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.     

Hyperfine Interaction Studies on Y, Zr, Nb, Mo, Rh, In and Xe in Co

Nuclear magnetic resonance on oriented nuclei and modulated adiabatic fast passage on oriented nuclei measurements were performed on several 4d and 5sp impurities in polycrystalline Co(fcc) foils and Co(hcp) single crystals. The hyperfine fields of Y and Zr in Co(fcc), the hyperfine fields of Y, Zr, Nb, Mo, Rh, In and Xe in Co(hcp), the electric field gradients of Zr, Nb and In in Co(hcp), and the nuclear spin-lattice relaxations of Zr, Nb, Rh and In in Co(hcp) were determined. The dependence of the hyperfine fields and electric field gradients in Co(hcp) on the angle between the magnetization and the c axis was investigated in most cases. The magnetic-field dependence of the spin-lattice relaxation was studied for Nb, Rh and In in Co(hcp), applying the magnetic field perpendicular to the c axis. The known hyperfine interaction parameters of the 4d and 5sp impurities in Co(fcc) and Co(hcp) are summarized. The new results provide a more detailed picture of the hyperfine interaction in Co.     

Pac study of the hyperfine interactions in154Gd3+ in rare-earth-garnets

The attenuation of the anisotropy of the γ-γ angular correlation for a rare-earth ion in magnetic iron and non-magnetic aluminum rare-earth garnets is governed by the static hyperfine electric field gradient (EFG) produced by the surrounding ions and by the static and fluctuating hyperfine magnetic fields produced by the 4f electrons of the ion. The latter effect depends upon the correlation time (τ_c) of the 4f electrons which, in turn, is determined by the interaction of the ionic spin with lattice vibrations and other magnetic ions. In an attempt to determine the significance of spin-spin relaxation on τ_c, perturbed angular correlation (PAC) measurements were performed on the S-state ion¹⁵⁴Gd³⁺ because spin-lattice interactions were expected to be small. The time-integral attenuation coefficients of the 1274/123 keV γ-γ cascade in¹⁵⁴Gd were measured in the temperature region 4.2–650 K for¹⁵⁴Gd³⁺ incorporated in GdAlG, GdIG, YAlG, and YIG. Rotation measurements were also made on the same cascade in the iron garnets at room temperature with an applied magnetic field in the range 0–15 kg. Employing independent measurements and calculated estimates of the static hyperfine magnetic field and EFG, the observed data could be interpreted in terms of reasonable values of τ_c. The correlation times were found to range from about 0.13 ns in YIG to about 1.30 ns in YAlG with GdAlG having a value of approximately 0.71 ns. In the case of GdIG, a self-consistent analysis of the data required a value of the lattice EFG which was larger than that estimated from the nearest neighbor point-ion model. The correlation time in GdIG was then found to be the same as in YIG.     

Hyperfine interactions in intermetallic compounds between Gd and 3d transition metals

Measurements of hyperfine interactions at ¹⁵⁵Gd nuclei in metallic compounds between Gd and 3d transition metals and at ⁶¹Ni in GdNi compounds by Mössbauer spectroscopy are reported. The results are discussed in terms of various models proposed for the electronic structure of these compounds. The Gd isomer shifts with respect to metallic Gd are at variance with the model of a strong d electron transfer from rare earths to transition metal ions. From the observation of a linear relation between magnetic hyperfine fields at Gd and at Dy nuclei in corresponding compounds it is inferred that crystal-field induced variations of Dy moments are neglible and that the conduction electron polarization induced by 4f moments is directly related to that caused by 3d moments.     

Time differential perturbed angular correlations of Gd160 implanted into ferromagnetic Gd and of Yb176 into Fe and Ni

Time differential perturbed angular correlation measurements of Gd¹⁶⁰ in ferromagnetic Gd at various temperatures and of Yb¹⁷⁶ in Fe and Ni have been performed following Coulomb excitation with a pulsed beam and recoil implantation. Using the theory of combined static magnetic and electric hyperfine interaction the measured time spectra are reasonably well reproduced, the magnetic fields and electric field gradients being in agreement with other works. A phase shift of the Larmor precession, however, points to anomalous hyperfine fields acting on the nuclei in a very short time interval after the beam pulse.     

