Jianmin Bai

Exchange coupled composite media for perpendicular magnetic recording

Exchange coupled composite (ECC) media has been shown to possess several major advantages relative to conventional perpendicular media, including a reduction in the switching field of approximately a factor two for the same thermal stability and greater insensitivity to easy axis distribution. In this paper, full magnetostatic interactions are included: this allows comparison between the behavior of multigrain thin films and that of isolated grains as presented earlier. Significant results include hysteresis loops for thin films under various conditions including inadequate and excessive intra granular exchange between the hard and soft materials. An important distinction is made between the coercivity and remnant coercivity as a function of angle between applied field and easy axis. A perpendicular magnetic recording head is used to compare the shape of effective fields for ECC and conventional perpendicular media. Written transitions in the ECC media appear to be similar to those written in perpendicular media at comparable densities.

Composite media (dynamic tilted media) for magnetic recording

We designed and fabricated a composite magnetic recording medium with exchange decoupled magnetic grains that consist of two vertically exchange-coupled magnetic regions (one is magnetically soft and one is magnetically hard) as an approach to alleviate the writing field limitation of perpendicular magnetic recording heads. A nonmagnetic layer with different thickness was put between the hard and soft layer to tune the exchange coupling. With proper coupling, significant drop of the coercivity field was observed for this composite medium while still maintaining good thermal stability. Better recording performance was obtained for such medium compared to perpendicular and longitudinal medium. The results have proved the possibility of fabricating a writable recording medium having an ultrahigh magnetic anisotropy constant (Ku) value.

High-magnetic-moment core-shell-type FeCo–Au∕Ag nanoparticles

We developed a physical technique combining an on-line sputtering/evaporation process with an integrated nanocluster deposition process to prepare core-shell-type nanoparticles. High-magnetic-moment (Fe60Co40)coreAushell and (Fe60Co40)coreAgshell superparamagnetic nanoparticles with controllable particle size of 10–20 nm and Au∕Ag shell thickness of 1–3 nm were prepared by using this method. Au shell is not only functional for the potential biocompatibility but also the key to prevent the oxidation of FeCo nanoparticles. Saturation magnetization of (Fe60Co40)coreAushell nanoparticles was found three times higher than that of iron oxide nanoparticles. This technique enables us to control independently the dimensions of core and shell and select individually materials for core and shell for any other core-shell-type nanoparticles.

In situ magnetic field alignment of directly ordered L10 FePt nanoparticles

Gas-phase prepared directly ordered FePt nanoparticles were shown to align with in situ magnetic fields. As shown by magnetic and x-ray diffraction data, a 3800Oe perpendicular field switched L10 FePt particles with a mean size of 6nm from in-plane arrangement in as-deposited state to out-of-plane orientation. A 5000Oe in-plane field successfully defined nanoparticles with in-plane texture. These results demonstrated the feasibility of preparing nanoparticle-based magnetic recording media and exchange-spring-type permanent magnets with desired magnetic orientation control. Only involving thermal fluctuation as the obstacle, this approach makes an ideal subject for theoretical understanding and further optimization.

Enhancement of quantum efficiency of organic light emitting devices by doping magnetic nanoparticles

Magnetic nanoparticles of CoFe are used as dopants to enhance the quantum efficiency of electroluminance in a single layer organic light emitting device (OLED). The enhancement of quantum efficiency increases with both increasing density of CoFe nanoparticles and external magnetic field. For a given OLED with 0.1wt% doping, the enhancement of the quantum efficiency reaches ∼27% and ∼32% without and with a magnetic field, respectively. The origin of these improvements could be attributed to the simultaneous increases of the portion of excitons among total charge carriers and the fraction of singlets among the total excitons

Frequency multiplying behavior in a magnetoelectric unimorph

We observe frequency multiplying behavior in a magnetoelectric (ME) unimorph: such a frequency-multiplied signal is generated when the input frequency (fin) of an alternating current magnetic field (Hac) is around 1/n (n denotes integer) of the mechanical resonance frequency (fr) of the device. We observe both odd and even harmonic signals when a direct current magnetic field (Hdc) is applied, whereas only even harmonic signals arise for Hdc = 0. This behavior results from the combined effect of the nonlinear character of the Metglas magnetostriction and a mechanical resonance phenomenon in the magnetoelectric unimorph.

