Magnetic nanostructures: radioactive probes and recent developments

We have investigated the magnetic properties of Cd atoms decayed from 111In at the interfaces of Fe/Ag evaporated films. In order to determine the magnetic properties of the films, we have measured 111Cd(←111In) time differential perturbed angular correlation (TDPAC) spectra at room temperature as a function of distance from the interface. Fe and Ag were evaporated using electron beam evaporation, and small amounts of radioactive 111In were evaporated by resistance-heating. The evaporated films had the following stacking orders on kapton polyimide film substrate: (1) polyimide/Fe 50 nm/111In/Fe x nm/Ag 20 nm (x=0.2,0.3,1,3,5,10), (2) polyimide/Fe 50 nm/Ag x nm/111In/Ag 20 nm (x=0,0.2,1). We used an Ag layer as a cap layer in order to prevent oxidation in air. TDPAC spectra were measured using a four-detector arrangement. The hyperfine magnetic field tends to be slightly larger when 111In is close to the interface of the Fe/Ag layer. The intensity of perturbation damps rapidly as 111In is close to the interface of Fe and Ag. These behaviors are thought to depend on electric field gradients and the influence of the surface roughness.

The magnetic hyperfine field at dilute99Ru impurities in ferromagnetic Gd has been investigated as a function of temperature by time differential perturbed angular correlation (TDPAC) measurements. The saturation field at 11 K is Hhf(Ru Gd)=51(2) KOe. This value fits well into the systematics of 4d impurity hyperfine fields in Gd. The magnetic hyperfine field of RuGd does not follow the magnetization of the host (Tc=290 K) but vanishes abruptly at about 70 K. A similar behaviour has previously been observed for the 5d impurity Os in Gd. From first TDPAC measurements of the hyperfine interactions in the intermetallic phases of the Ru-Sc system it can not be completely ruled out that the observed collapse of the hyperfine field at 70 K is due to the formation of the intermetallic compound RuGd3.

TDPAC study of the Hf-doped LaNi 5-hydrogen system

The time-differential perturbed angular correlation (TDPAC) method was applied to a study of the local environment at the 111Cd(←111In) probe nucleus introduced in 0.5 at. % Ga-doped ZnO. The TDPAC spectrum obtained at room temperature suggests that the doped Ga ions reside at the substitutional Zn site and they are locally associated in immediate proximity to the probe, reflecting strong attractive force between the probe and the dopants. The relative width of the magnitude of the electric field gradient produced at the probe nucleus shows that the doped Ga ion(s) adjacent to the probe is/are diversely distributed compared with doped In ion(s).

X-ray and time differential perturbed angular correlation measurements in ZrCr 2 and ZrCr 2H 3 Laves phase compounds

We use the linear muffin-tin orbital formalism, in the atomic sphere approximation, to investigate the trends of the electric field gradient (EFG) at the nucleus for the non-equivalent sites in Zr2T (T = Fe, Co and Ni) intermetallic compounds. As all those compounds crystallize in the same C16 crystallographic structure, they offer a rare opportunity to investigate electronic structure effects coming from transition metals on the EFG at Zr site. Those results are compared with EFG values obtained from quadrupole coupling constant measurements performed with the time differential perturbed angular correlation (TDPAC) technique, using the 191Ta probe.

In this work, we report on the hyperfine parameters of the foreign 181Ta probe in the rutile structure of the single crystal TiO2 using the e−γ and γ−γ time differential perturbed angular correlation (TDPAC) technique. We implanted 181Hf ions into a sample of single crystal rutile TiO2 in the Bonn Isotope Separator. The implanted sample was then thermally annealed at a temperature of 873 K for 315 min in a vacuum. The 181Hf radioisotopes decayed by β− emission, followed by a cascade to the ground of γ rays or conversion electrons into a stable state 181Ta. The 181Ta probe substitutes the Ti lattice site with a unique nuclear quadrupole interaction, allowing for the precise measurement of the largest electric field gradient (Vzz) and asymmetry parameter (η). The hyperfine parameters obtained from the e−γ TDPAC spectroscopy agree with those of the γ−γ TDPAC spectroscopy at room temperature, apart from a calibration factor, both from our experiments and the literature. This suggests that the electronic recombination following the internal conversion of the L shell electron takes less time (ps) than the intermediate lifetime of the metastable 181Ta state (ns).

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.

The electric field gradient (EFG) at the 181Hf→181Ta site in Ti2Rh C11 b -type compound was measured as a function of temperature using time-differential perturbed angular correlation (TDPAC) technique. The room temperature results show one EFG with the parameters of: ν Q =336(1) MHz→V zz =5.9×1017 V cm−2, η=0.1. Very week linear temperature dependence of this EFG was measured with the slope of 3.6 (2)×10−5 K−1. The results are compared with those for other isostructural compounds.

The contributions of 4f, 5d and 6s electrons to the saturation magnetic moments and magnetic hyperfine fields in the heavy rare earth metals are calculated using the model described in the previous paper. It is found that 4f shell moments are reduced from their free ion values by amounts varying from 0.05µB in Gd to several tenths of a Bohr magneton in Tb and Dy, in qualitative agreement with a recent published analysis of neutron diffraction results in Tb, but that the calculated total saturation moments in Tb and Dy are slightly larger than commonly accepted experimental values. After 6s contributions to magnetic hyperfine fields are determined by fitting observations in Gd, the predicted differences between the fields for metals and those for free ions are such that the estimated uncertainty ranges of the theoretical values overlap the experimental ranges. The 5d contribution in the model is negative, varying from about −40 kOe in Tb to −200 kOe in Er. Electric field gradients are also analysed. Observed results can be fitted if the average effective Sternheimer screening factorRd* for 5d electrons in the metals satisfies (1 —Rd*)≈0.7.

Exchange and hyperfine fields acting on dilute 153Eu In SmCo 5, Sm 2(Co 1-xFe x) 17 and Sm 2Co xM y( rmM = Fe, Cu, Zn, Zr)

Time differential perturbed angular correlation measurements were carried out using short lived isomeric 5/2+ state in 147Pm to investigate the magnetic and electric hyperfine interactions in Nd and Gd hosts at different temperatures. At 10 K the magnetic hyperfine fields at 147Pm in Nd and Gd hosts are 361(42) kG and 256(30) kG, respectively, and are very low as compared to the free-ion value while the electric field gradients are of comparable order. The magnetic hyperfine field in Gd is constant with temperature. The results are interpreted in terms of singlet ground state of Pm ion.

The investigations of the hyperfine interactions in Pt3Fe with the noncollinear magnetic order by means of ab initio methods are presented. It is shown that the ground state magnetic order of Pt3Fe is of noncollinear character. Electric field gradient and hyperfine magnetic field at 57 Fe nucleus were calculated and their dependence on the strength of Coulomb correlation interaction is discussed. The calculated values of electric field gradient correspond with the experimental data and the best agreement was found when the enhanced Coulomb correlation interaction for 3d-states of Fe was considered. Lattice, local valence electron (3d, 4p) and weakly bound 3p semicore electron contributions to electric field gradient are separated out and discussed.

Surface hyperfine magnetic field in nickel

PAC measurements on 111In(111Cd) implanted and thermally treated α-Fe have shown an indication for a cubic defect with the 111Cd probe in the centre of it. The measured room temperature (R. T.) magnetic hyperfine fields are B hf1 = −38.4(8) T for substitution and B hf2 = +11.5(3) T for the cubic defect. Additionally, probes with pure quadrupole frequency distributions were observed, which are incorporated in surface contaminations.