Scripta Metallurgica et Materialia最新文献

We have prepared mechanically alloyed samples of FeNi, FeNi₃, and (FeNi₃)_{1 − x}Ag_x. The production of nanometer size grains was confirmed by x-ray broadening and electron microscopy. X-ray measurements indicated that a small amount (~7%) of Ag could be alloyed into the Fe-Ni and that considerable lattice strain was present. Mössbauer and magnetic measurements found no evidence for superparamagnetism in these alloys.

Giant magnetoresistance (GMR) has been studied in electrodeposited CoNiCu/Cu multilayers. The value of the change of resistance with field, ΔR, is almost constant when the temperature is lowered to 15 K, whereas the $\text{GMR =}\text{ΔR}\text{R}$ is, of course, increased. The magnetic field dependence is unchanged with temperature, demonstrating that the GMR is not thermally activated. Lorentz electron microscopy was used to investigate the nature of the magnetic domains. The layers are antiferromagnetically coupled, and the domains extend without interruption for ≈ 200 nm along the layers.

(SiO₂)_{1 − x}Fe_x samples, with 0.2 ≤ x ≤ 0.4, were prepared by high energy ball milling. We studied in these samples the temperature dependence of the magnetization, coercive force and thermally activated demagnetization parameters. From all these results it was concluded the occurrence, in all the examined samples, of two different maxima in the blocking temperature distributions, identified as corresponding to the extremes of the particle size distribution.

The microstructure and magnetic properties of phase-separated (FeAg, CoAg) granular and multilayer Permalloy (Py)/Au magnetic thin films exhibiting giant magnetoresistance (GMR) havebeen investigated. Surprisingly, two Fe-Ag films of similar composition grown under identicalconditions and having substantially different microstructures yet display similar GMR. The microstructure of these films is characterized by Fe-rich or Co-rich regions respectively, 350–700 nm in extent and surrounded by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentration varies from 20 to 25 atomic % and the Co concentration is ~16 at%. Within these regions essentially pure fcc Co particles and bcc Fe particles are in parallel and rotated epitaxial alignment respectively with the fcc Ag matrix. The Co and Fe particles are ~15 to 25Å in diameter. It is these small particles which account for the GMR exhibited by these alloys. This suggests that a size distribution of magnetic particles, sharply peaked at the optimum size with limited bulk segregation might give rise to larger GMR values. On the other hand, high resolution electron microscopy (HREM) analysis of as-grown permalloy (Py)/Au multilayers in (111) orientation revealed that the Au and Py layers have sharp interfaces with defects such as twin boundaries and misfit dislocations. The density of twin boundaries decreases from the Pt-seed layer to the top surface of the multilayer. An average of 1 twin boundary/195 nm² has been observed. On mild annealing, the twin density decreases and the multilayer interfaces become rough. The magnetoresistance of the as-grown Py/Au multilayer decreased from 11% to 8% on annealing at 250 °C for 40 minutes. From the measured roughness and total thickness of Au layers it was found that Au preferentially intermixes with Py, which reduces the magnitude of GMR.

Microstructural influence on saturation magnetization, remanence, coercivity and giant magnetoresistance (GMR) of melt-spun Au_1−x-Co_x, (x = 0.65 and 0.284) ribbons in the as-quenched state and subsequently annealed at 480 °C has been investigated using analytical transmission electron microscopy. The evolution of the magnetic data with annealing time was found to be determined by the sequence of the formation of Co precipitates, their volume fraction, their particle size distribution and their morphology. GMR behavior was only measured for Au_73.6Co_28.4 and is clearly caused by a lamellar eutectic with a structure similar to multilayers.

Two approaches are discussed to understand the coercivity mechanism of multilayer ($\text{Co}\text{Pt}$) and ($\text{Co}\text{Pd}$) films with high coercivity and high perpendicular magnetic anisotropy. One approach follows the phenomenological model proposed by Kronmüller et al^(1,2), and the other is the one based on the relaxation mechanism⁽³⁾. The first approach provides the size of the pinning region. Using the measured values of coercivity, magnetization and perpendicular magnetic anisotropy constant as a function of temperature, and assuming the exchange constant and anisotropy of a pinning region to be a half the matrix value, and zero, respectively, one obtains a few Å for a pinning size in the multilayers. The second approach is based on the dependence of apparent coercivity on sweep speed of magnetic field. It is shown that the Barkhausen volume is of the order of 10⁻¹⁸cm³ for those multilayers. The results are compared to the case of Tb₂₆(FeCo)₇₄ amorphous films.

A review is given of the basic concepts which underlie the magnetostrictive behavior of ferromagnetic materials. The microscopic, i.e., atomic and electronic properties of magnetoelastic ferromagnets are discussed, with an emphasis on anisotropic magnetostriction. The latter is defined as the change in dimension of a ferro-magnet caused by rotation of the magnetization. Formulae for length changes along a given direction in terms of the magnetization direction are derived for cubic and hexagonal crystals. Approximations for the saturation strain of polycrystalline aggregates are discussed and compared with data on cubic crystals. Results of saturation magnetostriction measurements of rare-earth compounds and alloys are reviewed and tabulated. Finally, a brief discussion is given of the relationship between lattice structure and magnetostriction.

We present a simple closed-form expression for the reflectivity from a multilayer which includes the effects of absorption, refraction, surface and substrate reflections but neglects dynamical effects. This expression reproduces the exact dynamical calculation except for the regions near the critical angle and for intense Bragg reflections. The expression is generalized to include cumulative interface roughness which follows a $\text{t}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ power-law growth.

Hooke's law is used to calculate the elastic stress and strain in interstitial magnets. Emphasis is put on spherical and plate-like Sm₂Fe₁₇N_x particles with continuous and discontinuous interstitial concentration profiles, hypothetical high-performance Sm₂Fe₁₇N_3−δ/$\text{Fe}$Co composites, and ‘natural’ multilayers such as Nd₆Fe₁₃CuH_x, where the recently discovered large anisotropic lattice expansion is shown to arise from the crystal structure.

Das Hookesche Gesetz wird benutzt, um elastische Spannungen und Dehnungen in Zwischengittermagneten zu berechnen. Hauptgegenstand der Untersuchung sind kugelförmige und flache Sm₂Fe₁₇N_x Teil-chen mit kontinuierlichen und diskontinuierlichen Zwischengitterkonzentrations-profilen, hypothetische Sm₂ Fe₁₇N_3−δ/$\text{Fe}$Co Hochleistungskomposite und natürliche Vielfachschichten wie Nd₆Fe₁₃CuH_x, deren unlängst entdeckte groβe anisotrope Gitteraufweitung auf die Kristallstruktur zurückgeführt wird.

Large differences have been observed in the coercivity values of $\text{Fe}\text{SiO}{}_2$ and $\text{Fe}\text{BN}$ granular films with the coercivities being much lower in the BN matrix. We have used Mössbauer spectroscopy in an attempt to understand the differences in the magnetic hysteresis behavior. The Mössbauer data suggest a core/shell morphology for the $\text{Fe}\text{SiO}{}_2$ films with an Fe(Fe-Si) core surrounded by an Fe-Si-O shell whose surface consists of Fe²⁺-Si-O grains. In $\text{Fe}\text{BN}$ the data indicate the presence of amorphous Fe-B particles in a BN matrix. This hypothesized particle morphology can explain the high coercivity values in SiO₂ films and the low H_c in BN because of the low anisotropy of the amorphous phase.