Hyperfine quantum beat of short-lived beta-emitter8Li polarized by the tilted foil technique

To disclose the basic mechanisms of nuclear polarization creation realized by the tilted foil technique, hyperfine quantum beats of the nuclear polarization of the short-lived beta-emitter⁸Li(I =2⁺,T _1/2=840 ms) were measured as a function of the distance between two tilted foils. From the observed beats of the⁸Li nuclear polarization, it was found that the Fermi-contact interaction of the⁸Li nucleus with the ground 1s-electron in LiIII plays a main role in the final stage of polarization transfer from polarized orbital electrons to the⁸Li nucleus.     

Hyperfine interactions in Li2ZrF6 and Li2HfF6

The quadrupole hyperfine interaction in Li₂ZrF₆ and Li₂HfF₆ has been studied as a function of temperature. Both lattices are characterized by very similar low and symmetric electric field gradients at probe sites and no phase transitions were observed up to 770 K. On heating, between 700 and 830 K, the Li₂ZrF₆ undergoes chemical reactions which give rise to ZrO₂ and then to Li₃ZrF₇ (high temperature α-phase) and Li₂ZrO₃. On cooling, below 750 K, the α→β transition in Li₃ ZrF₇ already reported is found to take place. An analogous behaviour is determined for Li₂HfF₆. Accordingly, the same decomposition steps and exstence of high and low temperature phases for Li₃HfF₇ can be inferred.     

Hyperfine Interactions of 8Li in Ferromagnetic Single Crystal Fe

The hyperfine interactions of ⁸Li implanted in single crystal Fe have been studied using the β-NMR technique. One kind of hyperfine field B _hf was found. Since no dipolar field was observed, ⁸Li nuclei are suggested to sit in substitutional sites. The B _hf and the spin-lattice relaxation time T ₁ were observed as functions of temperature. The experimental B _hf and T ₁ were compared with the theoretical predictions from the first principles KKR-band-structure calculations. While predicted B _hf for the substitutional site is consistent with the experiment, predicted T ₁ cannot reproduce the experiment well. This revised version was published online in September 2006 with corrections to the Cover Date.     

Hyperfine fields in iron-metalloid ferromagnetic metals

A new coordination parameter Z_eff is defined at each inequivalent iron site in the interstitial iron-metalloid ferromagnetic metals which uniquely determines the hyperfine field at that site with considerable accuracy. The dependence of Z_eff upon bonding lenghts is defined and can be directly used to generate hyperfine field distributions for amorphous aggregates. It thereby promises to provide a fairly quantitative bridge between computer models and hyperfine field probes in iron metglasses.     

Hyperfine interactions in electrodeposited Fe x Ni1−x alloys

Mössbauer spectroscopy and X-ray diffraction have been used to study a set of Fe _x Ni_1?x samples obtained by electrodeposition onto graphite. The change of the crystalline structure from fcc to bcc was observed for Fe concentrations betweenx=0.57 and 0.69. Changes of the preferred direction of magnetisation of alloys were observed with the increase of Fe contents.     

Hyperfine field distributions in ferromagnetic quasicrystals

Recent magnetisation measurements indicated the presence of ferromagnetism in Al−Cu−Mn−Ge quasicrystals with T_c∼535K. In this paper we report the room temperature Mossbauer and X-ray measurements of a series of ferromagnetic AlCuMnFeGe alloys.     

Hyperfine interactions of181Ta in Ni1−x Hf x

Time-differential angular correlation technique was employed for studies of the system Ni_1?x Hf _x . The hyperfine interactions of¹⁸¹Ta especially in the intermetallic compounds Ni₅Hf, Ni₇Hf₂, NiHf, NiHf₂ and in solid solutions of Hf in Ni have been investigated.     

Hyperfine field of positive muon in ferromagnetic nickel

The electronic structure around a positive muon μ⁺ at an octahedral interstitial site of a ferromagnetic nickel is calculated from first principles. The hyperfine field experienced by μ⁺ at absolute zero is calculated to be ?0.72 kG in satisfactory agreement with the experimental value of ?0.64 kG. The calculation also explains the temperature dependence of the hyperfine field. It is shown that the temperature dependence of the hyperfine field coupling constant A(T) is predominantly determined by single particle excitations.     

Spin exchange polarization and measurement of the hyperfine splitting of short-lived8Li

Free⁸Li (T _1/2=0.84 sec) atoms were polarized by spin exchange with optically pumped rubidium vapour. The⁸Li nuclei were produced by the reaction⁷Li(d, p)⁸Li in a thin LiF target. They recoiled into helium which was used as a buffer gas for optical pumping as well. Nuclear polarization up to 2% was detected by β-decay asymmetry. Rf transitions between hfs levels of the atomic ground state yielded the hfs separation ΔW=382.543(7) MHz. The nuclear spin assignmentI=2 was confirmed.     

Hyperfine interactions in chromium

Conclusion Our experiments suggest that Tantalum in Chromium is non magnetic and does not clamp the S.D.W. The various transitions of chromium have been observed. On the other hand rhodium seems to couple with the S.D.W. but the interaction, if any, between rhodium and the S.D.W. does not strongly modify the shape of the S.D.W. TDPAC experiments are therefore a useful tool to investigate the magnetic properties of impurities dissolved in a chromium matrix. This is extremely interesting because the formation of a localized moment in an itinerant antiferromagnet as well as their interaction with the S.D.W.'s have never been studied. It is also interesting to note that S.D.W.'s could couple magnetic impurities at very large distances and then could give rise to a special type of spin glass.     

