A. P. Valanju

High‐frequency bias susceptibilities of ultrasoft CoB amorphous thin films

Resultats des premieres mesures a haute resolution des susceptibilites transversales de couches minces amorphes de CoB recuites sous champ rotatif, effectuees entre 2 et 200 MHz. Mesures systematiques de la variation avec le champ de polarisation des susceptibilites transverses χ t (0) et χ t (π/ 2 )

Magnetic and electrical properties of CoFeSiB:O thin films near the percolation threshold

Nanocomposite thin films with coexisting magnetic metal and magnetic nonmetal amorphous phases were synthesized by reactively sputtering CoFeSiB:O thin films with a large silicon (15 at.%), and varying oxygen concentrations. The microstructure, magnetization, and hysteresis loops were measured for films with resistivities near the metal–nonmetal transition. These data revealed that, below the metal–nonmetal transition, conductive transport was along the soft magnetic, metallic backbone of the percolation network; the nonmetal phase was a discontinuous, randomly distributed, hard magnetic oxide. The metallic resistivity and exchange anisotropy were both maximized in a nanocomposite with a resistivity at the metal–nonmetal transition.

Magnetic properties of sequentially sputtered amorphous Fe-Ge thin films

Fe/sub x/Ge/sub 100-x/ films with compositions in the range 50/spl les/x/spl les/82 at 300 K were deposited by rf diode, sequential sputtering from elemental targets. All the films were amorphous at 300 K indicating that the Fe concentration in the stable amorphous films was increased by more than 10 at. % over that obtained by vapor deposition. The sequentially sputtered Fe-Ge films exhibited a heteroamorphous morphology with nanoscale features that varied with composition. The variations in the 4/spl pi/M/sub s/ and T/sub c/ for the sputtered Fe rich films, agreed well with data extrapolated from measurements on Ge rich evaporated films. The improvement in the soft magnetic properties produced in these films by rotating magnetic field annealing (RFA) was shown to be correlated with changes in film morphology.

Soft magnetic characteristics of Co/Zr thin‐film layered structures (abstract)

We have extended the stable amorphous phase of the Co1−xZrx system to 7<x<52 by compositionally modulating very thin (five to ten atomic layers) layers of Co and Zr by rf diode sequential sputtering from the elemental targets onto rotating, water cooled substrates. This extension was attributed to a stable, two‐phase, hetero‐amorphous state similar to that previously reported for Co/B layered structures (Ref. 1). The measured compositional variation of Ms of our Co/Zr layered structures indicated that, as expected, the composition of the stable Co‐rich clusters in the heterogeneous state lies near the stable Co‐rich eutectic of the CoZr system. The composition variation of the coercivity of the as‐deposited Co/Zr structures was remarkably low (<1 Oe) over the entire composition range. The uniaxial anisotropies of 15–30 Oe were reduced to ≊2 Oe by a combination of bias sputtering and rotating field annealing. Although they have reduced Ms, the permeabilities of the Zr‐rich films are ≳1200, and constant (to...

Structural inhomogeneity and magnetic properties of amorphous Gd17Co83 compositionally modulated thin films

Amorphous Gd17Co83 compositionally modulated films (CMFs) with different bilayer periods were prepared by sequential deposition in a rf diode multitarget sputtering system. Morphological inhomogeneities with variable size, shape, and uniformity on the scale of 20–100 A were produced by variations in deposition parameters including substrate bias. Data from small‐angle electron scattering and Fourier digital image processing of TEM micrographs revealed that the morphological inhomogeneities were due to compositional fluctuations. The −65 V bias sputtered films (bilayer period <10 A) exhibited uniform small particles, while the unbiased films showed a wide distribution of morphological inhomogeneities. Magnetic domain observations indicated that the perpendicular magnetic anisotropy was determined by the size, shape, and distribution of the film inhomogeneities. The results suggested that the perpendicular magnetic anisotropy was a result of exchange coupling between uniformly shaped inhomogeneities.

Materials Issues in the Application of Ferromagnetic Films in Tunable High Q VHF/UHF Devices

Thin ferromagnetic films have narrow ferromagnetic resonance bandwidths, and large RF permeabilities that are attractive for tunable high Q device applications. They can also be processed at low temperatures and integrated with most RF and microwave materials. At VHF/UHF frequencies, where discrete devices can be realized, magnetic loss tangents (Q −1 ) less than 10 −3 can be obtained, if the effects of spin resonance and eddy current losses are reduced. The material figure of merit requires a tradeoff of the permeability, and Q. The highest FOM is obtained by maximizing the magnetization, anisotropy, resistivity, and minimizing the magnetic loss. This paper discusses the interplay of these properties, and the fill factor and magnetic bias required for tunable inductors with Q>1000 in the ∼50-500 MHz range. The most promising candidates for these applications are shown to be reactively sputtered nanocrystalline films. The large resistivity, magnetization, and thickness of these films can be used to attain large film permeances, and increased shape anisotropy fields from patterned film objects. The latter eliminates the need for using a large bias magnet to achieve Qs ∼1000 at VHF/UHF operating frequencies.