Effect of CrW underlayer on structural and magnetic properties of FePt thin films

L10 ordered FePt thin films with face-centered-tetragonal (001) texture have been prepared by magnetron sputtering on Cr100−xWx underlayer. The dependence of Cr100−xWx microstructure and FePt texture on the W content in the Cr underlayer was investigated. The addition of W element in Cr underlayer enhances the Cr (200) texture and increases the lattice constant of Cr, which is favorable for lowering the transformation temperature from fcc to fct phase because of the increase of the tensile stress along the FePt a axis. A good FePt (001) texture is obtained on the Cr85W15 underlayer with a substrate temperature of 400°C. The coercivity of FePt thin films on CrW underlayer is higher than on Cr underlayer and increases with increasing W content in the Cr underlayer because the formation of the CrW alloy inhibits the diffusion between FePt and CrW layer. A 5nm Pt intermediate layer was employed to suppress the diffusion between FePt and CrW underlayer further.

Cubic and Spherical High-Moment FeCo Nanoparticles With Narrow Size Distribution

FeCo nanoparticles with controllable shapes and narrow size distribution were fabricated by using a plasma-gas-condensation technique. Either cubic shape or spherical shape were obtained when the magnetic field distribution above the sputtering target were well controlled. Meanwhile, the size distribution of these FeCo nanoparticles was also controllable, and a narrow size distribution of 4.9% was obtained at the optimized conditions. The Fe-Co nanoparticles with different shapes showed different magnetic behaviors.

(FeCo)3Si?SiOx core?shell nanoparticles fabricated in the gas phase

A method of fabricating core?shell nanoparticles by using an integrated nanoparticle deposition technique in the gas phase is reported. The principle of the method is based on nanoparticle growth from the vapour phase, during which elements showing lower surface energies prefer to form the shells and elements showing higher surface energies prefer to stay in the cores. This method was applied successfully to the Fe?Co?Si ternary system to fabricate core?shell-type nanoparticles. The nanoparticles were exposed in air after collection to achieve oxidation. The analysis results based on transmission electron microscopy (TEM), Auger electron spectroscopy (AES), x-ray diffraction (XRD), and a superconducting quantum interference device (SQUID) showed that the core parts are magnetic materials of body-centred cubic (bcc) structured (FeCo)3Si of 15?nm in diameter, and the shell parts are amorphous SiOx of 2?nm in thickness. These core?shell-type nanoparticles show a magnetic anisotropy constant of about 7 ? 105?erg?cm?3 and a saturation magnetization of around 1160?emu?cm?3, which is much higher than that of iron oxide. After annealing at 300 ??C in air, (FeCo)3Si?SiOx core?shell-type nanoparticles showed a little bit of a drop in magnetic moment, while pure FeCo nanopariticles totally lost their magnetic moment. This means that the shells of SiOx are dense enough to prevent the magnetic cores from oxidation.

Composite perpendicular magnetic recording media using [Co∕PdSi]n as a hard layer and FeSiO as a soft layer

We fabricated the composite perpendicular magnetic recording (PMR) media successfully for the first time by combining a nanogranular FeSiO soft layer and a [Co∕PdSi]n hard layer. PdSi spacing layer (0–4nm) was used to study the exchange coupling effects between the FeSiO(5nm) and the [Co(0.26nm)∕Pd(1nm)]14 layers in the composite films. Proper coupling occurs when PdSi interlayer is ∼0.5nm. Significant lowering of the coercivity is observed for the composite PMR medium while still maintaining good thermal stability. The results prove the possibility to fabricate a writable PMR medium having an ultrahigh magnetic anisotropy constant Ku value.