Showing papers by "Seong-Cho Yu published in 2007"

Electron spin resonance and Raman studies of Mn-doped ZnO ceramics

Abstract: In the present work, the influence of annealing on structure, electron spin resonance (ESR), and Raman scattering (RS) spectra of x at. % Mn-doped ZnO (x=4 and 8) ceramic compounds has been systematically investigated. The samples were annealed at temperatures (Tan) between 400 and 1000°C for 12h. The obtained results revealed the strong dependence of x-ray diffraction, ESR, and RS spectra on the annealing temperature Tan. Mn2+ ions did not substitute into Zn2+ sites in samples annealed in the range of 400–600°C but started to substitute into Zn2+ sites for annealing temperatures Tan>600°C. The results of this investigation provide further insights into the physical processes occurring in Mn-doped ZnO materials due to annealing.

Review of the Magnetocaloric Effect in Manganite Materials

Abstract: A thorough understanding of the magnetocaloric properties of existing magnetic refrigerant materials has been an important issue in magnetic refrigeration technology. This paper reviews a new class of magnetocaloric material, that is, the ferromagnetic perovskite manganites (R1−xMxMnO3, where R=La, Nd, Pr and M=Ca, Sr, Ba, etc.). The nature of these materials with respect to their magnetocaloric properties has been analyzed and discussed systematically. A comparison of the magnetocaloric effect of the manganites with other materials is given. The potential manganites are nominated for a variety of large- and small-scale magnetic refrigeration applications in the temperature range of 100–375 K. It is believed that the manganite materials with the superior magnetocaloric properties in addition to cheap materials-processing cost will be the option of future magnetic refrigeration technology.

Magnetocaloric manganites: Progress and challenges

Abstract: Magnetic refrigeration based on the magnetocaloric effect (MCE) becomes a topic of internationally current interest. This enabling technology provides several advantages (e.g. high energy efficiency, small volume, non-pollution) over a conventional gas compression technique. It depends heavily upon the magnitude of MCE of magnetic substances that are used as working refrigerants. Therefore, current research is focused on a search for new materials that are low cost and exhibit small magnetic field-induced large MCEs. This paper aims to review the recent developments of a new class of magnetocaloric material, namely, ferromagnetic manganites (R 1-x M x MnΟ 3 , where R = La, Nd, Pr and M= Ca, Sr, Ba, etc.). The good magnetocaloric properties of ferromagnetic manganites in addition to their flexibility and cost-effectives make them very promising for active magnetic refrigeration (AMR) applications in the temperature range of 30-375 K.

The magnetization behavior and magnetocaloric effect in amorphous Fe–Nb–B ribbons

Abstract: The magnetization behavior and magnetic caloric effect have been analyzed for amorphous Fe 93− x Nb 7 B x (numbers indicate at.%; x = 9, 14, 20) alloys. An amorphous phase was formed after quenching by melt spinning with a copper wheel surface speed of 30 m/s. The magnetization measurements of the samples were performed by vibrating sample magnetometer. The Curie temperature increased from 299 to 419 K with increasing B concentration ( x = 9–20). Temperature dependence of the entropy change, Δ S M was calculated from the isothermal magnetization. The values of Δ S M are 1.44, 1.07 and 0.97 J/kg K at x = 9, 14 and 20, respectively. The maximum of Δ S M was found to appear in the vicinity of the Curie temperature in these alloy samples.

Room‐temperature magnetic entropy change in La0.7Sr0.3Mn1‐xMxO3 (M = Al, Ti)

Abstract: Magnetic entropy change in and above the room-temperature region has been measured for La 0.7 Sr 0.3 Mn 1-x M x O 3 (M = Al, Ti) by means of magnetization measurements in magnetic fields up to 6 T. The magnetocaloric effect (MCE) becomes strongest at the Curie temperature T c that is tuned to ∼300 K by the substitution of Al or Ti for Mn. While the substitution of Al for Mn drastically reduces the entropy change, it extends considerably the working temperature span and improves the relative cooling power (RCP). The MCE and RCP seem to be only lightly affected by Ti substitution. Although manganites are considered potential for magnetic refrigerants, the limitations for achieving strong MCE in the materials are also discussed.

