S. Nasu

Effect of Joule heating in current-driven domain wall motion

It was found that high current density needed for the current-driven domain wall motion results in the Joule heating of the sample. The sample temperature, when the current-driven domain wall motion occurred, was estimated by measuring the sample resistance during the application of a pulsed current. The sample temperature was 750 K for the threshold current density of 6.7×1011A∕m2 in a 10-nm-thick Ni81Fe19 wire with a width of 240 nm on thermally oxidized silicon substrate. The temperature was raised to 830 K for the current density of 7.5×1011A∕m2, which is very close to the Curie temperature of bulk Ni81Fe19. When the current density exceeded 7.5×1011A∕m2, an appearance of a multidomain structure in the wire was observed by magnetic force microscopy, suggesting that the sample temperature exceeded the Curie temperature.

Mechanical alloying of the high carbon FeC system

Abstract Mechanical alloying of iron and graphite powders was performed in composition range Fe1 − xCx (x = 0.17−0.90) by the use of a conventional ball mill. The structures of mechanically alloyed samples were examined by X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy, 57Fe Mossbauer spectroscopy and differential scanning calorimetry. The results from X-ray, TEM and Mossbauer measurements suggested the partial formation of amorphous phase. Amorphization was notable on the sample ball milled for about 200 h. After subsequent milling, formation of metastable carbides Fe3C for the powders with x = 0.17−0.25 and Fe7C3 for x = 0.29−0.70 was detected. Formation of fine paramagnetic particles was detected by Mossbauer spectroscopy for the powders having carbon content x = 0.80−0.90.

Propagation of a magnetic domain wall in magnetic wires with asymmetric notches

The propagation of a magnetic domain wall (DW) in a submicron magnetic wire consisting of a magnetic/nonmagnetic/magnetic trilayered structure with asymmetric notches was investigated by utilizing the giant magnetoresistance effect. The propagation direction of a DW was controlled by a pulsed local magnetic field, which nucleates the DW at one of the two ends of the wire. It was found that the depinning field of the DW from the notch depends on the propagation direction of the DW.

57Fe Mössbauer study under high pressure; ε-Fe and Fe2O3

Using diamond anvil cell, the57Fe Mössbauer spectra of pure iron foil and α-Fe2O3 powder under high pressure have been measured at room temperature.57Fe Mössbauer spectra of α-Fe were measured from 15 GPa to 45 GPa. Isomer shift value decreased and the quadrupole splitting slightly increased as the pressure increased.57Fe Mössbauer spectra of Fe2O3 under high pressure up to 72 GPa were observed. Above 52 GPa, the new lines appeared at the center portion of the spectrum corresponding to the new high pressure phase. The spectrum of new high pressure phase consisted of 6-line splitting and doublet, suggesting the existence of the two different kinds of iron states in it.

Mechanical alloying of Fe-B alloys

Mechanical alloying (MA) of Fe-B alloy systems using a conventional ball mill has been performed. The structure change of Fe100−xBx for the composition range 10≤x≤90 has been investigated using X-ray diffractometry, thermal analysis, transmission electron microscopy, scanning electron microscopy and Mössbauer spectroscopy. According to the X-ray diffraction analysis, the sequences of transformation have been classified into three groups,x≤20, 25≤x≤35 andx≥50 for Fe100−xBx. The amorphous phase whose composition is nearlyx=30, first appeared for the whole composition range. This amorphous phase changed into a tetragonal Fe2B-like compound on further milling for 1000 h. On further milling of thex= 50 sample, the Fe2B compound phase disappeared and orthorhombic FeB compound appeared.

Defects in metals

Recent study of defects in metals using Mössbauer spectroscopy is presented. Vacancy and interstitial atoms are focussed as defects in metals. General remarks for the study of defects are presented first and then, as examples of the study,119Sb and57Co Mössbauer measurements to monitor the trapping and detrapping process of quenched-in vacancies in Au are discussed with the aid of the complementary techniques which are resistivity measurement, computer simulation and positron lifetime measurement. Detailed results from these complementary techniques are described. Recent theoretical calculations for defect-associated states are also presented briefly.

Mössbauer study on mixing and kneading of metallic powders

Utilizing57Fe and119Sn Mössbauer spectroscopy, the microscopic behavior of metallic powders has been investigated during and after the mechanical alloying (MA) process. A conventional ball-milling method is employed and the following combinations of metallic powders have been studied for their MA; Al−Fe, Fe−Sn and Ag−Fe. Mössbauer spectra show the occurrence of mutual atomic dispersion during the MA process even for the Ag−Fe system which is known as mutually immissible even in the liquid state. Formation of an amorphous phase in the Al−Fe system has been detected.

197Au Mössbauer study of nano-sized gold catalysts supported on Mg(OH)2 and TiO2

We have studied nano-sized Au catalysts supported on Mg(OH)2 and TiO2 using 197Au Mössbauer spectroscopy. 197Au Mössbauer spectra observed for Au/Mg(OH)2 catalysts can be decomposed into one singlet with zero isomer shift and several doublets. One of the doublets shows an isomer shift that is typical for AuI, and other doublets are due to AuIII. The relative area of the AuI component shows the maximum value for a specimen calcined at 523 K, which also shows the highest catalytic activity.

57Fe mössbauer study of high-T c Y−Ba−Cu oxide superconductors

Abstract57Fe Mössbauer measurements have been performed for Fe-doped YBa2 (Cu1-x Fex)3O7-v (x=0.005–0.10) superconducting oxides. Fe atoms mainly substitute at Cul chain sites and the fraction of Fe at Cu2 plane sites is discussed thermodynamically. The spin direction of Fe at Cu2 was determined to be normal to thec-axis using an oxygen deficient specimen having a strong texture along thec-axis. A magnetically broadened spectrum at 4.2 K for Fe at Cul is compared with a simulated one which takes the distribution of hyperfine fields into account.

Mössbauer study of CaFeO3 under external high—Pressure

Abstract57Fe Mössbauer and X-ray diffraction measurements have been performed on a perovskite CaFeO3 under external high pressure upto 50 GPa at room temperature using a diamond anvil cell. Above 29 GPa the57Fe magnetic hyperfine splitting appears superimposing with usual paramagnetic pattern of CaFeO3. Magnitude of hyperfine field is 16 T and much smaller than 33 T of typical Fe4+ in SrFeO3 suggesting a transition from high-spin S=2 to low-spin S=1 state in CaFeO3.