摘要:

The composition and size dependence of the magnetic properties of crystalline Fe‐Co and Ni ultrafine particles has been investigated. Samples were prepared by sputter deposition onto sputter‐etched polyimide substrates. Little difference between the magnetic properties of crystalline and amorphous particles was found in Fe‐rich alloys, but we found a greater influence of magnetocrystalline anisotropy in Co‐rich alloys. When the particles are larger than 2500 A˚, the samples have negative anisotropyKu(easy direction parallel to the substrate plane). As the particle diameter decreases,Kubecomes positive, and the coercive fieldHcincreases from a few hundred oersteds to a maximum of more than 1000 Oe. Below a particle diameter of about 1000 A˚,Ms,Ku, andHcall drop rapidly toward zero, presumably due to the appearance of superparamagnetism. Annealing above about 200 °C leads to significant increases inHc,Ku, andMs, and also to significant changes in the line shapes of the x‐ray diffraction patterns, as a result of stress relaxation. A low‐temperature diffusion process leading to changes in the morphology of the subparticles may also be a cause of the increment of magnetic properties on annealing.

ISSN:0021-8979    

DOI:10.1063/1.340218

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The composition dependence of magnetic properties in ultrafine (Fe1−xNix)76Si8B16amorphous particles has been investigated. Samples were prepared by sputter deposition onto sputter‐etched polyimide substrates. The particle diameter was held constant at about 800 A˚, and theL/Dratio was approximately 4. The room‐temperature saturation magnetization (Ms) decreases with increasing Ni content and extrapolates to zero at aboutx=0.9. All the samples showed positive anisotropy (Ku). This anisotropy varied linearly withM2s, suggesting that its origin is largely the shape anisotropy of the particles. Furthermore, the measured perpendicular coercive field varied linearly withKu/Ms, as predicted by a simple Stoner–Wohlfarth single domain model. However, the absolute value ofHcand its angular dependence cannot be explained by the coherent rotation model.

ISSN:0021-8979    

DOI:10.1063/1.340219

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The design of sensor heads for fiber optic interferometric magnetometers is critical when sub‐nano‐Tesla fields are to be measured. Magnetically soft metallic glasses can exhibit large magnetostrictions and are candidates as materials for this application. However, their usefulness is limited at high frequencies through eddy current losses in bulk materials. This paper discusses the use of particulate metallic glasses in an inert binder. Measurements are presented on magnetization and magnetostriction of these composite materials in the quasi‐dc regime. Their dynamic behavior when used as magnetic field sensors is also described.

ISSN:0021-8979    

DOI:10.1063/1.340220

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have fabricated magnetic granular Fe‐SiO2solids over a wide metal volume fraction range 15%–100%. The microstructure of the samples has been characterized by using transmission electron spectroscopy, electron diffraction, and Mo¨ssbauer spectroscopy. Giant magnetic coercivity, as high as 2500 Oe, has been observed in samples with nanometer particle size. The behaviors of coercivity across the metal volume fraction range are attributed to the change of particle size and percolation effect.

ISSN:0021-8979    

DOI:10.1063/1.340195

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Rapidly quenched powders of Al74Mn20Si6were prepared by the ultrasonic gas atomization technique. Particles ranged from less than 40 &mgr;m to greater than 150 &mgr;m. The smaller particles were found to be primarily of the icosahedral phase. Larger powders showed an increasing proportion of the &bgr;‐Al‐MnSi phase. There was no evidence for the presence of &agr;‐AlMnSi commonly found in crystallized icosahedral AlMnSi alloys. Magnetic measurements showed the existence of a local Mn moment in all alloys. This moment was found to be 1.27 &mgr;Bfor the icosahedral phase and 0.63 &mgr;Bfor the &bgr; phase.

ISSN:0021-8979    

DOI:10.1063/1.340196

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The magnetic structure of small CoFe2O4particles has been investigated as a function of the particle size. Samples (in the 10–100 nm size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mo¨ssbauer spectra of the57Fe nuclei, obtained with a longitudinal magnetic applied field, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect of the crystallites that make up a particle is the origin of this phenomenon. A model is proposed in which the CoFe2O4crystallites that make up a particle consist of a core with the usual spin arrangement and a boundary surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependency of this structure is also examined.

ISSN:0021-8979    

DOI:10.1063/1.340197

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The magnetic properties of iron‐oxide particulates dispersed in silver (prepared by rf diode sputtering silver and iron oxide over a wide composition range) have been investigated using the Mo¨ssbauer effect, electron microscopy, and vibrating sample magnetometry. The sputtered films are nanocrystalline mixtures of immiscible particles of the two constituents (even when only a very small amount of silver is present). At room temperature, the Mo¨ssbauer effect data show that for most compositions (even those with low silver content) the majority of the film is superparamagnetic, with the fraction of material in the superparamagnetic state increasing with increasing Ag content. Consistent with these results, very narrow hysteresis loops were observed. On decreasing the temperature to near 100 K, the fraction of material in the superparamagnetic state decreased and the magnetically ordered component increased. Electron diffraction patterns clearly showed the presence of polycrystalline Ag and Fe3O4in the films, but composition analysis (0.03 &mgr;m resolution) was unable to separate these regions.

ISSN:0021-8979    

DOI:10.1063/1.341146

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Mercury‐based ferrofluids are prepared by using sodium amalgam to reduce iron into mercury while agitating mechanically and magnetically by an ordinary magnetic stirrer. The field dependence of the room‐temperature magnetization is measured and used to determine the particle size distribution and its aging behavior. Samples prepared by the conventional method of electroplating into a mercury cathode are also studied for comparison. It is shown that the samples prepared by the two methods are very similar and the small amount of sodium remaining in the samples prepared from sodium amalgam can enhance the stability.

ISSN:0021-8979    

DOI:10.1063/1.340198

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The magnetization process of ferrofluids with carrier fluids of water, paraffin, and alkylnaphtalene was investigated in a temperature range from 77 to 300 K as functions of the freezing rate and the intensity of cooling magnetic fields. A uniaxial magnetic anisotropy is induced by field cooling in frozen ferrofluids. This induced anisotropy which is caused by the formation of clustering of magnetic particles disappears on heating near the melting point. The magnetization of the frozen fluids exhibits an anomalous increase below the melting point of the carrier fluids. In a frozen state, no relative motion of particles, such as Brownian motion, occurs. With increasing temperature, the inhibited motion will be released gradually by the recovery of viscosity. The anomaly may be related to glassy transition and premelting. These magnetic properties are discussed in terms of mesoscopic phase transition between liquid and solid.

ISSN:0021-8979    

DOI:10.1063/1.340199

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Magnetotransport measurements provide an ideal probe to determine the various anisotropy energies in epitaxial magnetic films. The extraordinary Hall effect (EHE) can be used to determine the perpendicular or surface anisotropy energy while the anisotropic magnetoresistance (AMR) can be used to investigate the in‐plane anisotropy energies. The advantage of magnetotransport over more tranditional measurement techniques used to determine these anisotropy energies is the ease of the technique, the lack of a need for sophisticated equipment, and the insensitivity of the techniques to the magnetic properties of a semiconducting or insulating substrate. Both the EHE and the AMR have been used to study the magnetic properties of epitaxial iron films grown on GaAs substrates. The results of the EHE and the AMR study and how the various anisotropy energies compare with those determined by the more traditional techniques of ferromagnetic resonance and vibrating sample magnetometry will be discussed.

ISSN:0021-8979    

DOI:10.1063/1.340200

出版商:AIP    

年代:1988

数据来源: AIP