S. N. Hsiao

Effect of initial stress/strain state on order-disorder transformation of FePt thin films

Initial stress (σi) of a room-temperature deposited FePt films was manipulated to study the order-disorder transformation. We observed that, while σi was increased from −1.01 (compressive) to 0.18 GPa (tensile), the phase transformation activation energy decreased from 0.387 to 0.23 eV/atom. This causes a reduction in ordering temperature of about 100 °C. We also found that densification induces an increase in tensile stress of about 1 GPa before ordering. In the films with small σi, strong tension facilitates the nucleation of L10 FePt; however, if highly compressive σi cancels the densification tension, ordering is retarded causing higher ordering temperature.

Direct evidence for stress-induced (001) anisotropy of rapid-annealed FePt thin films

Roles of rapid thermal annealing (RTA) on the evolution of crystallographic anisotropy of single-layered FePt films have been characterized. We observed a huge biaxial tensile stress of 2.18 GPa induced with increasing heating rate from 0.5 to 40 K/s. The result is a transition of orientation from (111) to perfect (001) texture. The later then degrades at heating rates ≥80 K/s due to morphological variation. The advantages of RTA are to induce tensile stress by densification reaction within a very short time and to simultaneously impede thickness-dependent dynamic stress relaxation.

Effect of intrinsic tensile stress on (001) orientation in L10 FePt thin films on glass substrates

Single-layered FePt thin films were deposited on glass substrates and subsequently annealed at 800 °C for various times in a rapid thermal annealing (RTA) furnace. Near-fully-L10-ordered FePt films were obtained after RTA. The accumulation of the intrinsic tensile stress is mainly contributed by the densification reaction, which leads to the development of (001) preferred orientation. The relief of the tensile stress predominantly stems from the microstructural variation (from continuous to interconnected network state), resulting in a reduction of (001) texture. Enhanced perpendicular magnetic and crystalline anisotropy was obtained for the films annealed for 900 s, confirmed by a high Lotgering orientation factor of 0.99 and differential squareness of 0.5. The results provide direct evidence that intrinsic tensile stress prompts the (001) preferred orientation through suggested strain-induced grain growth.

Improvement of energy product in exchange coupled Fe49−xCoxPt51 (x=0.0,0.7,1.3,2.2) thin films

Magnetic properties and crystal structure of the Fe49−xCoxPt51 (x=0.0,0.7,1.3,2.2) thin films deposited on a quartz substrate heated at 500°C were investigated. A significantly enhanced energy product of 18.4MGOe (87% larger than that of the binary film) was obtained in the x=1.3 sample. This improvement can be attributed to the well exchange coupling between the Ll0 and the residual disordered FePt regions. Unlike the common exchange spring (coupled) magnets or films showing a well defined two-phase structure, the chemical ordering of Fe–Co–Pt films changes continuously from the ordered regions to the disordered regions. We consider that this continuous change of chemical ordering enhances exchange coupling. Furthermore, the addition of cobalt also accompanied the decrease of crystal domain (subgrain) size in FePt grains. The fine-grain dispersion also can be a reason for the high energy product.

Ordering Enhancement of FePt Thin Films by Introducing Mo Layers

Single-layer FePt, bilayer Mo/FePt and FePt/Mo, and trilayer Mo/FePt/Mo samples were prepared by rf magnetron sputtering. Each layer of FePt and Mo in the studied samples is 20 nm in thickness. Samples were deposited onto corning 1737 glass substrate at room temperature, then annealed at 350degC and 400degC for a period of 10-120 min. Coercivities of the Mo-layer introduced samples were higher than those of single-layer FePt. X-ray diffraction data also indicate higher degree of ordering for Mo-layer introduced samples. We suggest that the incoherent FePt/Mo interface may induce extra nucleation sites for ordering transformation, thus lower the threshold of ordering temperature

Magnetic property enhancement of Fe/sub 49-x/Co/sub x/Pt/sub 51/ (x=0.0,0.7,1.3,2.2) thin films

Magnetic properties of Fe/sub 49-x/Co/sub x/Pt/sub 51/ (x=0.0,0.7,1.3, and 2.2) thin films are investigated. Cobalt is found to enhance the magnetization and energy product of the films. A maximum energy product of 18.4 MGOe is obtained at x=1.3. This value is 87% larger than that of a binary FePt film. High-saturation magnetization of M/sub s/=1180 emu/cm/sup 3/ and remanent magnetization of M/sub r/=862emu/cm/sup 3/ are obtained as the sample with x=1.3 was annealed at 500/spl deg/C. The addition of cobalt increases the ordering temperature of the film, which retains more disordered phase in the film and thus produces higher magnetization than the binary sample. Exchange coupling is found to exist between ordered and disordered FePt phases. The intensive intergranular exchange coupling also makes the switching of magnetic moments in the thin film more cooperative. We consider that both the magnetization enhancement and the cooperated switching behavior in the thin film are responsible for the increased energy product.

