Self-assembled nano-size FePt islands for ultra-high density magnetic recording media

To find a method to form nano-size FePt alloy for ultra-high density magnetic recording media, this work concentrated on the formation mechanisms of nano-island FePt films on amorphous glass substrates. FePt films of different thicknesses (1-10 nm) were deposited on amorphous glass substrates and post-annealed at 700 °C for 10 and 30 min. The configuration of the film changed during the annealing process due to the surface energy difference between the glass substrate and FePt alloy. Investigation of the microstructures and magnetic properties of the ordered L1₀ FePt films revealed that the 1 nm FePt film annealed at 700 °C for 10 min had perpendicular magnetic anisotropy and good reproducibility of forming well-separated FePt nano-size islands for ultra-high density magnetic recording media.     

Magnetic properties and microstructures of single-layered Fe100 − xPtx films deposited onto heated substrates

The single-layered Fe_100 − xPt_x films of 30 nm thick with Pt contents (x) of 35-57 at.% are deposited on heated Si (100) substrate at a temperature (Ts) of 620 °C by magnetron co-sputtering. When the Pt content in the Fe-Pt alloy film is 35 at.%, the value of in-plane coercivity (Hc_//) is close to perpendicular coercivity (Hc_⊥) and both values are about 800 kA/m. The FePt films exhibit perpendicular magnetic anisotropy when the Pt content increases to the values of between 45 and 51 at.%. The perpendicular coercivity, saturation magnetization (Ms) and perpendicular squareness (S_⊥) for Fe₅₄Pt₄₆ film are as high as 1113 kA/m, 0.594 Wb/m² and 0.96, respectively. These magnetic properties reveal its significant potential as perpendicular magnetic recording media. Upon further increasing the Pt content to 57 at.%, the coercivity of the Fe-Pt film decreases drastically to below 230 kA/m and tends to be closer to in-plane magnetic anisotropy.     

Onset of hard magnetic L10 FePt phase with (001) texture

The perpendicular anisotropic magnetic properties of in-situ deposited FePt/Pt/Cr trilayer films were elucidated as functions of the deposition temperature and the sputtering rate of the FePt magnetic layer. Ordered L1₀ FePt thin films with perpendicular anisotropy and a (001) texture can be developed at a temperature as low as 300 °C with the sputtering of a FePt layer at a low rate. The larger Pt(001)[100] lattice induced an expansion of the FePt a- and b-axis, leading to the contraction of the FePt c-axis, enabling the epitaxial growth of the L1₀ FePt(001) texture to occur. A low rate of sputtering of the FePt thin film promotes the formation of the magnetically hard FePt(001) texture on the surface of the Pt(001) buffer layer at low temperature, while the high sputtering rate of FePt layer suppresses the phase transformation.     

Coercivity variation in exchange-coupled Fe/FePt bilayer with perpendicular magnetization

The soft/hard Fe/FePt film with perpendicular magnetization has been deposited on a glass substrate. The (001) oriented L1₀ FePt film was obtained when annealed by rapid thermal process at 800 °C and a Fe layer was deposited at room temperature with thicknesses of 2 nm to 20 nm. Controlling the Fe layer thickness allowed modification of the hysteresis loops from out-of-plane rigid magnet to in-plane exchange-spring like magnet due to the nanometer scale interface coupling. When the Fe layer thickness increased to 2 nm, the out-of-plane coercivity is reduced to 5.9 kOe but the remanence ratio (0.98) is still high. The Fe (2 nm)/FePt film shows perpendicular magnetization with linear in-plane hysteresis loop. The remanence ratio is reduced to 0.85 when the Fe layer thickness increased to 5 nm. When the Fe layer thickness was varied up to 10-20 nm, the in-plane hysteresis loop shows exchange-spring like behavior with two-step magnetization reversal processes. The films with perpendicular coercivity were moderated by the thickness of soft magnetic layer.     

Low temperature ordering of FePt films by in-situ heating deposition plus post deposition annealing

FePt thin films with 40 nm thickness were prepared on thermally oxidized Si (001) substrates by dc magnetron sputtering at the nominal growth temperature 375 °C. The effects of annealing on microstructure and magnetic properties of FePt thin films were investigated. The as-deposited FePt thin films show soft magnetic properties. After the as-deposited FePt thin films were annealed at various temperatures and furnace cooled, it is found that the ordering temperature of L1₀ FePt phase could be reduced to 350 °C. For FePt thin films annealed at 350 °C, the in-plane and out-of-plane coercivities of the films increased to 510 and 543 kA/m, respectively, and the films had hard magnetic properties. A highly (001) orientation was obtained, when FePt thin films were annealed at 600 °C. And the hysteresis loops of FePt thin films annealed at 600 °C show out-of-plane magnetic anisotropy.     

Effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers

FePt (50 nm) and [FePt(xnm)/AlN(1, 2, 3 nm)]₁₀ (x=2, 3 nm) films were prepared by RF magnetron sputtering technique, then were annealed at 550 °C for 30 min. This work investigates the effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers. Superlattice (0 0 1) peaks can be found in the grazing incidence X-ray diffraction of FePt and [FePt (3 nm)/AlN (1, 2, 3 nm)]₁₀ films, which indicate that the FCC phase has been partially transformed into ordered L1₀ phase. Compared with the single layer FePt film, superlattice (0 0 1) peaks of FePt/AlN multilayers are weak and wide, which indicates that the introducing of AlN hinders the growth of FePt particle, and also shows the introducing of AlN is not beneficial to the transformation from FCC phase to L1₀ phase. In addition, the low-angle XRD spectra show the layered structure of FePt/AlN has been broken after annealing. The coercivities, particle size, intergrain exchange interactions of FePt/AlN films are decreased with increasing AlN layer thickness.     

Influence of magnetic annealing on high-frequency magnetic properties of FeCoNd films

FeCoNd thin film with thickness of 166 nm has been fabricated on silicon (1 1 1) substrates by magnetron co-sputtering and annealed for one hour under magnetic field at different temperatures (T_a) from 200 °C to 700 °C. The As-deposited and annealed FeCoNd film samples at T_a ≤ 500 °C were amorphous while the ones obtained at T_a ≥ 600 °C were crystallized. We found that the perpendicular anisotropy field gradually decreases as the annealing temperature increases from room temperature to 300 °C. A well induced in-plane uniaxial anisotropy is achieved at the annealing temperature between 400 and 600 °C. The variation of the dynamic magnetic properties of annealed FeCoNd films can be well explained by the Landau-Lifshitz equation with the variation of the anisotropy field re-distribution and the damping constant upon magnetic annealing. The magnetic annealing might be a powerful post treatment method for high frequency application of magnetic thin films.     

Granular L10 FePt nanocomposite films fabricated by rapid thermal annealing

Granular (FePt)_100−x-(NiO)_x nanocomposite thin films with x = 0 − 42 vol% were fabricated on a natural-oxidized Si(100) substrate. It is found that both the coercivity and FePt domain size decrease with increasing NiO content for the (FePt)_100−x-(NiO)_x films. When the FePt-NiO composite film with NiO content of 10.4 vol% is post-annealed at 750 °C with a high heating ramp rate of 100 °C/s, the in-plane coercivity (H_c//) and perpendicular coercivity (H_c⊥) of the FePt films are 513 and 430 kA/m, respectively. On the other hand, we used conductive atomic force microscope (CAFM) to confirm that the NiO compound is distributed at boundary of FePt particles that will constrain the domain size of FePt and decrease the exchange coupling interactions between FePt magnetic particles.     

The effect of thickness on the texture and magnetic properties of single-layered FePt films by rapid thermal annealing

The single-layered FePt films with thickness in the range of 5 to 50 nm are deposited directly on Si(100) substrate without underlayer, then post annealed at 700 degrees C by rapid thermal annealing (RTA) technique. As the film thickness of FePt is over 20 nm, the L1(0) FePt(111) preferred orientation is presented and tended to in-plane magnetic anisotropy. However, the L1(0) FePt(001) texture is obtained and exhibited perpendicular magnetic anisotropy as the film thickness is decreased to 10 nm. Its perpendicular coercivity (Hc(perpendicular)), saturation magnetization (Ms) and perpendicular squareness (S(perpendicular)) are 14.8 kOe, 795 emu/cm3 and 0.79, respectively. On the other hand, both the grain size and domain size of FePt film decrease with decreasing the film thickness of FePt. The grain size for 10-nm FePt film is as small as 9.7 nm with domain size of 123 nm, which reveal its significant potential as perpendicular magnetic recording media for ultra high-density recording.     

Effects of AlN layer thickness on magnetic anisotropy and transport phenomena of CoPt/AlN layered structure

The magnetic anisotropy was studied as a function of the AlN layer thickness in [AlN(x nm)/CoPt(2 nm)]5/AlN(x nm) layered structure (x is AlN layer thickness, and 5 is the number of multilayer series). The multilayered film was deposited by a sputtering apparatus equipped with two pairs of facing targets. It was found that, in the range of AlN layer thickness below 30 nm, CoPt/AlN multilayers transform from an enhanced in-plane magnetic anisotropy to perpendicular magnetic anisotropy (PMA) through thermal annealing in vacuum, with an optimized AlN thickness of 10 nm for strong PMA. However, beyond this thickness range, the PMA did not occur, and thermal annealing only results in magnetic isotropy in both parallel and perpendicular directions. The related structure analysis revealed that smooth interface and good texture of CoPt (111) make positive contributions to interface anisotropy energy and magnetocrystalline anisotropy energy for producing PMA in CoPt/AlN layered structure. In addition, the transport phenomena were also studied by using a four-probe method.     