ESR study of La1-xPbxMnO3 (0.1 ≤x ≤ 0.5) perovskites

Abstract: Electron spin resonance (ESR) spectra of La1–xPbxMnO3 (0.1 ≤ x ≤ 0.5) compounds were recorded at different temperatures. Asymmetrical and distored resonance signals due to ferromagnetic correlations at temperatures T Tmin, where Tmin is the temperature corresponding to the narrowest ESR linewidth. The ESR linewidth with respect to temperature, ΔH (T), for the samples was fitted to the one-phonon process, ΔH (T) = A + BT. We found that B decreased from 5.45 Oe/K for x = 0.1 to 4.61 Oe/K for x = 0.5, indicating the decrease of lattice distortions with the Pb addition. The tem- perature dependence of the ESR intensity, I (T), for the samples was described well to an expression of I (T) = I0 exp (Ea/kBT). In the high-temperature region, 1/I (T) obeyed the Curie–Weiss law. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The discovery of the colossal magnetocaloric effect in a series of amorphous ribbons based on Finemet

Abstract: A large number of amorphous ribbons based on Finemet have been prepared by rapid quenching on a single copper wheel with linear speed of v = 25–30 m/s. The ribbons are 20–25 μm thick and 6–8 mm wide. All as-cast samples are amorphous. Two criteria producing the colossal magnetocaloric effect (CMCE) in magnetic materials working as magnetic refrigerants are high saturation magnetization and sharp ferromagnetic–paramagnetic phase transition. The Fe-based amorphous ribbons fit these cretia. Thermomagnetic curves as well as isothermal magnetization curves around the Curie temperature of all the studied samples have been determined. The results show that the magnetic entropy change, |Δ S m |, belongs to a class of materials with CMCE and the |Δ S m | max values have been obtained at a moderately low magnetic field change of 1.35 T, moreover |Δ S m | max occurred at quite high temperature.

Influences of annealing and wire geometry on the giant magnetoimpedance effect in a glass-coated microwire LC-resonator

Abstract: The influence of annealing and of wire geometry on the giant magnetoimpedance (GMI) effect in a glass-coated microwire LC-resonator were systematically investigated. It was found that annealing magnetic microwires significantly varied the permeability and hence the GMI ratio, whereas the magnetoimpedance response was sensitively altered with varying wire geometry of the LC-resonator. The magnitude of GMI is determined by the soft magnetic characteristics of the microwire and the effect of LC-resonance, while the multiple-peak GMI feature can be attributed to the LC-resonance circuit and the formation of standing magnetic waves within the sample. Interestingly, the GMI ratio reached the largest values of 400 000% and 270 000% at the resonance frequencies of 518.51 MHz and 146.32 MHz for Co83.2B3.3Si5.9Mn7.6 and Co67Fe3.8Ni1.4B11.5Si14.6Mo1.7 samples, respectively. The corresponding magnetic-field sensitivities of GMI are about 218 000%/Oe and 135 000%/Oe. These results are interesting for the development of a new family of ultra-sensitive and high-frequency magnetic sensors. The feature of a LC-resonance circuit can be used for improving the sensitivity of GMI-based magnetic sensors while selecting the working frequency.

Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets

Abstract: Nanocomposite hard magnetic materials (Nd,Dy)4.5Fe77.5B18 (No. 1) and (Nd,Dy)4.5Fe76B18Nb1.2Cu0.3 (No. 2) have been prepared by crystallizing amorphous ribbons, fabricated by single roll melt-spinning. The evolution of a multiphase structure was monitored by an x-ray diffractometer and by thermomagnetic measurement. We observed that, at annealing temperatures below 670??C, there is crystallization of soft phase Fe3B and a small amount of hard phase Nd2Fe14B. At annealing temperatures above 670??C, crystallization of ?-Fe and probably Dy2Fe14B phases with large magnetocrystalline anisotropy led to a drastic enhancement in the hard magnetic properties of the materials. The maximum value of HC is found to be 4.2?kOe for sample No. 1. For sample No. 2, with co-doping of Nb and Cu, nanostructure refinement yields a strong enhancement in exchange coupling between the component phases. Thereby, we obtained high reduced-remanence of 0.78, high remanence of 1.15 and a high (BH)max value up to 16.2?MGOe.

