The influence of small magnetic fields on the Mössbauer relaxation spectra of Au: Yb alloys

The influence of an external magnetic field on the hyperfine structure of the Γ₇ CEF ground state of dilute Yb impurities in Au is investigated through the Mössbauer effect. Strong changes in the shape of the hyperfine spectra are observed when small magnetic fields (?1 kG) are applied. The dependence of the hyperfine structure on applied magnetic fields is shown through a Breit-Rabi diagram. The electronic relaxation rate for this system is found to be independent of the fields applied. The nature of polarized radiation emitted by such sources is discussed.     

Surface and interface studies with ASPIC

Magnetic properties at surfaces and interfaces have been investigated performing Perturbed Angular Correlation (PAC) measurements in the UHV chamber ASPIC (Apparatus for Surface Physics and Interfaces at CERN) using different PAC probes. The results are: (i) determination of magnetic hyperfine fields of Se on Fe, Co, Ni which are explained by a theoretical study on the magnetic hyperfine fields of 4sp-elements in adatom position on Ni and Fe;(ii) static magnetic hyperfine fields in ultrathin Pd on Ni(0 0 1) which indicate an induced magnetic order in Pd;(iii) the observation of induced fluctuating magnetic interactions in Pd when thick Pd is in contact with Ni. Monolayer-resolved measurements of the magnetic hyperfine fields in magnetized Pd are in accordance with theoretical predictions of the layer dependence of the induced magnetic moments in Pd. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.

     

Temperature dependence of hyperfine interactions in incommensurate systems

One investigates the influence of thermally excited collective modes in incommensurate systems on the temperature dependence of hyperfine interactions. The observed hyperfine fields are reduced by a temperature-dependent Debye-Waller factor and the hyperfine field amplitude can eventually decrease to zero well below the transition temperature.     

Hyperfine fields at nonmagnetic impurities in ferromagnetic GdZn and GdCd

Hyperfine fields at sp impurities in ferromagnetic CsCl type compounds GdZn and GdCd have been measured by NMR method. The impurity hyperfine fields are always negative, indicating the negative polarization of s-like conduction electrons at impurities on the Zn(Cd) sites, and are smaller at the beginning and end of each sp series. This trend of the hyperfine fields is similar to that in Gd metal and is qualitatively understood by the Daniel-Friedel mechanism.     

Rare earth hyperfine interactions studied by perturbed angular correlations

This paper reviews the application of the perturbed angular correlation (PAC/D) technique to studies of rare earth hyperfine interactions. First, PAC/D measurements of the interaction of rare-earth nuclei with the large hyperfine fields of their own partially filled 4f electronic shell are discussed. It is shown that PAC/D spectroscopy is particularly useful for the investigation of 4f spin relaxation in liquid and solid environments, and for the study of the formation and stability of 4f magnetic moments in metallic hosts. The second part of the paper deals with the static hyperfine interaction of non-rare-earth impurity nuclei in the hexagonal, magnetically ordered rare-earth metals. The PAC/D contributions to the systematic investigation of impurity electric field gradients and magnetic hyperfine fields in rare earths are discussed. The importance of PAC/D as the main source of information on the temperature dependence of impurity hyperfine interactions in rare earths is stressed. The trends of the electric field gradients are compared to the behaviour of simpler non-cubic metal systems. The magnetic fields are discussed in the context of current theories.     

Hyperfine fields for119Sn in laves phases Rt2 (R=rare earth: T=Fe,Co, Ni)

The hyperfine fields acting on¹¹⁹Sn nuclei in the RT₂ (R=Sm?Lu; T=Fe, Co, Ni) have been measured by Mössbauer spectroscopy. It has been found that the hyperfine fields acting on¹¹⁹Sn nuclei in the RT₂ compounds are changed at some electronic occupation of the 4f shell of the R-component. The occupation of 4f shell is varied with the T-component. The sharp change of the hyperfine fields are connected with the change of the electronic band structure.     

A study of the magnetic moments and Fe site hyperfine fields in iron-chromium alloys

The magnetization of Fe-Cr alloys ranging from 1 to 15 atomic % of Cr has been measured at room temperature in order to study the relationship between the Fe site hyperfine fields and the magnetic moment. The average moment decreases linearly, at a rate of -2.36 μ_B per Cr atom, up to 10% Cr concentration. The Fe site hyperfine fields were measured in a previous study¹ using the same samples. It is found that the hyperfine fields measured are not proportional to the corresponding magnetic moments. The results are interpreted using a model previously developed for other binary alloys of iron².     

