Hiroyuki Hieda

2.5-inch disk patterned media prepared by an artificially assisted self-assembling method

Circumferential magnetic patterned media were prepared on a 2.5-inch-diameter glass plate and on a 3-in-diameter silicon plate. A Ni master disk possessing spiral patterns with 60-250-nm-width lands and a 400-nm-width groove was pressed into a resist film on a CoPt or CoCrPt film to transfer the spiral patterns. A diblock copolymer solution was cast into the obtained grooves and then annealed to prepare self-assembling dot structures aligned along the grooves. According to the dot patterns, the underlying magnetic films were patterned by ion milling to yield patterned media with a 40-nm diameter. Coercive forces and squareness ratios of the patterned media increased compared to those of the continuous media, probably due to the decrement of a demagnetizing field. Single magnetic domains with an almost perpendicular orientation were confirmed in each magnetic dot.

Thermally assisted magnetic recording on a bit-patterned medium by using a near-field optical head with a beaked metallic plate

A near-field optical head with a beaked metallic plate was used for writing marks on a Co∕Pd bit-patterned medium with a diameter of 20–25nm and a pitch of 30nm. Magnetic-force-microscope images of the medium show that the magnetizations of single bits were selectively reversed by the head. The light-utilization efficiency (defined as the ratio of the absorbed power in the medium to the incident light power) was estimated from the writing condition used and thermal modeling as about 5%.

Ar ion milling process for fabricating CoCrPt patterned media using a self-assembled PS-PMMA diblock copolymer mask

Etching the damage of a patterned media fabricated with an artificially assisted self-assembled mask is estimated. CoCrPt thin films were etched by ion milling into aligned dots with a diameter of 40 nm. The milling condition was optimized for reducing the etching damage. As a result, the damage to the crystal lattice and crystal orientation was estimated to be slight by transmission electron microscope (TEM) and scanning electron microscope (SEM) analyses. Though strong perpendicular anisotropy was induced by the patterning process, magnetic measurements and Landau-Lifshitz-Gilbert simulation revealed that the magnetic anisotropy energy was almost unchanged throughout this etching process.

Measurements of Electrostatic Double-Layer Forces Due to Charged Functional Groups on Langmuir-Blodgett Films with an Atomic Force Microscope

Using an atomic force microscope, we measured the forces between a Si3N4 tip and monolayers with different polar functional groups ( –NH2, –COOH, –CONH2, and –OH) prepared by the Langmuir-Blodgett method while varying the pH value of aqueous solutions. The obtained force vs distance curves were related to the surface charges of the tip and the dissociation of the functional groups from the pH dependency, and the charged state of functional groups could thus be discerned. In addition, the electrostatic origin of the force has been confirmed assuming constant potentials on both surfaces.

Electrical Double-Layer Forces Measured with an Atomic Force Microscope while Electrochemically Controlling Surface Potential of the Cantilever

The atomic force microscope was used to detect electrostatic forces and estimate surface potentials of samples in aqueous solution by applying voltage to a conducting tip. We controlled surface potential of a Au-coated cantilever electrochemically, and measured force vs distance curves on monolayers with different kinds of functional groups. As a result, we detected force variation with applied voltage to the tip, and the direction of force variation was found to agree with the polarity of surface charges due to the dissociation of the functional groups.

Fabrication and Characterization of FePt Exchange Coupled Composite and Graded Bit Patterned Media

Three methods to fabricate continuous FePt films with graded magnetic anisotropy for bit patterned media (BPM) were evaluated. Continuous FePt films with surface roughness of less than 0.3 nm were achieved in continuous FePt hard magnetic films, FePt/Fe exchange coupled composite (ECC) films and FePt/Fe based graded films. Depositing an Fe-rich film on FePt at high temperature was found to form large grains and cause the film surface very rough for BPM fabrication. Depositing Fe on FePt at room temperature and then annealing it to create graded anisotropy through the layer interdiffusion process was demonstrated to fabricate FePt/Fe based graded BPM. The continuous FePt films with hard layer only, ECC structure and graded magnetic anisotropy were patterned using a di-block copolymer self-assemble hard mask method with 25 nm dot size over 2-inch substrate. The switching field distribution (SFD) broadening and degradation of FePt BPM was studied. The reduction of SFD was achieved using a postannealing process. It was confirmed that the patterned graded BPM sample has smaller switching field and larger thermal energy barrier than the ECC sample.

5 Tdots/in 2 bit patterned media fabricated by a directed self-assembly mask

FePt bit patterned media (BPM) was fabricated with a self-assembled polymer mask with a feature size of 12 nm pitch (equivalent to 5 Tdots/in2 ). A 3.5 nm FePt film with high c-axis crystal orientation was prepared for the magnetic recording layer. A solvent vapor annealing process was applied for obtaining uniform directed self-assembling of polystyrene (PS)-polydimethylsiloxane (PDMS) diblock copolymer pattern. Pattern transfer from a polymer mask to FePt layer was achieved by employing a carbon hard mask. In spite of excellent magnetic characteristics of FePt layer, the fabricated FePt BPM showed small coercivity (Hc) of 6 kOe and large switching field distribution (SFD) of 21%. These results are due to the etching damage of FePt dots. Disordering of FePt L10 phase by the etching damage reduced magnetic anisotropy energy (Ku). The damaged portion became a nucleus of the magnetization reversal and reduced Hc. Distribution of the damaged volume and the extent of the Ku reduction contributed to large SFD. This model is supported by the experimental data of magnetic field angle dependence of switching field. The result suggests the domain wall motion type of magnetization reversal mode, where the domain wall is created at the interface between the damaged portion and the internal high-Ku region.

Study of high magnetic anisotropy Co∕Pd multilayers for patterned media

We have studied the perpendicular magnetic anisotropy of Co∕Pd multilayers for ultrahigh recording density patterned media. By using Pt as an underlayer, the magnetic anisotropy enhances significantly, which is considered to be due to a magnetostriction by inducing a tensile strain into the Co∕Pd multilayers. A patterned medium using the Co∕Pd multilayers has been successfully fabricated with the diameter of the dots and the distance between the center of the dots measuring 20 and 30nm, respectively. The coercivity of the nanopatterned medium is 9.5kOe.

Fluorescence quenching induced by injected carriers in organic thin films

The effect of injected charges on fluorescence of organic thin films consisting of the donor molecule tetra(N,N-diphenyl-4-aminophenyl)ethylene (TTPAE) was investigated. Fluorescence was efficiently quenched by charge injection. By comparison of fluorescence measurements and current measurements, it was found that hole injection had essential role in fluorescence quenching and electron injection did not. Coulomb interaction between photoexcited molecules and injected carriers was found to cause fluorescence quenching.

Charge injection and extraction on organic dot structures by atomic force microscopy

We describe the manipulation of the charged state of nanoscale organic ‘‘dot’’ structures using the atomic force microscope. Electric charges were injected into single dots by contact electrification from an atomic force microscope tip to which voltage was applied. On measuring the charge distribution two‐dimensionally, it was found that the injected charges were stably confined in the dot structures for an extended period. The number of injected charges could be controlled down to single elementary charge. Further, injected charges could be reextracted from the dots by using a tip with reduced applied voltage. However, complete extraction from highly charged dots was difficult, which suggests that some transfer of charge from dots to adjacent regions may occur.