Orbital Hyperfine Field Research Articles

Electronic structure and magnetic properties of dilute U impurities in metals

The electronic structure and magnetic moment of dilute U impurity in metallic hosts have been calculated from first principles. The calculations have been performed within local density approximation of the density functional theory using Augmented plane wave+local orbital (APW+lo) technique, taking account of spin–orbit coupling and Coulomb correlation through LDA+U approach. We present here our results for the local density of states, magnetic moment and hyperfine field calculated for an isolated U impurity embedded in hosts with sp-, d- and f-type conduction electrons. The results of our systematic study provide a comprehensive insight on the pressure dependence of 5f local magnetism in metallic systems. The unpolarized local density of states (LDOS), analyzed within the frame work of Stoner model suggest the occurrence of local moment for U in sp-elements, noble metals and f-block hosts like La, Ce, Lu and Th. In contrast, U is predicted to be nonmagnetic in most transition metal hosts except in Sc, Ti, Y, Zr, and Hf consistent with the results obtained from spin polarized calculation. The spin and orbital magnetic moments of U computed within the frame of LDA+U formalism show a scaling behavior with lattice compression. We have also computed the spin and orbital hyperfine fields and a detail analysis has been carried out. The host dependent trends for the magnetic moment, hyperfine field and 5f occupation reflect pressure induced change of electronic structure with U valency changing from 3+ to 4+ under lattice compression. In addition, we have made a detailed analysis of the impurity induced host spin polarization suggesting qualitatively different roles of f-band electrons on moment stability. The results presented in this work would be helpful towards understanding magnetism and spin fluctuation in U based alloys.

Field-induced-moment nuclear coupling for 59Co in a Heusler alloy Co2TiGa

The positive hyperfine field at Co nucleus in a Heusler alloy Co2TiGa is discussed to be not dominated by the orbital hyperfine field from a quite different point of view from former studies. Field-induced-moment nuclear coupling for 59Co via spin-orbit interaction on Co atom in Co2TiGa is discussed to be small from the observed high-field shift of the NMR of 59Co in the ferromagnetic state of only +0.83 % in contrast to the case of CoCl2.2H2O with +29%.

First-principles study of spin-orbit effects and NMR inSr2RuO4

We present a first principles study of NMR and spin orbit effects in the unconventional superconductor Sr2RuO4. We have calculated the uniform magnetic susceptibility, which agrees rather well with the experiment in amplitude, but, as in an earlier model result we found the calculated hard axis to be z, opposite to the experiment. We have also calculated the Knight shifts and the NMR relaxation rates for all atoms, and again found an overall good agreement, but important deviations from the experiment in same particular characteristic, such as the Knight shift anisotropy. Our results suggest that correlations in Sr2RuO4 lead to underestimations of the orbital effects in density-functional based calculations. We also argue that the accepted ``experimental'' value for the relative contribution of orbital polarization in susceptibility, 10-15%, is also an underestimation. We discuss the puzzling invariance of the the O and Ru Knight shift across the superconducting transition for all directions of the applied field. We show that this fact cannot be explained by accidental cancellations or spin-flip scattering, as it happens in some elemental superconductors. We also point out that large contribution of the dipole and orbital hyperfine field into the Knight shifts in Sr2RuO4, combined with the possibility of an orbital-dependent superconductivity, calls for a revision of the standard theory of the Knight shift in the superconducting state.

Theoretical investigation of the orientation dependence of the magneto-optical Kerr effect in Co.

The first-principles spin-polarized, relativistic linear muffin-tin orbital method has been applied to calculate electronic and magneto-optical properties of Co in both fcc and hcp structures. In particular, the magneto-optical properties have been calculated for several magnetization directions in order to study their magnetization orientation dependences. A pronounced anisotropy in the polar Kerr effect is found for hcp Co while the magneto-optical properties of fcc Co are predicted to be insensitive to the magnetization direction. Moreover, this magneto-optical anisotropy in hcp Co is found to be correlated with the presence of a large anisotropy in the orbital magnetic moment and orbital hyperfine field. Changing the structure from fcc to hcp gives rise to even greater changes in the magneto-optical properties of Co. Calculated Kerr angles and ellipticities are in reasonable agreement with the latest experiment.