The magnetic hyperfine field at Cd nuclei in the rare earth ferromagnets Gd,Tb, Dy,Ho, Er and Tm

The time differential perturbed angular correlation technique has been used to study the combined magnetic and electric hyperfine interactions at the site of a¹¹¹Cd impurity in the rare earth ferromagnets Gd, Tb, Dy, Ho, Er, and Tm at 4.2 °K. The following magnetic hyperfine fields at the site of¹¹¹Cd have been found: ¦H _hf ¦=340(7) kG in Gd, 275 (5) kG in Tb, 221 (4) kG in Dy, 116 (3) kG in Er and 60 (6) kG in Tm. In Ho two magnetically different sites were observed with magnetic fields of 159 (3) and 139 (3) kG. Both sites are equally populated. The coupling constantJ _5f of the conduction electron-4f interaction has been calculated for the different rare earth metals from the measured hyperfine fields by means of the RKKY theory.     

Ferromagnetic Semiconductors Studied by Hyperfine Interaction of Nuclear Probes

The ferromagnetic spinels CdCr₂Se₄ and CuCr₂Se₄ were investigated by PAC (perturbed angular correlations) after implantation of the probes ¹¹¹In(¹¹¹Cd), ^111mCd, ¹¹¹Ag(¹¹¹Cd) and ⁷⁷Br(⁷⁷Se). The site occupation of different probes was determined by magnetic and electric hyperfine interactions. Theoretical calculations of hyperfine parameters by the WIEN97 code gave satisfactory efg (electric field gradients) but the magnetic hyperfine fields are inconsistent with the experiments. This revised version was published online in September 2006 with corrections to the Cover Date.     

The electric field gradient in heavy rare earth metals

Estimates of the electric field gradient in heavy rare earth metals have been evaluated from experimental hyperfine interaction data. In addition, the magnetic hyperfine fields are analyzed. In the metals the effective radial integrals 〈r ^?3〉_4f of the magnetic and quadrupole hyperfine interaction are reduced at most by 10% compared with the free ion values. The electric field gradients due to the crystalline field have been found to be 200 times larger than those predicted from point charge calculations. This antishielding effect can be explained by an enhanced conduction electron density at the interstitial sites and an increase of the Sternheimer factor γ_∞ in the metallic environment.     

Precision measurements of nuclear quadrupole moments by muonic X-rays

The electric multipole hyperfine interaction in muonic atoms is discussed. In particular, the influence of the finite size of the nuclear electric multipole-charge distribution on the values of nuclear spectroscopic multipole moments that are extracted from muonic hyperfine measurements is considered. It is shown that nuclear electric quadrupole moments can be deduced from the observed hyperfine splittings of muonic M X-ray transitions with high precision and practically independently of the model nuclear quadrupole-charge density. Measurements of the ground-state electric quadrupole moments of 11 deformed nuclei in and near the rare-earth region are discussed.     

Mössbauer effect in iron and dilute iron based alloys

Fifteen years of Mössbauer Effect (ME) studies have significantly widened the insight into the physical properties of iron and iron based alloys. In this review article the various contributions to the hyperfine interactions as measured with the ME technique, namely the isomer shift, the magnetic hyperfine interaction and the quadrupole interaction are summarized. Further the impurity effects as the Friedel type oscillations in the charge and the RKKY type oscillations in the spin density distribution are discussed. Special attention is paid to the peculiar magnetic properties of metallic iron and its alloys. From a comparison of the magnetic hyperfine fields and bulk magnetizations as a function of the impurity concentration and from the temperature dependence of the magnetic hyperfine fields at the various sites in iron alloys, it is concluded that the 3d magnetic moments in iron are largely localized. Further the exchange interaction is provided by the remaining small percentage of itinerant 3d electrons. Finally, from a comparison of magnetic hyperfine field and isomer shift data in alloys an overall picture of the electronic changes involved in alloying has been developed.     

