I. Sakamoto

Ion-induced phase transformation in type 304 austenitic stainless steel by rare-gas ion irradiation

Foil specimens of austenitic stainless steel (type 304 stainless steel) were irradiated with 50– 400 keV Ar, Kr, and Xe ions in the fluence range of 1×1020–1×1021 ions/m2 at room temperature. It has been shown by glancing angle x‐ray diffraction (GXRD) measurement that the amount of ion‐induced phase transformation of γ →α’ in the foil specimens depends strongly on irradiation conditions such as ion species, energy, and fluence; the amount of the phase transformation at saturation increases linearly with increasing ion range. An ion‐induced new peak was found in the GXRD pattern, which is shown to be due to solidification of implanted rare‐gas atoms in the stainless steel. The solid phase inclusions of the rare‐gas atoms have a close correlation with the phase transformation. It is shown that the phase transformation is induced by the formation of highly pressurized rare‐gas inclusions, while the formation of the inclusions depends on a depth distribution of implanted rare‐gas atoms.

Magnetic structure and perpendicular magnetic anisotropy of rare‐earth (Nd,Pr,Gd)/Fe multilayers

The structural phase and the magnetic anisotropy of Nd/Fe, Pr/Fe, and Gd/Fe multilayers have been examined using magnetization measurements and conversion electron Mossbauer spectrometry. With decreasing Fe layer thickness dFe, the structural phase of the Fe layer changes from bcc crystalline to amorphous at the critical thickness dFec of 17–20 A via an intermediate phase, which contains a paramagnetic phase in the Nd/Fe and Pr/Fe systems, but not in Gd/Fe. At just above dFec, Nd/Fe and Pr/Fe multilayers exhibit perpendicular anisotropy, which originates from the interface anisotropy due to the one ion anisotropy of Nd and Pr. However, Gd/Fe multilayers show in‐plane magnetization because of the S‐ionic Gd. Below dFec, the magnetization increases from nearly zero with decreasing dFe because of the exchange interaction between Fe and the rare earth (Nd,Pr,Gd).

Magnetic properties and giant magnetoresistance of Fe/Au multilayers

Magnetic properties and magnetoresistance (MR) are examined for the Fe/Au multilayers (MLs) prepared on glass substrates by electron-beam evaporation techniques as a function of the thicknesses of Fe and Au layers; dFe and dAu. For small dFe, the perpendicular magnetic anisotropy is induced, and in the films of dFe≈5 A, the stripe magnetic domain patterns are observed. In dFe⩽3 A, however, a superparamagnetic nature and a granular-type giant magnetoresistance (GMR) appear at room temperature. In a series of [Fe(3 A)/Au(dAu)]40 MLs, the MR ratio increases with dAu and reaches the first peak at around 10 A. On the other hand, at 14 K, the ferromagnetic component having a perpendicular anisotropy becomes dominant, and the GMR arises from the random wall alignment due to the antiferromagnetic coupling between the adjacent Fe layers, causing the first MR peak at dAu≈10 A. The layered structure is obscured and the superparamagnetic Fe grains are created by annealing at 300–350 °C, followed by the crystalline gr...

Formation of nanosized Fe–Co alloys in α−Al2O3 crystals by ion implantation

Nanometer-sized Fe–Co composite clusters in α−Al2O3 matrices were synthesized by sequential implantation of Fe and Co ions with the projectiles’ energy at 100 keV. The synthesized clusters were shown to have bcc structure by the measurement of glancing angle x-ray diffraction. We have observed an apparent change in the internal magnetic field of the Fe–Co composite clusters with changing Co concentration, which exhibits the maximum at a concentration around 25 at. % Co as revealed by conversion electron Mossbauer spectroscopy. The magnetoresistance ratio of the granules was observed to increase with the Co addition to the Fe nanoclusters. The results provide clear evidences on the alloy formation of Fe–Co nanoclusters in α−Al2O3 layers synthesized by ion implantation.