射频驱动下电磁诱导透明窗口的分裂和增益的出现

在Λ型三能级系统的基础上引入两个共振射频场, 通过详细讨论系统的探测吸收特性随两个射频场Rabi频率取不同值时的变化规律, 得出电磁诱导透明(EIT)的分裂规律以及EIT上出现增益现象的产生条件.研究结果表明: 两个射频场对系统所起的控制作用不同, 控制基态精细结构能级之间跃迁的射频场对EIT的分裂起作用, 而控制激发态精细结构能级之间跃迁的射频场不会导致EIT的分裂; 而且, 只有当控制基态精细结构能级之间跃迁的射频场的Rabi频率大于控制激发态精细结构能级之间跃迁的射频场的Rabi频率时, 才能产生EIT与增益相叠加的新特性. 关键词： 射频场 电磁诱导透明 增益 精细结构能级     

Magnetic properties, Mössbauer effect and first principle calculations study of laves phase HfFe2

The magnetic properties and hyperfine interaction parameters for Laves phase HfFe₂ with C14 type structure are studied using SQUID magnetometer and M?ssbauer measurement. The saturation magnetization, remanent magnetization, coercive field, magnetic moment per unit formula and the hyperfine magnetic field at Fe site are reported. In addition, a detailed theoretical study of the electronic structure and hyperfine magnetic fields of the two possible HfFe₂ structures, C15 and C14, is presented. Using the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN 97 package, the equilibrium volume, bulk moduli, magnetic moments and hyperfine magnetic fields for the two structures are calculated. The obtained results are compared with the measured data.     

Investigations of “soft-landed” Cd surface atoms via nuclear methods: hyperfine-field sign determination

Using refined preparation techniques, cadmium guest atoms have been positioned at different sites on the surfaces of nickel crystals. The magnetic hyperfine fields and the electric field gradients at the Cd nuclei were measured by time-dependent perturbed angular correlation (TDPAC) spectroscopy of the emitted gamma radiations. By measuring the combined interactions, electric field gradients and magnetic hyperfine fields can be unambiguously attributed to each surface site. The signs of the magnetic hyperfine fields are determined by applying an external magnetic field and choosing the appropriate γ-ray detector configuration. The measured fields correlate with the number of neighbouring host atoms. Band structure calculations confirm this finding and predict magnetic fields for various sp elements from the band structure of the s-like conduction electrons. The quadrupolar interactions are manifestations of the balance in the occupation of the guest p-sublevels. These results provide new information on the structure and formation of electronic configurations of sp elements in different local environments and will contribute to understanding electronic effects on surfaces.     

Magnetic hyperfine fields of vacancy-associated119Sn in ion-implanted nickel

Magnetic hyperfine fields of¹¹⁹Sn impurity defects in nickel have been investigated by Mössbauer emission spectroscopy. Radioactive¹¹⁹Xe isotopes were implanted, annealing was performed after¹¹⁹Xe had decayed to¹¹⁹Sb. At least five different components with well-defined magnetic hyperfine fields, isomer shifts and Debye temperatures are identified in the rather complex spectra. One of these (B=2T) is known to be due to substitutional Sn. The hyperfine fields of the other components are pronouncedly larger (B=9T, B=15T, and B=17T, respectively, for single crystals). These defects are proposed to be Sn-multivacancy defects.     

Magnetic moments and hyperfine fields at Fe in 3d-transition metals

The magnetic moments and hyperfine fields at Fe sites in 3d-transition metals are calculated using the first principle discrete variational method in local density approximation. Although a large positive moment is retained at each Fe site, the hyperfine fields varied from large negative to large positive values. It is concluded that the absence of Mössbauer magnetic splitting does not necessarily imply the absence of local magnetic moments.     

微波驱动精细结构能级跃迁引起的电磁诱导负折射效应

本文通过建立Λ形四能级原子系统, 研究了微波驱动精细结构能级跃迁引起的电磁诱导负折射效应. 微波场作用于基态精细结构能级之间, 与不同精细结构能级之间的电偶极矩或磁偶极矩发生耦合, 使系统在某些频率处呈现负折射特性.同时, 两个耦合场各自激励一对基态和激发态之间的光学跃迁. 通过改变两个耦合场的频率失谐量控制负折射区域的频带宽度.结果表明, 耦合场失谐时出现负折射特性的频率范围比耦合场共振时迅速缩小, 而且耦合场负失谐和正失谐时的变化规律不同.     

