摘要:

The magnetic properties of &ggr;‐Fe2O3nanoparticles synthesized by vaporization condensation in a solar image furnace have been studied using both magnetic measurements and Mo¨ssbauer spectroscopy. The mean size of the particles turns out to be easily controlled by changing the pressure conditions in the growth chamber. The particles exhibit superparamagnetic behavior at room temperature. Magnetic measurements show the appearance of magnetic hysteresis in the low‐temperature range and from the evolution with temperature of the ferromagnetic ratio,MR/MS, we have determined the distribution of the blocking temperatures for the smallest particles that is fitted to a log‐normal distribution leading to a mean blocking temperature ⟨TB⟩=38±15 K. The size distribution of the magnetic unit is also determined from this fitting, as well as from the Mo¨ssbauer spectra, obtaining a mean particle volume of about 3.5×105 A˚3. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361125

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The microstructure, Curie temperature and magnetostriction of R(Fe1−xAlx)1.8alloys (R=Dy0.65Tb0.25Pr0.1,x≤0.3) have been investigated using optical microscopy, x‐ray diffraction, transmission electron microscopy, electron probe microanalysis, positron annihilation technique, vibrating sample magnetometer, and standard strain gauge technique. It was found that the alloys are essentially single phase up tox=0.30. The defects in R(Fe1−xAlx)1.8decrease with increasing Al content whenx≤0.1. The magnetostriction of the alloys decreases with increasing Al content in high applied magnetic fields, but it exhibits a peak whenx=0.05 in low applied magnetic fields (H≤160 k A/m). ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361126

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of anisotropy dispersion on the magnetization process in magnetoresistive sensor films is studied using micromagnetic calculation. It is assumed that the magnetization of the film is fixed at both ends to the track width direction, which corresponds to the average direction of the easy magnetization axis of the film. Magnetization distribution at different stages of the magnetization process is calculated by numerical integration of the Landau–Lifshitz–Gilbert equation. Our study shows that anisotropy dispersion causes buckling instead of coherent rotation of the magnetization. As dispersion increases, buckling becomes more pronounced. Furthermore, buckling does not completely disappear when the external magnetic field is weakened to a small value. The magnetization process is influenced by buckling, and this may lead to Barkhausen noise. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361127

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Measurements have been made of the time dependence of magnetization, under constant applied field conditions, of thin films of Tb–Fe–Co alloy. The results have been analyzed using a model in which it is assumed that magnetization proceeds by thermal activation of irreversible switching of the magnetic moments of identical elementary volumes. It is assumed that the activation energy for switching, intrinsically the same for all elementary volumes, varies according to local interactions. Values of the fluctuation field &Lgr; are derived and it is found that for the films examined, &Lgr; is independent of magnetizationMand the applied field. It is shown that information about the magnetic interactions between the elementary volumes, due to exchange and demagnetizing fields, may be derived from graphs ofdM/dtvsM. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361128

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A magnetization model is presented that is used to cover trilayers containing two magnetic layers that are exchange coupled via an intermediate nonmagnetic layer and that have different crystalline anisotropies. The interfaces are coupled to the bulk by a twisted magnetization configuration which is evaluated using the Ritz method. By minimizing the total energy, experimental magnetization curves of strongly coupled Co/Ru sandwiches can be reproduced with a good precision and with the same set of parameters in two perpendicular field directions. These physical parameters can be determined with a good reliability and are in agreement with the literature except for the bulk anisotropy of the Co layer first deposited, which is twice as large as the known bulk value. This originates in the magnetoelastic contributions due to lattice misfit and interface roughness. It is shown that the interlayer exchange coupling forces the magnetization of both layers to be along the same axis in the low‐field range notwithstanding the opposite sign of the anisotropy constants in most stacks. It is also demonstrated that the differences in the orientations of the moments in one Co layer are modest and depend on the various parameters. In particular, the bulk exchange constant is a decisive parameter that makes the calculated curves close to the experimental ones. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361129