Conversion electron Mossbauer study on iron-implanted α-Al2O3 with applied magnetic fields

Conversion electron Mossbauer spectroscopy with applied magnetic fields up to 0.4 T was used to study aggregation states of iron atoms in an α-Al 2 O 3 single crystal implanted with 57Fe+ ions at a dose of 1.5 x 10 17 ions/cm 2 . Consistent fits to all the spectra were achieved with five components for the iron states: spherical fine α-Fe particles that exhibit superparamagnetic relaxation, oblate ellipsoidal α-Fe particles exhibiting collective magnetic excitations, γ-Fe-like paramagnetic clusters, and two kinds of ferrous oxides. From the analysis of hyperfine field distributions of the component subspectra, particle-size distributions were obtained for the two α-Fe components. The average diameter of the superparamagnetic α-Fe particles was found to be about 3.5 nm, while that for the oblate ellipsoidal α-Fe particles was about 6.2 nm with a dimensional ratio (major axis/minor axis) of about 1.35, which suggests an anisotropic evolution of the precipitated α-Fe during the implantation.

CEMS study of Au/Fe multilayers in an annealing process

Abstract The magnetic and structural properties of Au/Fe multilayers (MLs) have been investigated in an annealing process by means of conversion electron Mossbauer spectroscopy (CEMS). In the [1.2 nm Au/0.3 nm Fe] and [1.2 nm Au/0.5 nm Fe] MLs, the CEM spectra show sextet magnetic splitting peaks, the broad band peaks and the doublet peaks with rising annealing temperature. These are suggested to be related to the change in the interface structure between Au and Fe layers and the formation of Fe islands having superparamagnetic nature in the annealing process.

Behavior of rare-gas atoms implanted in stainless steel

Abstract The behavior of rare-gas atoms in stainless steel after Ar, Kr and Xe ion irradiation has been investigated by means of glancing-angle X-ray diffraction, conversion electron Mossbauer spectroscopy, Rutherford backscattering spectroscopy (RBS) and particle-induced X-ray emission (PIXE). It has been shown by RBS and PIXE measurements that the retained amount of rare-gas atoms in stainless steel depends on the ion species, energy and fluence. In Kr and Xe ion irradiation, an increase in intensity and a decrease in position of Kr and Xe peaks with increasing ion energy and fluence have been observed. The pressure as estimated from the lattice parameter of Kr and Xe solids has been consistent with the occurrence of the ion-induced phase transformation ( γ -to-α phase) in stainless steel. We conclude that these results show the experimental evidence of the highly pressurized rare-gas inclusions formed in stainless steel.

Magnetoresistance and Hall effect in Co implanted GaAs hybrid magnetic semiconductors

The hybrid Co/GaAs ferromagnetic semiconductor has been prepared by the ion-implantation method, and the sample surface has been sputter-etched for examining the magnetic and transport properties as a function of the depth. Extraordinary Hall effect increases with depth until 120 nm etching, and the Hall resistance is proportional to the square of resistance, indicating the side-jump mechanism, in the depth region of 10–60 nm. The depth profiles of the saturation magnetization and the conductivity are estimated analytically, and then the Hall coefficient is evaluated as a function of etching depth.

Nano-clustering in α-Al2O3 crystals by ion implantation of iron and cobalt and their magnetic properties

Abstract Nano-composites of iron-based alloys in α-Al 2 O 3 matrices were formed by sequential implantation of Fe and Co ions. The implantation was done at room temperature up to a total dose of 2.0×10 17 ions/cm 2 with a projectile energy of 100 keV. We have observed an apparent change in magnetic properties of Fe–Co clusters with 0–60 at.% Co contents, which gives evidence of Fe–Co alloying. Mossbauer spectroscopy shows that hyperfine field at 57 Fe nuclei changes with Co substitution of 57 Fe in the nano-sized binary clusters while the blocking of superparamagnetic relaxation occurs when the Co concentration increases in the alloy clusters. The results are in good agreement with magnetization measurements.

Nano-clustering of iron and magnetic properties of the iron implanted in MgO

Iron precipitation and the accompanying magnetization and magnetoresistance (MR) were investigated in MgO(100) crystals implanted with Fe ions of 100 keV energy. Magnetization and MR properties were measured at room temperature. Conversion electron Mossbauer spectroscopy was used to identify physical states of the implanted irons. At doses less than 2 x 10 17 ions/cm 2 the measured magnetization exhibits non-hysteretic and unsaturated loops, i.e. superparamagnetic nature. The characteristics suggest the precipitation of implanted Fe ions as nano-granules in the MgO matrix. A significant GMR effect was observed at a dose of 2.0 × 10 17 ions/cm 2 and the obtained MR ratio was about 3.5% for fields of 12 kOe for the samples post-annealed at 300 °C in Ar. With increasing dose over 2.0 x 10 17 ions/cm 2 , a ferromagnetic component appears in the Fe implanted MgO samples.