Dieter Weller

Bio: Dieter Weller is an academic researcher from Western Digital. The author has contributed to research in topics: Magnetic anisotropy & Coercivity. The author has an hindex of 64, co-authored 321 publications receiving 20927 citations. Previous affiliations of Dieter Weller include Hitachi & IBM.

Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices

Abstract: 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.

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

Abstract: 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.

Thermal effect limits in ultrahigh-density magnetic recording

Abstract: 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.

Bit-Patterned Magnetic Recording: Theory, Media Fabrication, and Recording Performance

Abstract: Bit-patterned media (BPM) for magnetic recording provides a route to thermally stable data recording at >1 Tb/in 2 and circumvents many of the challenges associated with extending conventional granular media technology. Instead of recording a bit on an ensemble of random grains, BPM comprises a well-ordered array of lithographically patterned isolated magnetic islands, each of which stores 1 bit. Fabrication of BPM is viewed as the greatest challenge for its commercialization. In this paper, we describe a BPM fabrication method that combines rotary-stage e-beam lithography, directed self-assembly of block copolymers, self-aligned double patterning, nanoimprint lithography, and ion milling to generate BPM based on CoCrPt alloy materials at densities up to 1.6 Td/in 2 . This combination of novel fabrication technologies achieves feature sizes of 2 (roughly equivalent to 1.3 Tb/in 2 ) demonstrate a raw error rate -2 , which is consistent with the recording system requirements of modern hard drives. Extendibility of BPM to higher densities and its eventual combination with energy-assisted recording are explored.

Enhanced magneto-optical Kerr effect in spontaneously ordered FePt alloys: Quantitative agreement between theory and experiment.

Abstract: A chemical-ordering-induced peak in the polar magneto-optical Kerr effect spectra of FePt compounds is found to be in quantitative agreement with predictions based on ab initio magneto-optical calculations. Experimental spectra of epitaxial molecular-beam-epitaxy grown FePt(001) films reveal a more than twofold enhancement of the Kerr rotation if full chemical ordering is present. A spectral feature at 2-eV photon energy shows a peak Kerr rotation of up to \ensuremath{\sim}0.8\ifmmode^\circ\else\textdegree\fi{}, which is strongly correlated with the presence of perpendicular magnetic anisotropy in ordered compound films.

Chemistry and properties of nanocrystals of different shapes.

Abstract: The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Magnetic nanoparticles: Synthesis, protection, functionalization, and application

Abstract: This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications

Abstract: 1. Introduction 20642. Synthesis of Magnetic Nanoparticles 20662.1. Classical Synthesis by Coprecipitation 20662.2. Reactions in Constrained Environments 20682.3. Hydrothermal and High-TemperatureReactions20692.4. Sol-Gel Reactions 20702.5. Polyol Methods 20712.6. Flow Injection Syntheses 20712.7. Electrochemical Methods 20712.8. Aerosol/Vapor Methods 20712.9. Sonolysis 20723. Stabilization of Magnetic Particles 20723.1. Monomeric Stabilizers 20723.1.1. Carboxylates 20733.1.2. Phosphates 20733.2. Inorganic Materials 20733.2.1. Silica 20733.2.2. Gold 20743.3. Polymer Stabilizers 20743.3.1. Dextran 20743.3.2. Polyethylene Glycol (PEG) 20753.3.3. Polyvinyl Alcohol (PVA) 20753.3.4. Alginate 20753.3.5. Chitosan 20753.3.6. Other Polymers 20753.4. Other Strategies for Stabilization 20764. Methods of Vectorization of the Particles 20765. Structural and Physicochemical Characterization 20785.1. Size, Polydispersity, Shape, and SurfaceCharacterization20795.2. Structure of Ferro- or FerrimagneticNanoparticles20805.2.1. Ferro- and Ferrimagnetic Nanoparticles 20805.3. Use of Nanoparticles as Contrast Agents forMRI20825.3.1. High Anisotropy Model 20845.3.2. Small Crystal and Low Anisotropy EnergyLimit20855.3.3. Practical Interests of Magnetic NuclearRelaxation for the Characterization ofSuperparamagnetic Colloid20855.3.4. Relaxation of Agglomerated Systems 20856. Applications 20866.1. MRI: Cellular Labeling, Molecular Imaging(Inflammation, Apoptose, etc.)20866.2.

Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices

Abstract: 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.