Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.

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Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices

High K/sub u/, uniaxial magnetocrystalline anisotropy, materials are generally attractive for ultrahigh density magnetic recording applications as they allow smaller, thermally stable media grains. Prominent candidates are rare-earth transition metals (Co/sub 5/Sm,...), and tetragonal intermetallic compounds (L1/sub 0/ phases FePt, CoPtY,...), which have 20-40 times higher K/sub u/ than today's hexagonal Co-alloy based media. This allows for about 3 times smaller grain diameters, D, and a potential 10-fold areal density increase (/spl prop/1/D/sup 2/), well beyond the currently projected 40-100 Gbits/in/sup 2/ mark, Realization of such densities will depend on a large number of factors, not all related to solving media microstructure problems, In particular it is at present not known how to record into such media, which may require write fields in the order of 10-100 kOe. Despite this unsolved problem, there is considerable interest in high Ku alternative media, both for longitudinal and perpendicular recording. Activities in this area will be reviewed and data on sputtered and evaporated thin FePt films, with coercivities exceeding 10000 Oe will be presented.

High K/sub u/ materials approach to 100 Gbits/in/sup 2/

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in/sup 2/. This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at "stability ratios" (K/sub u/V/K/sub B/T)/sub 0//spl sime/35 /spl plusmn/ 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in/sup 2/ conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits.

Thermal effect limits in ultrahigh-density magnetic recording

The key methods for the preparation of magnetic nanoparticles are described systematically. The experimental data on their properties are analysed and generalised. The main theoretical views on the magnetism of nanoparticles are considered.

https://iopscience.iop.org/article/10.1070/RC2005v074n06ABEH000897/pdf

Magnetic nanoparticles: preparation, structure and properties

A shielded medical device implanted in a biological organism. The device has a magnetic shield, and the magnetic shield contains a layer of nanomagnetic material; such layer has a morphological density of at least 98 percent; and it is bonded to the medical device by means of an interlayer with a thickness of less than about 10 microns. The nanomagnetic material in such layer has a saturation magnetization of from about 1 to about 36,000 Gauss, a coercive force of from about 0.01 to about 5,000 Oersteds, a relative magnetic permeability of from about 1 to about 500,000, and an average particle size of less than about 100 nanometers.

Magnetically shielded assembly

This article reports on the properties of the media prepared on glass substrates which were used in IBM’s 10 Gbit/in.2 demonstration. In order to support a linear density of 315 kbpi and a track density of 33 ktpi, the remanant coercivity Hcr and remanant moment thickness product Mrt of the magnetic layer were 3450 Oe and 0.37 memu/cm2, respectively. The media used a NiAl seed layer, a CrV underlayer, a Co alloy magnetic layer, and a carbon overcoat protection layer. The magnetic film had a grain size of 12 nm as observed by transmission electron microscopy. The preferred orientation (PO) of the magnetic layer was (1010). This PO enables one to sustain high coercivities at low values of Mrt. It is observed that the c-axis in-plane texture of the magnetic layer is critical to achieve a low noise medium. Using a focused-ion-beam (FIB) trimmed giant magnetoresistance head and conventional partial response maximum likelihood channel, the on-track-error rates were measured at the 10−10 level.

10 Gbit/in.2 longitudinal media on a glass substrate (invited)

The microstructure and magnetic properties of CoPtCr thin films with 〈1120〉 restricted fiber texture grown on (100) planes of Cr single‐crystal films were characterized and subsequently correlated with the domain configuration during the magnetization reversal process. These films exhibited a bicrystal mode associated with two orthogonal easy axes of magnetization in the plane of the film. The domain configuration of these films during the magnetization‐reversal process consisted of two sets of domains extending in two orthogonal directions. These properties were compared with those of CoPtCr films with random crystallographic orientation.

Magnetic properties of CoPtCr thin films with 〈112̄0〉 crystal orientation

A thin film disk with laminated magnetic layers, for use in a disk drive, is described which exhibits a single switching behavior resulting in a smooth hysteresis loop. This is achieved by depositing a seed layer prior to depositing the Cr or Cr alloy underlayer. The seed layer material is selected to promote a [112] PO in the underlayer and subsequently a [1010] PO in the two or more laminated magnetic layers. The [1010] PO aids in maintaining minimal Hc variations between the magnetic layers. The seed layer can be a B2 type structure material such as NiAl or FeAl or any other material which results in the [112] PO in the underlayer. The underlayer is preferably Cr or a Cr alloy; the magnetic layers are preferably CoPtCr, CoPtCrTa or CoPtCrB. The spacer layer between the magnetic layers may be made from the same material as the underlayer, but may also be different, e.g. an hexagonal crystalline material such as Ru. The disks of the invention can be manufactured in standard high volume sputtering systems by virtue of the fact the magnetic layers have a reduced dependency on substrate temperature.

Laminated thin film disk for longitudinal recording

A thin film disk and a disk drive using the thin film disk are described. The disk has an onset layer between the underlayer and the boron containing magnetic layer, for example. The onset layer of the invention is useful because the boron containing magnetic layer material resists being deposited with the C-axis in plane. The onset layer material is selected to promote an in-plane C-axis orientation. When a boron containing magnetic layer is deposited on the onset layer the resulting in-plane PO is improved. The preferred onset layer is of hexagonal closed pack structured material which may be magnetic or nonmagnetic. Materials which are usable for the onset layer include a wide range of pure elements and cobalt alloys such as CoCr, CoPtCr, CoPtCrTa and CoCrB. The onset layer is particularly useful in allowing a ferromagnetic cobalt (Co) alloy containing a relatively high chromium and boron content to be deposited on nonmetallic substrates with the C-axis in the plane of disk without the need for negative bias during the sputtering of the underlayer.

Thin film disk with onset layer

CoPtCrTa films, with CrTi underlayer and a variety of crystallographic preferred orientations, were prepared on glass substrates. AlN, NiAl or Ta seed layers, deposited prior to deposition of the underlayer, were used to control the crystal orientation and grain size of the underlayer and thereby the magnetic layer. The correlation of magnetic and structural properties was studied. The media noise was found to depend primarily on grain size and grain size dispersion; the crystal orientation of the film had a secondary role. For a given CrTi underlayer, the [1010] preferred orientation was associated with the highest coercivity.

Mohammad Taghi Mirzamaani

Papers

10 Gbit/in.2 longitudinal media on a glass substrate (invited)

Magnetic properties of CoPtCr thin films with 〈112̄0〉 crystal orientation

Laminated thin film disk for longitudinal recording

Thin film disk with onset layer

Recording Performance of Thin Film Disks with Various Crystallographic Preferred Orientations on Glass Substrates