Size effects on the magnetic hyperfine fields at non-magnetic impurities in iron

An important contribution to the magnetic hyperfine fields at non-magnetic impurities in magnetic metals arises from the conduction-electron polarization (CEP). In the model of Stearns the CEP at the impurity site,Pce, arises from the exchange interaction of the conduction electrons at that site with the localized 3d host electrons. In this modelPce is a negative constant, independent of the impurity, so that an emperical volume-misfit correction has to be introduced to explain the observed positive hyperfine fields. The purpose of this paper is to show thatPce is not constant, but is strongly affected by the distortion of the lattice around the impurity. The correct form ofPce includes a volume term equivalent to that postulated by Stearns. The hyperfine fields for Cu, Zn, Sn, Sb and Au are calculated, taking the distortion effects into account. A fair agreement with the observed hyperfine fields is obtained. Furthermore, the effects of pressure on the CEP are discussed.Supported by the NSF Grant No. DMR 73-07665 AO 3     

Calculated hyperfine fields of light interstitials in Fe

Super-cell band structure calculations with the Korringa-Kohn-Rostoker method in the framework of the local-spin-density approximation are performed on ferromagnetic iron with typical elements (B, C andN) at the octahedral interstitial sites for the purpose of studying hyperfine fields of light interstitials. Lattice relaxations around the interstitial atoms are allowed in these calculations. Calculated hyperfine fields of these interstitial impurities are in better agreement with experimental values than the results obtained previously for unrelaxed lattices, showing that the inclusion of the lattice relaxation is crucial in these systems.     

Hyperfine interactions of111Cd in ion-implanted face centered cubic cobalt

The magnetic-dipole and electric-quadrupole hyperfine interactions of¹¹¹Cd probes in fcc cobalt after implantation of radioactive¹¹¹In⁺ ions have been investigated by PAC measurements with fast BaF₂ detectors. Six different sites of the probe atoms could be distinguished and characterized by their hyperfine parameters and annealing behaviour. Besides the substitutional site, three sites are assigned to In-vacancy complexes which are formed athermally in the implantation process or by thermally-activated trapping of lattice defects in annealing stage III. The remaining two sites are attributed to In located in stacking faults or hcp regions of the host. Systematic trends of impurity hyperfine fields in defect sites become evident from a comparison with other impurity-host combinations. The temperature dependence of the magnetic hyperfine fields has been measured between 20 K and 390 K. Large differences found for the various sites are discussed.     

A fully relativistic description of the hyperfine interaction in magnetic systems

Based on the spin-polarized version of the multiple scattering theory, a relativistically correct formulation for the Green’s function describing the electronic structure of magnetic metallic systems has been derived. By this way, orbital contributions to the magnetic moment and, for the first time, to the hyperfine fields, which are caused by an unquenching of the orbital momentum due to spin-orbit coupling, were accessible to a calculation. In the case of Fe, Co and Ni, it has been found, that the corresponding non-s-contributions to the hyperfine field of the conduction electrons stem nearly exclusively from the d-electrons, are positive and of the same order of magnitude as the s-part of the conduction band contribution to the total hyperfine field. In all three cases these large non-s-hyperfine fields compensate the relativistic enhancement of the (Fermi contact) s-hyperfine field compared to a non-relativistic calculation, leading to theoretical hyperfine fields which are below the experimental ones. At the moment, it cannot be decided whether this discrepancy is caused by the neglect of the non-s-contributions of the core electrons or shortcomings of the local spin density approximation.     

基于超精细结构下的激光光泵铯磁力仪的理论研究

激光光泵碱金属磁力仪具有很高的灵敏度,测量范围可以从地球磁场到生物磁场。给出了铯（Cs）光泵磁力仪的理论分析和系统设计以及磁场梯度测量原理,铯原子能级在I—J耦合时形成超精细结构,在外磁场的作用下超精细结构进一步产生塞曼分裂形成塞曼子能级,利用激光泵浦和射频磁场能够使电子在超精细结构中进行能级跃迁,产生光磁双共振的结果,最终通过共振频率就能够达到精确测量外磁场的目的。