Influence of Nb substituted for Fe on the microstructure and magnetic properties of Fe‐based nanocomposite alloy

Abstract: The influence of Nb substituted for Fe on the microstructure and magnetic properties including the magnetoimpedance effect of a Fe-based have been investigated. The nanocomposite structure composed of ultra-fine Fe(Si) grains embedded in an amorphous matrix was obtained by annealing the Fe-based amorphous alloy prepared by rapidly-quenched method. The measurements of thermomagnetic curves indicated that the Curie temperature of the amorphous phase of the samples decreases with increasing Nb content. The optimal heat treatment was performed at T a = 480 °C for 30 min and showed that the ultrasoft magnetic properties of nanocomposite materials were obtained. The magnetoimpedance (MI) of these samples has been studied in range frequency from 1MHz to 5 MHz and varying a dc magnetic field within 300 Oe. The correlation between the MI effect and the soft magnetic properties is discussed. The incremental permeability ratio (PR) showed the drastic changes of soft magnetic properties as a function of annealing temperatures.

Magnetocaloric effect in as-quenched and annealed Fe91-xYxZr9 (x = 0.5, 10) alloys

Abstract: The magnetization behaviours have been analyzed for as-quenched and annealed Fe 91-x Y x Zr 9 (x = 0, 5, 10) alloys. Samples were prepared by arc melting the high-purity elemental constituents under argon gas atmosphere and by single roller melt spinning. These alloys were annealed one hour at 500 °C. The magnetic properties of the samples were measured by VSM. The Curie temperature increases with increasing Y concentration (x = 0 to 10). Temperature dependence of the entropy variation ΔS M was found to appear in the vicinity of the Curie temperature. The results show that annealed Fe 86 Y 5 Zr 9 alloys have a greater magnetocaloric effect than that those in as-quenched alloys. The value is -1.23 J/kg K at AH = 1.5 T for annealed alloy and -0.89 for as-quenched alloy, respectively.

EXAFS Study for a Magnetic Shape Memory Alloy Ni-Mn-Ga

Abstract: The local structure related to the magnetic property during the martensite-autenite transition of a Ni2MnGa alloy has been studied by using a Ga K-edge (EXAFS) analysis for temperatures from 150 K to 300 K From the analysis, we found that the local structure was slightly distorted in the pre-martensite phase (PM) and that the third moment, ae (3) increased signiflcantly during the transition period The anharmonic Ga-Ni pair vibration is discussed in connection with the local structural distortion

Very large magnetoimpedance effect in a glass-coated microwire LC-resonator

Abstract: This paper reports on the high-frequency giant magnetoimpedance (GMI) effect in an LC-resonator built on a glass-coated amorphous Co67Fe3.8Ni1.4B11.5Si14.6Mo1.7 microwire with two cylindrical electrodes at the ends of the microwire. In the investigated frequency range of 100–600 MHz, the GMI effect observed first increases with increasing frequency, reaching a maximum at 146 MHz, and then decreases at higher frequencies. It has been demonstrated that the resonance in a glass-coated microwire LC-resonator occurs at a specific intensity of applied DC magnetic field. More noticeably, extremely large increases of GMI at near resonance frequency are observed. The maximum GMI response due to the resonance of LC components up to 90,000% is found by precise tuning of the frequency around 146.30 MHz. This result is very ideal for developing high-frequency and ultra-sensitive GMI-based magnetic sensor applications. The extreme GMI effect in a constructed LC-resonator originates from both the permeability changes of the microwire and the LC-resonance of the circuit.

The crystallization and properties of alloys with Fe partly substituted by Cr and Cu fully substituted by Au in Finemet

Abstract: The structure, crystallization and magnetic properties of ribbons obtained by first making amorphous ribbons and then objecting them to a crystallization annealing have been published elsewhere by us previously. In the present work the soft magnetic ribbons Fe 73.5− x Cr x Si 13.5 B 9 Nb 3 Au 1 (numbers indicate at.%, x = 1–5) are prepared by fast quenching on a single copper wheel. X-ray diffraction patterns show that the as-cast samples are amorphous. Differential scanning calorimetry analysis indicates that the crystallization temperature of the α-Fe(Si) phase is a little higher than that of pure Finemet. With the same annealing conditions, the crystallization volume fraction decreases with increasing Cr content substituted for Fe. Hysteresis loops of as-cast samples measured by Permagraph show that domain walls are pinned. After appropriate annealing, the ultrasoft magnetic properties of nanocomposite materials are established. The magnetic entropy change, |Δ S m |, of studied samples has been determined, and a giant magnetocaloric effect is found. Our materials could be considered as promising magnetic refrigerants working at high temperatures (several hundreds °C).

