Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.
We report a facile synthesis of monodisperse ferrimagnetic Co(x)Fe(3-x)O4 nanocubes (NCs) through thermal decomposition of Fe(acac)3 and Co(acac)2 (acac = acetylacetonate) in the presence of oleic acid and sodium oleate. The sizes of the NCs are tuned from 10 to 60 nm, and their composition is optimized at x = 0.6 to show strong ferrimagnetism with the 20 nm Co0.6Fe2.4O4 NCs showing a room temperature Hc of 1930 Oe. The ferrimagnetic NCs are self-assembled at the water-air interface into a large-area (in square centimeter) monolayer array with a high packing density and (100) texture. The 20 nm NC array can be recorded at linear densities ranging from 254 to 31 kfci (thousand flux changes per inch). The work demonstrates the great potential of solution-phase synthesis and self-assembly of magnetic array for magnetic recording applications.
Magnetic domain walls have been studied in micrometer-sized Fe20Ni80 elements containing geometrical constrictions by spin-polarized scanning electron microscopy and numerical simulations. By controlling the constriction dimensions, the wall width can be tailored and the wall type modified. In particular, the width of a 180• Néel wall can be strongly reduced or increased by the constriction geometry compared with the wall in unconstrained systems.PACS numbers: 75.60. Ch, 75.75.+a, 75.70.Kw For almost a century, the width of magnetic domain walls (DWs) has been believed to be determined by material properties only. However, recent investigations on DWs in nanometer-scale systems have revealed new physical properties due to the geometrical confinement of the magnetization. A reduction of the Bloch wall width has been predicted 1 and observed 2 in nanometer-sized constrictions. This effect is thought to be the origin of the large magneto-resistance measured in nanocontacts, and explained by ballistic transport through a narrow DW pinned within the contact.3,4,5 Furthermore, domain walls are now being investigated as tiny individual magnetic objects that can be manipulated in view of their potential for application in novel magnetic logic or memory devices.6 Of interest in this field are the possibilities of pinning DWs at constrictions and of displacing them using a magnetic field 7 or an applied current. 8,9,10,11,12 For all these phenomena, the key parameter is the magnetic structure of the domain walls. For a basic understanding as well as for potential applications, it is important to gain quantitative insight into how DW properties can be modified via the geometry.The prediction of DW narrowing in a constriction was based on a ferromagnetic model system containing a planar Bloch wall.1 Because dipolar contributions in the constriction were neglected, the problem was onedimensional and could be solved analytically. The vast majority of small elements, however, exhibits DWs of Néel type, with a nonvanishing magnetization component perpendicular to the wall. In these walls, the dipolar energy determines the wall profile to a large extent, and hence the problem is more intricate.In this paper, we investigate Néel-type walls in elements containing constrictions of controlled dimensions. The experimental results obtained by scanning electron microscopy with spin analysis (spin-SEM 13 or SEMPA 14 ) are compared with micromagnetic simulations. We demonstrate how the wall properties can be tuned both by the element size and the constriction dimensions. Constraining a DW in a micrometer-sized element strongly reduces the wall width compared with the width in an infinite film. By appropriately tuning the constriction dimensions, the Néel wall width can further be decreased, or alternatively, increased until the wall splits into two separate walls.The constrictions were fabricated in thin, micrometersized rectangular elements by using electron-beam lithography and Ar dry etching of Fe 20 Ni 80 thin films. These films w...
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 is comprised of a well ordered array of lithographically patterned isolated magnetic islands, each of which stores one bit. Fabrication of BPM is viewed as the greatest challenge for its commercialization. In this article we describe a BPM fabrication method which 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 (teradot/inch 2 ). This combination of novel fabrication technologies achieves feature sizes of <10 nm, which is significantly smaller than what conventional nanofabrication methods used in semiconductor manufacturing can achieve. In contrast to earlier work which used hexagonal closepacked arrays of round islands, our latest approach creates BPM with rectangular bitcells, which are advantageous for integration of BPM with existing hard disk drive technology. The advantages of rectangular bits are analyzed from a theoretical and modeling point of view, and system integration requirements such as provision of servo patterns, implementation of write synchronization, and providing for a stable head-disk interface are addressed in the context of experimental results. Optimization of magnetic alloy materials for thermal stability, writeability, and tight switching field distribution is discussed, and a new method for growing BPM islands from a specially patterned underlayer -referred to as "templated growth" -is presented. New recording results at 1.6 Td/in 2 (roughly equivalent to 1.3 Tb/in 2 ) demonstrate a raw error rate <10 -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.Index Terms-Bit patterned media, hard disk drive, block copolymer, self-assembly, double patterning, e-beam lithography, sequential infiltration synthesis, nanoimprint lithography, templated growth, thermal annealing, Co alloys, magnetic multilayers, interface anisotropy, magnetic recording, write synchronization, prepatterned servo, areal density.
A self-assembled magnetic recording medium was created using colloidal ferrimagnetic building blocks. Monodisperse cobalt ferrite nanoparticles (CoFe(2)O(4)) were synthesized using solution-based methods and then stabilized in solution using the amphiphilic diblock copolymer, poly(acrylic acid)-b-poly(styrene) (PAA-PS). The acid groups of the acrylate block bound the polymer to the nanoparticle surface via multivalent interactions, while the styrene block afforded the magnetic nanoparticle--polymer complex solubility in organic solvents. Moreover, the diblock copolymer improved the colloidal stability of the ferrimagnetic CoFe(2)O(4) nanoparticles by reducing the strong interparticle magnetic interactions, which typically caused the ferrimagnetic nanoparticles to irreversibly aggregate. The nanoparticle--polymer complex was spin-coated onto a silicon substrate to afford self-organized thin film arrays, with the interparticle spacing determined by the molecular weight of the diblock copolymer. The thin film composite was also exposed to an external magnetic field while simultaneously heated above the glass transition temperature of poly(styrene) to allow the nanoparticles to physically rotate to align their easy axes with the direction of the magnetic field. In order to demonstrate that this self-assembled ferrimagnet--polymer composite was suitable as a magnetic recording media, read/write cycles were demonstrated using a contact magnetic tester. This work provides a simple route to synthesizing stabilized ferrimagnetic nanocrystals that are suitable for developing magnetic recording media.
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