Nuclear orientation of82Br implanted into Fe

Nuclear orientation measurements are used to determine the hyperfine hamiltonian for⁸²Br implanted into Fe single crystals (dose 5×10¹⁴/cm^–2 implant energy 80 keV). Using a model based upon channeling measurements a good fit to the temperature-dependent gamma anisotropy is obtained for a pure magnetic interaction experienced by the 36±5% of Br which implants substitutionally of magnitudeB _hf (substitutional)=840±120 kG, withB _hf (non-substitutional)<150 kG. This hamiltonian is used to deduce unknown multipole mixing ratios in the daughter⁸²Kr decay. Hyperfine field systematics are shown to indicate a substitutionalFeBr field of 1000 kG, and the origins of this field and the smaller interstitial interaction are discussed.     

Nuclear orientation of154EuFe,166TmGd and169YbFe and169YbGd

Nuclear orientation measurements at low temperatures have been carried out on radioactive isotopes of¹⁵⁴Eu and¹⁶⁹Yb in Fe and of¹⁶⁶Tm and¹⁶⁹Yb in Gd. Gamma-rays from¹⁶⁹YbGd, obtained by the decay of implanted and melted sample of¹⁶⁹IuGd, were observed to have smaller anisotropies of opposite signs compared with those from¹⁶⁹YbFe. This may indicate that for Yb polarized in Gd electric quadrupole interaction is comparable with magnetic dipole one. Mixing ratios of the transitions in¹⁵⁴Gd,¹⁶⁶Er and¹⁶⁹Tm have been made for two levels in¹⁶⁶Er. Our analysis showed that the 3^– to 3⁺ beta decay to the 1128 keV level in¹⁵⁴Gd has 65(20)% j=0.     

Combination of emission channeling,photoluminescence and Mössbauer spectroscopy to identify rare earth defect complexes in semiconductors

Implanted radioactive ¹⁶⁷Tm / ¹⁶⁷Er and ¹⁶⁹Yb / ¹⁶⁹Tm impurities in Si and GaN were studied with emission channeling and photoluminescence spectroscopy. The effect of co-doping with oxygen on the rare earth (RE) lattice sites and their luminescence behavior was investigated. Tm and Yb occupy near-tetrahedral sites in Si and substitutional sites in GaN after room temperature implantation and annealing. O-RE complexes are formed upon co-doping with O resulting in modified luminescence signals. RE impurities remain substitutional in O-doped GaN, but are displaced from tetrahedral sites in O-doped Si. We discuss the feasibility of Mössbauer studies using ¹⁵¹Eu, ¹⁶⁹Tm and ¹⁶¹Dy to determine the RE valence state and to identify RE defect complexes.

     

Magnetic hyperfine fields in ultrathin Ni films on Cu(100)

The magnetic hyperfine field was measured at ¹¹¹In(¹¹¹Cd) probe atoms in ultrathin Ni films epitaxially grown on Cu(100) utilizing the perturbed -angular correlation (PAC) method. The behaviour of the hyperfine field as a function of temperature was studied for different film thicknesses ranging from 2 up to 10 monolayers. It was found that the strength of the hyperfine fields as well as the critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness. The orientation of the hyperfine field is discussed.     

Impurity lattice location and recovery of structural defects in semiconductors studied by emission channeling

Doping of semiconductors by ion implantation usually requires implantation doses below 10¹³ cm^–2 to obtain typical impurity concentrations of <10¹⁸ cm^–3. The lattice location of impurities as well as the defect recovery after such low dose implantations can be studied using the emission channeling technique. In this technique, single crystals are doped with radioactive probe atoms and the channeling effects of electrons, positrons or -particles emitted from these atoms are measured. We present a quantitative analysis of electron emission channeling measurements after heavy-ion implantation into Si and III–V compound semiconductors by comparison with calculated channeling profiles based on the dynamical theory of electron diffraction. For In atoms implanted into Si, complete substitutionality was found after rapid thermal annealing to 1200 K. For lower annealing temperatures, the observed channeling effects indicate small mean displacements (of about 0.2 Å) of the In atoms from substitutional sites, caused by residual implantation defects. For GaAs, GaP and InP implanted at low temperatures with In or Cd isotopes, pronounced recovery stages around 300, 400 and 350 K, respectively, were observed and substitutional fractions close to 100% were derived after annealing above the stage.     

