W.Y. Jeung

Magnetic properties and crystal structures of self-ordered ferromagnetic nanowires by ac electroforming

We have investigated the magnetic properties and crystal structures of α-Fe and Fe21Ni79 nanowire arrays fabricated by ac electroforming into self-ordered nanosized pores in anodic aluminum oxide (AAO). The nanopores were found to have diameters of 5–100 nm and aspect ratios of 300–1000 depending on the anodization conditions. X-ray diffraction patterns provide direct experimental evidence that the α-Fe and Fe21Ni79 nanowires have preferred crystallographic orientations, indicating that those were electroformed in the (221) and (220) orientations, respectively. For the α-Fe (Fe21Ni79) nanowires, coercivity Hc and squareness Mr/Ms were found to be ∼2.02 (∼1.20) kOe and ∼0.9 (∼0.99), respectively. It is observed that the coercivity in Fe21Ni79 nanowires increased up to 1.35 kOe with increasing annealing temperature.

HYDROGEN ABSORPTION AND DESORPTION BEHAVIOR IN PR-FE-B TYPE ALLOYS

In this study, the hydrogen absorption and desorption behavior of Pr–Fe–B type alloys with addition of Co, Ga, and Zr was investigated focusing on the disproportionation and recombination reaction in HDDR process. In Pr13B6Febal. alloy, the second hydrogen absorption reaction which corresponds to the disproportionation reaction in HDDR process occurs at about 620 °C, and the recombination reaction was not observed below 1000 °C in hydrogen atmosphere. In alloys with Co content of 12 at.%, the second hydrogen desorption reaction, which corresponds to the recombination reaction in HDDR process, occurs at about 955 °C and the occurring temperature of the recombination reaction is further decreased with increasing Co content. The Ga addition to Pr13B6Febal. alloy also reduces the recombination reaction temperature and has no significant influence on the disproportionation reaction. Moreover, the lowering of the recombination reaction temperature is remarkable when Ga and Co are added simultaneously. On the co...

Magnetic properties of Pr-Fe-B alloy powders prepared by mechanical grinding

The effect of ball milling on the magnetic properties of the Pr15Fe80B5 alloy, whose as‐cast structure was composed of matrix phase Pr2Fe14B and Pr‐rich and α‐Fe phases, has been investigated by measuring iHc, thermomagnetic curves, and x‐ray diffraction (XRD) patterns. By ball milling using a planetary ball mill up to 96 h, iHc decreased with the amorphization of the matrix and Pr‐rich phases. However, subsequent heat treatments gave rise to the marked increase of iHc. The alloy, after 48 h of ball milling and subsequent 600 °C annealing possessed iHc of 17.4 kOe, Br of 5.8 kG, and (BH)max of 6.5 MGOe. This alloy showed sharp diffraction peaks of Pr2Fe14B phase, while the α‐Fe phase peak, which persisted through the ball milling process, almost disappeared. The high iHc of the Pr‐Fe‐B alloy, subjected to optimum milling and subsequent annealing, was attributed to a very fine Pr2Fe14B phase precipitation from the amorphous state and to the effective solution of the α‐Fe phase into the matrix phase. Also, ...

Effect of Sn addition on the microstructure and magnetic properties of FePt thin film

An attempt has been made in this study to incorporate an alloying element to lower the order-disorder transformation temperature of Fe–Pt alloy and the effect of Sn addition was investigated for this purpose. The coercivity of the FePtSn film is measured to be about 5000 Oe, whereas that of FePt film is several hundred Oe at the annealing temperature of 300 °C. Therefore, Sn addition is effective to promote the L10 ordering and reduce the ordering temperature of FePt alloy. Analysis of crystal structure by x-ray diffraction reveals that the lattice expansion of cubic FePt phase is induced by Sn addition in as-deposited film. After the annealing from 300 °C to 600 °C, L10 ordered phase is formed in FePtSn films and lattice constant of FePtSn film is approached to pure FePt with increasing annealing temperature up to 600 °C. Difference in solid solubility of Sn between disordered and ordered phase exists and thus excess Sn will diffuse out from ordered FePt L10 phase. Segregated Sn is found at the surface a...

Relationship between the intrinsic coercivity and the phases of Nd-(Dy)-Fe-B sintered magnets

The magnetic properties of sintered Nd‐(Dy)‐Fe‐B magnets prepared with different annealing conditions have been investigated. Thermomagnetic analysis was carried out on the sintered and annealed Nd‐(Dy)‐Fe‐B magnets in the temperature range of room temperature to 320 °C. The coercivity of gas‐quenched Nd14.1Fe78.3B7.6 magnets after annealing for 1 h at temperatures between 600 and 900 °C is higher by 3 kOe than the furnace‐cooled one after annealing. Thermomagnetic analysis found that the sintered Nd14.1Fe78.3B7.6 magnet consists of the Nd2Fe14B phase as well as an unknown magnetic phase (A1) with Tc around 150 °C. The annealed magnets consist of two unknown magnetic phases (A1,A2) and the Nd2Fe14B phase with a Tc of 150, 240, and 305 °C, respectively. Thermomagnetic analysis also showed that the Mr value of the A2 phase in the gas‐quenched Nd‐Fe‐B magnet was higher than that of the furnace‐cooled one. These results seem to indicate that increase of coercivity is related to the formation of the A2 phase.

