Ikuo Nakai

The Magnetovolume Effect due to the Decrease of Local Magnetic Moment in Fe-Ni Invar Alloys

The local magnetic moment at the Curie temperature in the 35 at% Ni-Fe alloy retains still a half value of the magnetic moment at 0 K. Taking into consideration this fact, new formulas are proposed, which describe reasonably the phenomena of spontaneous volume magnetostriction and forced volume magnetostriction. The magnetovolume coupling constant determined by the new formulas is approximately 1.2 ×10 -8 cm 6 /emu 2 for Fe-Ni Invar alloys, which is independent of the Ni concentration.

Contribution of T2 Term to the Magnetization of a 35.4 at% Ni–Fe Invar Alloy

The magnetization of a single crystal of the 35.4 at% Ni–Fe Invar alloy has been investigated between 4.2 K and 280 K in external fields up to 16.9 kOe to an accuracy of about 5 parts in 10 5 . The spin-wave stiffness constant including its temperature variation and the higher-order term than the T 3/2 term due to the spin-wave excitations are determined from only the data of magnetization measurements. The observed decrease in magnetization excluding the spin-wave term depends on T 2 and seems to be associated with the Stoner-type excitations. The contribution of T 2 term is found to be as large as the spin-wave contribution.

Role of T2 Term in Temperature Dependence of the Magnetization for Fe-Ni Invar Alloys

Precise measurements of magnetizations for single crystals of FCC Fe–Ni alloys in the Invar region have been made at temperatures from 4.2 K to 300 K in external applied fields up to 16.9 kOe to a relative accuracy of 5×10 -5 . In an analysis of these observed results with iterative least-squares method, it is found that there exists a large Stoner-type contribution varying as T 2.00±0.05 besides the spin-wave one to the temperature change in magnetization and that the coefficient of the Stoner-type contribution increases rapidly with decreasing Ni concentration, showing a maximum at about 35 at%Ni, and decreases for the alloys containing less than 35 at%Ni. It becomes evident that ferromagnetism in FCC Fe–Ni alloys disappears at a critical concentration of about 31 at%Ni. An anomalous behavior of the magnetization at low temperatures is also shown.

Anisotropic absorption spectra of polyester films in the ultraviolet and vacuum ultraviolet regions

Reflection spectra of polyethylene terephthalate (PET) and polyethylene naphthalate films were measured for 50 to 600 nm, using a Seya–Namioka type monochromator at UVSOR, and were converted into absorption spectra by Kramers–Kronig formula. Dichroic nature of drawn films was clearly displayed in the whole region. Previous analysis was confirmed to be true that the third absorption peak of PET, which appeared in the limit of atmospheric measurement, consisted of two peaks of different polarization. Their VI and VII absorption bands at 13.3–15.5 eV originated in transitions associated with r electrons, and had absorption coefficients of 1:1–1:3 � 10 6 cm � 1 .

XAFS Study on Nitrogenation of CoTiSn

Gas-phase nitrogenation of CoTiSn compound with the MgAgAs-type structure has been examined by means of x-ray diffraction (XRD) and x-ray absorption fine structure (XAFS). The XRD suggests that the nitrogenation product is composed of the corresponding Cu2MnAl-type compound with expanded unit-cell volume comparable to that of Co2TiSn and subtle trace of Ti6Sn5 and Sn. Whereas the x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectra for the Ti, Co and Sn K-edges clearly indicate the coexistence of TiN and Co1+yTiSn phases in the product. It is also revealed that more Co atoms than expected from the chemical formulas are introduced into the vacancy site of the MgAgAs structure in Co1+yTiSn with 0≤y≤0.4.

Magnetic Excitations in FCC Fe–Ni Alloys Revealed from Detailed Magnetization Measurements

Detailed magnetization measurements for single crystals of Ni-rich Fe–Ni alloys have been carried out between 4.2 K and 300 K in external magnetic fields up to 16.9 kOe to a relative accuracy of 5×10 -5 . It is found by iterative least-squares analysis of these observed values that the temperature variation of magnetization consists of two parts: one is due to the spin-wave excitations and the other due to the Stoner-type excitations. The coefficient of the latter part has a considerable value at pure Ni, decreases with addition of Fe atoms to pure Ni, vanishing around 70 at%Ni, and reappears at about 65 at%Ni. A simple picture for metallic ferromagnetism at finite temperatures is proposed.

The Magnetization of Amorphous GdXY68-XCu32 Alloys

We report on the magnetization measurement of amorphous Gd X Y 68- X Cu 32 (17≦ X ≦68) alloys between 4.2 K and 300 K in fields up to 7 T. This system varies from ferromagnetic to cluster glass states with decreasing X . The high field magnetization in a cluster glass region as well as in a ferromagnetic phase is described well in terms of the spin wave T 3/2 law corrected for the applied field. The spin wave stiffness constant D thus deduced is independent of field and temperature. We have found the following features in some Gd-based alloys: an amorphous material shows a smaller D value than a crystalline counterpart; in both states, D is directly proportional to the paramagnetic Curie temperature. These facts are quantitatively understood by the nearest neighbor Heisenberg model with fluctuations in the exchange interaction.

Detailed investigation of the magnetization of an amorphous ferromagnet Fe83B17

We have carried out precise measurements of the magnetization on an amorphous metallic ferromagnet Fe 83 B 17 from 4.2 K to 340 K in a high field of 5.03 T. Detailed analysis of the observed data shows that the magnetization does not obey a simple spin wave T 3/2 law but follows a sum of the renormalized spin wave term and the Stoner-type contribution varying as T 2 rather than e - Δ / k B T . The spin wave stiffness constant is expressed as D = D 0 (1- D 2 T 5/2 ) with D 0 =120±5 meVA 2 and D 2 =(2.0±0.5)×10 -8 K -5/2 , which is roughly consistent with that determined directly from neutron scattering. This suggests no existence of the well-known anomaly of the spin wave stiffness constant reported in earlier measurements. We discuss behavior of the magnetization at finite temperatures in terms of the local magnetic moment.

Temperature Dependence of the Magnetization for Amorphous Gd65Co35

We present detailed investigation of the temperature dependence of the magnetization for amorphous Gd 65 Co 35 below 340 K in magnetic fields up to 5.27 T. The paramagnetic susceptibility follows the Curie-Weiss law. We experimentally find that T 2.00±0.05 is the functional form of another dominant contribution to the temperature dependence of the magnetization than the spin wave term T 3/2 . These T 2 and T 3/2 terms are interpreted as longitudinal and transverse fluctuations in the local moment derived from polarization of the spin bands. The longitudinal fluctuation comes from itinerant character of 3 d electrons in Co atoms.

POLARIZED ABSORPTION SPECTRA OF POLYETHYLENE TEREPHTHALATE AND POLYETHYLENE NAPHTHALATE FILMS IN SOFT X-RAY REGION

Abstract Using polarized nature of synchrotron radiation, core electron absorption spectra of thin films of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) were investigated. Aromatic rings in these films tend to lie in the film plane when the films are drawn uniaxially or, in particular, biaxially; previous interpretation of their core electron spectra based on their nature above was reinforced by recently observed spectra in higher resolution.