High-density Magnetic Storage Media Research Articles

Refining single-crystalline epsilon iron oxide nanorods via low-temperature aging

Refinement of ε-Fe2O3 phase in ε-Fe2O3/SiO2 composite powder is an urgent need in terms of its practical applications in high-density magnetic storage media. In this work, two types of refinement routes were employed for this purpose. It was found that short-time tempering treatment was not an ideal route for the refinement, because ε-Fe2O3 nanorods were ruptured to nanoparticles with an extremely wide size distribution, and their low coercivity of merely 1.0 kOe was not desirable for expected applications. By contrast, low-temperature aging route facilitated the formation of uniform ε-Fe2O3 nanorods with a length of about 200 nm and a width of about 40 nm. Because of their narrow size distribution and high crystallinity, these nanorods possessed large coercivity around 19 kOe, pronounced remanent squareness and prominent thermomagnetic behaviors, making them fairly desirable for further applications in magnetically-relevant field.

Arrangement of Maghemite Nanoparticles via Wet Chemical Self-Assembly in PS-b-PNIPAM Diblock Copolymer Films.

The structure and magnetic behavior of hybrid films composed of maghemite (γ-Fe2O3) nanoparticles (NPs) and an asymmetric diblock copolymer (DBC) polystyrene61-block-polyN-isopropylacrylamide115 are investigated. The NPs are coated with PS chains, which allow for a selective incorporation inside the PS domains at different NP concentrations. Upon incorporation of low amounts of NPs into the DBC thin films, the initial parallel (to film surface) cylinder morphology changes to a well ordered, perpendicularly oriented one. The characteristic domain distance of the DBC is increased due to the swelling of the PS domains with NPs. At higher NP concentrations, the excess NPs which can no longer be embedded in the PS domains, are accumulated at the film surface, and NP aggregates form. Irrespective of NP concentration, a superparamagnetic behavior of the metal oxide-DBC hybrid films is found. Such superparamagnetic properties make the established hybrid films interesting for high density magnetic storage media and thermoresponsive magnetic sensors.

Magnetic Characterization of Bio-magnetic Nanoparticles: ferritin

The magnetic properties of ferritin have brought various interests to dierent research fields such as high-density magnetic storage media, magnetic labeling, biomedicine field, etc. However, the magnetic behavior of ferritin is not completely understood. Its magnetization curves show dierent patterns compared to other well-known magnetic nanoparticles. In order words, the initial magnetization curves show a discrepancy from the Langevin model. We present a new model (3L) of the initial magnetization for ferritin to elucidate this discrepancy by assuming that the

Fabrication of surface magnetic nanoclusters using low energy ion implantation andelectron beam annealing

Magnetic nanoclusters have novel applications as magnetic sensors, spintronic and biomedicaldevices, as well as applications in more traditional materials such as high-density magneticstorage media and high performance permanent magnets. We describe a new synthesisprotocol which combines the advantages of ion implantation and electron beam annealing(EBA) to produce surface iron nanoclusters. We compare the structure, composition andmagnetic properties of iron nanoclusters fabricated by low dose 15 keV Fe implantation intoSiO2 followedby 1000 °C EBA or furnace annealing. Atomic force microscopy (AFM) and high resolution transmissionelectron microscopy (HRTEM) images together with superconducting quantum interferencedevice (SQUID) magnetometry measurements show that only EBA leads to the rapidformation of surface crystalline Fe spherical nanoclusters, showing magnetic moments perFe atom comparable to that of bulk bcc Fe and superparamagnetic properties. Wepropose a fabrication mechanism which includes e-beam enhanced desorption ofSiO2. This method has potential for fabricating nanoscale magnetic sensors integrated inmicroelectronic devices.

Exchange Bias in a Granular System of Antiferromagnetic Nanoparticles with Strong Magnetic Anisotropy Embedded in a Ferromagnetic Matrix

Recently, magnetic properties of nanoparticles have gained high interest from both technological and fundamental research, stemming partially from the applications in high-density magnetic storage media and biomedicine. In order to stabilize the recording units in nanometer scales at finite temperatures, the most challenge is to beat the superparamagnetism. Exchange bias, which represents a shift in the hysteresis loop induced by intimate contact between heterogeneous components, may be used to solve this issue. We present a nanogranular system with an antiferromagnetic-ferromagnetic cores-matrix morphology and study the interfacial-coupling and field-cooling dependence of exchange bias by conducting a modified Monte Carlo Metropolis method. When the interfacial coupling is antiferromagnetic, the competition between Zeeman and interfacial-coupling energies determines the strength and sign of exchange bias. Positive exchange bias appears when the cooling field overcomes the interfacial coupling. Whereas in the system with such a special morphology, the ferromagnetic anisotropy and exchange coupling may also determine the exchange bias behaviors to some extent when the interfacial coupling is antiferromagnetic and the strength of cooling field is intermediate. On the other hand, in the system with ferromagnetic interfacial coupling, exchange bias is negative constantly and linear roughly with not large interfacial coupling but independent of cooling field. The phenomena are clarified on the basis of the theories of surplus magnetization and pinning effect as well as by means of the microscopic spin configurations of system. [DOI: 10.1380/ejssnt.2011.126]

