Hyperfine magnetic fields at sp-sites in Co2YZ Heusler alloys

The systematics of hyperfine magnetic fields at sp impurities on the Z-sites in Co based Heusler alloys are investigated. New TDPAC measurements of Cd hyperfine fields are reported.     

Magnetic hyperfine fields of 5sp impurities in vacancy complexes in ferromagnetic metals

A survey is given of recent experimental results for the hyperfine interactions of 5sp impurities in vacancy complexes in ferromagnetic 3d host metals. Emphasis is put on changes of the impurity magnetic hyperfine fields in such complexes as compared to substitutional lattice sites. Systematic trends are presented, which are beginning to emerge from studies of Cd and Sn impurities in particular.     

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     

Self-consistent calculation of hyperfine fields and adiabatic potential of impurities in iron

Hyperfine fields of impurities of the atomic number Z=1–56 at the substitutional site and those of light impurities of Z=1–9 at the interstitial sites in ferromagnetic iron are calculated by the KKR method adapted to the system containing a single impurity atom. The potential of the impurity atom is determined self-consistently by use of the local spin density functional formalism. The results for nonmagnetic sp valence impurities agree with those of the previous nonself-consistent calculation by Katayama-Yoshida, Terakura and Kanamori except for a few cases, confirming their theory of the systematic variation of hyperfine fields. The calculation for magnetic impurities of transition elements is presented for the first time in this paper. The calculations mentioned so far assume that impurities are situated at the center of each site. For the purpose of discussing the stability of the impurity positions, the change of the adiabatic potential due to displacements from the center is calculated by carrying out similar self-consistent calculations for off-center impurity positions. It is concluded that positive muon and some light impurities including boron will be displaced from the center when trapped in a vacancy.     

Hyperfine fields amd electronic structure of non-magnetic impurities in iron

Hyperfine fields of non-magnetic impurities in iron are calculated using the local-density formalism in an embedded cluster model. The hyperfine fields are seen to result from a delicate balance between negative exchange polarized “bound-paired” states and positive “unpaired-band” contributions. Pressure and temperature dependence and effects due to lattice relaxation are discussed.     

Static and dynamic hyperfine interactions of Mn and Co impurities in the weak ferromagnet ZrZn2

We report the nuclear orientation of near-stoichiometric samples of ZrZn₂ containing trace impurities of radioactive⁵⁴Mn and⁶⁰Co in the temperature range 6.5–60 mK and in applied fields between 0.2 and 5.7 T. Analysis of the resullts indicates that in both cases, the transition-element impurities occupy two sites (the Zr and Zn sites) with about equal probablities. In one site, most probably that of Zr, both impurities have positive hyperfine fields of +12.0 T and +3.8 T for Mn and Co, respectively; in the other site, the corresponding hyperfine fields are negative, with the values −6.0 T and −6.9 T. The dynamic hyperfine interaction of Mn in ZrZn₂ has also been studied using the thermal cyclic method with a weak thermal link, and was analyzed for the two relaxation times corresponding to the different lattice sites.     

Hyperfine fields in 3d impurities in metals

The dynamic spin polarization of conduction electrons by an impurity spin is shown to provide a significant positive contribution to the impurity hyperfine field, accounting for the fact that the hyperfine field per unit impurity spin is only about half as large for impurities in metals as in insulators. The anomalously small hyperfine fields of Fe impurities in the noble metals are found to result from an orbital contribution. Quantitative analysis of this orbital contribution suggests that a strong dynamic Jahn-Teller effect is present. Similar orbital contributions can yield a strongly anisotropic hyperfine field for Fe or Cr impurities in hexagonal host metals, depending on the type of crystal-field orbital ground state.     

Effect of Lattice Relaxations on the Hyperfine Fields of Heavy Impurities in Fe

We present first-principles calculations of hyperfine fields of heavy impurities in bcc Fe. In particular, the effect of lattice relaxations on the calculated hyperfine fields are studied. The calculations are based on a full-potential Korringa–Kohn–Rostoker Green's-function method for defects and employ the local spin-density approximation for the exchange and correlation effects. The calculated lattice relaxations around the 5sp and 6sp impurities in Fe are found to be relatively small, for the first and second nearest neighbor shells less than 6% of the nearest neighbor distance. Also the estimated relative volume changes induced by the sp impurities are found to be around 60–95% of the elementary volume of Fe, surprisingly small in view of the large atomic volumes of the impurities. The calculated hyperfine fields of 5sp and 6sp elements are compared with available experimental data and it is shown that the inclusion of lattice relaxations in the calculation improves the agreement with experiments.     

