Fe Host Research Articles

IMP AC in-beam and out-of-beam;g-factors and pre-equilibrium effects following ion-implantation

An out-of-beam IMPAC technique has been developed to measure theg-factors of the 2 l + states in the neutron-deficient Pt isotopes. The hyperfme field for Pt in Fe in those measurements was close to the field obtained in spin-echo and radioactivity measurements, in contrast with in-beam IMPAC measurements for194,196,198Pt, where smaller field strengths were observed. These data, and static fields from in-beam IMPAC measurements for other ions in the Pt region, show a correlation with the lifetime of the probe state, consistent with the static field being absent for about 10 ps after the ion has come to rest. New and previously published IMPAC data for76Os,77Ir,78Pt and79Au in Fe hosts, are examined to assess whether such measurements may be sensitive to the thermal spike created at the end of the range of an implanted ion.

Spin dependence of impurity scattering in ferromagnetic metals

Modern results of band structure calculations are applied to calculate the spin dependence of impurity resistivity within the two-current model of Fert and Campbell. Non-magnetic and transition-metal impurities in Ni and Fe hosts are considered and a qualitatively satisfactory description of the experimental situation is obtained. The adequacy of the Anderson and s-d exchange models is discussed. Effective s-d exchange integrals for impurities with well-defined magnetic moments are estimated. The important role of conduction electron spin polarization in explaining the experimental data is demonstrated. On this basis, the role of d-electrons in conductivity is discussed.

Calculated oscillation periods of the interlayer coupling in Fe/Cr/Fe and Fe/Mo/Fe sandwiches.

We have performed an ab initio study of the oscillation periods of the interlayer coupling in sandwiches of [100] layers of body centered cubic Cr, respectively, Mo, with up to 20 monolayers thickness embedded in an Fe host. Our derived values for the oscillation periods of the interlayer coupling agree well with those expected from the respective Fermi surface calipers and are in excellent agreement with experiment. We compare the interlayer coupling across a nonenhanced paramagnetic Cr spacer with the interlayer coupling across a Cr spacer in the spin-density wave state. We find that the spin-density wave gives rise to an additional contribution to the interlayer coupling. The origin of the observed and calculated 18 \AA{} period of the interlayer coupling can be explained as due to a caliper of the Fermi surface of Cr in the CsCl structure. In the case of the Mo spacer we investigate the influence of the lattice constant on the interlayer coupling and also the dependence of the interlayer coupling amplitude on the Fe magnetic moment.

Measured static hyperfine magnetic fields following implantation of Pt into Fe interpreted as evidence for pre-equilibrium effects

The static hyperfine magnetic field present at Pt nuclei implanted in ferromagnetic Fe has been measured using the ion-implantation perturbed angular correlation (IMPAC) technique following Coulomb excitation. The present measured precessions agree with earlier data, but more recent information on the transient field correction leads to an inferred static field strength that is ∼ 25% smaller than obtained previously. Comparisons are made between the static fields measured by various techniques for Pt and neighbouring ions in iron. From these comparisons, we show that the IMPAC data are consistent with a scenario in which (i) the static field takes about 10 ps to reach its equilibrium value, following recovery from dynamic structural damage caused by the ion-implantation process, and (ii) following equilibration, a large fraction (∼ 90%) of the implanted ions have final positions on lattice sites of the Fe host.

Calculated oscillation periods of exchange coupling in Fe/Mo/Fe sandwiches

We have performed an ab initio study of the oscillation periods of the exchange coupling in sandwiches of bcc (100) layers of Mo embedded in an Fe host. We find three periods for the exchange coupling, which are in close agreement with the corresponding periods extracted from the appropriate Fermi surface calipers. We show that the short period is dominated by RKKY behaviour whereas the longer periods are dominated by QW behaviour.

Isoelectronic early n-d impurities in Fe: Magnetic and hyperfine field properties

The hyperfine fields and the local magnetic moments in a series of alloys of isoelectronic transition impurities diluted in iron are discussed. We intend to show that, even within a very simplified model, period effects for isoelectronic impurities at the beginning of an n-d series (e.g., Sc, Y, and Lu) are dominated by the charge difference effect, inducing a very strong repulsive potential. We find that the local impurity magnetic moments are very close to each other, leading to hyperfine fields, in fair agreement with experimental data. Our calculation accounts for the general trends of the local impurity moments in ferromagnetic Fe, despite quantitative deviations that do occur, due to the very simple structure of this model. We point out, however, that our calculation is a starting point for a more refined one, dealing with rare-earth impurities (such as Eu, Gd, and Tb) placed in the Fe host, where the strong charge effects are present, together with f-spin scattering.