Transferred hyperfine fields in rare-earth-substituted YFe2 and YNi2

In the intermetallic compound YFe2, the metallic hyperfine field at the Y nucleus, as measured by 89Y NMR spectroscopy, is affected by the substitution of one or more neighbour Y atoms by different rare-earth impurities. In order to understand the underlying mechanisms for the transferred hyperfine interaction, we have performed first-principles calculations, within the local spin-density theory, for embedded clusters representing a Y atom and its vicinity in the compounds Y1-xRxFe2 (R=Gd, Tb, Ho, Tm or Lu) and Y1-xGdxNi2. The contact magnetic hyperfine field and the dipolar field were obtained; the resulting total held was found to increase with the spin of the rare-earth impurity. Orbital hyperfine fields were not considered. The contact transferred field was found to arise from direct polarization of the s electrons by the rare-earth spin.

197Au NMR study of Au impurities in Fe and FCC Co

NMR measurements of 197Au in Fe and FCC Co alloys with 0.1 at.% Au have been made using powdered specimens in zero external magnetic field at 1.7K. Electric quadrupole splitting has been found in the spectra. The spacing between the quadrupole-split component lines of FeAu alloy, Delta nu Q, has been determined by the steady-state method to be 0.65+or-0.03 MHz, and the resonance frequency of the central peak nu 0 to be 93.25+or-0.01 MHz, yielding a quadrupole coupling of e2qQ/h=1.30+or-0.06 MHz and a hyperfine field of Hhf=1258.0+or-0.1 kOe. Analysis of the hyperfine structure anomalies between Au isotopes has given a non-contact field, i.e. an orbital hyperfine field, of +161+or-26 kOe at Au nuclei in iron. Because q will be independent of the isotope, the present and previous results of e2qQ/h for Au isotopes in iron have given the nuclear moments 198Q=0.88+or-0.08 b and 199Q=0.64+or-0.06 b. For CoAu alloy, Delta nu Q=0.75+or-0.2 MHz and nu 0=58.0+or-0.05 MHz, corresponding to e2qQ/h=1.5+or-0.4 MHz and Hhf=782.5+or-0.7 kOe, respectively, have been deduced.

Mossbauer effect observation of an induced hyperfine field in YbAl3

Mossbauer absorption measurements on the 84.25 keV gamma ray of 170Yb in YbAl3 have been made from 1.2K to 130K in applied magnetic fields up to 75 kOe. The isomer shift measurements show that Yb has a temperature independent valency of 2.7+or-0.3. An excess hyperfine field, induced by an applied magnetic field, is explained in terms of an ionic orbital hyperfine field generated by a strongly correlated 4f electron band.

Nuclear Magnetic Resonance Study of MnB

We report the observation of the nuclear magnetic resonance of the Mn 55 nuclei in the ferromagnetic monoboride MnB in a static external magnetic field. These measurements show that the easy axis of magnetization is perpendicular to the C-axis, with a field anisotropy of about 13 kOe at 4.2 K and that the hyperfine field is negative. The spin-lattice relaxation time yields a value of 1.8×10 6 Oe for the orbital hyperfine field.

NMR evidence for a transferred orbital hyperfine field on near neighbours of Fe in Cu

Satellite NMR data taken on CuFe powders (from 1.4 to 300 K) show that the anisotropy deduced by Stakelon and Slichter from single crystal experiments at 300 K cannot be explained by a pseudodipolar coupling. Temperature independent contributions to both the isotropic and anisotropic parts of the local field tensor are evidenced. A calculation of the spin polarization induced by the impurity orbital magnetization through spin orbit coupling gives an order of magnitude agreement with the data.

Nuclear Magnetic Resonances of Mo95, Mo97 and Fe57 in Ba2(FeMo)O6, Sr2(FeMo)O6 and Ca2(FeMo)O6

Nuclear magnetic resonances of Mo 95 , Mo 97 and Fe 57 in Ba 2 (FeMo)O 6 , Sr 2 (FeMo)O 6 and Ca 2 (FeMo)O 6 with ordered perovskite structure were studied in their ferromagnetic state by spin echo method. The Mo 95 and Mo 97 spin echo signals at 4.2°K were observed between 33.0 MHz to 65.6 MHz, 30.5 MHz to 81.2 MHz and 31.5 MHz to 79.6 MHz, respectively. The Mo 95 spin echo peak frequencies of these compounds (54.4 MHz, 67.6 MHz and 63.4 MHz, respectively) are higher in the compound with higher ferromagnetic Neel temperature. In Ba 2 (FeMo)O 6 , the Mo 95 spin echo peak shifts to higher frequencies with increasing external static magnetic field. The measured hyperfine fields of the Mo 5+ ions in these compounds are far smaller than the value of the orbital hyperfine field which is expected when the magnitude of the effective orbital angular momentum is 1. The Fe 57 spin echo signal in Ba 2 (FeMo)O 6 at 4.2°K was found at 79 MHz.

