Applications In Magnetic Recording Media Research Articles

Heat-treatment influence on the microstructure and magnetic properties of rare-earth substituted SrFe12O19

Rare-earth substituted strontium ferrite nanopowders SrFe12-xRxO19 (R = La, Gd and Er; x = 0.2, 0.5 and 1) were prepared by sol-gel-autocombustion method and subsequent heat treatments. Structural and magnetic properties of SrFe12-xRxO19 powders heat treated at 800, 900 and 10000C, for various times, were characterized with an X-ray diffractometer, a vibrating sample magnetometer and a scanning electron microscope. The results of X-ray diffraction measurements showed the M-type hexagonal structure formation by heat treatments. Magnetic properties, such as specific saturation magnetization σs, specific remanent magnetization σr and coercivity Hc, as well as microstructure depend on the heat treatment conditions (temperature and time). The coercivity Hc exhibits a great increase after a critical heat treatment time. When the heat treatment time increases, one obtains an increase in Hc after a shorter heat treatment time. This jump of Hc was explained by a transition from the superparamagnetic state to normal state of the single domain nanoparticles. The occurrence of an agglomerated structure composed of magnetically interacting ultrafine crystallites also contributes to the increase of Hc.The heat treatment determines a reduced grain growth due to the internal stress generated by R ions. With increasing R content the σs and σr decrease due to the dissolution of R ions into the hexaferrite lattice. We believe that by selecting the time and temperature of the heat treatment, microstructure and magnetic properties suitable for magnetic recording media application can be obtained. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic Properties and Microstructure of TbFeCoAg Amorphous Thin Films

The effect of Ag content on the magnetic properties and microstructure of Tb19.05Fe22.01Co58.94-xAgx film (x=0–18.51 at. %) were investigated. As judged from X-ray diffraction and transmission electron microscopy analyses, as-deposited Tb19.05Fe22.01Co58.94-xAgx films were amorphous. It was found that the addition of a small amount of the nonmagnetic element Ag into TbFeCo increases the saturation magnetization (Ms) and perpendicular coercivity (Hc⊥) of the film. For a Tb19.05Fe22.01Co55.75Ag3.19 film, the Ms is 268 emu/cm3, the perpendicular squareness is 0.94 and the Hc⊥ is 2058 Oe at room temperature. The Hc⊥ of the Tb19.05Fe22.01Co55.75Ag3.19 film decreases to approximately 40 Oe as temperature is increased to 200 °C. This film has potential for heat-assisted magnetic recording media applications.

Two-Step Growth of Goethite from Ferrihydrite

Goethite (alpha-FeOOH) is an antiferromagnetic iron oxyhydroxide that is often synthesized by precipitation from homogeneous, aqueous solution followed by aging. This paper addresses goethite growth by phase transformation of six-line ferrihydrite nanoparticles to goethite followed by oriented aggregation of the goethite primary particles. Data tracking goethite nanocrystal growth as a function of pH, temperature, and time is presented. In general, goethite growth by oriented aggregation is faster at higher pH and at higher temperature even as growth by coarsening becomes increasingly important as pH increases. In addition, particle size measurements demonstrate that the primary nanoparticles grow by Ostwald ripening even as they are being consumed by oriented aggregation. Finally, the use of a microwave anneal step in the preparation of the precursor six-line ferrihydrite nanoparticles substantially improves the homogeneity of the final goethite product. Final goethite nanoparticles are unaggregated, acicular crystals in the tens of nanometers size range. These particles may be ideal for mineral liquid crystal and magnetic-recording media applications.

A double templated electrodeposition method for the fabrication of arrays of metal nanodots

Using a double template method, we have fabricated large areas of either isolated or interconnected arrays of Ni, Co, Pt and Au dots with variable diameters and separations. This was achieved by the electrochemical deposition of the metal inside spherical cavities formed within macroporous poly(pyrrole) secondary templates which were themselves prepared by electrodeposition around templates formed of self-assembled poly(styrene) spheres assembled on evaporated Au substrates. The advantage of using poly(pyrrole) films as a secondary template is that the conductivity of the polymer can be electrochemically destroyed either temporary or permanently. Permanent destruction of the conductivity of the poly(pyrrole) film allows it to be used as an insulating template over a wide potential range so that it is possible to deposit a wide range of metals, alloys, semiconductors or oxides as nanoscale patterned arrays. This new approach represents a simple approach to the fabrication of two and three-dimensional submicron patterned arrays which may find application in magnetic recording media, photonic crystals and biosensors.