Systematics of Hyperfine Fields at Impurity Nuclei in Ferromagnetic Rare Earth Compounds GdZn and GdCd

The hyperfine (hf) fields at the nuclei of N, Ge, Tl, V and Fe impurities in ferromagnetic GdZn and GdCd have been measured by NMR and Mossbauer methods. The results, together with the reported ones, nearly complete the systematics of the hf fields at s p and 3 d impurities in GdZn(Cd). The hf fields at 4 s p , 5 s p and 6 s p impurities are always negative, exhibiting a similar variation with increasing the impurity valence. The trend of the hf fields at 3 d impurities shows local moment contributions to the hf field at the middle of the 3 d series, indicating finite local moments on Mn and Fe impurities. These results are found to resemble the systematics of the hf fields at impurities in Gd and are qualitatively explained by the theory of the hf field developed for Fe and Ni hosts.

Low temperature nuclear relaxation of dilute Tc and Nb in iron

Using the method of thermal cycling of oriented nuclei (TCON) applied to highly dilute, recoil-implanted samples, we have determined the nuclear spin-lattice relaxation (SLR) constants of Nb and Tc in Fe host in the 10 mK range. The results are comparable with earlier experiments using resonant methods, and confirm the discrepancies with theory.

Calculated oscillation periods of exchange coupling in Fe/Cr/Fe sandwiches

We have performed an ab initio study of the oscillaton periods of the exchange-coupling in sandwiches of layers of Cr embedded in an Fe host, by means of a spin-polarized Green's function technique, based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations. We treat Fe/Cr/Fe sandwiches with up to 12 monolayers of Cr and find that the calculated exchange coupling oscillates with a short-range period of 2 monolayers and a long-range period of approximately 8 monolayers (11 Å). This is in qualitative agreement with recent scanning electron microscopy measurements.

Bloch enhancement of electric field gradient in Fe and Cu host alloys

The Bloch enhancement factor α(k f ) of the electric field gradient has been evaluated for the half-filledd-core Fe host metal and completely filledd-core Cu host metal in single orthogonalized plane wave (OPW) approximation. For this purpose the radially-dependent antishielding factors,γ(r) have been calculated in non-orthogonal Hartree-Fock perturbation theory (NHFPT). The results show that the contributions of antishielding to α(k f ) from the plane wave-plane wave part and the core part of the OPW state are individually large but opposite in sign and thereby lead to partial cancellation. The net effect of antishielding on α(k f ) is found to be − 5.6% in Fe and 14% in Cu.

Investigation of spin-neutrino correlation in decay of polarized 56Co nuclei

A new technique has been developed for investigation of the spin-neutrino angular correlation in electron capture. The technique is based on measurement of Doppler shifts in the energy of gamma-quanta resulted from the decay of polarized nuclei. This technique was used to investigate the decay of 56Co nuclei polarized by the magnetic field of the Fe host at the temperature 17 mK. A Doppler shift + 12 ± 1 eV has been obtained for the 2598.5 keV gamma-line. It allows to admit that the magnetic moment of the 56Co nucleus has a positive sign and to estimate the behaviour of the recoil ion of 56Fe in the Fe crystal lattice.

New aspects of transient magnetic fields

Strong attenuations of transient magnetic fields are observed in Fe and Gd host for Coulomb excited2HSi(21+)- and24Mg(21+)-ions in their single-electron charge state as well as for multi-electron62Ni(21+) ions. These effects are shown to be induced by the heavy ion beams used. The perturbations are found to be dynamic in nature and are attributed to a momentary loss of ferromagnetism of the host. This feature is also present in many earlier measurements using heavy ion beams but was not identified. The magnitude of the attenuations seems to be correlated with the energy loss of the beam ions in the ferromagnet.

Theoretical and experimental investigation of the magnetic x-ray dichroism of early 5d impurity atoms dissolved in ferromagnetic Fe

Polarization-dependent x-ray absorption is now going to be established as a new probe to study the magnetic and electronic structure of multicomponent systems. New experimental results are presented for early 5d impurity atoms dissolved in ferromagnetic Fe. The observed spectra clearly reflect the antiferromagnetic coupling of the local spin magnetic moment of the early 5d impurity atoms to the magnetization of the Fe host. From the data it is concluded that the change to a ferromagnetic coupling occurs at a higher atomic number in the 5d transition-metal series than deduced earlier from neutron scattering measurements. This conclusion is supported by first-principle calculations for the absorption spectra, which are based on the spin-polarized relativistic Korringa–Kohn–Rostoker–Green function method of band-structure calculation. The theoretical approach presented here, which has recently been applied to calculate the spectra of some late 5d impurities in Fe, results in a very satisfying agreement with experiment and allows a detailed, parameter-free discussion of the observed spectra.