Optical Detection of Spin-Lattice Relaxation and hfs in the ExcitedE¯(E2)State ofV2+andMn4+inAl2O3

The hyperfine structure (hfs) and spin-lattice relaxation in the excited $\overline{E}(^{2}E)$ states of ${\mathrm{V}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ have been studied by optical-detection techniques. This is an extension of an earlier study of EPR (electron paramagnetic resonance) in the excited $\overline{E}(^{2}E)$ state of ${\mathrm{Cr}}^{3+}$ in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$. The experimental results for all three isoelectronic ions in the same host lattice allow us to make meaningful comparisons with theoretical ideas of hfs and spin-lattice relaxation. A well-resolved hfs is found for ${\mathrm{V}}^{2+}$ and ${\mathrm{Mn}}^{4+}$, which is in contrast to the absence of hfs in the case of the isoelectronic ${\mathrm{Cr}}^{53}$ ion studied earlier. The values of the hyperfine-splitting parameter for the three ions can be adequately explained by considering the combined effects of the corepolarization hyperfine field, orbital hyperfine field, and dipolar hyperfine field. The experimental data are fitted to a simple spin Hamiltonian for an effective spin $S=\frac{1}{2}$ with $|{g}_{\ensuremath{\parallel}}|=2.2198\ifmmode\pm\else\textpm\fi{}0.001$, and $|{A}_{\ensuremath{\parallel}}|=(46.3\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for ${\mathrm{V}}^{2+}$, and $|{g}_{\ensuremath{\parallel}}|=3.0959\ifmmode\pm\else\textpm\fi{}0.0006$, and $|{A}_{\ensuremath{\parallel}}|=(123\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for ${\mathrm{Mn}}^{4+}$. Since ${g}_{\ensuremath{\perp}}\ensuremath{\sim}0$ for both ions, ${A}_{\ensuremath{\perp}}$ could not be determined. In the temperature range in which ${T}_{1}$ could be measured (1.4-2.15\ifmmode^\circ\else\textdegree\fi{}K for ${\mathrm{V}}^{2+}$, 6-9\ifmmode^\circ\else\textdegree\fi{}K for ${\mathrm{Mn}}^{4+}$), the spin-lattice relaxation times ${T}_{1}$ were found to follow an Orbach process: ${T}_{1}=c\mathrm{exp}(\frac{\ensuremath{\Delta}}{\mathrm{kT}})$, where $\ensuremath{\Delta}$ is the $2\overline{A}\ensuremath{-}\overline{E}$ splitting of the $^{2}E$ level. $\ensuremath{\Delta}=12.3 \mathrm{and} 80$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for ${\mathrm{V}}^{2+}$ and ${\mathrm{Mn}}^{4+}$, respectively, and the measured values of $c$ were 5.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ and 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec, respectively. For ${\mathrm{Cr}}^{3+}$, where $\ensuremath{\Delta}=29$ ${\mathrm{cm}}^{\ensuremath{-}1}$, the value of $c$, obtained earlier, is 3.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec. The parameter $c$ is related to the direct-process relaxation time for the spontaneous transition between the non-time-reversed states $2\overline{A}$ and $\overline{E}$, ${T}_{2\overline{A}\ensuremath{\rightarrow}\overline{E}}$, in which a phonon of energy $\ensuremath{\Delta}$ is emitted. For the case where $\ensuremath{\Delta}\ensuremath{\gg}\mathrm{kT}$, this direct-process relaxation time should vary inversely as ${({V}^{(1)})}^{2}{\ensuremath{\Delta}}^{3}$, where ${V}^{(1)}$ is the orbit-lattice coupling parameter which can be determined from static-strain measurements, and the measured relaxation time is found to be so governed. In the vanadium experiment, the EPR signal was so weak that several new experimental techniques had to be used to extract the signal from the noise. In the case of manganese, a circular-polarization method to detect the excited-state EPR signal had to be developed. This technique of detecting excited-state EPR by monitoring circularly polarized light should be applicable to a wide variety of materials characterized by inhomogeneously broadened emission lines.