Nearly perfect (001)-oriented Ag/[CoPt/C]5/Ag composite films deposited on glass substrates

Sandwich Ag/[CoPt(3 nm)/C(3 nm)]₅/Ag films were deposited on glass substrates by magnetron sputtering. After annealing at 600 °C for 30 min, a nearly-perfect (001)-oriented L1₀ CoPt film with extremely high perpendicular anisotropy was obtained when the thickness of Ag top- and underlayer were both equal to 5 nm. The strain energy caused by the Ag layer together with the diffusion of Ag and C atoms, resulted in the enhancement of the ordering degree of the L1₀ CoPt phase and the development of the (001) texture of the films.     

Diffusion induced columnar structure, high perpendicular anisotropy and low transformation temperature in thick FePt films

Thick FePt films (800 nm) were deposited by electroplating using Ag electrode. 2 at.% Ag doping into the electrolyte can lead to a columnar structure after annealing. The annealed film shows a high coercivity and perpendicular anisotropy. The additive of Ag can also significantly reduce fct-phase ordering temperature to 400 °C, comparing with an ordering temperature of 700 °C without Ag doping. The diffusion from Ag electrode and dopant is attributed to the formation of columnar structure, perpendicular anisotropy and reduced ordering temperature.     

Thickness dependence of microstructures and magnetic properties for CoPt/Ag nanocomposite thin films

Ag underlayer (30 nm) has improved the degree of ordering and perpendicular magnetic anisotropy of CoPt films (7.5-10 nm). After annealing at 600 °C and 700 °C, the perpendicular coercivity of CoPt/Ag films has been raised as the thicknesses of CoPt layers are increased. The magnetic easy axis of CoPt/Ag films would change from a random orientation to an out-of-plane orientation. It is found that Ag underlayer with thickness of 30 nm can improve the perpendicular magnetic properties of CoPt layers with thicknesses in the range of 7.5-10 nm. The CoPt/Ag films would be a candidate for perpendicular magnetic recording media.     

Influence of underlayers on the c-axis distribution in Co80Pt20 thin films

A series of approximately 40 nm thick Co₈₀Pt₂₀ thin films have been sputter-deposited onto a combination of Ta, Pt and Ru underlayers grown at different layer thicknesses. The addition of a Ta seed layer to the Pt and Ru underlayers caused the {0002} hexagonal close packed (hcp) Co₈₀Pt₂₀'s c-axis dispersion's full width at half maximum to narrow from approximately 12° to approximately 2°. In-situ stress measurements taken during deposition showed that the Ta seed layer reduced the growth stresses for the Pt and an initial 1 nm of growth for the Ru underlayers. The Ru layer thickness controlled the c/a ratio of the hcp Co₈₀Pt₂₀ film which regulated the degree of magnetic easy-axis alignment in the Co₈₀Pt₂₀ film. The optimal underlayer material stack for Co₈₀Pt₂₀ with a narrow c-axis dispersion and a high degree of magnetic easy-axis alignment was 5 nm Ta/10 nm Pt/5 nm Ru.     

Microstructure and magnetic properties of (0 0 1)-oriented L10 FePt films: Role of Ag underlayer and Fe/Pt ratio

FePt multilayer films were deposited on Si(1 0 0) substrate with thermally grown SiO₂ film and sputtered Ag underlayer at room temperature by dc magnetron sputtering and subsequently annealing in vacuum. Experimental results suggest that proper thickness of Ag underlayer and slightly rich of Fe content can effectively induce the (0 0 1) texture of FePt films. A Fe_57.4Pt_42.6 thin film on the 8 nm Ag underlayer exhibits a large perpendicular coercivity of 7.6 kOe with magnetic remanence close to 1.     

Hydrothermal in-situ growth of Tris(8-hydroxyquinolate) aluminum nanorods thin films

Tris(8-hydroxyquinolate) aluminum(Alq₃) thin films assembled with large-scaled nanorods have been fabricated on Al substrates through hydrothermal in-situ growth method assisted by the surfactant of sodium dodecylbenzenesulfonate. The obtained Alq₃ thin film is composed of uniformly sized (500-800nm × 4-10 μm) nanorods with regular hexagonal cross section, which are assembled to form dense nanorod arrays perpendicularly to the Al substrate. X-ray diffraction revealed that the prepared Alq₃ nanorods were the α-phase. Photoluminescence spectra showed that the Alq₃ nanorods thin film possessed a spectral blue-shift (10 nm) compared with the Alq₃ solution. The hydrothermal growth mechanism of nanorods was studied, which implied that the hydrothermal in-situ growth process on the metal substrate played an important role in the formation of the Alq₃ nanorods thin film. This simple hydrothermal method provides a convenient fabrication approach for nanocrystalline functional organic/metal interface.     

