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For bit patterned media (BPM) applications, while significant progress has recently been made in demonstrating high-resolution dot patterning using e-beam direct writing, and followed by the successful imprint process, many serious issues in fabrication still remain. This paper will only discuss the key challenges in the BPM lithography, including 1) the commercial availability of a high-resolution rotating stage e-beam system; 2) the approach and limitation of directed polymer self-assembly for resist pattern quality improvement and resolution enhancement; 3) the difficulties and limitations in the fabrication and replication of a 1 x template with a density beyond 1 Tbit per square inch (Tb/[in.sup.2]), while the defectivity, lifetime, and damage of the template are still questionable; 4) the tight requirements of size uniformity and placement accuracy; and 5) the needs of advanced metrology associated with the very small dot size and defect characterization. In this paper, we will address the above challenges and present some experimental data along with possible solutions for the challenges. Index Terms--Advanced lithography, bit patterned media, directed self-assembly, nanoimprint lithography.

A complex transverse ac susceptibility ([[chi].sub.T]) technique based on magneto-optical detection has been developed with sufficient sensitivity for thin-film recording media. The technique has been used to measure the dispersions in grain size and the anisotropy field of perpendicular magnetic recording media. Experiments have been designed to study the key factors contributing to the anisotropy field distribution ([sigma][H.sub.k]) in perpendicular media. The correlation between anisotropy dispersion, stacking faults, and recording performance have also been studied. A good correlation between stacking faults and [sigma][H.sub.K] has been established. Lower stacking faults result in smaller anisotropy distribution and better media signal-to-noise ratio. The main factors controlling stacking faults are studied. Theoretical studies of the effect of interactions on the [[chi].sub.T] are also presented. Index Terms--AC suseptibility, anisotropy distribution, magnetic recording, perpendicular media, stacking faults.

Due to the limits of conventional perpendicular magnetic recording, it appears that alternative technologies are needed at areal densities >500 Gb/[in.sup.2]. Heat-assisted magnetic recording (HAMR) is a promising approach to extend areal densities to 1 Tb/[in.sup.2] and beyond. All of the unique components necessary for a working HAMR system have been demonstrated. Although HAMR permits writing on high Hc media with lower magnetic fields and can produce higher write gradients than conventional magnetic recording, head/media spacing and the development of high Hc media with small grains remains challenging. Index Terms--Heat-assisted magnetic recording (HAMR), high-density recording.

A complex transverse alternating-current (ac) suspectibility [X.sub.T] technique based on magneto-optical detection is developed with sufficient sensitivity for thin film recording media. The technique is used to measure the dispersions in grain size and anisotropy field in perpendicular magnetic recording media.

The selected aspects of the recording physics of magnetic data storage specific to writing and reading in patterned medium are discussed. The influence of a recording layer design on playback resolution is evaluated, which offers guidelines for the design and optimization of patterned magnetic recording medium.

Exploring new Co-alloy composition to achieve both chemical segregation and suitable magnetic properties is a priority task for the development of the future perpendicular magnetic recording media. With proper interlayer (IL) design, columnar Co-alloy grains with narrow orientation distribution ([DELTA][[theta].sub.50] < 6[degrees]) have been obtained on thin IL (4 nm thick). We demonstrate CoCrPtB-alloys with full remanence squareness and large negative reverse field ([H.sub.r] < -1.5 kOe) at low IL and magnetic layer thickness of 4 and 11 nm, respectively. It is also shown that carbon overcoat thickness reduction results in a signal-to-noise ratio gain of about 0.5 dB/nm. Index Terms--Co-alloy, microstructure, perpendicular magnetic recording media, recording performance.

Numerical and analytical calculations of rotational process in perpendicular recording media are presented. The work supports recent experimental studies that suggest that the measurement of rotational magnetization processes can be used to determine the value of the anisotropy constant. An expression for the rotational magnetization for a noninteracting system is derived taking into account the dispersion of K and the easy-axis orientation. The calculations show that the experiments determine the mean value of [H.sub.K], essentially independent of the angular dispersion. A numerical (Monte-Carlo based) micromagnetic model is used to study the effects of magnetostatic and exchange interactions at nonzero temperatures. It is shown that for small values of KV/kT, irreversible magnetization processes take place, which precludes the use of the rotational magnetization method to determine K values. This effect is enhanced by the presence of the magnetostatic interaction. However, the presence of exchange interactions is found to enforce coherent rotation in small fields, reducing the irreversible processes and allowing the determination of [H.sub.K]. Under these circumstances, it is shown that the exchange does not significantly affect the value of [H.sub.K], and that a well-defined demagnetization correction of 4[pi] M is appropriate. Finally, a comparison with experimental data gives good agreement for multilayer and granular media and shows the role of domain formation on the rotational magnetization process. Index Terms--Anisotropy, micromagnetics, perpendicular media, rotational magnetization.