Contributions to theV51Nuclear Magnetic Resonance Frequency Shift and Susceptibility in Vanadium Sesquioxide

The ${\mathrm{V}}^{51}$ NMR frequency shift ($\frac{\ensuremath{\Delta}\ensuremath{\nu}}{\ensuremath{\nu}}$) in metallic ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ has been measured in the temperature range of 175 to 575\ifmmode^\circ\else\textdegree\fi{}K the frequency shift at these two temperatures being -0.61% and +0.1%, respectively. Various contributions to the frequency shift and magnetic susceptibility are determined by constructing a ($\frac{\ensuremath{\Delta}\ensuremath{\nu}}{\ensuremath{\nu}}$)-versus-$\ensuremath{\chi}$ diagram. The $d$-band spin susceptibility ${\ensuremath{\chi}}_{d}$ is found to obey a Curie-Weiss law down to the transition temperature of about 160\ifmmode^\circ\else\textdegree\fi{}K. A large temperature-independent orbital susceptibility is found to be ${\ensuremath{\chi}}_{\mathrm{VV}}\ensuremath{\simeq}1.52%$. From the temperature dependence of ${\ensuremath{\chi}}_{d}(T)$ and the corresponding frequency shift ${(\frac{\ensuremath{\Delta}\ensuremath{\nu}}{\ensuremath{\nu}})}_{d}$, a negative hyperfine field of ${{H}_{d}}^{\mathrm{hf}}\ensuremath{\simeq}\ensuremath{-}140$ kOe/Bohr magneton, resulting from core polarization, a temperature-dependent orbital hyperfine field, and a dipolar hyperfine field, is found. With decreasing temperature the ${\mathrm{V}}^{51}$ NMR signal abruptly disappears at the transition temperature, suggesting that ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ undergoes antiferromagnetic ordering for temperatures less than 160\ifmmode^\circ\else\textdegree\fi{}K. Contributions to the ${\mathrm{V}}^{51}$ NMR linewidth are discussed.

Nuclear Magnetic Resonance of Ni61 in Nickel-Ferrite

The nuclear magnetic resonance (NMR) of Ni 61 in Nickel-ferrite at zero external field has been observed at liquid air temperature by using the pulsed NMR method. The spin echo signal could be observed in the frequency range from 31 Mc/sec to 38 Mc/sec. The center frequency (34.5 Mc/sec) corresponds to the value of about 97 KOe of hyperfine field, which could be accounted for as the sum of the core polarized and the orbital hyperfine fields of Ni 2+ ion. This value is consistent with the hyperfine coupling constant of paramagnetic Ni 2+ ion diluted in Al 2 O 3 .

Interpretation of Knight Shifts and Susceptibilities of Transition Metals: Platinum

Measurements of the magnitude, sign, and temperature dependence of the Knight shift and susceptibility in platinum are used to determine the contributions to each arising from the spin paramagnetism of the $s$ and $d$ bands, the orbital paramagnetism and the core diamagnetism. A complete expression for the orbital contribution to the Knight shift in a transition metal is derived, including spin-orbit coupling. We show that in the tight-binding limit, as in the free ion, a simple relation exists between the orbital susceptibility and the orbital hyperfine field. The relation of the enhancement of the $d$-spin susceptibility over its specific heat value to the possible occurrence of superconductivity is discussed.

Electron-Nuclear Double Resonance ofNi61inAl2O3and Variation of hfs Through an Inhomogeneous Line Due to Random Crystal Fields

The electron-nuclear double resonance spectrum of Ni/sup 61/ in Al/sub 2/ O/sub 3/ was measured and the absolute value of the Ni magnetic moment was determined to be 0.746 plus or minus 0.007 nm. Inconsistencies in previous measurements of the magnetic moment were explained. It is shown that orbital hyperfine fields play a crucial role in nominally orbital singlet ground-state ions such as Ni/sup 2+/. A variation in hyperfine field was observed through an inhomogeneously broadened line. This arose from a variation of orbital hyperfine field in the different spin packets associated with slightly different axial crystal fields. (C.E.S.)