Motohiko Yamada

Non-collinear spin structure and magnetization process of Tm2Fe14B

Abstract The spin structures and magnetization curves of R 2 Fe 14 B (R: rare earth atom) are calculated on a basis of a simplified Hamiltonian. The B −2 2 O −2 2 terms in the crystal field potential cause a non-collinear spin structure whenever the magnetization does not lie along the c-axis of the tetragonal structure. Neutron diffraction study on Tm 2 Fe 14 B has revealed a non-collinear spin structure at low temperatures where the magnetization vector lies in the c-plane. Favorable parameters are found that reproduce the observed spin structure, the magnetization curves and the spin reorientation temperature of Tm 2 Fe 14 B.

Mössbauer study of the intermetallic compound Nd2Fe14B. II. Temperature dependence and spin reorientation

Abstract The 57 Fe Mossbauer effects of Nd 2 Fe 14 B were measured in a temperature range of 4.2−300 K. Below the spin reorientation transition temperature T sc = 148 K, the spectra were satisfactorily analyzed with twelve Zeeman sextuplets due to splitting of six crystallographic Fe-sites into twelve non-equivalent sites. It was shown that the magnetic moments of the Fe and the Nd atoms are non-collinearly coupled in the magnetic structure with canted moments below T sc . The directions of the moments at 4.2 K are inclined at 27° for Fe and at 58° for Nd from the c -axis to the [110] direction. The average moments are 2.27μ B for Fe and 3.3μ B for Nd at 4.2 K. The increase of the average hyperfine field with decreasing temperature is suppressed below T sc , and its value at 4.2 K is reduced by 1% from the value of 337 kOe which is observed in Y 2 Fe 14 B and also estimated for Nd 2 Fe 14 B by extrapolating the values above T sc . On the other hand, the Nd moment increases abruptly around T sc as the temperature decreases. The directions of the principal axes of electric field gradients on the six distinct Fe-sites were also obtained. The anomalous temperature dependence of quadrupole splittings and isomer shifts was observed around T sc . They were discussed in a framework of the changes in the band structure and the lattice parameters incidental to the spin reorientation transition.

Metamagnetic Phase Transitions in FeTiO3

Magnetic susceptibilities and magnetization isotherms have been measured for single crystal FeTiO 3 . The susceptibility parallcl to the c -axis has a large and sharp peak at T N =58.0±0.3 K, while the perpendicular susceptibility is almost independent of temperature below T N . Magnetization isotherm at 4.2 K with fields applied along the c -axis exhibits a typical metamagnetic transition at H c =80.3±0.1 kOe, being the first observation of metamagnetism in 3 d transition metal oxides. The transition is of the first order up to about 35 K, above which it is of the second order. The experimental data are well reproduced by molecular field calculations with the best fit parameters of the anisotropy field H A =150 kOe, the ferromagnetic intrasublattice exchange interaction of 2 z 1 J 1 =28.5 K, and the antiferromagnetic intersublattice interaction of 2 z 2 J 2 =-6.67 K.

First order magnetization process in Pr2Fe14B single crystal

Abstract High-field magnetization process in Pr 2 Fe 14 B single crystal has been studied in static magnetic fields up to 230 kOe. The spontaneous magnetization is along the [001] direction of the tetragonal structure down to 1.5 K. When the field is applied along the [100] and [110] directions at low temperatures, magnetization jumps are observed at about 130 kOe and 160 kOe, respectively, which are considered to correspond to the first order magnetization process (FOMP). Observed features of the magnetization curves including FOMP and their temperature dependence are well reproduced by the calculation based on a simplified Hamiltonian including the crystalline electric fields and the FePr

Experimental investigation of the buckling of shallow spherical shells

Abstract In the present paper, the buckling behavior of clamped thin shallow spherical shells under external pressure is studied. Seventy-nine plastic shells formed by thermovacuum process were tested. The distributions of initial geometrical imperfections and vertical displacements were minutely measured with a differential transformer. It was possible to control the symmetrical initial geometrical imperfection of each specimen. Results indicate that the buckling phenomena of shallow spherical shells vary greatly with the symmetrical initial imperfection parameter η. In the case of the geometrical parameter λ larger than 5.5, the amplitude of the asymmetrical displacement component with the bifurcation buckling wave calculated by Huang becomes large immediately before buckling. The validity of Huangs theory for an initially perfect shell is experimentally demonstrated.