We present a novel experimental method to determine the switching field distribution in perpendicular thin-film magnetic recording media. The method assumes a constant demagnetization factor independent of the magnetic state during switching and a log-normal switching field distribution. The new technique allows a deconvolution of the effects of the switching field distribution and demagnetization, which is important for the development of low-noise perpendicular magnetic recording media. Index Terms--Magnetic characterization, magnetization reversal, perpendicular recording, switching field distribution.

The problem of energy shortage and environmental pollution is becoming remarkably important and has attracted much attention. Driven by this situation, self-powered devices are highly desirable in all areas. Herein, a broadband photoresponse, self-powered, polarization-sensitive photodetector based on stacked two-dimensional type-II GaSe/ReS2 heterostructures is demonstrated. Because of the formation of the type-II band alignment, GaSe/ReS2 heterostructures display a self-powered photoresponse with an Ion/Ioff ratio of about 8.3 × 102 and exhibit a high responsivity of 54 mA/W under illumination with 532 nm laser light. A maximum photoresponsivity of 170 mA/W and an external quantum efficiency of 10% can be achieved by illumination with a 340 nm laser. Additionally, benefiting from the highly in-plane anisotropic atomic arrangements of ReS2, the photodetector shows a polarization-sensitive photocurrent. The dichroic ratio and polarization sensitivity are determined to be 1.73 and 3.03, respectively, and the highest photoresponsivity of the photodetector can reach up to 94 mA/W when the direction of the incident polarized laser is parallel to the b-axis of ReS2. Importantly, the self-passivating behavior of GaSe can maintain the performance of the heterojunction for 1 week. The present findings open new opportunities for developing air-stable, high-performance van der Waals heterostructure photodetectors. [ABSTRACT FROM AUTHOR]

Electron–phonon coupling plays a key role in affecting the properties of the semiconducting nanostructures, such as providing the possibility for obtaining higher superconducting transition temperatures. Here, using Raman, temperature‐dependent and polarized Raman scattering measurements, ultra‐strong electron–phonon coupling in 1D CdS nanowires and 2D graphene heterostructures is demonstrated. The intensity ratio of 2LO/1LO mode in CdS nanowires provides a spectroscopy‐based method to quantify electron–phonon coupling, the strength of which is temperature and polarization dependent. The intensity ratio mode of 2LO/1LO in heterostructure reached up to 8.95 when the incident laser polarization is parallel to the c‐axis of the nanowire. It is ≈2.37 times higher than in an individual nanowire. In addition, in situ and time‐resolved photoluminescence spectra demonstrate the dynamics of the exciton recombinations, providing a comprehensive understanding of the enhancement of electron–phonon coupling in heterostructures. Via optical waveguiding characterization, the graphene layer is demonstrated to not only be an ultrafast carrier transfer channel but also a low Fermi level channel that induces the formation of the built in electrical field, elevating the electron–phonon coupling. Such new mixed dimensional heterostructures illustrate a straightforward approach to enhance the electron–phonon coupling, which may be applied to many integrated superconducting photonic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Design considerations for areal densities beyond 20 Gb/in.(super 2) are discussed. The demonstration profited by more advanced channels, modeling and experiment showed that it was not required to reduce the bit-aspect ratio.

Selected aspects of the recording physics of data storage systems based on patterned magnetic recording medium are discussed. The micromagnetic study of the magnetization reversal in patterned magnetic recording medium is presented. The effects of bit size, head/medium misregistration, bit-to-bit spacing, soft underlayer (SUL) patterning, and side wall roughness on magnetization reversal are explored. The results offer the guidelines for the design and optimization of patterned magnetic recording medium. Index Terms--Bit magnetization reversal, micromagnetics, patterned medium recording.

We report on the fabrication of 1 Tdot/in² bit patterned media with high coercivity (HC) and narrow intrinsic switching field distribution (iSFD) based on nanoimprint from a master pattern formed by e-beam guided block copolymer assembly onto a carbon hard mask and subsequent pattern transfer via etching into a thin CoCrPt perpendicular anisotropy recording layer. We demonstrate that an additional vacuum annealing step after pattern transfer into the CoCrPt layer and after Carbon hard mask removal not only yields recovery from undesired damage of the island edges, but actually transforms the islands into a magnetically more favorable compositional phase with higher HC, lower iSFD/HC, and three-fold increased thermal stability. Energy filtered transmission electron microscopy analysis reveals that the diffusion of Cr from the island cores to the periphery of the islands during post-annealing is responsible for the transformation of the magnetic bits into a more stable state. [ABSTRACT FROM AUTHOR]

The noise performance of amorphous FeCoB soft underlayers (SULs) with radial magnetic anisotropy is examined. The internal stress declines with thermal annealing as the increase in annealing power reduces the dc noise and the SUL-induced dc noise drops to the electronic noise floor.