Unique magnetic behaviour of TmFeAl

The magnetic properties of the compound TmFeAl have been studied using⁵⁷Fe and¹⁶⁹Tm Mössbauer spectroscopy, magnetic measurements and neutron depolarization (ND). The compound exhibits a complex magnetic behaviour. At 4 K it contains a Tm moment of 7 _B and a distribution of Fe moments with an average of 0.5 _B. Between 10 and 60 K a distribution in magnetic ordering temperatures is observed. ND displays the growth of small magnetic domains when the temperature decreases towards 4 K. The structural disorder of the Fe atoms in the lattice combined with the competing exchange interactions between the different types of atoms and the high anisotropy of the Tm moment gives rise to the observed magnetic behaviour. Results of high field magnetization and⁵⁷Fe Mössbauer spectroscopy are presented also for GdFeAl and YFeAl.     

Perturbed angular correlation and Mössbauer study of the hydrogen distribution near Ru,Rh and Pd in palladium hydride

Mössbauer experiments with the 90 keV resonance of⁹⁹Ru as well as PAC measurements using the 353 and 528 keV - 90 keV cascades in⁹⁹Ru and the 84 keV - 75 keV cascade in¹⁰⁰Rh were combined in a study of the hyperfine interactions produced by the hydrogen distribution near Ru, Rh and Pd atoms in -PdH_x (0.6x1.0). The electric field gradients produced by the non-cubic nearest neighbour hydrogen distribution were found to be very small. This shows that the protons are very strongly shielded and justifies an interpretation of the Mössbauer spectra in terms of isomer shift distributions only. By combining the two methods one thus can obtain a rather detailed picture of the hydrogen distribution near the probe atoms.     

Magnetic interaction in Tm3Fe5O12 studied by means of169Tm and57Fe Mössbauer spectroscopy

The temperature dependence of the hyperfine parameters of thulium iron garnet (Tm₃Fe₅O₁₂) powder was studied from 90 to 550 K using¹⁶⁹Tm and⁵⁷Fe Mössbauer spectroscopy (MB). The spectra were analyzed by least mean square fits to the transmission function. The temperature dependence of the magnetic fields of the thulium nuclei is well described by the mean field model. The coupling constants between the magnetic lattice occupied by the thulium atoms and the magnetic lattices occupied by the iron atoms were derived.     

Hyperfine interactions of 111Cd in TiO2 (Rutile) studied by perturbed angular correlations

The time-differential perturbed angular correlation for ¹¹¹Cd nuclei has been measured after ¹¹¹In implantation into polycrystalline TiO₂. The observed perturbation functions are characterized by a well-defined electric field gradient with the quadrupole coupling constant ν_Q = 105(1) MHz and the asymmetry parameter η = 0.18(1). We attribute these hyperfine parameters to ¹¹¹Cd on the (distorted) substitutional cation site in rutile. The PAC results are compared with those in SnO₂ rutile as well as with X-ray diffraction, RBS channeling experiments and point charge model calculations including relaxation of the probe atom surrounding.     