Multilayer CoFe/NiFe Films for Microwave Applications

ewidths are very narrow indicating a high degree of magnetic homogeneity. The films are easily magnetized in-plane since the shape anisotropy is much larger than any other anisotropy, even for small Fe-thickness (n =3ML). The temperature dependence of the second and fourth order magnetic anisotropies’ has been studied for a specific sample, Fe4V4. The special interest for this sample is that its Fe-thickness is very close to the critical thickness of n=3 ML where an isotropic in-plane behavior has been recently reported.’ A reorientation of the easy axis from the [IOO] to the [ I I O ] in-plane direction as a result of competition between volume K4V// and surface K4S// fourfold anisotropies favoring the [ 1001 or [ 1 IO], respectively, is expected to occur at T=450 K. Finally. a very small stepinduced in-plane uniaxial anisotropy K2// is recorded indicating the existence of very large atomically flat terraces on the sample, supporting the XRD data.

Microstrip resonator for GHz measurements of microscale magnetic objects (abstract)

The integration of thin ferromagnetic films for applications above 1 GHz is expected to require micron scale, thin film objects (∼100 μm×100 μm×0.1 μm) with volumes less than ∼10−15 m3. With maximum relative susceptibilities of ∼100, the measurement of these micro-objects by small perturbative techniques is experimentally challenging. Here, we describe the design, calibration, and performance of a microstrip resonator with a cavity volume of <10−6 m3 and a cross-sectional area <1 mm2. We have designed and built a gap-coupled, transmission-type, linear microstrip resonator. At even (odd) harmonics of the fundamental resonance, maxima of the electric (magnetic) field are established at the midpoint of the linear resonator. This resonator was designed to measure the magnetic responses of ensembles of micron scale ferromagnetic objects in the 0.4–10 GHz frequency range. The lowest resonant frequency (400 MHz) intentionally overlapped the upper operating frequency range of our lower frequency measurement syste...

GHz ferromagnetic resonances in laminated microscale objects (abstract)

Recent research1 has indicated that ferromagnetic laminates are potentially useful in the design of integrated monolithic microwave integrated circuit (MMIC) devices operating at GHz frequencies. Laminates with n=10–50 insulated, uniaxial magnetic films can have large permeabilities and device fill factors, and are process compatible with monolithic integration. The effect of lamination disappears, however, for frequencies greater than the frequency of a dimensional resonance (DR) associated with the easy axis dimension of the laminated object.23 Thus, when the ferromagnetic resonance (FMR) occurs at a frequency greater than the DR, the FMR spectra of the magnetic films may be distorted or, possibly, not observed. The bandwidths of both the DR and the FMR will be significantly broadened, if either lies above the eddy current relaxation (ECR) frequency of the individual magnetic films. Prior models23 of the permeability of laminated objects were developed for magnetic recording head laminates operating at ...

In-Situ, Time Resolved, Kinetics of Reactions in Co-Ge thin Films

We report the first kinetic study of metal-semiconductor interface reactions using in-situ, time resolved, laser interferometry. Diffusion couples with Co/Ge thicknesses of 1500 A/1500 A were sputter deposited on silicon wafers, and vacuum-annealed at temperatures between 300°C-400°C. Under these conditions polycrystalline CoGe was expected to form [1]. Real time laser (HeNe 6328 A) interferograms for each anneal were recorded in-situ . These data were supplemented by information from AES and X-ray. For temperatures below 400°C the diffusion controlled formation of CoGe was observed. The composition was confirmed by Auger depth profiling that showed uniform Co and Ge concentrations when the reaction went to completion. The well defined interferences fringes were formed by the dissolution of amorphous Ge. The activation energy = 1.6 eV for the formation of CoGe were determined with precision from the temperature dependence of the time required to anneal the fixed λ/4 distance between adjacent minima and maxima of the interferogram. We discuss the evidence for formation of an intermediate Co-rich compound following the initial diffusion of Co into Ge. The results of these experiments indicate that optical interferometry will be a valuable adjunct to other techniques used to study metal-semiconductor interface reactions.