Y. J. Chen

Fabrication and characterization of bit-patterned media beyond 1.5?Tbit/in2

We fabricated bit-patterned media (BPM) at densities as high as 3.3 Tbit/in(2) using a process consisting of high-resolution electron-beam lithography followed directly by magnetic film deposition. By avoiding pattern transfer processes such as etching and liftoff that inherently reduce pattern fidelity, the resolution of the final pattern was kept close to that of the lithographic step. Magnetic force microscopy (MFM) showed magnetic isolation of the patterned bits at 1.9 Tbit/in(2), which was close to the resolution limit of the MFM. The method presented will enable studies on magnetic bits packed at ultra-high densities, and can be combined with other scalable patterning methods such as templated self-assembly and nanoimprint lithography for high-volume manufacturing.

High coercivity in SiO2-doped CoFe2O4 powders and thin films

Mossbauer and magnetic studies have shown that 1–2 wt % of SiO2 could be dissolved in the CoFe2O4 structure. Changes in magnetization and Curie temperature were observed. The presence of SiO2 might enhance magnetic anisotropy. Coercivity values of up to 3.5 kOe were measured for mechanically alloyed CoFe2O4/SiO2 powders. High coercivities were also achieved in SiO2-doped Co–ferrite thin films prepared by sputtering technique. The Co–ferrite thin film deposited on a silicon wafer using a 5 wt % SiO2/Co–ferrite target possessed a coercivity of 7.4 kOe. Spring-magnet behavior was observed, indicating the possible presence of remanence enhancement due to exchange coupling because of nanocrystalline structure.

Magnetic domain structures of Co22Ag78 granular films observed by magnetic force microscopy

A magnetic force microscope (MFM) was used to image the topography and magnetic microstructures of Co22Ag78 granular films. The observed morphology shows isolated nanometer-scale cobalt particles (granules) embedded in the silver matrix. Stripe magnetic domains with much larger size (typically of ∼100 nm wide) than that of cobalt particles are resolved clearly on MFM micrographs for the annealed samples. It is demonstrated that the domain width and the relative magnetic force strength first increases and then decreases with annealing temperature with a maximum at about 600 K. We suggest that the appearance of the stripe domains is attributed to magnetic correlation among many of the isolated single-domain cobalt particles and is dependent on the microstructure of the samples.

Periodic magnetic nanostructures on self-assembled surfaces by ion beam bombardment

We demonstrate that periodic magnetic nanostructures can be fabricated by depositing perpendicular magnetic films on self-assembled surfaces by ion beam bombardment. Similar stripe domain patterns and reduced coercivity are observed for the as-deposited sample as compared with those for a reference sample with unbombarded substrate. Further processing steps such as etching and polishing will be needed to isolate the magnetic elements from each other, in order to make ultrahigh density magnetic storage media.

A study of multirow-per-track bit patterned media by spinstand testing and magnetic force microscopy

We propose and demonstrate a concept of multirow-per-track bit patterned media (BPM) recording to overcome the problems encountered with the conventional one-row-per-track design. Focused ion beam was used on continuous granular perpendicular magnetic media to fabricate the prototype structures consisting of birow tracks with sub-100-nm single domain magnetic islands with the two rows of islands to be interleaved along the track direction, as well as an additional nonmagnetic spacer band between adjacent birow tracks for further bit aspect ratio (BAR)≥2 adjustment. Readback from such birow tracks with a two-row-wide read head was performed by dynamic spinstand testing. The proposed concept BPM provides many advantages including higher linear recording density (under the same lithographic limit), therefore enabling a higher data rate and a greater BAR≥2 for better integration with head design and servocontrol, as well as allowing the use of wider thus larger recording heads to improve writing efficiency fo...

Ion implanted nanostructures on Ge(111) surfaces observed by atomic force microscopy

Epi-ready Ge(111) surfaces were implanted with cobalt ions to doses of 1016–5×1017 ions/cm2 at accelerating voltages of 40–70 kV. Cellular nanostructures were observed by contact mode and tapping mode atomic force microscopy (AFM). These are similar (at higher resolution) to those reported in earlier scanning electron microscope measurements. Image distortions observed in contact mode AFM are attributed to not only the effect of the tip size but also the change of the effective tip shape due to the softness and stickiness of the implanted surface layer. The variation of the root-mean-square roughness with ion dose (1016–1017 ions/cm2), accelerating voltage (40– 70 kV), and mean beam current density (15–150 μA/cm2) is presented and explained in terms of ion range and surface temperature.

Direct observation of domain wall evolution at a bifurcation in magnetic network structures

We report on the magnetization dynamics at a bifurcation in a dual-branch magnetic network structure. When a transverse domain wall (DW) propagates through the network, interaction with an edge defect at the bifurcation leads to the transformation of the DW from transverse to vortex. The topological charge is conserved as the DW moves through the bifurcation, and this charge conservation is intrinsically linked to a −1/2 topological defect in the system. Magnetic force microscopy (MFM) imaging enables the direct observation of defect displacement during DW transformation, which induces a selective switching in the branch of the network structure.

Characterization of L10-FePt/Fe based exchange coupled composite bit pattern media

L10-FePt exchange coupled composite (ECC) bit patterned media has been considered as a potential candidate to achieve high thermal stability and writability for future high density magnetic recording. In this paper, FePt based ECC bit patterned structures with 31 nm bit size and 37 nm pitch size were fabricated using di-block copolymer lithography on 3 inch wafer. Remanant states were tracked using magnetic force microscopy (MFM). DC demagnetization (DCD) curves were plotted by counting the reversed bits in the MFM images. Magnetic domains in which the magnetizations of the neighboring bits were aligned to the same direction were observed in the MFM patterns. Thermal decay measurement was performed for the samples to obtain the thermal stability and gain factor. The thermal barrier was found around 210 kBT with a gain factor up to 1.57 for the bit patterned structure FePt(4 nm)/Fe(4 nm).

Thermal effects of heated magnetic disk on the slider in heat-assisted magnetic recording

In this paper, the real time thermal effect of the heat-assisted magnetic recording (HAMR) on the slider is investigated with the HAMR platform. The net temperature rises of the slider caused by laser absorption and heat radiation of the locally heated recording magnetic disk are studied by monitoring the calibrated read sensor resistances in different rotation speeds. The maximum slider temperature increase of about 3.5°C caused by the locally heated recording disk is obtained when the media net temperature increase is 85°C at 2000rpm rotation speed. A higher temperature case is also discussed.

Smooth discrete track media fabricated by focused ion beam

We have successfully fabricated discrete track media reproducibly using the focused ion beam (FIB). 1 pA and 10 pA FIB beam currents were used effectively for discrete track fabrication; at Ga+ doses of ∼4.2×1016ions∕cm2 and below, media magnetic properties are effectively suppressed, whilst damage to the morphology was minimal. Larger land/groove magnetic contrast was achieved through increasing the ion bombardment rates within this range. Beyond this dosage range, magnetic suppression leveled out, and physical grooves were etched in the media.