Search for resonant absorption of solar axions by atomic nuclei

Search for resonant absorption of solar axions by ⁵⁷Fe and ¹⁶⁹Tm nuclei, leading to the excitation of the first nuclear level: ⁵⁷Fe: A + ⁵⁷Fe → ⁵⁷Fe* → ⁵⁷Fe + γ and A + ¹⁶⁹Tm → ¹⁶⁹Tm* → ¹⁶⁹Tm + γ, has been performed. To this end, the energy spectra of the Si(Li) detector with ⁵⁷Fe and ¹⁶⁹Tm targets have been measured. The detector and targets were located in a low-background setup equipped with passive and active shields. A new upper limit on the axion mass: m _A ≤ 330 eV (90% confidence level) has been established from the spectrum of ⁵⁷Fe (30 days). Measurements with a ¹⁶⁹Tm target during 8 days gave a new upper limit on the product of the axion-photon coupling constant and the axion mass: g _Aγ (GeV^?1) × m _A (eV) ≤ 2.9 × 10^?4 (90% confidence level).     

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.     

Emission channeling

Hyperfine interaction techniques like Mössbauer effect or perturbed angular correlation are commonly applied to study the structure and properties of impurity-defect complexes in solids. It is often difficult to resolve a certain defect structure unambiguously with these techniques, because an absolute determination of the lattice site of the probe atoms is not straight-forward. The emission channeling technique allows the direct determination of lattice sites of radioactive impurity atoms, incorporated into single crystalline solids. The channeling effects of electrons, positrons or alpha particles, emitted from radioactive impurities are measured along different crystal axes and planes. From the measured anisotropic emission distributions the lattice sites of the emitting atoms can be determined. Emission channeling can be applied to a large variety of different probe atoms. Also, rather low impurity concentrations, comparable to those typically required for hyperfine interaction techniques, are sufficient. In this contribution, the principles of the emission channeling technique, the experimental requirements and the quantitative analysis of emission channeling spectra are reviewed. The capabilities and possibilities, which the emission channeling technique offers, are highlighted by three recent experimental studies. First, studies of the diffusion of Ag in CdTe using transmutation doping with the electron emitting isotopes^107mAg and^109mAg are described. Second, lattice location studies of As in diamond, which is a potential n-type dopant in this material, will be discussed. Third, an experiment is described to study the lattice location of oversized impurities after low dose implantation into Fe. In this experiment, the unique decay properties of²²¹Fr and²²¹ Ra are utilized to determine the lattice sites of five different impurity atoms in a single emission channeling measurement.     

Paramagnetic fluctuations in Yb3Ga5O12 investigated with 172Yb PAC probe

¹⁷²Yb PAC measurements were carried out on Yb₃Ga₅O₁₂ at temperatures ranging from 14 to 1100 K. The time dependent hyperfine interaction is clearly evident below room temperature. It is attributed to spin fluctuations of Yb ion in the ground state of the ²F_7/2 multiplet. Above 500 K, the temperature dependence of the electric field gradient is shown to result from a varying population of the crystal electric field levels. This revised version was published online in August 2006 with corrections to the Cover Date.     

implantation of Bi into GaP. II. channeling studies

The channeling technique has been used to investigate the properties of Bi-implanted Gap. Measurements of the crystal disorder for 100 keV room temperature implants indicate a damage vs dose curve corresponding to ～13000 displacements/ion in the linear region and saturation at ～1.5 × 10¹³ Bi ions/cm². Annealing of the radiation damage has been observed and indicates two annealing steps at ～450°C for light damage and ～750°C for implants in the 1 × 10¹⁴/cm² range. Difficulties associated with the thermal decomposition of the implanted area have been overcome with the use of SiO _x coatings. The experimental details associated with the use of the SiO _x layer and with the use of a C¹² beam to obtain better depth and mass resolution in the backscattering spectrum are discussed. The lattice location measurements of the Bi impurity show ～50 per cent of the Bi atoms to be along the 〈110〉 string after a 900°C anneal for a 7.5 × 10¹³/cm² implant. In addition, the spectra show ～25 per cent of the Bi atoms have diffused to the surface. Correlations of these lattice location results with measurements of the photoluminescent intensity of the GaP (Bi) isoelectronic trap show an agreement in trend with anneal temperature but indicate a factor of ～10 more substitutional ions in the channeling measurement as compared to the photoluminescence results.     