Hyperfine fields in ferromagnetic compounds PtMnSn and PtMnSb

Hyperfine fields at various nuclear sites have been measured in ferromagnetic compounds PtMnSn and PtMnSb by spin echo method. The ‘giant’ fields at platinum sites in these two compounds are respectively found to be 800 kOe and 651 kOe at 4.2 K.     

Hyperfine fields and neutron scattering in ferromagnetic metals

The origin of ferromagnetism in the transition metal ferromagnets, iron, cobalt, and nickel is discussed, from an ab initio band structure point of view, with proper attention to the explicit roles of exchange, correlation and hybridization effects. The influence of these effects and all the mechanisms such as direct, exchange core polarization and many-body effects that have been found important for the hyperfine properties of atomic systems are included in attempting to understand the experimentally observed hyperfine fields at the nuclei in these metals. Spin-density distributions using calculated spin polarized band wave-functions are used to make comparisons with experimental neutron scattering data. The impact of the results of analyses of hyperfine fields at the nuclei and spin density distributions on the origin of hyperfine fields at muon sites is discussed. This talk, and the corresponding article for the proceedings of this conference, will deal with the theoretical understanding of the hyperfine fields at the nuclei and neutron scattering form factors in the three ferromagnetic metals, iron, cobalt and nickel and the impact of this understanding on that of the origin of the hyperfine fields at positive muon sites in these metals. With these aims in mind, the plan of my talk will be the following.

1. Discussion of a first-principle principle procedure to obtain the energy bands and electronic wave-functions in these metals and the understanding of the origin of their ferromagnetism from a band point of view.
2. Mechanisms contributing to hyperfine fields in atomic systems and their relevance for ferromagnetic metals.
3. The mechanisms for the origin of hyperfine fields in these metals, corresponding theoretical results and comparison with experiment.
4. Comparison between calculated spin-density distributions and experimental results from neutron scattering data.
5. Remarks on the origin of hyperfine fields at muon sites in these metals.

     

Hyperfine interaction of rare earth impurities in ferromagnetic metals

The present knowledge concerning rare earth impurities implanted in iron and nickel is reviewed. New Mössbauer spectra on the isotopes¹⁶¹Dy,¹⁶⁶Er and¹⁶⁹Tm are presented. In the case of¹⁶¹DyFe and¹⁶¹DyNi both the annealing behaviour and the temperature dependence have been studied. It is shown that in almost all cases about half of the rare earth impurities end up in substitutional sites, whereas the other part probably is associated with vacancies. The temperature dependence for these systems is in accordance with the generally accepted local moment picture, with fast relaxation behaviour on the electronic moment at the substitutional sites. For the iron host the exchange interaction dominates the cubic crystalline electric field, but for nickel both interactions are of comparable magnitude, leading to a considerable decrease of the hyperfine interaction.     

Hyperfine field systematics of nonmagnetic ions in ferromagnetic alloys

Hyperfine fields at interstitial and substitutional nonmagnetic ion sites in ferromagnetically ordered binary and tenary alloys have been studied within the framework of an extension of the phenomenological model due to Daniel and Friedel. It is concluded that the effects associated with the local Hartree potential of the impurity and the spin-split conduction band of the host are largely responsible for the observed systematic change of sign of the hyperfine field as a function of the impurity valence. The strong correlation noted by Balabanov and Delyagin between hyperfine fields and the atomic number of the impurity, in a fixed column of the periodic table, is shown to be largely due to the orthogonalization of the conduction electron wave functions to the occupied impurity core orbitals. Theoretical results for hyperfine fields at a large number of impurity sites in several ferromagnetic hosts are compared with the most recent experimental data and a number of predictions are given. The present charge screening model is critically compared with other theoretical model calculations and improvements that should be made on the present calculations are outlined.     

Hyperfine interactions in isolated ions

Conclusion It has been demonstrated that the effect of a tilted multifoil array on an ensemble of nuclei approaches a regular precession in a field of magnitude pH in the direction of the tilt axis. The magnitude of the precession can, under certain circumstances, be considerably larger than in a transient field, the only other method available today for measurements in the picosecond range. The technology of multifoil arrays has not been fully mastered to date, the immediate objective being the construction of arrays which keep their shape and spacings for many hours in a particle beam.     

Hyperfine interactions in cubic perovskites

ABO₃ perovskites display several physical properties determined by the characteristics of A and B cations. These compounds have cubic structure at high temperature. Lower symmetry cells that are distorted cubes are found at low temperature. Defects modify the properties of these compounds. Under standard conditions oxygen vacancies are produced. Cation substitution also alters the characteristics of perovskites. These materials have been studied by Perturbed Angular Correlation (PAC) spectroscopy and other hyperfine techniques. In this way abundant information is available to determine charge distributions close to probes. In the cubic phase perturbations were detected that are produced by the interaction of probes with defects. To show up these effects we analyze the quadrupole interaction at ¹⁸¹Ta in several compounds: ABO₃ with A=Ca, Sr and Ba, BaTi_1−x Hf _x O₃ and PbZr_1−x Ti _x O₃ for 0⩽x⩽1. Three different quadrupole interactions were found and are interpreted in terms of distinct probe-oxygen vacancy configurations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hyperfine interactions of muonium in alpha-quartz

We present the properties of muonium which are connected with the presence of the muonium quadrupole moment and discuss the results of experimental studies of the quadrupole interactions of muonium with the alpha-quartz lattice.     

Hyperfine structure of Li like bismuth

The hyperfine structure of lithium-like ²⁰⁹Bi⁸⁰⁺ was calculated in the dynamic-correlation model and compared with other theoretical results. The values obtained within this model are considerably smaller than those obtained in perturbation calculations. This revised version was published online in August 2006 with corrections to the Cover Date.