Influence of stoichiometry and growth temperature on the crystal structure and magnetic properties of epitaxial L10 Fe-Pd (001) films

To investigate the crystallographic structure and the magnetic properties as a function of chemical composition (40–62 at. %) and substrate temperature (Ts = 300–550 °C), we fabricated epitaxial FePd films of thickness 20 ± 2 nm by sputtering on MgO (100) single-crystal substrates. The ordering parameter, measured by x-ray diffraction with synchrotron radiation, and the perpendicular anisotropy of the Fe54Pt46 films increased with increasing substrate temperature. For FePd films (Ts = 550 °C) with varied Fe content, a direct correlation between the ordering parameter and the magnetocrystalline anisotropy field is found. The greatest ordering parameter, 0.84, and magnetocrystalline anisotropy field, ∼20 kOe, were obtained for FePd films of Fe concentration 54 at. %, indicating that an off-stoichiometric L10 FePd might be preferable for engineering and applications.

Effect of Annealing Process on Residual Strain/Stress Behaviors in FePt Thin Films

We have characterized the dependence of residual strain/stress on annealing process (post- and in-situ annealing) in single-layer FePt films prepared by sputtering onto amorphous glass substrates. A remarkable difference of evolutions in residual strains between post-andin-situ annealed samples was observed by Sin2 ψ method using synchrotron radiation. The onset of ordering temperature for both series samples is almost identical (~ 350 °C), verified by x-ray diffraction. Crystalline domain size, measured by x-ray peak breadths, of FePt films indicates a difference between these two series samples, which is associated with evolution of residual strain. We believe that the dynamic stress relaxation is the major factor in discrepancy of the residual stress behavior, since the atomic mobility of adatoms during film deposition for in-situ annealing samples are much higher than that for post-annealed films. It is further deduced that residual strain mechanisms may influence the ordering behaviors and related microstructure of FePt films.

Effect of Ag Segregation on Reversal Behavior of (FePt)

Effect of Ag segregation on magnetic and structural properties of (FePt)77 Ag23 single-layer films were studied. Thin films were first sputtered on the heated glass substrates and cooled to room temperature, a post annealing at temperatures (Tp) from 200 degC to 400 degC for 60 min was then applied. Ll0 structure formed as substrate temperature (Ts) equals to 600 degC and 700 degC. For those ordered films, Ag was observed to precipitate as particles on the film surface and grow with the increasing Ts. After post annealing at 400 degC for 60min, those silver particles dissolved into the film, leading to changes in reversal behavior. The optimum condition is obtained in the Ts=600degC, Tp=400degC sample. Drastic reduction in coercive squareness ratio (from >0.8 to 0.41) and enhanced coercivity (from 4.7 to 6.4 kOe) indicate the reversal behavior alters from cooperative to independent. Negative deltaM curve suggests that this independent rotation results from the reduction of exchange interactions

_{77}

Fe_100-xPt_x (x = 20, 25, and 30) thin films with nominal thickness 50 nm were prepared by radio-frequency (RF) magnetron sputtering onto glass substrates. Crystallographic structure and magnetic properties of such films were investigated by varying Fe content, annealing temperature (400-700degC), and heating process. Crystallographic structure obtained by X-ray diffraction (XRD) indicated that ordering of L1₂ Fe₃Pt phase takes place for Fe₇₅Pt₂₅ postannealed above 400degC. Ordering parameter of the Fe₇₅Pt₂₅ sample increases from 0.29 to 0.94 as annealing temperature rises from 400degC to 700degC. Saturation magnetization 1270 emu/cm³ was obtained in Fe₇₅Pt₂₅ films annealed at 700degC. Fe₇₀Pt₃₀ films annealed at 700degC show similar characteristics with those of Fe₇₅Pt₂₅, while Fe₈₀Pt₂₀ films exhibit paramagnetism due to disordered Fe-rich face-centered cubic (fcc) structure. Postannealing seems necessary for formation of pure L1₂ Fe₃Pt phase while in situ annealing leads to formation of a metastable tetragonal Fe₃Pt phase, as confirmed by XRD.