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Notes: A fiber-optic sensor was fabricated with a magnetostrictive multilayer thin-film coating synthesized to have a combination of properties favorable for resolving mm scale inhomogeneities in low level magnetic fields (∼100 mOe) varying at MHz rates. High-frequency detection was achieved by mixing signal and local oscillator fields varying at frequencies f and f+fmr/2, into the mechanical resonance of the sensor at fmr. These sensors are expected to be useful for the noninvasive detection of RF magnetic fields, and for mapping the eddy current induced magnetic-field anomalies in nondestructive test evaluation. Since fiber-optic sensing techniques have high immunity to electromagnetic interference, the sensor reported here is compatible with integration into mapping arrays. © 1997 American Institute of Physics.

Notes: 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 systems, to permit comparison and calibration. The center conductor of the microstrip was photolithographically patterned in a 6 μm thick gold film electroplated on a high quality microwave alumina substrate. A quasi transverse electric mode (TEM) was launched on the microstrip from a well-matched transition to a 3.5 mm coaxial connector. The microstrip resonator was excited by coupling to the microstrip transmission line through two 50 μm capacitive gaps (∼0.07 pF) in the center conductor. The measurements were performed with HP 8510 network analyzer. The equivalent circuit of the microstrip transmission resonator and coupling capacitors were used to compute the 22 transmission resonances expected in the 0.4–10 GHz range. The results compare well with the measured resonances indicating the applicability of the quasi static approximation to 6 GHz. A perturbation theory for calculating the permeability from the measured cavity parameters was formulated for the microstrip resonator. Initial measurements on microscale magnetic objects are in good agreement with those obtained from other measurements.© 1997 American Institute of Physics.

Notes: The effect of interface mixing on the magnetic properties of multilayer thin films consisting of N Co50Fe50/Ni80Fe20 bilayers was investigated for N=1–25. A fixed 50 Å Ni80Fe20 thickness was used in all the multilayers, but the Co50Fe50 layer thickness was varied as 1260 Å/N. With increasing N, the saturation magnetorestriction (λs) of the multilayer films increased from 40 to 58 ppm, but both the anisotropy field (Hk) and the coercivity (Hc) decreased, from 86 and 30 Oe, to 20 and 2.5 Oe, respectively. λs varied linearly with the 1/modulation thickness (D), confirming that the increase was due to interface mixing in the bilayers. From x-ray data, and the variations of the saturation magnetization (4πMs) and resistivity (ρ) of the multilayers, it was determined that the interfacial layer resulted from the diffusion of Fe from the NiFe layers into the CoFe layers. The sensitivity (dλ/dH), and the permeability of the multilayers also increased with N. The utility of this combination of properties for magnetostrictive sensor applications has been demonstrated in a related work.© 1997 American Institute of Physics.

Notes: Nanometer-scale thin-film heterogeneities are important for determining the magnetic properties of amorphous, soft magnetic films.1 The remanent-magnetization distributions in ultrasoft films appear to result from the balance of magnetostatic and exchange energies associated with the nanometer-scale heterogeneity.2 In sputtered films this balance is determined by the deposition and annealing parameters. Previous work focused on CoB binary-alloy films which were compositionally inhomogeneous with a two-phase amorphous structure.3 To determine whether compositional inhomogeneities are a requirement for ultrasoft-film behavior, we studied amorphous, Co-rich alloy (Co87Zr5Nb8) films not expected to be compositionally inhomogeneous in the as-deposited state. Electron-diffraction-scanning analysis in a scanning-transmission-electron microscope showed that the morphologies of the as-deposited films were featureless. On the other hand, the morphology of films annealed in a rotating field coarsened and detailed electron probe analysis (EPMA) confirmed that the annealed films were compositionally inhomogeneous with two amorphous phases. Magnetic-hysteresis and rf-susceptibility measurements clearly revealed that the onset of ultrasoft-film response was correlated with the latter. We discuss the possible magnetostriction in these inhomogeneous films and its effect on the experimentally observed high-frequency susceptibilities.

Notes: Multilayered Co–B films with various bilayer periods (BP=TCo+TB, TCo=4×TB) and ratios of bilayer thickness were ion beam mixed with 150 keV 1016 Ar ions/cm2. The morphologies of both the as-deposited and the ion beam mixed films changed with BP. The ion beam mixed films exhibited amorphous structures, and the morphologies differed as a function of layer thickness in the as-deposited state. The morphological differences appeared to influence magnetic properties such as in-plane anisotropy and coercivity. Magnetization measurements revealed that the compositional inhomogeneities resulted in the anomalous extension of ferromagnetic composition range compared to melt-quenched films.

