W. B. Muir

Formation, structure, and crystallization of Al‐rich metallic glasses

The formation, structure, and the crystallization of Al85YxNi15−x are studied using x‐ray diffraction and differential scanning calorimetry. The results show two distinct glasses depending on composition. Y‐rich glasses (x≥8) are homogeneous with a well‐defined glass transition. The x‐ray diffraction patterns have a single main peak. These glasses crystallize through a nucleation and growth process. Y‐poor glasses (x<8) do not show a glass transition and have a shoulder on the high‐angle side of the main peak in their x‐ray diffraction patterns. We show that the shoulder peak is due to quenched‐in Al nuclei. These glasses are shown to crystallize through the growth of these nuclei. Y‐rich glasses (x≥8) are more stable as demonstrated by the presence of the glass transition and their higher crystallization temperature, enthalpy, and activation energy. The occurrence of a prepeak for all compositions is attributed to Y‐Y pairs.

Effect of annealing on the giant magnetoresistance of sputtered Co/Cu multilayers

Co/Cu multilayers with the form of glass/Cu 50 A[Co11.5 A/Cu(tCu)]30/Cu50 A, prepared by rf triode sputtering, exhibit oscillatory magnetoresistance (MR), as a function of the Cu layer thickness with a period of ≂12 A and maximum MR values of 36.8% and 22.3% at 77 and 295 K, respectively. In order to study the effect of annealing on the structural and magnetic properties, five samples with Cu thicknesses between 9 and 34 A were heat treated at temperatures up to 300 °C and analyzed by low‐ and high‐angle x‐ray diffraction, MR, and mageto‐optic Kerr effect measurements. Annealing at moderate temperatures for the samples with Cu thicknesses around the second and third MR peaks leads to an initial increase in the MR. In contrast, annealing causes only a monotonic MR decrease for the sample at the first peak with a Cu thickness of 9 A.

Electrical resistance and crystallization characteristics of Fe80B20

The electrical resistance of amorphous Fe80B20 has been measured as a function of time at various temperatures during an isothermal crystallization process. The results fit a universal curve when ΔR/ΔRI is plotted against t/tI. The value of tI as a function of annealing temperature fits the Johnson‐Mehl‐Avrami equation with n = 3 changing to 1.4 for t/tI≳1.4.  The resistance of a series of partially crystallized samples was measured between 4.2 and 300 K. dR/dT at 300 K and dR/dlogT below the resistance minimum were both found to be linear funtions of ΔR/ΔRI. That is, both the above quantities were found to be strictly proportional to the amount of crystalline phase present. This may pose some difficulty for the structural tunneling model of the low temperature resistance minimum in metallic glasses.

Direct determination of cobalt site preferences at infinite dilution in iron‐based intermetallic compounds (invited)

Extremely low doping levels (∼1 ppm) and unambiguous interpretation combine to make the Mossbauer‐source technique an ideal method for determining cobalt site preferences in intermetallic compounds. Data on Gd2Fe17 and Nd2Fe14B are presented and compared with earlier work using Mossbauer spectroscopy, NMR, and neutron diffraction.

The thermopower of FeB metallic glasses

The thermoelectric power of FeB metallic glasses for B concentration the range 13 to 22 at.% B has been measured from 4K to 500K. The thermopower is extremely sensitive to B concentration and very non-linear with temperature. The data cannot be explained on the basis of the Faber-Ziman theory but require the inclusion of magnetic scattering.

Cumulative interface roughness and magnetization in antiferromagnetically coupled NiCo/Cu multilayers

Cumulative interface roughness and its influence on the magnetization process in antiferromagnetically coupled (Ni80Co20/Cu)×N multilayers is studied. In these multilayers, Cu and Ni80Co20 thicknesses are fixed at 20 and 15 A, respectively, in order to obtain the antiferromagnetic coupling at the second oscillation peak of giant magnetoresistance (GMR) versus Cu thickness. Low‐angle x‐ray reflectivity measurements show that cumulative interface roughness increases with increasing bilayer number N. In‐plane magnetization hysteresis measured with both SQUID and surface magneto‐optic Kerr effect (SMOKE) magnetometers are compared. When the cumulative interface roughness is significant, SMOKE hysteresis loops, which are sensitive to the top 5 or 6 magnetic layers, display a nonlinear plateau region at small fields. Comparison of low‐angle x‐ray, and SMOKE results show that interfaces of relatively high quality in top layers only exist for sputtered multilayer with N<10.

Thermopower of Fe-Zr metallic glasses

Abstract The thermopower of Fe-Zr metallic glasses for Fe concentrations in the range 20 to 43 at.% has been measured from 4K to 500K. The form of the thermopower changes dramatically from a nearly linear temperature dependence when the glasses are non magnetic to strongly non-linear dependence when the glasses become magnetic. The data for the non magnetic glasses can be explained using the usual Faber-Ziman approach. For the magnetic glasses inelastic magnetic scattering must be specifically included.

Evolution of structure and magnetoresistance in granular Ni(Fe,Co)/Ag multilayers: Dependence on magnetic layer thickness

Structural and magnetoresistance results on annealed sputtered (Ni81Fe19, Ni66Fe16Co18)/Ag granular multilayers are presented. Structural evolution has shown that highly (111) textured, discontinuous layered structures can persist on annealing up to 400 °C. The average magnetic particle size is controlled by the annealing temperature and the initial magnetic layer thickness. No giant magnetoresistance was observed in the as‐deposited films, while significant MR was found after annealing between 300 °C and 400 °C. Magnetoresistance over 30%, together with a small saturation field, was found at 4.2 K for a starting magnetic thickness of 4 A. Increasing the magnetic layer thickness to 20 A greatly improves the magnetic thermal stability, and leads to high magnetoresistive sensitivities of up to 0.35%/Oe in a field of 10 Oe at room temperature. The magnetization hysteresis, anisotropy, and magnetic interaction in such a granular multilayer are also discussed.

Composition and temperature dependence of coercivity of Nd‐Fe‐B alloys crystallized from the amorphous state (abstract)

As a first step to study the role of second phases in achieving excellent hard magnetic properties in Nd‐Fe‐B permanent magnets we have prepared Nd‐Fe‐B alloys, near the Nd2Fe14B composition, with a maximum variation of 10 at. % Nd and 5 at. % B. The alloys were first quenched into an amorphous state by melt spinning, as previous studies on crystallization kinetics suggest better control on the microstructure and hence better hard magnetic properties of the recrystallized alloys. For all compositions with Nd concentrations 1 at. % greater than that of Nd2Fe14B the intrinsic coercivity Hci is above 1.2 T at 100 °C. For a constant Nd/B ration, Hci increases monotonously with Nd constant while for compositions where even small amounts of α. Fe form as a crystallization product Hci shows a sharp drop. On the other hand, it appears that B content is much less significant in determining Hci. The temperature dependence of Hci is almost linear for all compositions between 20 and 200 °C and is much better than tha...

Abstract: Transport properties of amorphous metals

It is convenient to divide amorphous metallic alloys into two general categories :metal‐metalloid systems, such as Co‐P and the ’’metglasses’’, and metal‐metal systems such as Tb‐Ag or Y‐Ni. All these materials display such interesting transport properties as negative temperature coefficients and/or low temperature minima in the electrical resistivity. In this paper we review existing data and analyze them using both magnetic and non‐magnetic models. Data for the thermo‐power of metal‐metalloid alloys will also be presented. In the second part of the paper, we will discuss specific metal‐metal alloys. particular attention will be given to Yttrium‐based alloys, Cu‐Zr alloys, rare‐earth transition metal and rare‐earth noble metal alloys.