Saburo Nasu

Hyperfine field distribution in Co-Cu alloy: 59Co nuclear magnetic resonance

Abstract The spin-echo spectra at 77 K for the Co-1, 3, 5 and 8% Cu alloy solid solution consist of a sharp main line at 216.9 MHz, and one or two satellite lines which are centered at 199 MHz and 180 MHz respectively. The main line corresponds to the sharply defined NMR line in the pure f.c.c. Co metal, but its center frequency changes slightly with the Cu concentration. The two satellite lines correspond to the 59Co NMR associated with perturbed Co atoms which have one and two Cu impurities in their nearest neighbor sites, respectively; resonance frequencies of these satellite Unes are in agreement with those estimated from empirical equation. The integrated intensity ratios of the satellite lines to the main line were in disagreement with the values estimated previously reported relations I 1 I 0 = 12c (1 − c) and I 2 I 0 = 66c 2 (1 − c) 2 . These intensity ratios were fairly expressed with the following relations as I 1 I 0 = 2c (1 − c) and I 2 I 0 = 11c 2 (1 − c) 2 . The concentration dependence of the resonance frequency of the main line is also discussed.

Mössbauer evidence against the existence of magnetically “dead” layers

Abstract 57 Fe Mossbauer spectrum was measured for 57 Co source deposited on the surface of cobalt metal by electrolytic means. The result showed that the d-spin polarization in the surface is not decreased and it was evidenced that the magnetically “ead” layers do not exist.

High-Pressure 57Fe Mössbauer Spectroscopy of LaFeAsO

The electronic properties of an oxypnictide, LaFeAsO, pressurized in a diamond-anvil cell, were investigated by 57 Fe Mossbauer spectroscopy and electrical resistance measurements at pressures up to 24 and 35 GPa, respectively. The Neel temperature gradually decreased from ∼140 K at 0.1 MPa to ∼50 K at 20 GPa, and fell below 8 K or disappeared at 24 GPa. The hyperfine field at 8 K decreased from 5.3 T at 0.1 MPa to 2.2 T at 20 GPa. On the other hand, the onset of superconductivity occurred at ∼9 K at 2 GPa. The superconductivity peaked at ∼21 K at 12 GPa, and then began a perceptible decline, disappearing at ∼35 GPa. This suggests that suppression of antiferromagnetic order plays an important role in the emergence of pressure-induced superconductivity.

Nuclear magnetic resonance of Co69 nuclei in precipitated cobalt particles

Abstract The Co 59 nuclear magnetic resonance signals in precipitated Co-rich particles have been successfully observed by a transient (spin-echo) NMR method using heat-treated Cu-3.52 at. % Co alloy powders. The observed resonance spectra consist of a main intense sharp line and a week satellite line, and the resonance frequencies of both lines at 77°K are 217 ~ 219 MHz, depending on the heat treatment, and 198 MHz respectively. The resonance frequency of the main line indicated that the crystal structure of the precipitated particles is face centered cubic as has been expected. Careful measurements of the center frequency of the main line make it possible to discuss the internal pressure, acting on the precipitated particles, as a function of the effective particle size. The satellite line is attributed to the Co atoms located on the nearest neighbor sites of the Cu atoms dissolved in the precipitated particles. From the intensity ratio of the main line to the satellite line, the Cu concentration in precipitated particles has been estimated to be an extremely small value of 0.5 % and is independent of the aging time and the particle size. This is probably the first report of the NMR study of the precipitated particles and the pulsed NMR technique is found to be one of the most effective methods to get microscopic and detailed informations of the precipitation phenomena.

Deconvolution of Transmission Mössbauer Spectra Using Conjugate Gradient Method

A numerical method to extract the absorber resonance cross-section from the transmission Mossbauer spectrum has been presented. The convolution equation which relates to both of the transmission spectrum and the cross-section is expressed by linear algebraic equation and solved by conjugate gradient method. Applying this method to the examples simulated numerically, the resonance cross-section spectra have been successfully reproduced. Present method is powerful to determine the intensities and their ratios of the absorption lines. The effects due to the statistical fluctuations of the data and the non-resonant radiation have been also discussed.

Study of precipitation phenomena in alloys by nuclear magnetic resonance

The nuclear magnetic resonance of Co59nuclei in precipitates from Cu-3.52%Co and Cu-19%Ni-18%Co alloy powders aged at 600°C after solution heat treatments has been observed with a pulsed technique. The observed spinecho spectra for the Cu-3.52%Co alloy indicate that the precipitates have f.c.c. structure and their Cu concentrations are extremely small. In the case of the Cu-19%Ni-18%Co alloy, the observed distribution of resonance frequencies suggests that a completely random solid solution cannot be obtained by quenching and that concentration fluctuations exist in this alloy even immediately after quenching from 1075°C.