Microstructure, optical and photoluminescence properties of simple thermal evaporated CdS nanocrystalline films

Abstract: CdS nanocrystalline films were deposited on glass substrates by thermal evaporation at 300 K and 573K. All the films exhibited wurtzite structure with crystallite size varied from 10 - 20 nm. The sharp sudden fall of optical transmittance spectra shifted from 485 nm to 455 nm with increasing substrate temperature. The excitation wavelength of all films is about 500 nm and emission wavelengths 545 nm and 570 nm for all films. The possible optical and photoluminescence mechanisms are proposed in this paper. The photoluminescence of CdS nanocrystalline films enables them to have important applications in optoelectronic devices.

Ultrasoft magnetic properties of Co–Fe–Hf–O nanocomposite films

Abstract: This paper reports on the excellent high-frequency magnetic performance of Co-Fe-Hf-O thin films, which were deposited on Si(100) substrates by the oxygen reactive rf-sputtering method. It was shown that the films possessed not only high electrical resistivity but also large saturation magnetization and hard-axis anisotropy field. Among the compositions investigated, Co 19.35 Fe 53.28 Hf 7.92 O 19.35 exhibited the ultrasoft magnetic properties of high saturation magnetization, 4πM s ∼19.86 kG, and low coercivity, H c ∼1.5 Oe. The magnetic permeability remained almost constant up to 3 GHz and reached a maximum at the ferromagnetic resonant frequency of 4.024 GHz. These properties of this film together with a high electrical resistivity of 3569 μΩcm make it ideal for producing micromagnetic devices for high-frequency applications.

Ferromagnetic Behavior above Room Temperature of Fe-Ion-Implanted ZnO

Abstract: Zinc-oxide (0001) single crystals with a 0.5 mm thickness were prepared, and then 80 keV Fe ions with a dose of 3 × 10 ions/cm were implanted into the ZnO single crystals at 350 ◦C. The implanted samples were post-annealed at 700, 800 and 900 ◦C by rapid thermal annealing in an N2 atmosphere for 5 min to remove the ion-implantation damage. The structure and the magnetic properties of Fe-ion-implanted ZnO were investigated by using X-ray diffraction and a superconducting quantum interference device magnetometer. The carrier transport properties were measured in the range from 5 K to 300 K. The X-ray diffraction results showed an island peak near 37 ◦C, which was identified as ZnO2. The magnetization curve showed hysteresis loops at 5 K in the samples annealed at 700 and 800 ◦C, showing ferromagnetism. Hysteresis loops were observed at 300 K in the samples annealed at 700 ◦C. The temperature dependence of the magnetization was taken in field cooling (FC) and zero field cooling (ZFC) in the samples annealed at 700 ◦C. The difference magnetization (∆M ) between the FC and the ZFC magnetizations did not converge to a zero value even at 340 K, showing ferromagnetism above room temperature. The magnetoresistance (MR) curve showed negative curvature up to 150 K which could be explained with the delocalized d state of ZnO implanted with Fe ions.

Influence of film thickness on the electrical and magnetic properties of Co-Fe-Al-O films

Abstract: Research Center for Advanced Magnetic Materials, Chungnam National University, Daejeon 305-764, South Korea Received 7 May 2007, revised 30 October 2007, accepted 31 October 2007 Published online 18 December 2007 PACS 75.70.Ak, 85.70.Kh Influence of film thickness (t) on the electrical and magnetic properties of Co–Fe–Al–O films has been systematically investigated via means of vibrating sample magnetometry (VSM), permeability spectra and magneto-optical Kerr effect (MOKE). It is found that the electrical resistivity (ρ) decreases as the film thickness increases; ρ = 412.5 µΩcm for the t = 600 nm sample decreases to ρ = 368.2 µΩcm for the t = 1200 nm sample. The coercive force, measured along the easy-axis direction, decreases as the film thickness increases. Interestingly, along the hard-axis magnetization direction, the magnetic hysteresis loop is reversed and the coercive force is negative for the t = 600 and 800 nm samples. However, this pe-culiar feature disappears as t reaches 1200 nm, which is probably attributed to the microstructural change with respect to changes of film thickness. At a frequency of 1GHz, the hard-axis effective permeability decreases from 1252 to 1000 as the film thickness increases from 600 to 1200 nm, respectively. The VSM and MOKE results reveal an increase in magnetic anisotropy in the near-surface region of the film.

