line. Mainly, the following two explanations were discussed: (i) The additional broadening is due to lattice inhomogeneities. (ii) The EFG depends on the direction of the magnetization with respect to the crystal axes. To explore whether the second possibility was the origin for this broadening, Aiga and Itoh prepared single crystal alloys of Fe-0.3% Ir by the stress annealing method. The NMR measurements yielded exactly the same spectra as observed for the polycrystal samples [5]. Thus Aiga and Itoh concluded that the quadrupole interaction is almost independent of the direction of the magnetization, and even if there is some anisotropy, it is of the order of, or less than, a few hundred kHz. Johnston and Stone performed NMR on oriented nuclei (NMR-ON) measurements on 192

The orbital moment and the noncubic charge distribution in ferromagnetic transition metals with cubic lattice symmetry are investigated within the tight-binding model. By combining the tight-binding approximation, perturbation theory, and the Green's function formalism for impurity scattering, approximate expressions for both effects are derived that depend only on the spin-orbit coupling strength and the density of states of the system without spin-orbit coupling. The basic relations between the orbital moment, the noncubic charge distribution, and the band structure are derived from the form of these expressions and from their application to various model band structures: We explain in this way the scaling with the spin-orbit coupling strength and bandwidth, the typical order of magnitude, the variation as a function of the band filling, the sensitivity to band structure details, and the role of the splitting between spin-up and spin-down states. For the noncubic charge distribution we derive the form of the dependence on the direction of the magnetization and show how the sign and magnitude of this anisotropy are related to the different energy distributions of e g and t 2g states. This tight-binding analysis is finally applied to the 5d impurities in Fe. The local densities of states without spin-orbit coupling are obtained by self-consistent augmented plane-wave calculations using a supercell method. The special features of the 5d impurities in Fe with respect to the band structure, the orbital moment, and the noncubic charge distribution are discussed. The general trend of the systematics is interpreted as a band filling effect. The prevailing sign of the anisotropy is ascribed to the concentration of the e g states near the Fermi energy. The results of the tight-binding analysis are compared with the experiment and a more rigorous calculation.

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The spectroscopic quadrupole moments of the neutron deficient radioactive Ir isotopes Os), for which K 0 and 1 is expected. In the framework of the rotational model the K-quantum number can be determined from the spectroscopic quadrupole moment Q which is connected with the intrinsic quadrupole moment Q 0 byThus, for a low-K state of a nucleus with prolate deformation, a negative quadrupole moment is expected. For 186 Ir, this was confirmed by the measurement of the spectroscopic quadrupole moment with quadrupole-interaction nuclear orientation (QI-NO), which yielded Q 22. 41͑20͒ b [3]. This value indicated a larger nuclear deformation than expected from the extrapolation of heavier Ir isotopes. In this context, it is an interesting question whether the nuclear deformation of the low-K anomalous ground state 186 Ir g is enhanced by the specific properties of the p1͞2 2 ͓541͔ proton intruder state coming down from the ph 9͞2 orbital. This can be tested by a measurement of the quadrupole moment of the low- Os), for which K 4 and 5 is expected. Recently, from spectroscopic investigations, the configuration and even the assignment of I 5 for 184 Ir was doubted [7]. Here, in addition to precision measurements of quadrupole moments, we present a new method for the measurement of ground state spins with resonance precision. It is based on the following features: For the case of a combined magnetic-dipole plus electric-quadrupole hyperfine interaction the resonance spectrum consists of 2I subresonances which are separated equidistantly around the magnetic hyperfine splitting. The frequency offset of the subresonance with the largest amplitude to the magnetic hyperfine splitting depends on I. Thus I can be determined by frequency measurements. This method was applied to 184 Ir, with the unambiguous result I 5. The questions addressed above can be answered by measuring the quadrupole splittings in hcp-Co, from which-without the exact knowledge of the electric field gradient (EFG) of Ir in hcp-Co-highly precise ratios of quadrupole moments are obtained. We also present, however, results of quadrupole-interaction-resolved NMR on oriented nuclei (QI-NMR-ON) measurements on 187 Ir known from muonic x-ray spectroscopy [9], we are able to determine 5016 0031-9007͞96͞77(25)͞5016(4)$10.00

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