Magnetic hyperfine field of129I impurities in Pd2MnSb

Conclusions In the Heusler alloy Pd₂MnSb (Z) agreement has been found between the experimental values of magnetic hyperfine fields of 5sp shell impurities Z that substitute Sb and the theoretical predictions of Blandin and Campbell |2|. The decrease of the field when the 5sp shell is nearly full lends support to conduction electron polarization models based on the Daniel Friedel |4| type of theory. In view of these results, the rise of the field of 5sp impurities in iron right up to shell closure remains puzzling.     

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.     

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.     

Nontransition element impurities in ferromagnetic transition metals

Effects of nontransition element impurities on the ferromagnetism of transition metals are discussed in the light of our theoretical analyses of the hyperfine interaction data of implanted nuclei which have been carried out in recent years. A detailed discussion of the effective filling of the d band by impurity valence electrons is given after presenting a brief summary of our theory of the hyperfine interaction. The change of the saturation magnetization is discussed in particular. It is pointed out that an sp valence impurity at an interstitial site in iron can increase the magnetic moments of surrounding host atoms.     

First application of the real space linear muffin-tin orbital atomic sphere approximation method for calculating Fermi contact hyperfine fields

In this work we apply the recently developed first-principles real space linear muffin-tin orbital atomic sphere approximation (RS-LMTO-ASA) scheme to calculate the hyperfine field for 3d impurities (V, Cr, Mn andFe) in Cu. We obtain the Fermi contact term at the impurities andat four shells of host atoms around the impurities. We compare our results with theoretical values obtained using the KKR-Green-function method andwith NMR measurements. The overall agreement is excellent, confirming the reliability of the RS-LMTO-ASA approach as means of obtaining information on hyperfine fields.     

Nuclear orientation of60Co in concentrated CoAu alloys

We report β-particle and γ-ray angular distribution measurements from⁶⁰Co oriented in CoAu at low temperatures in applied fields up to 72 kOe and Co concentrations of 0.95 to 11 at.%. The results are explained by assuming a random molecular field interaction between groups of Co impurities. The hyperfine field on Co nuclei in groups of three or more nearest neighbors is negative and has a value near ?180 kOe. The induced field on Co-pair nuclei is small and probably also negative. Complete polarization of the Co moments in the more concentrated alloys requires a very large applied field.     

The hyperfine field on impurities in Gd

Predictions are given for the hyperfine fields on s-p impurities in Gd.     

Hyperfine Fields of Light Interstitial Impurities in Ni

The magnetic hyperfine interaction of light interstitial impurities in Ni have been studied by means of the Korringa–Kohn–Rostoker (KKR) band structure method. This method allows to deal with the impurity problem by solving the corresponding Dyson equation for the Green’s function. It also allows to account for lattice relaxations. For this purpose a new technique was developed that allows to handle in principle arbitrary lattice distortions. Corresponding calculations have been performed for the magnetic hyperfine fields of the light interstitial impurities H to Ne in Ni. By minimising the force on the nearest neighbour host atoms their equilibrium position was determined. The resulting hyperfine fields for the equilibrium configuration are found to be in rather good agreement with available experimental data.     

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.     

Hyperfine fields at non-magnetic sites in metals

The conduction electron polarization contribution to the hyperfine fields at s-p sites in magnetic metals is discussed using an RKKY-like model in which the effect of the charge at the s-p site is included explicitly. It is shown that this mechanism alone can provide a simple explanation for observed hyperfine systematics in a variety of systems, in particular for s-p elements in Heusler alloys and for s-p impurities in Fe, Ni or Pd hosts.     

A Mössbauer effect study of Y(57Fe:Mn)12

The intermetallic compound, YMn₁₂, doped with 0.5at.%⁵⁷Fe has been investigated using the⁵⁷Fe Mössbauer resonance. The data show that the Fe impurities occupy preferentially the f-site at the expense of the i- and j-sites. Whereas the i- and j-sites are expected to carry identical local magnetic moments in the ordered state, low temperature hyperfine fields of 50.08(±0.05)T and 2.90(±0.03)T are measured for the i- and j-sites respectively. It is proposed that large transferred hyperfine fields are responsible for this disparity.