Wind loads and wind-induced dynamic behavior of a single-layer latticed dome

This paper describes the results of some basic investigations on wind loads and wind-induced dynamic behavior of a rigidly jointed single-layer latticed dome with a long span. First, the wind pressures were measured simultaneously at many points on a dome model in a turbulent boundary layer which simulated the natural wind over typical urban terrain. In the wind tunnel experiments, nine models with different rise/span and wall-height/span ratios were tested. In order to investigate the structure of the pressure field on the dome, the proper orthogonal decomposition (POD) technique was applied to the pressure data. The effect of dome geometry on the structure of the pressure field is briefly discussed. Then, the dynamic response of nine latticed domes with a span of 120 m was analyzed in the time domain. The equation of motion for the structural frame of the dome was numerically integrated by using the Newmarks β method; the wind loads, expressed as the concentrated loads at the interior nodes of the network, were generated by superimposing the first eight eigenfunctions of pressure, which were obtained from the above mentioned POD analysis. The results indicate that the dynamic response is dominated by several vibration modes; that is, the first three or four axisymmetric modes and a few lower asymmetric modes with small wave numbers both in the circumferential and in the radial direction.

Magnetic properties of R2Fe14B compounds

Abstract On single crystals of R 2 Fe 14 B compounds (R = Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm), magnetization measurements have been made in magnetic fields up to 20 kOe at 4.2 K and the values of magnetic moments for the Fe and the R ions and anisotropy constants are estimated. The magnetic anisotropy is discussed with crystal-field parameters for the R 3+ ions.

Wind-induced dynamic behavior and its load estimation of a single-layer latticed dome with a long span

Abstract This paper describes the results of some basic investigations on the wind-induced dynamic behavior and the resultant load estimation of a single-layer latticed dome with a long span. The wind pressures at many locations on dome models were measured simultaneously in a wind tunnel. The dynamic response of several dome models was analyzed in the time domain by using a finite element method. For generating a time history of the wind load vector, which consisted of concentrated loads at all internal nodes, the proper orthogonal decomposition (POD) technique was applied to the wind pressure data obtained from the wind tunnel experiment. The nodal displacements and the resultant member stresses, both axial and bending, were computed at each time step. Based on the results, the characteristics of the wind-induced dynamic response of the dome are discussed. In the analysis, special attention is paid to the vibration modes that dominate the dynamic response. Furthermore, a brief discussion is made of the load estimation for the dome. A simple model for estimating the design wind load, expressed as an effective load, is proposed.

Some effects of free-stream turbulence on the flow past a cantilevered circular cylinder

Abstract Flow around a cantilevered circular cylinder has been studied experimentally in the subcritical and critical Reynolds-number ranges. Particular attention was paid to the free-stream turbulence effects on the steady aerodynamic characteristics of the cylinder. Measurements were made of the surface pressure distribution, flow separation line and vortex shedding frequency in grid-generated turbulent flows. The aerodynamic characteristics of a cylinder were only minutely affected by the free-stream turbulence in the subcritical flow. The critical Reynolds number was expressed as a function of the Taylor number Tx defined in terms of the longitudinal integral scale of the free-stream turbulence. The mean drag coefficient and the position of the flow separation point were fairly well-correlated with the parameter Re 1.34 Tx , with Re being the Reynolds number. Furthermore, it was found that some steady aerodynamic coefficients of the cylinder were closely correlated among themselves.

Effects of the corner shape of high-rise buildings on the pedestrian-level wind environment with consideration for mean and fluctuating wind speeds

Abstract A series of wind-tunnel experiments have been carried out on the effects of the corner shape of high-rise buildings on the pedestrian-level wind environment around them. This study is concerned with high-rise apartment houses which have often been constructed in suburban areas of Japan. Four types of corner shapes were tested. The distributions of the mean and rms wind speeds around each model were measured by a hot-wire anemometer for various wind directions at the pedestrian level in a turbulent boundary layer. The wind environment was evaluated based on the distribution of an “ effective wind speed ” in which both the effects of the mean and the fluctuating wind speed are considered. The results inducate that the flow characteristics around the building generally change with a small change of the buildings corner shape, and that the degree of the effect depends on the wind direction. The most desirable corner shape regarding the wind environment was suggested for each wind direction.