Magnetotransport and micro-x-ray absorption near-edge structure studies of glass-coated Fe – Ni – Cu microwires

Magnetotransport and micro-x-ray absorption near-edge structure (micro-XANES) studies of glass-coated 20Fe–20Ni–60Cu microwires have been carried out on both as-cast and heat-treated samples. The micro-XANES spectroscopy data were collected at the K edges of Cu, Ni, and Fe with the x-ray microprobe beamline at the Center for Advanced Microstructures and Devices. Comparison of the Fe K-edge spectra from the microwires to standard spectra reveals that the Fe atoms in the as-cast sample are in a face-centered cubic (fcc) configuration and they remain in the fcc phase throughout the annealing processes. Giant magnetoresistance (GMR) has been observed in the microwires and it reaches ∼6.5% at 300 K in a field of 9 T. The MR decreases to ∼2.5% as the annealing temperature increases to 500 °C. The loss of GMR upon annealing is attributed to the growth of Fe–Ni rich magnetic particles. Significant reduction in the number of extremely small particles is obtained after annealing at 500 °C, which shows MR characteri...

Magnetotransport and micro-x-ray absorption near-edge structure ͑micro-XANES͒ studies of glass-coated 20Fe-20Ni-60Cu microwires have been carried out on both as-cast and heat-treated samples.The micro-XANES spectroscopy data were collected at the K edges of Cu, Ni, and Fe with the x-ray microprobe beamline at the Center for Advanced Microstructures and Devices.Comparison of the Fe K-edge spectra from the microwires to standard spectra reveals that the Fe atoms in the as-cast sample are in a face-centered cubic ͑fcc͒ configuration and they remain in the fcc phase throughout the annealing processes.Giant magnetoresistance ͑GMR͒ has been observed in the microwires and it reaches ϳ6.5% at 300 K in a field of 9 T. The MR decreases to ϳ2.5% as the annealing temperature increases to 500 °C.The loss of GMR upon annealing is attributed to the growth of Fe-Ni rich magnetic particles.Significant reduction in the number of extremely small particles is obtained after annealing at 500 °C, which shows MR characteristics that are different from the as-cast and 300 °C annealed microwires.© 2000 American Institute of Physics.

͓S0021-8979͑00͒57708-1͔
Recently glass-coated alloy microwires have attracted great interest since they exhibit a rich variety of magnetic properties.The diameter of the microwires is in the range of 5-20 m and the thickness of the glass coating is around 2-10 m.Most of the studies have focused on their soft magnetic properties.2][3] Recently, glass-coated Fe-Ni-Cu microwires prepared by the Taylor technique have been shown to exhibit negative GMR. 4 There is no significant difference in the magnetoresistance between IЌH and I ʈ H configurations, which is characteristic of granular GMR.Both dc and low frequency ac ( f Ͻ1 kHz) measurements exhibit essentially the same values of magnetoresistance.The observed magnetoresistance in Fe-Ni-Cu microwires is of the same origin as that found in granular GMR materials and is distinguishable from the giant magnetoimpedance commonly seen in soft magnetic microwires.It is believed to originate from the spin dependent scattering of electrons by the magnetic nanoparticles imbedded in the nonmagnetic Cu rich matrix.This speculation, however, has yet to be experimentally verified and their magnetotransport properties need to be investigated further.In this work, structural examinations by micro-x-ray absorption near-edge structure ͑micro-XANES͒ and magnetotransport studies have been conducted on both as-cast and heat-treated samples.
The glass-coated Fe-Ni-Cu microwires were prepared using the Taylor technique described in Ref. 4. They were extracted from the lower end of a glass tube in which a Fe-Ni-Cu alloy ͑Fe 20%, Ni 20%, and Cu 60%͒ was melted by an induction heating coil.The thickness of the glass coating is 10 m, and the diameter of the metal core is 5 m.The microwires were later annealed for 1 h at 300, 500, and 600 °C, respectively.The x-ray diffraction data of all samples show the three major peaks due to the ͑111͒, ͑200͒, and ͑220͒ reflections of face-centered-cubic ͑fcc͒ lattices.As will be discussed later, two fcc phases, Ni-Fe rich ␥ 1 and Cu rich ␥ 2 , co-exist near the composition Fe:Ni:Cu ϭ20:20:60.The lattice parameters of the two fcc phases are close and not distinguishable in our x-ray diffraction data.