Compositionally controlled FePt nanoparticle materials

High temperature solution phase decomposition of Fe(CO)/sub 5/ and reduction of Pt(acac)/sub 2/ in the presence of stabilizers, oleic acid and oleyl amine, are employed to produce 4 nm diameter FePt nanoparticles. The Fe and Pt composition of the nanoparticle materials can be tuned by adjusting the molar ratio of Fe(CO)/sub 5/ to Pt(acac)/sub 2/, and the compositions ranging from Fe/sub 30/Pt/sub 70/ to Fe/sub 80/Pt/sub 20/ are obtained. The nanoparticle materials are easily dispersed into alkane solvent, facilitating their self-organization into nanoparticle superlattices. As synthesized FePt nanoparticles possess a disordered fcc structure and show superparamagnetic behavior. Thermal annealing induces a change of internal particle structure and thus of the magnetic properties of the particles. Composition dependent structure analysis shows that an annealed FePt nanoparticle assembly with a composition around Fe/sub 55/Pt/sub 45/ will lead to the highly ordered fct phase. This Fe/sub 55/Pt/sub 45/ nanoparticle assembly yields high coercivity, and will be a candidate for future ultra-high density magnetic recording media applications.

Structural and magnetic properties of HCP-CoCrPt–SiO 2 granular media

Granular HCP-(CoCrPt) 100− x (SiO 2) x thin films with Cr underlayers have been fabricated by sputtering multilayers followed by post-deposition annealing. Magnetic and structural properties of the films for potential applications in magnetic recording media have been investigated in detail. In as-deposited films coercivities exceeding 2.5 kOe have been obtained with SiO 2 varying from 8 to 16 vol%; high coercivity of 5.6 kOe and anisotropy of 4.6×10 6 erg/cm 3 have been achieved at low M r t value (about 0.4 memu/cm 2) in the post-annealed films. VSM measurements showed that the magnetic moment lies well in the film plane under proper preparation conditions. Grain isolation in the magnetic layer was improved by segregating SiO 2 into grain boundaries and further enhanced by post-deposition annealing. The rapid increase of the coercivity upon annealing is most likely due to the significant decrease in intergranular exchange coupling, as shown by the δM measurement in which the peak value of δM curves changed from a positive value to a negative value upon annealing. Magnetic reversal properties of the films have also been systematically studied. These results show that the HCP-CoCrPt–SiO 2 granular film is a promising candidate for ultra-high-density recording media up to 100 Gbit/in 2 or beyond because of its low Pt content and desirable properties.

Magnetic and structural properties of hexagonal-close-packed-CoCrPt–C granular media for high areal density recording

Magnetic and structural properties of hexagonal-close-packed-(CoCrPt)100−xCx thin films for potential applications in magnetic recording media have been investigated. CrTi and Cr underlayers were used to improve the in-plane preferred orientation. High coercivity of 4.4 kOe and anisotropy of 3.6×106 erg/cm3 have been achieved in films with a CrTi underlayer after postdeposition annealing. The addition of Ti to the Cr underlayer results in lower annealing temperature and shorter annealing time, while retaining higher coercivity. Vibrating sample magnetometer measurements showed that the magnetic moment lies fairly well in the film plane under proper preparation conditions; while x-ray diffraction measurements showed that the CrTi underlayer promoted Co (10.0) in-plane preferred orientation. Grain isolation in the magnetic layer was improved by the segregated C and further enhanced by postdeposition annealing. The peak value of δM curves decreased upon annealing and changed to a negative value when higher annealing temperatures were applied, indicating that strong exchange coupling in as-deposited film decreased and changed to dipolar interaction dominance upon annealing.

Ultrahigh coercivity of granular CoCrPt–SiO2 thin films

Magnetic properties and microstructure of granular CoCrPt–SiO2 thin films with Cr underlayers have been investigated for potential applications in magnetic recording media. The coercivities exceeding 2500 Oe have been obtained in the as-deposited films with SiO2 varied from 8 to 16 vol %. Remarkable improvement in magnetic properties was achieved by postdeposition annealing in vacuum. The ultrahigh coercivity up to 5640 Oe was obtained in the lower SiO2 content film (CoCrPt)96(SiO2)4 which was annealed at 580 °C for 10 min. The annealing did not induce significant grain growth. The average grain size varied from 10 to 20 nm, depending on SiO2 concentration. Grain isolation in magnetic layer was achieved by the segregated SiO2 and further enhanced by annealing. The ac demagnetized magnetic clusters, observed by magnetic force microscopy, became very fine after annealing. The peak value of delta M curves changed from positive to negative value, indicating a great reduction of intergrain magnetic exchange coupling after annealing.