Direct electrogeneration of FePt nanoparticles into highly ordered Inorganic NanoPattern stabilising membranes

ZrO2 Inorganic NanoPatterns have been prepared on conductive electrodes by the direct block copolymer templating method associated with sol–gel dip-coating followed by thermal treatment. They have then been successfully utilised to direct the co-electrodeposition of FePt nanoparticles into the 2D well-ordered arrays of nanoelectrodes with controlled dimension and periodicity. By this method, the FePt nanodomains are confined and stabilised into the well-ordered cavities of the ZrO2 membrane. While this system constitutes the first steps towards high-density magnetic storage media, the present fully bottom-up method can be easily scaled up and generalised to infinite nanocomposite combinations.

Atomic Migration in Bulk and Thin Film L1<sub>0</sub> Alloys: Experiments and Molecular Dynamics Simulations

Ferromagnetic L10 ordered alloys are extensively studied nowadays as good candidates for high density magnetic storage media due to their high magnetic anisotropy, related to their chemical order anisotropy. Epitaxial thin bilayers NiPt/FePt/MgO(001) have been grown at 700 K and annealed at 800 K and 900 K. At 800 K, the L10 long-range order increases without measurable interdiffusion. At 900 K, the interdiffusion takes place without destroying the L10 long-range order. This surprising observation can be explained by different diffusion mechanisms that are energetically compared using molecular dynamics simulations in CoPt in the second moment tight binding approximation. In addition, the frequencies of the normal modes of vibration have been measured in FePd, CoPt and FePt single crystals using inelastic neutron scattering. The measurements were performed in the L10 ordered structure at 300 K. From a Born-von Karman fit, we have calculated the phonon densities of states. The migration energies in the 3 systems have been estimated using the model developed by Schober et al. (1981). The phonon densities of states have also been used to calculate several thermodynamic quantities as the vibration entropy and the Debye temperature.

Investigation of copt and copt + ZrOx thin films for magnetic storage media using high-resolution analytical electron microscopy

Materials for very high density magnetic storage media, with capacities of 10 Gbits/in2 and beyond, require high coercivity and high signal to noise ratio. To achieve storage densities of this level engineering of the material to produce very fine, magnetically decoupled domains is necessary. We have characterized the microstructure and microchemistry of 10 nm thick CoPt and CoPt + ZrOx thin films, as deposited and annealed, using transmission electron microscopy (TEM) and nanometer-scale energy dispersive x-ray spectroscopy (EDS).CoPt has a very high coercivity (Hc) when annealed to produce the ordered Ll0 phase. Annealing also increases grain size which reduces the signal to noise ratio. Co-sputtering CoPt with ZrOx was intended to reduce grain size. TEM micrographs in Figure 1 show that grain growth was dramatically reduced in the CoPt + ZrOx films. Essential to the development of optimum material properties are quantified grain size measurements, yet owing to the small grain size and the complexity of TEM images due to diffraction contrast there are few systematic studies of grain size in thin films.

Two-dimensional equalization: theory and applications to high density magnetic recording

Two dimensional, separable signal models are proposed for modeling the effects of intersymbol and inter-track interference in high-density magnetic storage media. The optimum (minimum mean-squared error) linear and decision feedback equalizers based on these models are developed. The use of a two dimensional LMS algorithm for adaptive updating of the equalizer coefficients is discussed and analytical results presented. Finally, simulation evidence is provided to predict the performance improvements achievable based on these models vis-a-vis one dimensional equalization.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Correlation of Texture With Magnetic Properties of Thin Film Storage Media

Thin coherent metal films of Co–Cr alloys on Si or glass substrates were used for high density magnetic storage media. They were made by sputtering or evaporation. Variation of the deposition parameters will change the micro structure especially the texture of the thin magnetic layers. We could relate the crystallographic data (rocking curves and pole figures) with the magnetic data (hysteresis loops). Correlations between deposition parameters and crystallographic and magnetic investigations will be discussed in detail.