Showing papers by "Seong-Cho Yu published in 2004"

Large magnetocaloric effect in a La0.7Ca0.3MnO3 single crystal

Abstract: We report the results of a thorough study of the magnetocaloric effect (MCE) in a La0.7Ca0.3MnO3 single crystal, which undergoes a first-order magnetic phase transition at ∼227 K. The magnetic entropy change ΔSM and the adiabatic temperature change ΔTad reach, respectively, ∼6.42 J/kg K and 4.76 K for ΔB=5 T and even if both are smaller than those exhibited by gadolinium (∼9.8 J/kg K and ∼11.7 K), the ΔSM distribution here is much more uniform than that of gadolinium and polycrystalline manganites. This is desirable for an Ericson-cycle magnetic refrigerator. The MCE is larger in the single-crystalline manganite compared to the polycrystalline one. The manganite single crystal has large ΔSM induced by low magnetic field change, which is beneficial for the household application of active magnetic refrigerant (AMR) materials. All these make the lanthanum manganite single crystal an attractive candidate as a working substance for AMR. The molecular field model provides a fairly good description of the magnet...

Large magnetocaloric effect in La0.845Sr0.155Mn1−xMxO3 (M = Mn, Cu, Co) perovskites

Abstract: We present the results of an investigation on the magnetocaloric effect in the perovskites of La{sub 0.845}Sr{sub 0.155}Mn{sub 1-x}M{sub x}O{sub 3} (M = Mn, Cu, Co). It is found that there was a large magnetic entropy change, i.e. a large magneto-caloric effect, in all these samples. Among them, the magnetic entropy change reaches a maximum value of 2.67 J/kg K at the applied field of 13.5 kOe for the Cu-doped sample, suggesting that this material would be a suitable candidate for the advanced magnetic refrigeration technology. The large magnetic entropy change produced by the abrupt reduction of magnetization is attributed to the strong coupling between spin and lattice that occurs in the vicinity of the ferromagnetic-paramagnetic transition temperature (T{sub C}) - which is experimentally verified by electron paramagnetic resonance study. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Enhanced GMI effect in a Co70Fe5Si15B10 ribbon due to Cu and Nb substitution for B

Abstract: We present here, the results of an investigation on giant magnetoimpedance (GMI) effect in both annealed and as-quenched Co 70 Fe 5 Si 15 B 10 and Co 70 Fe 5 Si 15 Nb 2.2 Cu 0.8 B 7 ribbons. Substitution of Cu and Nb for B in an initial Co 70 Fe 5 Si 15 B 10 composition forming the Co 70 Fe 5 Si 15 Nb 2.2 Cu 0.8 B 7 composition improves both GMI effect and its field sensitivity. The GMI effect was more pronounced in the annealed samples. The field sensitivity of both the longitudinal permeability ratio and the magnetoimpedance ratio for the annealed Co 70 Fe 5 Si 15 Nb 2.2 Cu 0.8 B 7 ribbon increase exponentially as the testing temperature is increased, indicating that the magnetic permeability is very sensitive to the temperature. The results obtained are of significant importance in developing quick-response magnetic sensors.

Luminescent properties of Eu-doped Y2O3 nanophosphors

Abstract: Nanophosphors of 5 mol% Eu-doped Y2O3 were fabricated using the combustion method. X-ray diffraction analyses confirmed a cubic structure for the synthesized samples. Based on the X-ray diffraction patterns, the average grain size of the samples was estimated to be in the nanometer range. Using an appropriate synthesis route, the high photoluminescence intensity of the nanophosphor samples was significantly improved by varying the glycine-to-metal nitrate (G/M) molar ratio and the flame temperature. The luminescence lifetime of the 5D0–7F2 transition at 610 nm was ∼1.4 ms for the sample having the smallest average particle size of about 5 nm. By using the time-resolved luminescence spectrum technique, two novel spectrum bands were observed of P1 and P2 at 613.2 and 622.8 nm, respectively. The influences of glycine and flame temperature on the grain size and luminescent properties were clarified. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin dynamics and spin-glass state in Fe-doped cobaltites

Abstract: A thorough study of the magnetic and transport properties of La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds has been made. The Fe substitution destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. The charge localization due to Fe substitution is likely to have its origin in the electronic configuration rather than in its ionic size. The hole-poor regions, corresponding to the Fe-rich regions, would also dilute the magnetic lattice and thereby prevent the occurrence of long-range order. Spin-glass behavior was observed for x⩾0.5 compositions and is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co3+ and Co4+, and the antiferromagnetic interactions like Co-O-Fe and Fe-O-Fe. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates a finite spin-glass phase-transition temperature Tg≈85 K and a dynamic exponen...