Notes: Compared to single-layer films, CoZrNb/SiO2 multilayers with amorphous, soft magnetic films exhibit increased high-frequency response (to about 100 MHz) that is not understood. We studied single and multilayer films in this system and observed three distinct types of magnetic bias and frequency responses (phases I–III). The high-frequency responses of phase II and III films were reduced from that of phase I. Phase changes were produced in the single-layer amorphous CoZrNb films by varying the film thickness, and in double-layer (N=2) and multilayer (N〉2) films by varying the magnetic layer thickness. The phase boundaries in the former were shifted by magnetostatic coupling of the perpendicular component of M. These studies indicated that phase I films had uniform planar magnetizations, while phase II and III films had perpendicular components. The results are summarized in the form of a phase diagram (film thickness versus inverse film separation), which suggests that the frequency response of multilayer thin films is enhanced when the individual magnetic layers are sufficiently thin to insure a planar magnetization (phase I).

Notes: Artificially layered structures have attracted attention because of their unique structural and magnetic characteristics. It has been reported that in these structures the anisotropic distribution of atomic pairs can play an important role in determining their magnetic anisotropy.1 Our previous work showed that the application of substrate bias in a rf diode sputtering system was effective in producing dense and stable amorphous compositionally modulated films (CMFs).2 In this work, we studied the interrelationships of the structural and magnetic properties of amorphous Tb-Fe CMFs deposited under various substrate bias conditions (0, −70, −90 V) in a multitarget rf diode sputtering system. Although considerable research has been conducted on the effect of ion bombardment during deposition on the film structure, it has been difficult to independently control the film structure and the composition in such experiments because of the preferential backsputtering of atoms.Accordingly, in this research we systematically prepared Tb-Fe CMFs by independently controlling the rf voltage ratio of both the Tb and Fe targets, thereby allowing the composition of the film to be held constant as the substrate bias was changed. In contrast to alloy films, atomic clustering was observed in ultrathin layered films and produced anomalous structural and magnetic property changes, e.g., a shift in the room-temperature compensation composition (25 at. % Tb for −70 V biased films and 23 at. % Tb for −90 V biased films), higher Curie temperatures, and an increased compositional range over which amorphous phase was stable. The atomic arrangements in layered films were modified by a variation of deposition parameters such as layer periodicity, substrate bias, and composition. Although all the bias-sputtered CMF films, except those with small Tb content (〈18 at. % Tb), had a dense and featureless amorphous structure, electron diffraction revealed significant differences in atomic short-range ordering for different Tb compositions. The results indicated that the magnetic moments of the Fe atoms in the CMF were increased by an enhanced pair ordering of like atoms produced by the alternate deposition of ultrathin layers. This result was consistent with the observed anomalies in magnetic properties.

Notes: We investigated the effect of bias sputtering on the oxidation resistance of amorphous Tb32Fe68 compositionally modulated films (CMFs). The columnar morphologies of aged, unbiased films were more diffused than those of as-deposited films. On the other hand, the morphologies of the −70 V bias sputtered, aged film were as dense and featureless as those of as-deposited films. The substrate bias also influenced the magnetic and magneto-optical characteristics of aged films. The Kerr rotation obtained from the film surface was significantly increased by the formation of a transparent, nonmagnetic oxide layer, and by optical interference between this layer and the unoxidized amorphous matrix. The coercivity of the amorphous matrix in unbiased films increased markedly with aging time. These changes are attributed to compositional changes enhanced by the easy diffusion path of oxygen in the columnar void structure. In contrast, the coercivity of −70 V bias sputtered films was unchanged with aging, and exhibited the B-H loops of exchange coupled double layers. The results indicated that the intrinsic stability of Tb-Fe CMFs was strongly influenced by the film microstructure and was improved by the application of substrate bias during deposition.

Notes: 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(ring) 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(ring)) 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.

Notes: Sequentially cosputtered, amorphous thin films with an average composition of 〈Co61B39〉 were found to have an anomalously high Curie temperature (〉575 K) compared to those reported for their bulk counterparts. Melt-spun Co-B ribbons, for example, with only 30-at. % B(max) have a measured Tc ≈500 K. Tc of ≤300 K would be expected for ribbons with 39-at. % B. The sputtered films crystallized at ≈675 K, but became extremely soft when annealed in a rotating field at 500 K≤Tc. The coercivity of the as-deposited films was reduced from 1 to 2 Oe to approximately 0.01 Oe and the large uniaxial anisotropies of 5–10 Oe vanished; the annealed films were isotropic. Extremely large initial permeabilities of 8000–10 000 were measured to over 10 MHz. The observed anomalies indicate that the center regions of the columnar structure of the sputtered films were enriched by ≈10% Co compared to the boundaries.