Michio Shimotomai

Study of vacancies in the intermetallic compound Ni3Al by positron annihilation

Specimens of the intermetallic compound Ni3Al were irradiated by 2 MeV electrons at room temperature or quenched from 900 degrees C to about 1300 degrees C and subsequently annealed isochronally. The behaviour of the vacancies was studied using the positron lifetime. The vacancies annealed out around 215 degrees C. The formation energy of a vacancy was determined to be EvF=1.6+or-0.2 eV. The activation energy for the motion of the vacancy was estimated to be EvM=1.2+or-0.2 eV. The annealing of the vacancies was found to be a second-order reaction.

Magnetic characteristics of Laves phase compound PrFe2

The magnetic properties of Laves phase compound PrFe2, synthesised at high pressure, were studied by Mossbauer spectroscopy and saturation magnetisation. The ferromagnetism in PrFe2 has been confirmed. The rotation of the easy axis of magnetisation on the (110) plane to (111) with respect to (001) was observed as temperature increased. The authors have calculated the single-ion crystalline field magnetic anisotropy and have found this model to account for the experimental results in principle.

Characterization of Graphites by Positron Lifetimes

Positron lifetimes have been measured in different types of graphites such as glassy carbon, nuclear graphite, pyrolytic graphite and natural graphite. An analysis based on the trapping model shows that positrons preferentially monitor structural defects such as the internal surfaces between crystallites and surfaces around pores or voids. The lifetime of free positrons delocalized in the perfect graphite lattice is estimated to be 215?10 ps. The lifetime of 400?20 ps, observed in well-crystallized pyrolytic graphite and natural graphite, is interpreted as the lifetime of positrons trapped at the internal surfaces between crystallites. It is possible to characterize different types of graphites by the positron lifetime data.

Vacancy in Graphite: Positron Srudies

The positron techniques of lifetime and Doppler-broadening were applied to vacancies in graphite. Vacancies were introduced into highly-oriented pyrolytic graphite by irradiation with 2 MeV electrons and fission reactor neutrons. The lifetime of positrons trapped at the vacancies was determined as 245±5 ps. Lifetime studies revealed two distinct vacancy annealing stages at 600–900°C and at 1300–1500°C. The Doppler-broadened lineshapes of the annihilation gamma-ray were taken in the directions perpendicular and parallel to the c -axis of the graphite lattice. Momentum distribution for an electron-positron pair at a vacancy was deduced and compared with the results of calculation based on the defect molecule model in the framework of LCAO-MO theory.

Crystal-field effects in Nd2Fe14B compounds

Abstract An attempt has been made to treat the rare-earth contribution to the magnetocrystalline anisotropy in Nd2Fe14B with a single ion model using a Hamiltonian of the form H = λL · S + H CF + 2 μBHex · S. It is found that Nd+3 ions on all the four inequivalent sites prefer the uniaxial anisotropy through the combined action of crystal-field and exchange field.

Mobilities of quenched-in defects in NiAl

Abstract Abstract Samples of the intermetallic compound NiAl have been quenched from 800–1400°C and their recovery behaviour studied by positron annihilation techniques. Two distinct recovery stages were found by Doppler broadening measurements; one at around 280°C and the other at around 500°C. It is shown that vacancies anneal out at the latter stage. The former stage is ascribed to the migration of divacancies to form voids.

Lattice parameter evidence of intermediate valence of Pr atoms in laves phases PrxR1−xFe2 (R = Sm, Tb and Y)

Abstract The pseudo-binary Laves phases Pr x R 1− x Fe 2 (R = Ce, Sm, Tb and Y) were prepared using high pressure technique for the first time. Lattice parameters for Pr x Sm 1− x Fe 2 , Pr 2 Tb 1− x Fe 2 and Pr x Yin1− x Fe 2 show negative deviation from Vegards linear relationship. We have discussed the results in terms of intermediate valence of Pr atoms under lattice pressure.

Phil. Mag. Letters: Differential scanning calorimetric study of quenched-in vacancies in NiAl

Abstract Differential scanning calorimetry on specimens of monocrystalline Ni-47·3 at.% Al, quenched from 1400°C, has revealed annealing stages at 320 and 480°C in good agreement with the previous positron annihilation study. The activation energy and the order of reaction for each stage have been determined by a kinetic analysis. The low-temperature stage is second order, while the high-temperature stage is first order.

Magnetic properties of laves phases PrxSm1-xFe2

Abstract The pseudo-binary Laves phases Pr x Sm 1- x Fe 2 were prepared and their magnetic properties were studied by the Mossbauer effect and magnetization measurements. The usual single-ion crystalline electric field model gives a good account of the magnetic anisotropy up to x = 0.42.

Correlation of57Fe hyperfine interactions with Fe-Fe distance in Laves phase compounds RFe2 (R=rare earth)

The57Fe Mössbauer isomer shifts and magnetic hyperfine fields in Laves phase compounds RFe2 (R=Pr, Nd and Sm) are studied with particular reference to the effect of the Fe−Fe interatomic distance on the hyperfine interactions. It is shown that the charge density at the Fe nuclei scales linearly with fractional volume change up to 20%. The57Fe hyperfine field corrected for the influence of rare-earth moment shows a systematic variation with the distance, which can be understood in terms of the Bethe-Slater curve arguments. The similarity of the atomic volume dependence of the57Fe hyperfine interactions in Lves phase compounds to those in iron with close-packed structure is emphasized.