Hideki Watanabe

Effect of Spin Fluctuations on Magnetic Properties and Thermal Expansion in Pseudobinary System FexCo1-xSi

Both magnetization and thermal expansion measurements have been made on the same samples of Fe x Co 1- x Si alloys. The magnetization data is analyzed on the basis of the spin fluctuation theory developed by Takahashi to obtain information on the nature of spin fluctuations of this system. We found that concentration variation of magnetic behaviors of the ferromagnetic alloys, such as the Arrott plots and the temperature dependence of spontaneous magnetization, is quantitatively well explained by the theory. Magnetovolume coupling constant is also estimated from thermal expansion coefficient on the basis of the SCR theory over the wide concentration range. The magnetovolume effect is also in good accordance with the predictions of Takahashis theory, even quantitatively.

Helical Spin Resonance and Magntization Measurement in Itinerant Helimagnet FexCo1-xSi(0.3≤x≤0.85)

Measurements of electron spin resonance and magnetization have been performed on the cubic B-20 Fe x Co 1- x Si (0.3≤ x ≤0.85). The magnetization process on Fe 0.8 Co 0.2 Si indicates that this sample is a helimagnet with the cone-ferro-magnetic transition under a low magnetic field. In this sample, the helical spin resonance and the critical field resonance have been observed at 9 GHz at low temperatures, and are similar to those in MnSi observed by Date et al . The helical spin resonance data of Fe 0.8 Co 0.2 Si and MnSi are reasonably explained by the recent magnetic resonance theory for the helical spin structure due to the anti-symmetric exchange interaction existing in the cubic B-20 structure because of the lack of inversion symmetry. In every sample of Fe x Co 1- x Si, ferro- and para- magnetic resonances have been observed at 9 and 24 GHz, and are briefly discussed.

Ferromagnetic Resonance in (NiFeCo/Cu/Co)-Multilayers.

Ferromagnetic resonance (FRM) measurements were made on [Ni 0.8 Fe 0.15 Co 0.05 /Cu/Co] multilayers in order to observe the coupling between ferromagnetic layers across Cu spacer. The resonant fields vary oscillatory with Cu spacer thickness. The FMR results were analyzed by the resonant model of a magnetically coupled two-layer system. By using a modified RKKY theory for interlayer exchange interaction and FMR results, the coupling constant, J , was determined as a function of Cu spacer thickness. The resonant field can be predicted for Cu spacer thickness and are well compared to the experimental values. In samples of antiferromagnetic coupling, the resonance mode of the magnetization flopping were observed at small applied fields.

Helical spin resonance in the cubic B-20 Fe0.8Co0.2Si single crystal with a long-period helical spin structure

Electron spin resonance has been studied at various frequencies (6–24 GHz) on B-20 Fe 0.8 Co 0.2 Si single crystal, which has a long-period helical spin structure (HSS) with a cone-ferromagnetic transition at a critical field H c . Three resonance absorptions have been observed clearly: (a) behaving like a ferromagnetic resonance, (b) occuring around H c and (c) assigned to be a helical spin resonance. The resonance frequency of (c) decreases with increasing the magnetic field. Such dependence does not agree with the resonance theory for a HSS resulting from a competition between symmetric exchange interactions (SEIs), but it agrees well with that for a HSS resulting from a competition between the SEI and the antisymmetric exchange interaction. The present results together with our previous ones give the evidence that the HSS in Fe 0.8 Co 0.2 Si is of the latter type.

Electron spin resonance in Fe1−XCoXSi

Abstract ESR was observed in the ferromagnetic region of Fe 1− X Co X Si. The magnitude of the g -shiftδsfnc;Δ g | becomes very large in the higher Fe concentration region. The stability of helimagnetism and Δ g are discussed.

Thermal Expansion in Weak Itinerant Ferromagnets FexCo1-xSi (x=0.48 and 0.77)

Both thermal expansion and magnetization measurements have been made on the same samples of Fe x Co 1- x Si alloys with x =0.48 and 0.77 in the temperature range from 4.2 to 340 K and under magnetic fields of up to 50 kOe. Magnetic contribution to a thermal expansion coefficient was estimated by subtracting the nonmagnetic contribution expressed by Gruneisens equation from the observed one. It was found that the magnetic thermal expansion coefficient has a positive and approximately constant value above the Curie temperature, and that the observed magnetovolume effect of these alloys is quantitatively well explained by the spin fluctuation theory developed by Moriya and Usami.

Effect of spin fluctuations in FexCo1−x

Magnetic behavior of weak itinerant ferromagnet Fe x Co 1− x Si is studied on the basis of the recent spin-fluctuation theory. The magnetization data are analyzed with a hypothesis that the magnetic carriers are Co atoms alone. It is insisted that the Co atoms dominantiy contribute to the magnetic behavior in the Fe-rich region.