Pac studies of spin deviations,pressure induced hyperfine field shifts,and temperature variation of supertransferred hyperfine fields

A PAC study of¹¹¹Cd substiuted antiferromagnetic transition metal salts is reported. Supertransferred hyperfine fields at¹¹¹Cd nuclei were used for the first time to observe three effects: zero-point spin deviations, temperature variation, and pressure shift of the hyperfine field. Comparison of KNiF₃ and RbMnF₃ with their corresponding quadraticlayer fluorides K₂NiF₄ and Rb₂MnF₄ yields an estimate for the magnitude of the zeropoint spin deviation in doped antiferromagnets. The temperature dependences of the Cd hyperfine fields in RbMnF₃/Cd and MnF₂/Cd have been determined. The shift in hyperfine field observed for -MnS/Cd under external pressure further supports our model for the origin of the supertransferred hyperfine fields.This work was supported by the US Energy Research and Development Administration.     

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.     

Excited levels of111In and113In

Levels and transitions in¹¹¹In and¹¹³In have been studied in the¹¹¹Cd(p, n)¹¹¹In and¹¹³Cd(p, n)¹¹³In reactions. By means of- coincidences,-angular distributions, relative excitation functions and conversion electron measurements more than 20 levels below 2 MeV have been established in each nucleus. Several negative-parity states were found above 1 MeV. Two low-spin positive-parity states with features similar to the possible rotational band in the heavier In nuclei are observed below 1,400 keV in both nuclei.     

Quadrupole interaction at111Cd nuclein in single-crystalline thallium

The quadrupole interaction of¹¹¹Cd nuclei in a single crystal of hexagonal thallium has been investigated for different geometries and sample temperatures employing the perturbed - angular correlation technique. The observed variation of the electric field gradient with temperature is attributed to local modes of thermal vibrations of the Cd impurities. An analytical relationship has been derived from lattice-dynamical models, expressing the temperature dependence in terms of impurity-host mass ratios and local force-constants.     

Precision PAC measurements in Er2O3 and Ho2O3 single crystals and structure refinement

The structure of C-form Ho₂O₃ and Er₂O₃ single crystals and powder samples was investigated by the electric quadrupole hyperfine interaction of¹¹¹In(EC)¹¹¹Cd probe ions using the perturbed - angular correlation method (PAC). The resulting set of refined atomic coordinates is compared to X-ray data and used to calculate the orientations of the electric field gradients (EFG) which are reproduced by the PAC measurements in single crystals. The temperature dependence of the coordinates was measured for both substances.     

DPAC measurement of the hyperfine fields in100RhCo and100RhFe

Using the 85–75 keV -cascade of¹⁰⁰Rh, we report the first DPAC measurement of the hyperfine field in¹⁰⁰RhCo and¹⁰⁰RhFe. Our results were made possible by continued improvement in time resolution using BaF₂ scintillators. At room temperature we obtained fields of 420.6(1.3) kG and 540.4(1.5) kG for¹⁰⁰RhCo and¹⁰⁰RhFe, respectively. The former revises an earlier spin echo result by Kontani and Itoh by 9%; the latter is consistent with the work of Matthiaset al. using resonant destruction of angular correlations.     

Defect trapping by100Pd in fcc metals

Lattice defects in Al, Cu, Ag and Au were studied by the perturbed angular correlation technique (PAC) using the probe atom¹⁰⁰Pd/¹⁰⁰Rh. The comparison of data obtained on interstitial trapping in Cu and Au at different probe atoms (¹⁰⁰pd,¹¹¹In) allows defect characterisation less affected by the respective probe. The trapping efficiency of¹⁰⁰Pd for vacancy like defects is quite different to that of¹¹¹In atoms.     

Damage studies on the system^{111} \mathop {In}\limits^ \to \underline {Ni}

Lattice damage after implantation of¹¹¹In in Ni has been studied applying the DPAC technique to the 171–245 keV -ray cascade in the daughter nucleus¹¹¹Cd. Implantations were carried out at 10 K and at 300 K. The low temperature implantation yields a higher regular substitutional fraction (80%) than the room temperature implantation (40%). The annealing behaviour of both implants above RT is the same. In addition, two distinct defect-associated sites were observed in isochronal annealing sequences. A microscopic model for these defects is presented, which takes into account magnetic and electric hyperfine interaction strengths, binding energies and site populations as a functions of temperature.     

