Young Oh

Atomic migration in Ni–Co ferrite

Ni–Co ferrite has been studied with Mossbauer spectroscopy and x‐ray diffraction. The crystal structure for this system is spinel, and the lattice constant is in accord with Vegard’s law. The Mossbauer spectra consist of two six‐line patterns corresponding to Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Neel temperature increases linearly with Ni concentration, suggesting the superexchange interacion for the Ni–O–Fe link is stronger than that for the Co–O–Fe link. It is found that Debye temperatures for the A and B sites of CoFe2O4 and NiFe2O4 are found to be θA=734 K, θB=248 K, and θA=378 K, θB=357 K, respectively. The intensity ratio of the A to B patterns is found to increase at low temperatures with increasing temperature due to the large difference of Debye temperatures of the two sites and to decrease at high temperatures due to migration of Fe3+ ions from A to B sites. Atomic migration of CoFe2O4 starts near 400 K and increases rapidly with increasing temperature to such a degree that ...

Structure and magnetic properties of Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta//

The perovskite type Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// (x=0.0, 0.25, 0.5, 0.75 and 1.0) system has been studied by X-rays, Mohrs salt analysis, and Mossbauer spectroscopy. From the results of X-ray diffraction measurement the structure of the Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// system is orthorhombic with x values of 0.0, 0.25 and 0.5, and cubic with the x values of 0.75 and 1.0. Mossbauer spectra of Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// have been taken at various temperatures ranging from 13 to 800 K. Analysis of Mossbauer spectra for this system demonstrated the existence of the mixed valence states of iron and the coordination state of Fe/sup 3+/ and Fe/sup 4+/ ions. The Neel temperature decreases linearly with Sr concentration, suggesting that the superexchange interaction for Gd-O-Fe link is stronger than that for Sr-O-Fe link. >

Fabrication and Magnetic Properties of Nonmagnetic Ion Implanted Magnetic Recording Films for Bit-Patterned Media

We have investigated the magnetic M-H loop characteristics of CoCrPt-SiO2 perpendicular recording media as influenced by nonmagnetic ion implantations. We have also developed patterned media via ion implantation using a convenient polymer nanomask approach for local control of coercivity of magnetically hard [Co/Pd]n multilayer film with a [Co 0.3 nm \Pd 0.8 nm] 8/Pd 3 nm/Ta 3 nm layer structure. The CoCrPt-SiO2 magnetic layer having perpendicular magnetic anisotropy is sputter deposited on a flat substrate. The [Co/Pd] n multilayer film with vertical magnetic anisotropy is deposited and the regions corresponding to the magnetic recording bit islands are coated with polymer islands using a nanoimprinting technique. Subsequent ion implantation allows patterned penetration of implanted ions into the [Co/Pd]n multilayer film, thus creating magnetically isolated bit island geometry while maintaining the overall flat geometry of the patterned media.

Magnetic Properties of Antidot Patterned Co/Pd Multilayer Film

Co/Pd multilayer media with an antidot structure have been fabricated on hole-patterned Si substrates. Si nanoholes were created by dry etching using a self-assembled diblock copolymer (DBCP) pattern as an etch mask. To enhance dry etching selectivity, thin Cr film was vacuum deposited on DBCP surface at an oblique incident angle. In comparison with the Co/Pd multilayer on a flat Si substrate, the magnetic hysteresis loop of the antidot structured Co/Pd multilayer revealed an enhanced coercive force largely independent of the amount of reactive etching time, while the shape of hysteresis loop was substantially altered as a function of dry etching time. In an optimized condition, high perpendicular magnetic anisotropy was obtained while no significant parallel magnetic anisotropy was seen, which is desirable for perpendicular recording media or pseudobit-patterned media application.

Planarization of Discrete Track Recording Media to Improve Flyability of Magnetic Recording Sliders

Discrete track recording (DTR) media which consist of discrete tracks are presently considered a new paradigm to achieve ultra-high areal density in hard disk drives (HDDs). However, due to the presence of the grooves, stable flying of magnetic recording sliders over DTR media is a question of great concern. In this study, planarization of DTR media with hydrogen silsesquioxane (HSQ), a nonmagnetic material, is investigated. Scanning electron microscopy (SEM) of planarized disks showed very good ldquogap fillingrdquo properties of HSQ in the groove area during the planarization process. Atomic force microscopy (AFM) measurements indicated that the residual groove depth of planarized DTR media decreases with decreasing rotating speed of the disk during spin-coating. Improved flyability of magnetic recording sliders over DTR media was observed after planarization.

Di-block copolymer directed anodization of hexagonally ordered nanoporous aluminum oxide

Porous anodized aluminum oxide (AAO) nanostructures have been extensively investigated as versatile templates for nanodots and nanowires for many applications. Such self-ordered AAO structures are often achieved by so-called two-step anodization. Ordered pore arrangements can be obtained in the second step after removing the AAO layer formed in the first anodizing step, during which hexagonally ordered, concave-pored Al surface is formed so as to serve as vertical pore nucleation sites for the subsequent anodization step. Although such a two-step anodization process has proven useful for bulk Al surface, the relatively large amount of Al material that needs to be used up to obtain a well ordered AAO template is an issue when the starting material is a thin film layer of Al rather than a bulk Al foil. In this paper, we demonstrate successful fabrications of ordered and vertically aligned AAO nanopore patterns directed by a hexagonally patterned poly(styrene-b-4-vinylpyridine) di-block copolymer layer place...

Diameter-Reduced Islands for Nanofabrication Toward Bit Patterned Magnetic Media

Thin films deposited on a flat substrate, if the adhesion is not strong, can be made to ball up and form discrete islands upon heating to elevated temperatures due to the surface energy difference. We have applied this useful process to electron beam lithography (EBL) and nano-imprinting lithography (NIL). By combining the ball-up processes of Ni thin film with e-beam lithography followed by reactive ion etching, Si nano islands and vertical nanopillars as small as 10 nm in diameter have been realized. There is a strong correlation between the initial thickness of Ni mask layer and final Ni island diameter obtained after ball-up. The smallest island diameter is obtained using ~ 5-nm initial Ni layer thickness. Below 3-nm initial layer thickness, the intended ball-up reaction does not occur. With more than ~ 15-nm initial metal film thickness, the Ni layer is broken up into nonspherical and highly irregular structures. [Co/Pd]n multilayer magnetic thin films deposited on prepatterned substrate by nano imprinting lithography with versus without island ball-up process have been investigated for bit patterned media studies. The coercivity of magnetic islands can be enhanced by island diameter reduction using the ball up process.