Apparent giant hyperfine anomaly of nuclear levels in the indium isotopes — A puzzle

The magnetic hyperfine fields of In in Fe, Co and Ni were measured by PAC using¹¹⁷In, and by NMR-ON using^114mIn and¹¹⁴In. There is a disparity of about 8% (4%) between the fields derived from PAC on¹¹⁷In (NMR-ON on¹¹⁴In) and the values that have been measured using isomeric 1g_9/2 proton states in^109,111,115In. This is tentatively attributed to a giant hyperfine anomaly.     

Local spin correlations in partially frustrated amorphous Fe-Mn

Mössbauer measurements in a-(Fe_0.765Mn_0.235)₇₈Sn₂Si₆B₁₄ have been performed with⁵⁷Fe and¹¹⁹Sn. Both hyperfine fields show a kink near 70 K, indicating a freezing of the transverse spins. The ratio of the two hyperfine fields contains information about the correlation of the transverse spins. The ratioB _hf(Sn)/B _hf(Fe) stays constant aboveT _xy and rises belowT _xy . This indicates an alignment of the transverse spins on a local scale, as was also found inAuFe and a-FeZr.     

Spatial spin-modulated structure and hyperfine interactions of 57Fe nuclei in multiferroics BiFe1–x T x O3 (T = Sc,Mn; x = 0, 0.05)

The results of the Mössbauer studies on ⁵⁷Fe nuclei in multiferroics BiFe_1–x T _x O₃ (T = Sc, Mn; x = 0, 0.05) in the temperature range of 5.2–300 K have been presented. The Mössbauer spectra have been analyzed in terms of the model of an incommensurate spatial spin-modulated structure of cycloid type. Information has been obtained about the effect of the substitution of Sc and Mn atoms for Fe atoms on the hyperfine parameters of the spectrum: the shift and the quadrupole shift of the Mössbauer line, the isotropic and anisotropic contributions to the hyperfine magnetic field, and also the parameter of anharmonicity of the spatial spin-modulated structure.     

Effects of nitrogen and vanadium on the57Fe hyperfine field of RFe12−x V x N y (R=Y,Nd)

Magnetization and⁵⁷Fe Mössbauer measurements were carried out on RFe_12–x V _x N _y compounds (R=Y and Nd,x=1.7 and 2.2) and the effects of nitrogen and vanadium atoms on the⁵⁷Fe hyperfine fields at the different iron crystallographic sites were investigated. The hyperfine field decreases with increasing number of vanadium neighbour atoms at all the iron sites. The hyperfine field is strongly enhanced in the nitrogen composition withy>1 where the compound tends to transform into an amorphous-like solid. The iron moment deduced from the hyperfine field increases more upon nitrogenation for the 8i-site than for the other sites, and exceeds the moment of bcc iron.     

Mössbauer study of the magnetic and electric hyperfine interaction of dilute57Fe impurities in rare earth metal hosts

The hyperfine interaction of dilute⁵⁷Fe in the rare earth (RE)metals Gd to Lu was investigated by Mössbauer measurements with⁵⁷Co doped RE sources. In all hosts well split, 2-lines spectra were observed at room temperature, with slight asymmetries of the line intensities in some cases. The quadrupole splitting eQV_zz/2 increases from 0.29 mm/sec for Gd to 0.50 mm/sec for Tb, and decreases by less than 10 % between Tb and Lu. Only about 10 % of the corresponding electric fieldgradient (EFG) can be accounted for by the ionic EFG on a substitutional RE site. The temperature dependence of the EFG was measured in the case of Tb. No variation within 3 percent was found between 300 K and 700 K. Measurements of the magnetic hyperfine interaction at low temperatures were carried out in Tb. The saturation field of⁵⁷Fe in this host is H_hf(FeTb;4.2 K)=25(2) KOe. The temperature dependence of the magnetic hyperfine field does not follow the host magnetization (T_c=220K) but vanishes at about 80 K. Similar anomalies of H_hf(T) have previously been observed for other transition element impurities in the RE ferromagnets.