Hyperfine field of156Gd in Tb(Zn1−xCux) with x=0, 0.36, 0.46

Time-integral - perturbed angular correlation measurements have been made on¹⁵⁶Gd nuclei in TbZn_1–xCu_x using two Ge(Li) detectors. The hyperfine fields at¹⁵⁶Gd nuclei in TbZn_1–xCu_x at 77 K are found to be –177±26, –104±24 and –27±38 kOe for TbZn, TbZn_0.64Cu_0.36 and TbZn_0.54Cu_0.46, respectively. These values indicate that the transferred hyperfine fields at Gd nuclei are positive due to the exchange interaction of s-f type.     

Structural changes in nickel alloys subjected to gamma irradiation in a gaseous medium

The structuring action of rays from⁶⁰Co and⁵⁷Co sources on dilute nickel alloys containing elements with a high affinity for oxygen was studied. It was shown that after irradiation in the alloys at room temperature there is a prolonged redistribution of the elements which is accompanied by the precipitation of dispersed oxides.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 75–82, February, 1996.     

Mössbauer effect study of charge and spin transfer in Fe-Cr

The influence of temperature and time of annealing on hyperfine fields and isomer shifts has been studied for a range of Fe–Cr alloys containing 1–45 at% Cr. It has been revealed that up to 15at% Cr neither time nor temperature of annealing practically does affect the hyperfine parameters. For more concentrated samples, however, both temperature and time of annealing are important. In particular, the Mössbauer spectrum of Fe–45.5 at% Cr annealed at 700°C for 5 h was a single-line indicating that the sample was paramagnetic. The observed changes of the hyperfine fields and the isomer shifts have been interpreted in terms of a spin and charge trasfer, respectively.Strong linear correlations between the following quantities have been revealed: the hyperfine field H(0,0) and the isomer shift IS(0,0); the average hyperfine field $\text{H}$ and the average isomer shift $\text{IS}$; the average hyperfine field $\text{H}$ and the average number of Cr atoms in the first two coordination spheres, $\text{N}$. It has been calculated from the first two correlations that a) a change of polarization of itinerant s-like electrons of one electron is equivalent to a change of the hyperfine field of 1602 kOe, and b) on average, a unit change of s-like electron polarization is equivalent to 3277 kOe. The two constant are very close to theoretical estimations, which can be found in literature. Correlation between the hyperfine field and the isomer shift led to a conclusion that the substitution of Fe atoms by Cr ones decreases the density of spin-up electrons on average by 0.026 electrons per one Cr atom in a unit cell. These electrons are most likely trapped by Cr atoms, because the hyperfine field at neighbouring Fe nuclei decreases and the density of charge at those nuclei increases at the rate of 0.029 electrons per one Cr atom in a unit cell.Based on the results obtained, a formula describing the dependence of the average hyperfine field on Cr contents and on the heat treatment has been postulated for Fe–Cr alloys.     

Magnetic properties of Pm in PrNi

A magnetic property of Pm as an impurity in PrNi single crystal was investigated by means of the low-temperature nuclear orientation of^143,144Pm. The angular distribution of -ray anisotropies revealed that the direction of hyperfine field experienced by the nuclei lie in the (a, c) plane andmade an angle of 60° (5) or 240° (5) with respect to the crystallinec-axis. From the temperature dependence of the anisotropies the strength of the hyperfine field of Pm in PrNi was deduced to be 185(22)T. The hyperfine field of Co at the Ni site of PrNi was found to be less than 4T.     

Supertransferred hyperfine magnetic fields at111Cd(←111In) in ferrimagnetic oxides with the spinel structure

The supertransferred hyperfine magnetic fields,H _STHF, at¹¹¹Cd(¹¹¹In) on the tetrahedral sites of the spinel oxides Fe_3-xM_xO₄ (M = Ni or Co) were measured by means of the time differential perturbed angular correlation. The observedH _STHF were analyzed as the sum of the fields transferred from individual B site magnetic ions: 11 kOe/Fe³⁺, 8 kOe/Fe²⁺, 4 kOe/Ni²⁺ and 4 kOe/Co²⁺. A brief discussion is presented on these results in comparison with previous ones onH _STHF at¹¹¹Cd(^111mCd) in rocksalt-type oxides and the one at¹¹¹in in NiFe₂O₄.     