A new band in Cr3+-doped MgAl2O4 natural spinel at room temperature

Abstract: Time resolved photoluminescence spectra of natural MgAl2O4 spinel doped with Cr3+ ions have been investigated at room temperature in the wavelength range between 380 and 800 nm. Using several excitation sources from the different lasers we have observed a new band, namely K, with a peak of 15,547 cm−1 (643.2 nm) and the transition process of photoluminescent lines among the electronic levels of the Cr3+ ion. In terms of the experimental results and the Tanabe-Sugano diagram, we strongly propose that the K-band originates from the electric dipole transition, 2Eg 4A2g in the strong crystal field with Dq/B > 2.3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence properties of Cr3+-doped MgAl2O4 natural spinel

Abstract: We present, here, the results of a study of photoluminescence spectra of Cr3+-doped MgAl2O4 natural spinel in a given wavenumber range between 26,300 and 12,500 cm-1. By using several excitation sources, we observed a new broadband, namely K, with a peak at 15,547 cm-1 and the transition process of luminescence lines among the electronic levels of the Cr3+ ion, such as R, Ni (i = 1-4) lines. In terms of the experimental results and the configuration coordinates, we strongly propose that the K-band might originate from the electric dipole transition 2Eg 4A2g, in a strong crystal field with Dq/B 2.3. The decay-time and luminescence-rise-time curves on the zero-phonon lines and the K-band were also recorded.

The structural and magnetic properties of Fe-Si and Fe-C solid solutions as a function of milling times

Abstract: We have studied the formation of metal-metalloid alloys for Fe-Si and Fe-C as a function of milling times. These alloys were produced using mechanical alloying. The effect of milling time on local structural changes of Fe-Si and Fe-C has been investigated by means of Mossbauer spectrometry, EXAFS study and XRD. Saturation magnetization was also measured by VSM. XRD pattern from mechanically alloyed Fe-Si and Fe-C powders indicates the formation of solid solution. The different variation of lattice parameters could be analysed from the different substitution of Si and C atoms into the Fe structure. The Mossbauer spectrum showed typical sextets in the 1 h milled sample corresponding to alpha-Fe spectrum. Increasing the milling time, the sextets became broader due to appearance of disordered Fe atoms in both solid solutions. The hyperfine field distributions were decreased as increasing milling time, which is similar trend with magnetization distribution.

Chemical properties and GMR improvement of specular spin valves with nano-oxide layers, formed in ambient mixed gases

Abstract: Specular spin valves (SVs) containing nano-oxide layers (NOLs) structured as substrate/seed/AF/P1/NOL/P2/Cu/F/NOL, have been fabricated. The NOLs were formed by natural oxidation in different ambient atmospheres of pure oxygen, oxygen/nitrogen and oxygen/argon gas mixtures. The fabrication conditions were optimized to enhance the magnetoresistance (MR) ratio, to suppress the interlayer coupling fields (Hf) between the free and pinned layers, to suppress the high interface density of the NOL, to ease the control of the NOL thickness and to form a smooth NOL/P2 interface for promoting specular electron scattering. The characteristics of our specular SVs are the MR ratio of 14.1%, the exchange bias field of 44–45 mT, and Hf weaker than 1.0 mT. The optimal conditions for oxidation time, total oxidation pressure and the annealing temperature were found to be 300 s, 0.14 Pa (oxygen/argon = 80/20) and 250°C, respectively. Also, the origin of thermal stability of MMn-based (M = Fe, Pt, Ir, etc) specular SVs has been explained in detail by chemical properties of NOL using secondary-ion mass spectroscopy and x-ray photoelectron spectroscopy depth profile analyses. Thermal stability turns out to be caused by a decrease in MR ratios at high temperatures (>250°C), which is a serious problem for device applications using the SV structure as a high density read head device.

Influence of terminal electrodes on LC resonance frequencies in glass-coated Co83.2B3.3Si15.9Mn7.6 microwires

Abstract: A new super-giant magnetoresonance effect was found in LC resonators consisting of a glass-coated microwire with capacitive terminal electrodes. The changes of resonance frequencies were examined as a function of the electrode dimensions in amorphous Co 83.2 B 3.3 Si 5.9 Mn 7.6 microwires The widths of two capacitive electrodes at the end of the microwire were varied from 3 to 6 mm, and the gap lengths between the electrodes were also varied from 6 to 9 mm. The shape of the impedance curves as a function of a dc magnetic field changed dramatically near the LC resonance frequency. The phase angle was also strongly dependent on this field. The sudden change of impedance by as much as 6400% with a change of phase angle as large as 180° is a new phenomenon, which is evidence for the occurrence of the magnetoresonance at a given intensity of the external dc field.