Electron diffraction was used for examining the stacking faults (SFs) and fcc-phased grains in perpendicular media. CoCrPtB alloys possess greater amount of SFs and fcc grains in comparison to the CoCrPt alloy.

Electrostatic interaction due to the work function difference between the tip of a magnetic force microscope (MFM) tip and the sample surface competes with the magnetic interaction at a lower tip–sample distance. This results in the superposition of sample topographic and magnetic images. As a consequence, the interpretation of MFM images becomes difficult, and the spatial resolution is limited. In this work, it is demonstrated that by applying an external voltage to the tip, the electrostatic tip–sample interaction can be compensated and the influence of topography on the magnetic images can be significantly reduced. The minimum tip–sample distance without topography influence is dramatically reduced and the detected signal is increased by 50%. The impact on the spatial resolution of the MFM is discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A micromagnetic model is developed describing the influence of the domain wall pinning in a magnetic thin film consisting of a granular microstructure with perpendicular magnetocrystalline anisotropy and a film thickness in the range of 5-16 nm. It is stated that the prismatic domain wall pinning centers is varied between 2 and 10 nm.

A technique that can directly fabricate L1(sub 0) phase FePt nanoparticles is reported. FePt nanoparticles are generated through gas-phase aggregation using a magnetron-sputtering-based nanocluster source.

Reducing the tip-sample distance results in topographical information superimposed onto magnetic force microscopic (MFM) images. Impact on the spatial resolution of the MFM is discussed.

Monodispersed FePt nanoparticles are synthesized by reduction of iron acetylacetonate and platinum acetylacetonate with 1,2-hexadecanediol as the reducing reagent in the polyol process. Transmission electron microscopy deviation about annealing at 650 degree Celsius for 30 min in AR atmosphere where the oxygen level is less than 1 ppm.

The anisotropy field of perpendicular media with a well-defined demagnetizing state has been determined with the help of magneto-optical measurements. The theoretical studies indicate that the effects of easy axis angular dispersion are small, about 1% error for 5 degrees dispersion, and the rotation method measures the mean value of anisotropy field H(sub k) for media with anisotropy field distribution.

Thermally stable CoCrPtB media has been fabricated at low magnetic layer thickness and interlayer thickness. The media is well oriented and has high magnetic anisotropy and small grain size.

The thickness dependence and long-term stability of the magnetic properties of epitaxial (Cr0.5Mn0.5)2GaC MAX phase films on MgO (111) were investigated. For 12.5- to 156-nm-thick films, which corresponds to 10-125 c-axis unit cells, samples were found to be phase pure with negligible c-axis lattice strain of less than 10−4 nm even for the thinnest films. No influence of the interface layers on the magnetic anisotropy, the magnetization or the para- to ferromagnetic phase transition was observed. All samples remained stable for more than one year in ambient conditions. The complex temperature- and magnetic field-dependent magnetism of electrically conducting (Cr0.5Mn0.5)2GaC MAX phase films is environmentally robust over one year and independent on interface effects. [ABSTRACT FROM AUTHOR]

Abstract: High‐efficiency multichannel waveguiding components are desirable for integrated photonic systems to realize optical information processing and communication interconnection. Although different branched micro‐/nanostructures have been used as nanoscale multichannel waveguides, the propagation losses from their junctions are very high, which limits their applications in on‐chip photonic systems. Here, a new cadmium sulfide (CdS) nanowire branched heterostructure, with tin (Sn) nanoparticles implanted in its junctions, is achieved via a single‐step vapor deposition method. The self‐organized formation process of this structure is well investigated, through step‐by‐step observations during the growth. The implanted Sn nanoparticles in the junctions can act as strong light‐scattering centers, greatly improving the optical transportation from the backbone to the branches of the structure. Contrasting experiments demonstrate that the light‐guiding efficiency in the heterostructured nanowire branch waveguide is 20 times higher than that of the branched structures without Sn nanoparticles. Theoretical simulations further demonstrate the existence of Sn nanoparticles can greatly enhance the light scattering at the junctions and improve the light‐guiding performance. This kind of Sn‐CdS nanowire branched heterostructures may find promising applications for integrated photonic devices and systems. [ABSTRACT FROM AUTHOR]

