Bo Bian

Fabrication and nanostructure of oriented FePt particles

Thin films of oriented tetragonal FePt particles separated by amorphous alumina have been fabricated by electron beam evaporation. The ordering of the FePt particles without coarsening can be tailored by annealing conditions. The value of coercivity of the annealed film reached as high as 4.4 kOe. The perpendicular magnetic coercivity of the annealed film was slightly larger than in-plane coercivity. Some of the tetragonal FePt particles were found to have {111} twins and stacking faults. From our high-resolution electron microscopy observations, it was determined that central region of the ordered FePt particles tended to have c-axis perpendicular to the film plane.

Annealing effect on exchange bias in Ni/sub 81/Fe/sub 19//Cr/sub 50/Mn/sub 50/ bilayers

The effect of annealing on the exchange bias in Ni/sub 81/Fe/sub 19//Cr/sub 50/Mn/sub 50/ bilayers was investigated. A large increase of the exchange field and coercivity was observed in the samples annealed at temperatures higher than 310/spl deg/C. The annealing causes the Mn atoms in the Cr/sub 50/Mn/sub 50/ layer to diffuse into the Ni/sub 81/Fe/sub 19/ layer and form an antiferromagnetic NiFeMn phase, which coats on the Ni/sub 81/Fe/sub 19/ grain surface and contributes to the improvement of exchange bias. An increase of interface roughness due to the Mn diffusion also results in the exchange bias enhancement. The enhanced exchange field and coercivity were explained by the reversible and irreversible motions of NiFeMn moments of the inhomogeneous grains.

Exchange biasing in sputtered NiFe/PtMn bilayers

The exchange coupling between ferromagnetic Ni81Fe19 and antiferromagnetic (AF) PtxMn1−x films prepared by rf and dc magnetron sputtering has been investigated. The Pt content in the PtxMn1−x film is in the range of 0 at. %<x<20 at. %. The exchange field and coercivity were found to depend strongly on the deposition conditions and the AF film composition. X-ray diffraction measurements and transmission electron microscopy measurement showed a γ-PtMn phase with a disordered fcc structure when the PtxMn1−x was deposited on top of the Ni81Fe19 layer. The exchange bias was found to depend on the texture and film composition of the γ-PtMn layers.

Stacking faults and their effect on magnetocrystalline anisotropy in Co and Co-alloy thin films

Stacking faults in (101~0) unicrystal Co and Co-alloy thin films have been analyzed by the electron diffraction technique. As predicted no diffraction contrast of stacking faults could be observed when the beam is parallel to the [101~0] direction. However, both by plan-view TEM observation and electron diffraction along the [112~0] direction reveal that the stacking fault density in biased pore Co (101~0) Bin films is much lower than that in unbiased ones. The addition of Cr in Co films significantly reduces stacking fault density whereas the addition of 8 at% Pt leads to a considerable increase in stacking fault density. Our results show that stacking faults in Co thin films significantly, and negatively, affect the anisotropy energy density and its temperature dependence.

Synthesis and structure of isolated L1/sub 0/ FePt particles

Isolated FePt particles were fabricated by taking advantage of overgrowth of Fe on Pt particles and subsequent annealing at 873 K. The annealing lead to the formation of ordered tetragonal (L1/sub 0/) FePt particles with three orthogonal variants. The average size of FePt particles could be adjusted to less than 10 nm and no significant grain coarsening occurred upon annealing due to the existence of an amorphous Al/sub 2/O/sub 3/ layer. Nano-beam electron diffraction revealed that the lattice constant ratio c/a was approximately 0.97 for the annealed FePt particles with Fe composition close to 50 at.%.

Effects of stoichiometry on the magnetic and structural properties of perpendicular barium ferrite thin film media

The effects of stoichiometry factor n on the magnetic and structural properties of perpendicular barium ferrite thin films with composition of BaO.n(Fe/sub 2/O/sub 3/) were systematically investigated. Within the range 2.2<n<6, the c-axis for barium ferrite thin films can be oriented perpendicularly by the use of a Pt underlayer and with optimized processing conditions. Perpendicular c-axis orientation becomes worse as n decreases from 6 to 2.2. The average grain size was greatly reduced from about 2000 /spl Aring/ to 300 /spl Aring/ with the decrease of n. The decrease of the grain size can be related to the existence of nonmagnetic phase BaFe/sub 2/O/sub 4/, which was observed by X-ray diffraction.

High coercivity Co-alloy thin films on polymer substrates

Films of (101~0) textured CoCrPt have been sputtered on polymer substrates for use as thin film tape media. Underlayers of NiAl combined with various intermediate layers were employed to obtain this orientation without thermal damage to the polymer. It was found that NiAl underlayers sputtered on tape substrates have a [112] growth texture, like they do on glass. Film coercivity varied as a function of intermediate layers used, with the highest value of coercivity (3800 Oe) being obtained with a composite intermediate layer of CrMn/CoCrMn. TEM observation showed that the CoCrMn component of these intermediate layers was hexagonal with a (101~0) texture and low stacking fault density. The CoCrPt layers deposited on these CoCrMn layers had uniform grains with a lower stacking fault density, possibly due to grain-to-grain epitaxial growth of CoCrPt on CoCrMn.

Improved grain isolation of Co/sub 80/Pt/sub 20/ films via grain boundary diffusion of Mn

Post-deposition rapid thermal annealing (RTA) was performed on the Cr-less Co-20 at%Pt alloy with a Mn diffusion layer on top. A Ku value of about 8/spl times/10/sup 6/ erg/cm/sup 3/ was obtained for the as-deposited film and it decreased only up to about 10% after annealing. The high intergranular exchange coupling of the as-deposited CoPt film was suppressed by RTA and the coercivity increased dramatically from about 2200 to 6000 Oe. The energy-filtered TEM plane-view images showed depletion of Co and aggregation of Mn at the CoPt grain boundaries. This is a direct evidence supporting preferential diffusion of materials from adjacent layers into magnetic grain boundaries.

Effects of a Pt underlayer and interlayer on the growth texture of strontium ferrite thin films

The effects of a Pt underlayer and a Pt interlayer on the textured growth of the sputtered strontium ferrite (SrM) films were investigated. The structural and magnetic properties of SrM films on the Pt underlayers were compared with SrM films on thermally oxidized Si substrates. It was found that much better [001] texture was obtained on the films with Pt underlayers, as indicated by the strong [006], [008] and (0014) reflections from X-ray diffraction (XRD) measurement. A Pt interlayer was also found to further improve the [001] texture.

Barium and strontium ferrite perpendicular thin film media with a sendust soft magnetic underlayer

The possibility of using sendust (FeAlSi) as the soft magnetic underlayer for hexagonal barium ferrite (BaM) and strontium ferrite (SrM) was studied. Sendust films were found to retain their soft magnetic properties with a rapid thermal anneal (RTA) at 800 C for 60 s, which is necessary to crystallize the as-deposited amorphous BaM and SrM films. Several film structures with sendust films as the soft underlayer were studied and compared. Strong hcp (001) texture and bcc (110) texture were obtained on the BaM(SrM)/Pt/buffer/FeAlSi/Si multi-layer structure films, as indicated by the strong hcp (006), (008) and (0014) reflections for BaM(SrM) layer and bcc (110) reflection for the sendust underlayer from X-ray diffraction measurements.