Local structure and magnetic properties of mechanical alloyed Co-C compositions

Abstract: Formation of nanocrystalline Co particles embedded in an amorphous C matrix produced by the mechanical alloying process was studied The formation of the Co5C95 alloy was examined by the x-ray diffraction (XRD) and extended x-ray absorption fine structure methods XRD analysis displayed the decrease in the Co grains size, the transformation from the face-center-cubic (fcc)+hexagonal close-packed (hcp) phases to the hcp phase and an amorphization of carbon matrix with milling time At the early stage of milling, the coercivity, HC, increased because of the higher hcp phase HC than that of the fcc phase Partial HC increase could be also caused by the decrease of Co particles size The subsequent powder milling led to the decrease in the HC due to thermal process and/or due to the transformation of hcp-Co to a random close-packed phase under a long-term milling Magnetization of the compositions decreased gradually with milling The size of single domain particles after the 60 h of milling was about 10 nm

Thermal treatment effects on high-frequency giant magnetoimpedance in glass-coated Co83.2B3.3Si15.9Mn7.6 microwires

Abstract: The thermal treatment effect on high-frequency giant magnetoimpedance (GMI) in a glass-coated Co 83.2 B 3.3 Si 5.9 Mn 7.6 microwire was investigated. The high-frequency GMI measurement in the form of LC resonators for the microwires annealed at 100 °C, 150 °C, 200 °C, and 250 °C for 1 h in a vacuum was performed on samples with 3 mm in width of electrodes and 9 mm in gap length between electrodes in the frequency range 100 MHz-1 GHz.

Giant Magnetoimpedance in C0 67 Fe 4 Mo 1.5 Si 16.5 B 11 Metallic Glass Ribbon

Abstract: Giant magneto-impedance (GMI) effect in zero-magnetostrictive Co-based amorphous ribbons samples in their as-quenched and stress-released states as well as with intentionally induced magnetic anisotropy were investigated. Magnetic and impedance properties of the samples exhibiting different anisotropy were compared and the optimum operation conditions for the studied samples from the view-point of their utilization as a sensor element have been determined. A design of a model of magnetic field sensor and characteristics of the constructed prototype are presented.

EPR study of Pr0.7Sr0.3−xBaxMnO3 lanthanum manganites

Abstract: Among the wide variety of colossal magnetoresistance materials the Pr-compositions occupy some special places. Namely, in zero magnetic field the Pr1−yCayMnO3 samples are insulators for any level of Ca doping and show low field hysteresis. Pr0.7Ba0.3MnO3 composition is also insulating at low as well as at high temperatures. The combined Pr- and (Sr, Ba)-doped perovskite-like manganites should show interesting properties due to the large difference between the Pr and Sr, Ba ionic size. In this work we present an electron paramagnetic resonance (EPR) study of Pr0.7Sr0.3−xBaxMnO3 perovskite (x = 0.00, 0.15 and 0.30). We observed that the intensity of the EPR line decreases exponentially with temperature. The activation energy values derived from the temperature dependencies of EPR line intensity equals 0.16 eV, 0.09 eV and 0.08 eV for the x = 0.00, 0.15 and 0.30 compositions, respectively. The temperature dependence of the EPR linewidth showed a minimum near the TC. The temperature and concentration dependencies of EPR linewidth and line intensity will be discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of mixed gases on characteristics of specular spin‐valves containing oxide layers

Abstract: Specular spin valves containing oxide layers (OXLs), structured as substrate/seed/AF/P1/OXL/P2/Cu/ F/OXL, have been fabricated. The OXLs were formed by natural oxidation in different ambient atmospheres of pure oxygen, of oxygen/nitrogen, and of oxygen/argon gas mixtures. The fabrication conditions were optimized to enhance the magnetoresistance (MR) ratio, to suppress the interlayer coupling fields (Hf) between the free and pinned layers, to suppress the high interface density of the OXL, to ease the control of the OXL thickness and to form a smooth OXL/P2 interface for promoting specular electron scattering. The characteristics of our specular spin valves are as follows: the MR ratio of 14.1%, the exchange bias field of 44–45 mT and Hf weaker than 1.0 mT. The optimal conditions for oxidation time, total oxidation pressure and the annealing temperature were found to be of 300 s, 0.13 Pa (oxygen/ argon = 8/2), and 250 °C, respectively. The results suggest that specular spin-valves containing OXLs satisfy the requirements of read head performances in high density (100 Gbits/in2) recording devices. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic and transport properties of La0.5Sr0.5(FexCo1−x)O3 (0 ≤ x ≤ 0.2) perovskites

Abstract: The magnetic and transport properties of La 0.5 Sr 0.5 (Fe x Co 1-x )O 3 (0 ≤ x ≤ 0.2) compounds were investigated. Tc decreased slightly and the resistivity increased, even with a very small extent of Fe substitution. The Fe substitution induced additional antiferromagnetic Fe-O-Fe interactions that suppress strongly the ferromagnetism and conductivity, thus causing a separation between two phases at around 124 K for both x = 0.1 and 0.2 compositions. Spin-glass behavior was observed for the Fe-doped samples. The resistivity, p(T), can be described by the Mott's variable range hopping model in the low temperature region.