A survey of various measurement techniques, including the rotation method, the hard-axis hysteresis loop, and the torque measurement, is presented using Co/Pt multilayer recording media. It is found that for highly exchange-coupled samples, the results from the rotational method are consistent with those from the hard-axis hysteresis loop and torque measurements. Such anisotropy measurements are applied to determine the interfacial anisotropy in Co/Pt multilayers (MLs) by studying a series of Co/Pt ML samples with Co thickness ranging from 0.1 to 2 nm. The Pt thickness is fixed at either 1 or 0.5 nm. It is found that the multilayer with 1 nm Pt layer shows a larger interfacial anisotropy constant (KS∼0.8 erg/cm2) than the media with 0.5 nm Pt layer (KS∼0.6 erg/cm2), corresponding to about 80% and 60% of the interfacial anisotropy of the ML made by molecular-beam epitaxy, respectively [D. Weller et al., Mater. Res. Soc. Symp. Proc. 313, 791 (1993)]. Such thickness dependence is supported by the x-ray-diffraction superlattice peak measurements that indicate the discontinuous growth for the case with thinner Pt. [ABSTRACT FROM AUTHOR]

In this paper we report a technique that can directly fabricate L10 phase FePt nanoparticles. FePt nanoparticles were generated through gas-phase aggregation using a magnetron-sputtering-based nanocluster source. Following the source chamber, an online halogen-lamp heater was used for the L10 phase formation during the particles' flight in vacuum. Transmission electron microscopy and vibrating-sample magnetometer data verified the successful fabrication of the L10 phase FePt nanoparticles. The coercivity value at 300 K is 1100 Oe for the nanoparticles with online heating. Neon carrier gas was applied to manipulate FePt nanoparticle size and to enhance particle size uniformity. The size dependence of nanoparticle ordering was investigated. [ABSTRACT FROM AUTHOR]

Magneto-optical measurements have been applied to determine the anisotropy field of perpendicular media with a well-defined demagnetizing state. The rotation of the perpendicular magnetization component is measured as a function of the in-plane field by the Polar Kerr effect. The field range is sufficiently small to avoid domain formation and ensure coherent magnetization rotation. By fitting to a simple response function, the effective anisotropy field (H'[SUBK1] = H[SUBK1] - 4πM[SUBS]) with a well-defined 4πM[SUBS] demagnetization correction can be determined. This also enables accurate determination of the ratio of short-time coercivity H[SUB0] over anisotropy field H[SUBk], an important parameter for perpendicular recording. The H[SUB0]/H[SUBk] ratio is found to decrease with increasing intergranular coupling strength, in good agreement with our micromagnetic results. It drops from about 0.8 for weakly coupled media to 0.5 for strongly coupled media. Theoretical studies show that the effects of easy axis angular dispersion are small, about 1% error for 5° dispersion, and the rotation method measures the mean value of H[SUBk] for media with anisotropy field distribution. [ABSTRACT FROM AUTHOR]

Stacking faults (SFs) and fcc-phased grains are studied by electron diffraction in perpendicular media. Co-alloy compositions, such as CoCrPt, CoCrPtB, are compared on different underlayers and deposition conditions. It is found that CoCrPtB alloys tend to have greater amount of SFs and fcc grains than the CoCrPt alloy. Underlayers as well as low deposition rate and base pressure help to reduce the amount of SFs and fcc phase. [ABSTRACT FROM AUTHOR]

We study the noise performance of amorphous FeCoB soft underlayers (SULs) with radial magnetic anisotropy. 200 nm thick FeCoB films are sputter deposited and optionally postannealed for 8 s at different annealing powers. The correlation of SUL read-back noise with the magnetic and structural properties is studied using spin stand testing, in-plane magneto-optical Kerr effect measurements, magnetic force microscopy, and x-ray diffraction. The effects of annealing to achieve low read-back noise are examined. It is found that as-prepared films show large dc noise associated with stripe domains due to stress-induced perpendicular anisotropy. Thermal annealing reduces the internal stress and the films become magnetically anisotropic in the radial direction. The SUL-induced dc noise drops to the electronic noise floor, dc noise is found to decrease with an increase in annealing power until the films start to crystallize. [ABSTRACT FROM AUTHOR]

An anomalous enhancement of the coercive field ( Hc ) and of the perpendicular magnetic anisotropy (PMA) of CoCr10Pt10 (at.%) alloy thin films is observed when the magnetic layer is deposited on an ultrathin Pt interlayer (IL). The dc magnetic properties of Pt/CoCr10Pt10 are studied as a function of the magnetic alloy thickness (4–8 nm) and the Pt IL thickness (up to 3 nm). Both the coercive field and the interface anisotropy ( Ks ) exhibit a non-monotonic dependence on the Pt thickness, showing a characteristic peak value at an intermediate thickness, with a rise in the PMA up to 25%. Similar thickness dependence of Hc is observed when using Pd and Ir ILs, but with a more moderate enhancement. This approach provides a means to control and engineer the PMA of CoCrPt alloys, and may find an application in data storage and low-power spinelectronics. [ABSTRACT FROM AUTHOR]