XANES spectroscopy was performed to elucidate the structure of the microwires.This technique was chosen due to its ability to provide local symmetry and coordination information in an element-specific manner.Micro-XANES measurements were performed using the x-ray microprobe at the Center for Advanced Microstructures and Devices ͑CAMD͒. 5The experiments were performed with the CAMD electron storage ring operating at 1.3 GeV with an average current of ϳ150 mA.The synchrotron radiation was monochromatized using Si͑111͒ crystals in the Laboratorio Nacional de Luz Sincrotron ͑LNLS͒ double-crystal monochromator. 6,7The x-ray beam was focused using a Kirkpatrick-Baez focusing system 8 with gold mirrors set at grazing angles of 6 mrad.The sample stage was scanned while imaging with the appropriate K␣ fluorescence signal to locate the focused x-ray beam (40 mϫ40 m) on the sample.XANES spectra were measured at the K edges of Fe ͑7112 eV͒, Ni ͑8333 eV͒, and Cu ͑8979 eV͒ in fluorescence mode by monitoring the K␣ fluorescence line with an energy dispersive germanium detector.Scans were performed over a range of Ϫ30 to Ϫ80 eV with respect to the appropriate absorption edge, with a step size of 0.5 eV and an integration time of 20 s.The total elapsed time for each XANES scan was approximately 1 h, and between two and six scans were performed on each sample.The spectra were summed and smoothed with a seven-point adjacent averaging algorithm to produce the spectra presented.
The results of the micro-XANES measurements are presented in Fig. 1.The XANES spectra can be used to identify the local structural environment based on ''fingerprinting'' of the spectra.The Ni and Cu K-edge spectra of the polycrystalline fcc Ni and Cu standards, respectively, can be used to identify the main features of the XANES spectra for these transition metals in a fcc structure.This consists of a preedge peak on the left shoulder of the edge ͑related to a 1s to a mixed p-d state͒ and a double feature at the edge ͑in the multiple scattering region of the x-ray absorption fine structure spectrum͒, and is followed by the first extended x-ray absorption fine structure oscillation ϳ50 eV past the edge.This can be contrasted to the body-centered-cubic ͑bcc͒ signature shown in the Fe K-edge spectrum of the Fe standard ͑a polycrystalline bcc Fe foil͒.By comparison to these standards, it can be seen that all the sample spectra reflect the fcc structure.For the Cu K-edge and Ni K-edge data, this is not unexpected.Cu atoms can be expected to be in a fcc environment, whether alloyed or segregated.Similarly, Ni atoms would be expected to be in a fcc environment.
The results at the Fe K edge are more interesting.If Fe atoms segregate out of the matrix, the Fe nanoclusters would be expected to have a bcc structure, which would be reflected in the XANES spectrum.However, the local coordination of the Fe atoms is clearly fcc in the as-cast and the annealed microwires.The magnetotransport behaviors of the microwires indicate the presence of magnetic nanoparticles dispersed in a nonmagnetic matrix. 4In the Fe-Cu system, such properties are due to the precipitation of nanoclusters of bcc Fe in the Cu rich matrix. 9,10The Fe K-edge XANES results indicate that Fe does not precipitate as bcc grains in this system.Rather, the data suggest that the magnetic phase is a Ni-Fe alloy.This is consistent with the ternary phase diagram, which shows that fcc Ni-Fe ͑␥ 1 phase͒ and fcc Cu ͑␥ 2 phase͒ co-exist around composition the 20Fe-20Ni-60Cu over the temperatures at which the annealing was conducted.3][14][15] Our micro-XANES study suggests that the Fe atoms remain in the fcc ␥ 1 phase throughout the annealing processes.It should be pointed out that Fe atoms have been found to exist in the bcc phase in an annealed 10Fe-10Ni-80Cu melt-spun ribbon as indicated in its Mo ¨ssbauer spectrum. 16Although the melt-spun ribbon has a slightly different composition than our microwires, the phase diagram alone could not explain why the two have different forms of Fe.