Bulk Spin Torque-Driven Perpendicular Magnetization Switching in L10 FePt Single Layer

Due to its inherent superior perpendicular magnetocrystalline anisotropy, the FePt in L1₀ phase enables magnetic storage and memory devices with ultrahigh capacity. However, reversing the FePt magnetic state, and therefore encoding information, has proven to be extremely difficult. Here, it is demonstrated that an electric current can exert a large spin torque on an L1₀ FePt magnet, ultimately leading to reversible magnetization switching. The spin torque monotonically increases with increasing FePt thickness, exhibiting a bulk characteristic. Meanwhile, the spin torque effective fields and switching efficiency increase as the FePt approaches higher chemical ordering with stronger spin–orbit coupling. The symmetry breaking that generates spin torque within L1₀ FePt is shown to arise from an inherent structural gradient along the film normal direction. By artificially reversing the structural gradient, an opposite spin torque effect in L1₀ FePt is demonstrated. At last, the role of the disorder gradient in generating a substantial torque in a single ferromagnet is supported by theoretical calculations. These results will push forward the frontier of material systems for generating spin torques and will have a transformative impact on magnetic storage and spin memory devices with simple architecture, ultrahigh density, and readily application.     

Effect of carbon additive on microstructure evolution and magnetic properties of epitaxial FePt (001) thin films

FePt:C thin films were deposited on CrRu underlayers by DC magnetron co-sputtering. The effects of C content, FePt:C film thickness and substrate temperature on the microstructural and magnetic properties of the epitaxial FePt (001) films were studied. Experimental results showed that even with 30 vol.% C doping, the FePt films could keep a (001) preferred orientation at 350 °C. When a FePt:C film was very thin (< 5 nm), the film had a continuous microstructure instead of a granual structure with C diffused onto the film surface. With further increased film thickness, the film started to nucleate and formed a column microstructure over continuous FePt films. A strong exchange coupling in the FePt:C films was believed to be due to the presence of a thin continuous FePt layer attributed to the carbon diffusion during the initial stage of the FePt:C film growth. Despite the presence of a strong exchange coupling in the FePt:C (20 vol.% C) film, the SNR ratio of the FePt:C media was about 10 dB better than that of the pure FePt media. The epitaxial growth of the FePt:C films on the Pt layers was observed from high resolution TEM cross sectional images even for the films grown at about 200 °C. The TEM images did not show an obvious change in the morphology of the FePt:C films deposited at different temperatures (from 200 °C to 350 °C), though the ordering degree and coercivity of the films increased with increased substrate temperature.     

Temperature effect on structure and surface morphology of indium tin oxide films deposited by reactive ion-beam sputtering

Indium tin oxide (ITO) films were deposited by reactive ion-beam sputtering. The relationship among the surface morphology, the resistivity ρ of the films, the substrate temperature T_S and the film thickness t_F was investigated. The heat power from the ion source during the sputtering was 265 W. T_S increased from 30 to 145 °C with an increase of t_F. The films thinner than 187 nm at T_S lower than 120 °C were amorphous, the film surface was as smooth as the substrate. The films deposited at T_S in the range between 135 and 145 °C were polycrystalline. So, the films thicker than 375 nm were in a multilayer structure of a polycrystalline layer on an amorphous layer. The surface of the polycrystalline films became rough. ρ of the films suddenly decreased at t_F of 375 nm, where the structure of the films changed. Next, the amorphous films with t_F of 39 nm were annealed in the atmosphere. The film structure changed to a polycrystalline structure at annealing temperature T_A of 350 °C. However, the surface roughness of all the films was almost same. As a result, the substrate temperature during the sputtering was important for the deposition of the films with a very smooth surface.     

L10 ordered FePt based double-layered perpendicular recording media with (002) oriented FeCo films as a soft magnetic underlayer

The introduction of the soft magnetic underlayer (SUL) in perpendicular recording technology is to further increase the recording areal density. However, problems such as growth of the uncontrollable recording layer and additional media noise contributed from the SUL could be resulted. In this work, a synthetic antiferromagnetically (SAF) coupled (002) oriented Fe₆₅Co₃₅ film as an SUL was developed for L1₀ ordered FePt based double-layered recording media. The crystallography of hetero-epitaxially grown double-layered media CrRu/(Ru/FeCo)₂/Pt/FePt/Ru was demonstrated. The L1₀ ordered FePt based double-layered perpendicular recording media with SAF coupled FeCo films as the SUL were developed.