Yong-Goo Yoo

Chemical states of Co and Fe in a specularly reflective oxide layer in spin valves

In an examination of near-edge x-ray absorption fine structure spectra and magnetoresistance responses of a spin valve with a partially oxidized Co90Fe10 layer, we found that oxygen preferentially reacted with the solute Fe rather than with the solvent Co, and the very oxidized Fe likely enhanced specular reflections leading to an increase in the magnetoresistance value. Most Fe oxides transformed into α-Fe2O3, which further improved the magnetoresistance response after the spin valves were annealed. Our results show that obtaining appropriate oxide phases and controlling their quantity are the key to optimizing the magnetoresistance performance and magnetic properties of spin valves with specularly reflective oxide layers.

Annealing Temperature Dependence of Exchange Bias Effect in Short Time Annealed NiFe / NiMn Bilayer Thin Film by FMR Measurement

The NiMn/NiFe bilayer structure which was short time annealed in order to induce unidirectional anisotropy were studied as a function of annealing temperature. The maximum exchange bias field of NiMn/NiFe bilayer was presented at 250℃ after short time annealing process with no external field. The appearance of exchange bias was due to phase transformation of NiMn layer. In plane angular dependence of a resonance field distribution which measured by FMR was analysed as a combined effect of unidirectional anisotropy and uniaxial anisotropy. The resonance field and the line width from FMR measurement were also analysed with annealing temperature.

EXAFS study for mechanically alloyed Co50C50

Co 50 C 50 alloys were fabricated by mechanical alloying process from the mixed powder of pure cobalt and carbon. The variations of the local structure and magnetic properties were examined by EXAFS analysis and the VSM magnetometer. It is shown that the alloying was progressed as the milling time increases. The significant changes of the magnetization and the structure has been occured in 12 h milling. EXAFS spectrum and its Fourier transform confirms that the final product is in the amorphous state. The EXAFS spectrum of the alloy milled for 24 h was fitted to the conventional EXAFS equation. The interatomic distances obtained from the fitting were 1.94 ±0.02 A for Co–C pair and 2.52 ±0.02 A for Co–Co pair, respectively. The magnetization was almost saturated in 24 h milling.

Structural evolution of Fe80C20 alloy with alloying times

The structural evolution of mechanically alloyed Fe-C alloys was studied as a function of alloying times. The effect of alloying time on local structural changes of Fe-C has been investigated by means of Fe57 Mossbauer spectrometry, extended x-ray-absorption fine structure (EXAFS), and x-ray diffraction (XRD). XRD pattern from 24h alloyed Fe-C powder indicates at least the mixture of bcc-Fe and Fe3C phases. Mossbauer spectra analysis reveals that bcc-Fe decreases to the detriment of Fe3C phase with increasing alloying time, while both carbon-containing bcc-Fe and amorphouslike phase assigned to Fe located in grain boundaries (estimated at two atomic layers) remain alloying time independent. The variation of Fe3C phase content is in a good agreement with that observed by EXAFS analysis.

Two magnetic phases in the Fe/sub 20/Co/sub 30/Cu/sub 50/ solid solution

The magnetic property of supersaturated Fe/sub 20/Co/sub 30/Cu/sub 50/ solid solution, prepared by mechanical alloying, has been investigated by using magnetometry, Mossbauer spectroscopy and inelastic neutron scattering measurements. Magnetization measurement of the alloy could be analysed by ferromagnetism only. However, Mossbauer spectroscopy and inelastic neutron scattering experiment showed that not only a ferromagnetic phase but also a paramagnetic phase of about 12% coexisted in the alloy. Spin wave stiffness constant obtained from the analysis of the temperature dependence of magnetization was similar to the value obtained from analysis of inelastic neutron scattering experiment.

Structural and Magnetic Properties of Fe₅₀Cr₅₀ Alloys Prepared by Mechanical Alloying Method

Fe50Cr50 metastable alloys were prepared by the mechanical alloying method with milling periods of 1, 2, 4, 6, 12 and 24 hours, respectively. The structural evolution was analyzed by the extended x-ray absorption fine structure (EXAFS). In this work, the EXAFS analysis provided the local structural information around Fe central atom. The saturation magnetization was also measured by VSM. The magnetization decreased as the process mechanical alloying progressed. The magnetic property was related to the local structural variation as a function of processing time. The analysis showed that the diffusion Cr atoms into Fe clusters caused the reduction of magnetization. EXAFS analysis exhibited that the local ordering of magnetic atoms caused the magnetic ordering. Also, EXAFS analysis showed that the long range order of Fe atoms was destroyed completely in 24 hour milling.

Local Ordering Study of Nanostructure FeMnAl Alloys

Fe55Mn10Al35 alloys were prepared by mechanical alloying. Local ordering around the central atom was examined with milling time by using extended X-ray absorption fine structure. The structural and magnetic evolution was analyzed by EXAFS and Mossbauer study as a function of milling time. The first shell of the Fourier transformed spectra changed with milling time, indicating a change in the local ordering around the central Fe atom. X-ray diffraction patterns indicate a bcc phase after 24 h milling time. For 1 h milled sample, Mossbauer spectra analysed three sextets as bcc-Fe phase, broad sextet and Fe surrounded Al and Mn phase. The magnetization showed decrease with milling time corresponding to magnetic dilution.

Structural and Magnetic Properties of Ni

The magnetic properties and structure of Ni45Al45C10 nanocrystalline alloys prepared by the mechanical alloying process are studied as a function of milling time. The milling times are 1, 2, 4, 6, and 12 h. The effect of milling time on structural characterization was investigated by an X-ray diffractometer (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS). Both XRD and EXAFS studies showed that the alloying process occurred after 4-h milling. The nano particle size and shape are examined by scanning electron microscopy. Magnetic properties are examined by vibrating sample magnetometer. Magnetic saturation is decreased with milling time, which seemed be due to substitution of Ni atoms by Al or C atoms resulting in magnetic dilution and/or decreasing of particle size. The coercivity (Hc) is decreased with milling time indicating the increase in single-domain size owing to the crystallite size reduction.

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In this work we analyzed the local structure of GexSi1−x (x = 0.5 and 0.8) ultra thin film alloys deposited on silicon substrate. The local structural parameters for the thin films were compared to the values for a bulk sample. The coordination numbers for the thin films were similar to the value of a bulk sample but the interatomic distances were different. Also, the use of a germanium solid state detector was important for EXAFS analysis of ultra thin film alloys.

Al

The structural and magnetic properties of mechanically alloyed (FexCo1-x)75C25(FexCo1-x)75C25 (x=0x=0, 0.2, 0.5, 0.8, 1) alloys were studied as a function of Fe content. The local structural change of FeCo–C has been investigated by means of XRD, EXAFS and Mossbauer spectrometry. XRD pattern from 25h alloyed FeCo–C powders indicated cohenite like phases. EXAFS analysis showed (FeCo)3C(FeCo)3C phase formation in x=0.2x=0.2, 0.5 and 0.8, respectively. Mossbauer analysis reveals that Fe(Co)33C phase as dominant phase decreased with the Fe content decrease, in a good agreement with EXAFS analysis.