Applications of γ-γ-Directional correlation measurements in studies of trace elements in plants

Perturbed --directional correlation measurements were used in a study of the uptake and transport of trace elements in plants. The results from experiments with^111mCd in plant cell systems, cytoplasm and intact tomato plants indicate that Cd, applied as Cd²⁺, is rapidly bound to cell walls.     

The magnetic hyperfine field at Hg impurities in ferromagnetic Gd

The magnetic hyperfine field at dilute Hg impurities in Gd has been investigated by the conversion electron (e ^–)--time differential perturbed angular correlation (TDPAC) technique. The radioactivities^197m Hg and¹⁹⁹Tl were implanted into Gd foils by means of an isotope separator. TDPAC measurements were performed with the 165 keV-L-conversion electron—134 keV--cascade of¹⁹⁷Hg at different temperatures and with the 334 keV--158 keV-K-conversion electron cascade of¹⁹⁹Hg at 200 K.The regular site occupation probabilities were found to be 15(3)% for an annealed^197m HgGd sample and 29(5)% in unannealed¹⁹⁹TlGd samples.From the magnetic hyperfine interaction frequencies measured for the regular sites at 200 K the magnetic hyperfine fields |H _hf(¹⁹⁷HgGd; 200 K)|=256(13) kG and |H _hf(¹⁹⁹HgGd; 200 K)|=267(7) kG were deduced.On leave from the University of Lisboa, Portugal     

Study ofγ radiation from100Pd decay

This paper presents a study of the rays from the electron capture decay of 3.63d ¹⁰⁰Pd. Singles and coincidence measurements provided an accurate determination of-ray intensities and an improved knowledge of the¹⁰⁰Rh level scheme. Experimental evidence supports excitation of levels in¹⁰⁰Rh at 86-, 136- and 154 keV, not previously reported from the decay of¹⁰⁰Pd.On leave from Departamento de Fisica, Comision Nacional de Energia Atomica, Buenos Aires, Argentina     

Nuclear orientation of48V in Fe and Co and NMR of48VFe

Observations of the anisotropies of the 984 and 1312 keV -rays from⁴⁸VFe and⁴⁸VCo in fields up to 4 T and temperatures down to 4 mK show extremely long nuclear spin-lattice relaxation times (3–4 h in Fe, 8 h in Co host) and indicate negative hyperfine fields in both hosts. The Fe measurements show a magnetic moment for the⁴⁸V 4⁺ ground state considerably larger than previously reported. This is confirmed by NO/NMR of⁴⁸VFe, giving (4⁺)=2.012(11) _N. The advantages of pulsed NMR techniques, especially in systems with slow relaxation, are discussed.     

Investigation of Hyperfine Interactions in CeIn3 byTDPAC

Magnetic hyperfine fields (mhf) at ¹¹¹Cd and ¹⁴⁰Ce nuclei, dilutely substituting the In and Ce sites, respectively, have been measured in the intermetallic compound CeIn₃ using perturbed angular correlation technique. A pure electric quadrupole interaction with an axially symmetric electric field gradient was observed at ¹¹¹In(EC)¹¹¹Cd probe nuclei at room temperature while a combined magnetic dipole and electric quadrupole interaction is observed below 10K. Below the ordering temperature, only a magnetic interaction is observed at ¹⁴⁰La(^–)¹⁴⁰Ce probe. The values of mhf measured experimentally as a function of temperature are discussed in terms of critical behavior.     

Super transfer in doped LaFeO3

The effects of doping the orthoferrite LaFeO₃ with Ga, Ni, Cr, Sc and Al on the hyperfine fields at the⁵⁷Fe nuclei are investigated and discussed. The dependence of the magnetic hyperfine field at 4.2 K on the kind of nearest neighbour can be described in terms of a supertransfer model based upon the LCAO method.     

Hyperfine interactions of interstitial12B in Fe,V and Ta studied by use of the asymmetric β decay

Magnetic hyperfine fields and spin-lattice relaxation times of interstitial and substitutional¹²B nuclei in Fe are studied in a temperature range, 100 K<T<1200 K, by use of the asymmetric decay from the spin polarized nuclei and NMR detection. In order to infer information regarding the location of¹²B nuclei and expansion in the nearest Fe surrounding following recoil implantation, hyperfine interactions of¹²B in non-magnetic bcc V and Ta crystals are studied.