The magnetoresistance of the microwires was measured in a Quantum Design physical property measurement system ͑PPMS͒ system up to a maximum magnetic field of 9 T. Magnetoresistance MRϭ⌬R/Rϭ(R (H) ϪR (0) )/R (0) , where R (H) and R (0) are resistances at an applied field H and H ϭ0, respectively, were measured in fields both perpendicular and parallel to the wire orientations.There is no significant orientation dependence in any of the samples.Figure 2 shows the room temperature MR of the as-cast, 300, and 500 °C annealed microwires when the field was applied perpendicular to the wires.Data sets were not obtained for the microwires annealed at 600 °C due to the formation of severe cracks upon annealing.At room temperature and at 9 T the MR is about 6.5% for the as-cast sample, and it decreases slightly upon annealing at 300 °C for 1 h.The loss of GMR upon annealing is attributed to the growth of ␥ 1 phase magnetic Ni-Fe grains.It has been shown that grains of both ␥ 1 and ␥ 2 phases undergo progressive coarsening upon annealing at increasingly higher temperatures and longer durations. 15The grain growth upon annealing has been demonstrated in our microwires in the magnetic susceptibility versus temperature plots where the blocking temperature increases with the annealing.The blocking temperature, which is proportional to particle volume, is less than 50 and about 130 K, respectively, for the as-cast and annealed ͑at 300 °C͒ microwires.The grain growth effectively reduces the interface region between ␥ 1 and ␥ 2 phases which is mainly responsible for the GMR.This is further illustrated by the MR data of the microwire annealed at 500 °C, where the MR is substantially reduced to about 2.5%.The data also suggest that, although the grain growth starts as low as 300 °C, rapid coarsening occurs at a higher temperature, somewhere between 300 and 500 °C.An interesting feature of the MR data taken from the microwires annealed at 500 °C is that, in addition to the much reduced magnitude, MR exhibits a large positive curvature ͑Fig.2, low field region͒, which is absent from the as-cast and 300 °C annealed samples.The lack of any sign of saturation in MR in very high fields has been attributed to the extremely small magnetic particles typically present in granular materials; see, for example, Ref. 17.This is the situation for the as-cast and 300 °C annealed microwires.Upon heating to 500 °C, grain growth reduces the number of very fine particles, and the high field MR decreases significantly.Even in this sample the magnitude of MR increases almost linearly with the field in the high field region, which suggests that significant particle size distribution still exists in the 500 °C annealed microwires. 17hen et al. 18 reported a MR study on 20Fe-20Ni-60Cu and found that the MR of the sample annealed at 450 °C for 3 min was higher than the as-quenched sample.Upon further annealing for a longer time ͑30 min at 450 °C͒ or at a higher temperature ͑30 min 650 °C͒, the MR decreases again.The initial increase in MR after short annealing may be due to decomposition into the ␥ 1 and ␥ 2 phases from some single phase region that was retained as a result of the quenching.The annealing conducted in our study was of 1 h duration and thus may not reveal any possible decomposition ͑if any single phase region remains in the as-cast samples͒ that might occur after a shorter annealing time.In addition, the starting materials and quenching conditions were different.Our starting samples were obtained by quenching from melt to room temperature directly as opposed to quenching from the solid single phase region. 18It should also be mentioned that the MR in Chen et al.'s report 20Fe-20Ni-60Cu was attributed to interface scattering where both the particles and matrix are ferromagnetic.
In summary, 20Fe-20Ni-60Cu microwires were studied with micro-XANES spectroscopy, which shows fcc symmetry of the Fe atoms in the as-cast and annealed samples.The Fe exists mainly in the Fe-Ni rich fcc ␥ 1 phase, which grows in particle size upon annealing.The magnetoresistance of the annealed samples shows a corresponding decrease in magnitude as the magnetic particles grow.A significant reduction in the number of small particles is obtained after annealing at 500 °C, which shows MR characteristics that are different from the as-cast and 300 °C annealed microwires.

FIG. 1 .
FIG. 1. Micro-XANES spectra collected at the Fe, Ni, and Cu K edges.The micro-XANES spectra for the as-cast wire and for wires annealed at 300, 500, and 600 °C ͑collected in fluorescence͒ are shown, along with transmission spectra from polycrystalline Fe, Ni, and Cu standards.