Heavy ion beam induced attenuations of transient magnetic fields

Measurements of transient magnetic fields for Coulomb-excited 28Si(2 +) and 62Ni(2 +) ions recoiling in Fe(Gd) host with high and low velocity, respectively, gave clear evidence that the field strengths are substantially reduced due to momentary loss in ferromagnetism when using Ni-beams for excitation. There are strong indications that many earlier measurements suffered from the same effect and must be therefore reviewed.

Gyromagnetic ratios of excited states in 103Rh

Transient-field precessions were measured simultaneously for levels in 103Rh and the first 2 + states in even Pd isotopes as their ions traversed polarized Fe hosts, Gyromagnetic ratios were inferred for the negative-parity levels in 103Rh up to an excitation energy of ∼1 MeV. E2/M1 multipolarity mixing ratios were determined and/or confirmed for some transitions from measured unperturbed particle-γ-ray angular correlations. The measured static and dynamic electromagnetic moments are compared with present interacting boson-fermion model (IBFM) calculations which reproduce the main features of the spectrum of both the positive- and negative-parity levels in 103Rh below an excitation energy of ∼1 MeV.

Disparity in transient field characteristics for Au ions traversing Fe and Gd hosts through measurements ofg-factors in197Au

Transient field precessions were measured for levels in the nuclides of182,184,186W and197Au as their ions simultaneously traversed polarized Fe and Gd hosts. Consistent sets of gyromagnetic ratios were inferred for the lowest 5/2+ and 7/2+ states in197Au using these two ferromagnetic host media, provided that (i) the transient field for Au ions in Gd is calibrated using the W in Gd precessions (simultaneously measured) and, (ii) the transient field for Au in Fe is calibrated using the field strength and velocity dependence for Pt in Fe (previously measured). The presentg-factor results are: g(5/21+;197Au)=0.296±0.023, g(7/21+;197Au)=0.241±0.021, and g(5/21+;197Au)=1.2±0.2.

Transient fields for W ions traversing Fe hosts and for Os ions traversing Fe and Ni hosts

Transient field strengths were measured for184, 186W ions traversing thin, magnetized Fe foils with velocities in the range 1.8 ≲ υ/υ0 ≲ 5.7 (υ0 = Bohr velocity) and for188, 190, 192Os ions traversing polarized Ni hosts with average velocities 〈υ/υ0〉 ∼ 4. The present measured transient field strengths, together with previously measured results for W, Os ions, are compared with transient field strength parametrizations, and discussed in terms of microscopic models of the transient field.

Measurements of theg-factor ratio of the first 3? and 5? states in40Ca and the lifetime of the 5? state

[Theg-factor ratio of the first excited 3− and 5− states in40Ca was measured to beg 3/g 5=1.01(10) employing the implantation perturbed angular correlation technique. The static hyperfine fields (SF) in Fe and Gd hosts were used. In addition the lifetime of the 5− state was measured to be τ=426(7)ps. The values of the SF in Gd and Fe hosts were deduced and compared with systematics in this element region.

Transient-field strength measurements for52Cr traversing Fe hosts at high velocity and polarization transfer mechanisms

Transient-field strengths were measured for52Cr ions traversing polarized Fe hosts at velocities up to 12υ0 (12υ0 =c/137 = Bohr velocity). The results are compared with predictions of various transient-field parametrizations and discussed in terms of possible mechanisms by which polarization might be transferred from the Fe host to inner vacancies of the moving Cr ions. Theg-factor of the first 2+ state of52Cr was also measured by the transient-field technique and found to be in accord with shell model calculations.

Transient magnetic fields at Ne ions in Fe and Gd hosts and a critical assessment of 20Ne(4 +) g-factor measurements

A large transient magnetic field (TF) of B TF = 621(108) T was found for Ne ions traversing Gd at v ion = 3.1v 0 (v 0 = c 137 ) . The measurements were performed using the (α, α') reaction on Ne targets. Contrary to a previous measurement the beam-bending at the 2 + and 4 + resonances in the 12C( 12C, α) reaction at bombarding energies of 34.7 MeV and 32.6 MeV, respectively, was found to be the same and small in magnitude. A critical assessment of all g-factor measurements of the 20Ne(4 +) state is given.