The Effects of CrV Underlayer on the Structure and Magnetic Properties of FePt Thin Film

An attempt is made in this study to employ vanadium containing chromium (CrV) alloy underlayer to control the microstructure and ultimately to facilitate the in-situ ordering of Fe-Pt thin film. CrV alloys with V contents ranging from 0 to 15 at% were investigated to evaluate their effects on the magnetic properties and structural modification of FePt thin film. Addition of V in Cr underlayer results in the formation of FePt L10 phase with (001) preferred orientation. Analysis of XRD and HR-TEM results reveals that the lattice expansion of Cr underlayer induced by V addition increases the lattice misfit strain between CrV underlayer and FePt magnetic layer. Consequently well aligned FePt (001) grains and in-situ ordering of FePt L10 thin film can be obtained. Magnetic property measurements show that FePt/Pt/5.8 at.%V-Cr multilayer has the maximum out-of-plane coercivity (4000 Oe) and squareness (0.95). Due to in-situ ordering, good perpendicular magnetic properties with higher coercivity and squareness were achieved from FePt/Pt/Cr94.2V5.8 multi-layers at the processing temperature of 350°C without further annealing treatment.

The effects of the /spl infin/-Fe precipitation on the coercive force of the rapidly quenched Pr-Fe-B-Zr alloys

It has been found that the magnetic properties of the optimally heat-treated overquenched ribbons are inferior to those of the ribbons optimally quenched directly from the molten alloy. An attempt is made to elucidate the difference in coercive force between the two states. X-ray diffraction and thermomagnetic analysis confirm that the alpha -Fe phase is precipitated during the underquenching and the annealing after overquenching. This indicates that the degradation of coercive force of the annealed overquenched ribbons originates from the alpha -Fe precipitates. The existence of alpha -Fe is less detrimental to the coercive force than expected, suggesting that the grain boundary suppresses the propagation of domain walls. >

The Effects of the Pd-TiN Seed Layer on the Magnetic Properties of Co/Pd Multilayered Media

We investigated a Pd-TiN thin film as a seed layer for a [Co/Pd] multilayered perpendicular magnetic recording media. The 15-nm Pd-TiN thin film and [Co(0.2 nm)/Pd(0.8 nm)] multilayered media were fabricated by dc magnetron sputtering without heating the substrate. The Pd-TiN(20 vol.%) granular seed layer with apparent grain boundaries is considered to provide suitable seeds to [Co/Pd] multilayer and reduced the magnetic exchange coupling of [Co/Pd] grains causing an increase of the perpendicular coercive field and a decrease of the magnetic cluster size for the [Co/Pd] multilayered media. These results suggest that the Pd-TiN seed layer contributed to the formation of physically well-separated [Co/Pd] multilayered grains, improving the magnetic properties of [Co/Pd] multilayered media.

Improvement of Magnetic Properties and Texture of FePt Thin Films on MgO Substrates by Sn Addition

In this work, we studied the effects of Sn addition on the ordering temperature of FePt thin film. The coercivity of FePtSn film was about 1000 Oe greater than the coercivity of FePt film for an annealing temperature of 600℃. Therefore, Sn addition was effective in promoting the L1? ordering and in reducing the ordering temperature of the FePt film. From our X-ray diffraction results, we found that in the as-deposited film, the addition of Sn induced a lattice expansion in disordered FePt thin films. After the annealing process, the excess Sn diffuses out from the ordered FePt thin film because of the difference in the solid solubility of Sn between the disordered and ordered phases. The existence of precipitates of Sn from the FePt lattice was deduced by Curie temperature measurements of the FePt and FePtSn films. Therefore, the key role played by the addition of Sn to the FePt film can be explained by a reduction in the activation energy for the L10 order-disorder transformation of FePt which originates from the high internal stress in the disordered phase induced by the supersaturated Sn atoms.

Ordinary Magnetoresistance of an Individual Single-crystalline Bi Nanowire

We report the magneto-transport properties of an individual single crystalline Bi nanowire grown by a spontaneous growth method. We have successfully fabricated a four-terminal device based on an individual 400-nm-diameter nanowire using plasma etching technique to remove an oxide layer forming on the outer surface of the nanowire. The transverse MR (2496% at 110 K) and longitudinal MR ratios (38% at 2 K) for the Bi nanowire were found to be the largest known values in Bi nanowires. This result demonstrates that the Bi nanowires grown by the spontaneous growth method are the highest-quality single crystalline in the literatures ever reported. We find that temperature dependence of Fermi energy () and band overlap () leads to the imbalance between electron concentration () and hole concentration () in the Bi nanowire, which is good agreement with the calculated from the respective density of states, N(E), for electrons and holes. We also find that the imbalance of plays a crucial role in determining magnetoresistance (MR) at T and at T, while mean-free path is responsible for MR at T>75 K for and T>205 K for .