Giant Magnetoimpedance Effect in a Co-Based Microwire for Quick-Response Magnetic Sensor Applications

Abstract: Development of autobiased linear field sensors based on asymmetrical giant magnetoimpedance (AGMI) effect in Corich amorphous microwires upon the application of a biasing dc current is approached. Upon biasing dc currents, the highest field sensitivity of AGMI of 20%/Oe was found at a biasing dc current of 10 mA. The reduction of the AGMI under a biasing dc current of 25 mA and a frequency of 10 MHz has been observed. The result indicates that an optimum design of autobiased linear field sensors based on AGMI can be achieved by applying the biasing dc current of 10 mA and in the frequency range of 100 kHz–5 MHz. A stress-induced change in AGMI has also been found in these microwires and this offers a new approach to the development of stress sensors. All these features make the Co-rich amorphous microwire a multifunctional and smart material that can be used for different purposes of sensing applications.Copyright © 2004 by ASME

Temperature dependence of high-frequency giant magnetoimpedance in glass-coated Co67Fe3.8Ni1.4B11.5Si14.6Mo1.7 microwires

Abstract: The investigation of the low-temperature dependence of the high-frequency magnetoimpedance (MI) effect is very important for scientific interests and thermal stability of magnetic sensors. Also, using high frequencies may be more profitable for microwires due to the very short penetration depth. In this study, the giant magnetoimpedance (GMI) effect in the high-frequency range from 100 MHz to 1 GHz was investigated in a glass-coated amorphous Co 67 Fe 3.8 Ni 1.4 B 11.5 Si 14.6 Mo 1.7 microwire measured at various temperatures from 10 K to 300 K. The values of the magnetoimpedance ratio (MIR) became larger with an increase of the temperature and also increased with an increase of the frequency. The MIR ratio is shown to be extremely stable for magnetic sensors at low temperature.

The effect of cyclic annealing treatments on magnetoresistance in IrMn‐pinned top and bottom spin valves

Abstract: IrMn-pinned top spin valve (TSV) and bottom spin valve (BSV) films were prepared by DC magnetron sputtering onto thermally oxidized Si (111) substrates at room temperature under a magnetic field of above 100 Oe. The effect of cyclic annealing treatments on magnetoresistive and structural properties in the films was investigated. The exchange coupling field (Hex) of TSV and BSV is stabilized after the second annealing cycle and it is thought that these films reveal high thermal stability. The magnetoresistance (MR) ratio and Hex of TSV were 5.6% and 430 Oe, respectively. For BSV the Hex value (1180 Oe) is much higher while MR ratio (3.6%) is reduced with respect to TSV. The blocking temperatures of the TSV and BSV were 250 °C and 270 °C, respectively. The results concerning magnetic properties together with structural investigations suggest the Hex is mainly dependent on the quality of fcc (111) crystalline texture while MR is sensitive to both crystalline texture and interface roughness. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetoresistance of antiferromagnetic Ir22Mn78-pinned spin filter specular spin valves

Abstract: Specular spin valves (SSVs) having the spin filter layer (SFL) in contact with the ultrathin free layer of composition Ta 3 /NiFe 2 /IrMn 7 /CoFe 1 /(NOL1)/CoFe 2 /Cu 1.8 /CoFe(t F )/Cu(t SF )/(NOL2)/Ta 3.5 (in nm) deposited by magnetron sputtering were studied. For these antiferromagnetic Ir 22 Mn 78 -pinned spin filter specular spin valve (SFSSV) films, an optimal magnetoresistance (MR) ratio of 11.9% was obtained when both the free layer thickness (t F ) and the SFL thickness (t SF ) were 1.5 nm, and a MR ratio higher than 11% was maintained even when t F was reduced to 1.0 nm. This was due to an increase of specular electrons by the nano-oxide layer (NOL) and of current shunting through the SFL. Moreover, the interlayer coupling field (H int ) between the free layer and pinned layer could be explained by considering the RKKY and magnetostatic coupling. The coercivity of the free layer (H cf ) was significantly reduced as compared to traditional spin valves (TSV), and remained as low as 4 Oe when t F varied from 1 to 4 nm. It was found that the SFL made it possible to reduce the free layer thickness and enhance the MR ratio without degrading the soft magnetic property of the free layer.

Adiabatic polaron transport in La0.9Pb0.1MnO3 manganites

Abstract: A study of single-phase La0.9Pb0.1MnO3 compositions is presented. The temperature dependence of the EPR line intensity and resistivity evidence a spin–lattice interaction in the paramagnetic region near the Curie point. In the high temperature region a spin–spin interaction is dominant. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)