High coercivity CoCrPt-C nanogranular thin films with Cr (CrTi) underlayer for magnetic recording

Magnetic and structural properties of hcp-(CoCrPt)/sub 100-x/C/sub x/ thin films have been investigated for potential applications in magnetic recording media. High coercivity has been achieved by varying the C content and post-deposition annealing. Addition of Ti to Cr underlayer results in lower annealing temperature and shorter annealing time, while remaining higher coercivity. VSM and XRD measurements showed that the magnetic moment lies fairly well in the film plane under proper preparation conditions. Grain isolation in magnetic layer was improved by the segregated C and further enhanced by post-deposition annealing. The peak value of /spl delta/M curves decreased upon annealing and changed to a negative value when higher annealing temperatures were applied.

CoPt/Ag nanocomposites for high density recording media

Nanocomposite CoPt/Ag films have been successfully fabricated and their microstructural and magnetic properties have been investigated for potential applications in magnetic recording media. This was done by first making sputtered CoPt/Ag multilayers with the face-centered cubic (fcc) structure which is magnetically soft and then annealing the multilayers to transform the fcc phase to the highly anisotropic face-centered tetragonal phase (fct) which is magnetically hard. The final nanocomposite structure consists of CoPt nanoparticles with the hard fct phase embedded in a fcc Ag matrix. Large values of coercivity in the range of 1–17 kOe were achieved with grain sizes in the range of 7–100 nm, respectively.

Internally oxidized cobalt-rich amorphous alloys for potential magnetic recording media application

A new class of nanostructured magnetic materials which hold potential for novel recording media is described. Methods for minimizing intercrystalline interactions and reducing wear and corrosion by suitable coatings are obtained using the process of internal oxidation and crystallization of amorphous Co 95.26− x Fe 4.74(BSi) x alloys. Such a process leads to solid state dispersions of single-domain isotropic cobalt particles coated with a borosilicate layer which provides compositional separation between the cobalt grains. Dramatic microstructural differences were observed between samples of different metalloid compositions. The ribbons produced from compositions Co 75.26Fe 4.74Si 2B 18 and Co 74.26Fe 4.74Si 13B 8 proved to be the most promising for recording media application in terms of magnetic properties (coercivity 1200–1400 Oe) as well as morphological properties (size less than 300 Å). High coercivities obtained from these alloys are attributable to the increased surface anisotropy due to interactions between the Co particles and the borosilicate layer.

Time effects in magnetic recording particles

Time-dependent magnetic phenomena in fine particles used or proposed for application in magnetic recording media have been studied by measuring the change in magnetization in the presence of a constant field. The observed evolution of the magnetization decay, and of the derived parameters useful for studying the time effects with the temperature and the addition of doping ions, are discussed in relation to the evolution of the magnetic stability and of the magnetization reversal mode.

Applications of TEM to microstructure, epitaxy, and preferred orientation in magnetic thin films

J. Kent Howard, some years ago, published an extensive review of the phenomenon of what he referred to as “grain-to-grain” or “polycrystalline” epitaxy in thin films. Of the six or so different applications of this phenomenon, from superconductors to magnetics, none has greater technological significance than applications to magnetic thin films. This derives from the magnetic anisotropy of these thin films which has such a profound influence on their magnetic properties. In particular, magnetocrystalline anisotropy is often used to engineer thin films with the desired orientation of the easy axis of magnetization for magnetic record ing media applications. Magnetocrystalline anisotropy is intimately connected to the the crystallographic preferred orientation of the grains composing the magnetic layer which can be controlled through the skillful use of epitaxy with a suitable sublayer. The techniques of AEM, especially elongated probe microdiffraction (EPMD), have proven themselves invaluable for characterizing the micro-structure of such thin films.

Ultra fine magnetic metal particles--Research in Japan

Research on ultra fine particles in Japan with emphasis on the magnetic application is reviewed. Magnetic fine particles prepared with both conventional and reactive gas evaporation methods show favorable characteristics for application in magnetic recording media.