Anomalous magnetoimpedance in Co–Fe–Al–O thin films

Abstract: The high-frequency giant magnetoimpedance (GMI) effect in a Co47.5Fe21.9Al17.2O15.2 thin film was systematically investigated. In the frequency range of 100–500 MHz, GMI profiles showed a double-peak characteristic, reflecting an evident existence of the transverse magnetic anisotropy in the film, which coincided with the magnetization data. Interestingly, there was an anomalous transformation in the GMI profile from “positive” to “negative” and this was associated with the complex phase transformation of permeability. The GMI ratio decreased with decreasing film-thickness, due to an increase of the electrical resistivity and a decrease of the magnetic softness. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The substitution effect of boron on reentrant behavior of rapidly solidified Fe–Mn–Zr alloys

Abstract: We have studied the magnetic properties of boron-substituted amorphous reentrant Fe–Mn–Zr alloys. The magnetic characterization of a-Fe 82 Mn 8− x B x Zr 10 (numbers indicate at.%) alloys has been done through the studies of thermal variation of magnetization, ac susceptibility and ferromagnetic resonance. Low-temperature spin freezing was observed at 54 K with large coercivity and low magnetic moment for a-Fe 82 Mn 8 Zr 10 . The large coercivity at low temperatures and high freezing temperature for this sample is attributed to magnetic hardening. With an increase of the concentration of B, it is observed that the Curie temperature increases and the spin-glass-like transition observed at low temperature moves to lower temperatures and finally vanishes at x = 8 at.%. A single resonance peak in FMR spectrum was observed below T C and at least two distinct line shapes were observed well beyond the T C for two of the compositions, namely, x = 4 and 6. These observations could be qualitatively understood in terms of a finite spin clusters plus an infinite ferromagnetic matrix model. It can also be concluded that the magnetic disorder is reduced as boron concentration is increased.

X-Ray Absorption Fine Structure Study for Fe60Ni40 Alloy

Abstract: Fe60Ni40 alloys were fabricated by the mechanical alloying process with process periods of 1, 2, 4, 6, 12 and 24 hours, respectively. The formation of alloy and the structural evolution of the alloy were examined by X‐ray diffraction and extended X‐ray absorption fine structure methods. With increase of alloying time the BCC phase of iron was changed significantly during the mechanical alloying process. The alloying was activated in about 6 hours and completed in about 24 hours.

Influence of film thickness on the soft magnetic properties of sputtered Co-Fe-Hf-O films

Abstract: Influence of film thickness on the soft magnetic properties of Co19.35Fe53.28Hf7.92O19.35 films, which were deposited on Si(100) substrates using the oxygen reactive RF-sputtering method, has been thoroughly investigated. It is found that as the film thickness (t) increases from 50 to 430 nm, the easy-axis coercivity (HCE) strongly decreases from 26.2 to 1.5 Oe, while the hard-axis anisotropy field (HKH) significantly increases from 68.7 to 84.8 Oe and the saturation magnetization (4πMs) tends to increase from 16.8 to 19.7 kG. As t exceeds 430 nm, HKH gradually decreases, while both HCE and 4πMs remain almost unchanged. It reveals that a strong phase separation appears to occur in films with t > 430 nm, resulting in the decrease of HKH. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Longitudinal and transverse incremental permeability of Co19.35Fe53.28Hf7.92O19.35 films

Abstract: We present here, the results of a thorough study of the longitudinal permeability ratio (LPR) and transverse permeability ratio (TPR) in sputtered Co 19.35 Fe 53.28 Hf 7.92 O 19.35 films with varying thicknesses (t = 1, 1.5, 1.8 and 2.4 μm) in the frequency range of 100 - 1000 MHz. It is found that for all samples investigated, the LPR/TPR first increases with increasing frequency, reaches a maximum at 710 MHz, and then decreases at higher frequencies. This is related to the ferromagnetic resonance. The LPR and TPR profiles measured along the easy axis of the film show a single-peak feature, whereas those measured along the hard axis of the film show a double-peak one. This indicates an existence of magnetic anisotropy and its dispersion in the films. It reveals that the study of LPR and TPR spectra provides an alternative way for the evaluation of the high-frequency magnetic softness of film-type materials.