Perturbed Angular Correlation Study of Magnetic and Electric Hyperfine Interactions at 111Cd in GdNi2 and SmNi2

The magnetic hyperfine field B _hf of the closed-shell probe nucleus ¹¹¹Cd in the C15 Laves phase RNi₂ (R= Gd, Sm) has been investigated as a function of temperature by perturbed angular correlation (PAC) spectroscopy. The saturation magnetic hyperfine fields at 10 K are B _hf=7.7(2) and 3.9(1) T for GdNi₂ and SmNi₂, respectively. Although the probe nucleus resides on the cubic rare-earth site, a strong axially symmetric electric quadrupole interaction (QI) is observed in the paramagnetic phase at T300 K. The possible relation of this unexpected QI to the structural instability of RNi₂ is discussed.     

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.     

Spin and temperature dependence of the magnetic hyperfine field of Cd in the rare earth-aluminum Lavesphase compounds RAl2

Perturbed angular correlation spectroscopy has been used to investigate the combined magnetic and electric hyperfine interaction of the probe nucleus ¹¹¹Cd in ferromagnetically ordered rare earth (R)-dialuminides RAl₂ as a function of temperature for the rare earth constituents R=Pr, Nd, Sm, Eu, Tb, Dy, Ho and Er. In compounds with two magnetically non-equivalent Al sites (R=Sm, Tb, Ho, Er), the magnetic hyperfine field was found to be strongly anisotropic. This anisotropy is much greater than the anisotropic dipolar fields, suggesting a contribution of the anisotropic 4f-electron density to magnetic hyperfine field at the closed-shell probe nucleus. The spin dependence of the magnetic hyperfine field reflects a decrease of the effective exchange parameter of the indirect coupling with increasing R atomic number. For the compounds with the R constituents R=Pr, Nd, Tb, Dy and Ho the parameters B₄, B₆ of the interaction of the crystal field interaction have been determined from the temperature dependence of the magnetic hyperfine field. The ¹¹¹Cd PAC spectrum of EuAl₂ at 9 K confirms the antiferromagnetic structure of this compound.     

Hyperfine Fields of 181Ta in UFe4Al8

The γ–γ Perturbed Angular Correlation technique was used to study the hyperfine interaction of ¹⁸¹Ta at the Hf site(s) in UFe₄Al₈ at room temperature and 12 K. The data at room temperature are well described by two electric field gradients, while at low temperature two combined hyperfine interactions have to be considered, one with the magnetic hyperfine field collinear with the c-axis and another with the magnetic hyperfine field in the basal plane. The results are compared with previous Mössbauer and neutron diffraction experiments and the lattice site of Hf 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.     

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using ¹¹¹In→ ¹¹¹Cd and ¹⁴⁰La→ ¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. The PAC results for ¹¹¹Cd show two well-defined electric quadrupole frequencies above T _C assigned to probes occupying Gd and In sites, with ~50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below T _C the measurements for ¹¹¹Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by ¹⁴⁰Ce below 100 K.     

The Nonanuclear [Mo(IV){(CN)Fe(III)(3-ethoxy-saldptn)}8]Cl4 Complex Compound Exhibits Multiple Spin Transitions Observed by Mössbauer Spectroscopy

The hyperfine interactions at ¹⁸¹Ta ions on Fe³⁺ sites in α-Fe₂O₃ (hematite) were studied in the temperature range 11–1100 K by means of the perturbed angular correlation (PAC) technique. The ¹⁸¹Hf(β⁻)¹⁸¹Ta probe nuclei were introduced chemically into the sample during the preparation. The hyperfine interaction measurements allow to observe the magnetic phase transition and to characterize the supertransferred hyperfine magnetic field B_hf and the electric field gradient (EFG) at the impurity sites. The angles between B_hf and the principal axes of the EFG were determined. The Morin transition was also observed. The results are compared with those of similar experiments carried out using ¹¹¹Cd probe. ^aAlso at Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Argentina.     

Surface studies with hyperfine probes

The current status of surface hyperfine fields measured with NMR, Mössbauer spectroscopy and PAC is reviewed. Emphasis is put on experiments with well-characterized, free surfaces and on results on surface electric field gradients. Applications to detection of probe sites and diffusion phenomena are discussed.This article is dedicated to the memory of our late colleague Dr. Wolfgang Keppner     

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.