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The ‘‘hyperfine magnetic field’’‐Hhpfat the nuclei of the diamagnetic atoms119Sn was measured in Heusler alloy Ni2MnSn by Mo¨ssbauer absorption spectroscopy for pressures up to 10.8 GPa at room temperature. The pressure dependences ofHhpfSndoes not show hysteresis and can be fitted by a linear functionHhpfSn(P)/HhpfSn(0) =1+kH⋅Pwith the coefficientkH=−0.095±0.015 GPa−1, whereHhpfSn(0)=4.60±0.12T. Diamond anvil cells were used to obtain high pressure at room temperature. The Curie temperature (TC) has been measured (up to 5.2 GPa) in the ‘‘toroid’’ high pressure device using thermal ac susceptibility measurements. That pressure dependence was linear with the coefficientdTC/dP=7.44 K/Gpa [TC(0)=341.3 K]. The compressibility of the sample has been measured by means of resistive strain gauges at hydrostatic pressure up to 9 GPa. The equation (V−V0)/V0=−a⋅P+b⋅P2has been fitted to the experimental data with the coefficientsa=8.64⋅10−3GPa−1,b=1.13⋅10−4GPa−2. The drop of theHhpfSnin pressure range 0–10.8 GPa can be explained on the basis of the empirical theory developed by Delyagin, Krylov, and Nesterov [JETP76, 1049(1980)]. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361130

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Theoretical results recently obtained for polarizability of intersecting spherical particles have been compared with those for polarizability of intersecting cardioidal particles resembling the spherical shape. Calculations based on a modified Parseval Rule have been performed for a number of metals in a dielectric matrix in a quasistatic approximation. A large difference in the polarizabilities has been found in spite of a striking similarity in their shapes. An unexpected small value for both the real and the imaginary part of the polarizability of cardioidal shaped metal particles occurs at wavelengths where simple metal spheres absorb strongly. A Maxwell‐Garnett model has been used to compare the optical properties of composite materials containing the cardioidal and intersecting spherical particles of interest. It has been found that a composite containing cardioid shaped particles in a dielectric matrix can have an unusual response. Its absorption is very close to zero, while the refractive index is close to that of the fully dense host. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361131

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Raman spectra of single‐crystal CoSb3and RhSb3and of polycrystalline IrSb3have been studied. We have also studied four different polycrystalline‐filled skutterudite samples, Ir4LaGe3Sb9, Ir4NdGe3Sb9, Ir4SmGe3Sb9, and Fe4CeSb12, where rare‐earth ions occupy the voids in the skutterudite structure. These void‐filling ions interact with some of the lattice vibrations in this structure and produce a shift and broadening of the observed lines. The most prominent lines in all of the samples are the Sb4ring‐breathing modes. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361132

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report here an investigation of selectively delta‐doped strained InGaAs/GaAs quantum wells. Electronic structures of the systems were calculated by self‐consistently solving the Schro¨dinger and Poisson equations and the calculations revealed a systematic variation of the band structure as the delta sheet moved away from the center of the well to the edge and finally to the barrier. The results were found to be in agreement with our photoluminescence (PL) measurements. For center‐doped samples, band‐gap renormalization was found to be strong from the PL data, and our realistic random‐phase approximation calculation for the heavily doped sample is in excellent agreement with the PL data. The radiative lifetimes were measured to be around 450 ps for all the samples, and surprisingly they vary very little from sample to sample although the wave‐function overlap was considerably different for some samples. We also report Shubnikov–de Haas (SdH) measurements on the two barrier doped cases. For the heavily doped sample (A12132), two oscillation signals were detected and they were identified as two upper subbands. The measured electron densities were in very good agreement with the self‐consistent calculation. Illumination did not make any difference to the measured densities. For the low‐doped sample (A12025), however, the measured electron density before illumination is much smaller than the calculated, and illumination was found to make a large difference. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361133

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Using dark‐field transmission electron microscopy images of ordered GaInP samples, we show how the ordering domain size depends on the growth temperature. Samples with different average domain sizes are compared with regard to their photoluminescence (PL) and excitation spectra. We find a close correlation between the size of the ordered domains and the relative intensity of the PL peak from band–band recombination compared with the rapidly shifting, below‐band‐gap luminescence emission. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361099

出版商:AIP    

年代:1996

数据来源: AIP