F.T. Yuan

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.

Effect of interfacial diffusion on microstructure and magnetic properties of Cu∕FePt bilayer thin films

The crystal structure, microstructure, and magnetic properties for a series of Cu∕FePt bilayer films were investigated. The samples were prepared by depositing a Cu top layer on a highly ordered L10 FePt film. To promote interdiffusion, the bilayer samples were annealed at a temperature Td ranging from 300to800°C. X-ray diffraction data indicate that observable diffusion occurs at 400°C. The maximum coercivity thus obtained is 14.0kOe, which is 24% larger than that of the ordered FePt film without a Cu top layer. The high Hc can be attributed to the diffusion of copper atoms through the grain boundaries of the magnetic films, which may produce extra pinning sites for domain-wall movement. The ΔM data measured from the Henkel plots of annealed Cu∕FePt films change from negative to positive values as Td is raised from 400to800°C. This can result from the effects of demagnetization coupling and exchange coupling and is further explained from the variation of squareness ratios of hysteresis loops.

Exchange bias in sputtered FM/BiFeO3 thin films (FM = Fe and Co)

Magnetic properties of sputter-deposited ferromagnetic (FM)/BiFeO3 (BFO) films on Pt/Ti/SiO2/Si(100) substrate (FM = Co and Fe) have been investigated. Isotropic perovskite BFO single phase is obtained for 200-nm-thick BFO films deposited at 300–450 °C and BFO films at 400 °C with thickness of 50–400 nm. Large exchange bias field (HEB) of 308–400 Oe and coercivity (Hc) of 1201–3632 Oe at RT are obtained for polycrystalline Co/BFO bilayers. The roughened surface induced by high deposition temperature and increasing thickness of BFO layer enhances localized shape anisotropy of FM layer, resulting in the increase of Hc the improved crystallinity and roughened surface of BFO/Co interface might be responsible for the HEB enhancement. Additionally, comparison on the HEB in polycrystalline Co/BFO and Fe/BFO systems is also discussed.

Sputter-prepared BiFeO3(001) films on L10 FePt(001)/glass substrates

The preparation of BFO films by sputtering at a temperature as low as 450 °C on glass and commercial Pt/Ti/SiO2/Si(001) substrates have been studied. The underlayers with different orientations were prepared on the glass substrates including strongly textured Pt(111) and L10-FePt(001) induced by rapid thermal annealing process. Isotropic perovskite BFO grains with size of about 200 nm formed on the commercial substrates, showing larger surface roughness. Pt(111) suppresses BiFeO3 phase. Single phase perovskite BFO with strong (001) texture, reduced surface roughness and fine grain size was formed on the L10-FePt(001) buffer layer. Considerable enhancement of ferroelectric properties was achieved as compared to the films grown on commercial substrate.

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.

High quality multiferroic BiFeO3 films prepared by pulsed laser deposition on glass substrates at reduced temperatures

Phase structure and ferroelectric properties of BiFeO3 (BFO) films grown on Pt(111) buffered glass substrates at various growth temperatures and oxygen pressures have been studied. The optimization of the 20-nm-thick Pt(111) bottom electrode enables BFO films to form perovskite single phase structure at reduced temperatures between 400–500 °C. Microstructure of fine grains and smooth surface is obtained, which is favorable for applications. Multiferroic properties including 2Pr of 15.1–124 μC/cm2, Ec of 351–680 kV/cm, and Ms of 5–8 emu/cm3 are achieved within the oxygen pressure range of 10–75 mTorr. The optimized ferroelectric properties are comparable to the reported values of BFO samples using Si and MgO single crystal substrates. The results of this study may broaden the possibility of the use of BFO films.Phase structure and ferroelectric properties of BiFeO3 (BFO) films grown on Pt(111) buffered glass substrates at various growth temperatures and oxygen pressures have been studied. The optimization of the 20-nm-thick Pt(111) bottom electrode enables BFO films to form perovskite single phase structure at reduced temperatures between 400–500 °C. Microstructure of fine grains and smooth surface is obtained, which is favorable for applications. Multiferroic properties including 2Pr of 15.1–124 μC/cm2, Ec of 351–680 kV/cm, and Ms of 5–8 emu/cm3 are achieved within the oxygen pressure range of 10–75 mTorr. The optimized ferroelectric properties are comparable to the reported values of BFO samples using Si and MgO single crystal substrates. The results of this study may broaden the possibility of the use of BFO films.

Effect of Ba substitution on the multiferroic properties of BiFeO3 films on glass substrates

Effect of Ba substitution on the multiferroic properties of non-epitaxially grown polycrystalline Bi1−xBaxFeO3 (BBFO) films on refined Pt(111) electrode buffered glass substrates is studied. The structural analysis shows that a pure perovskite phase is present for BBFO films (x = 0.05-0.15), and (110) preferred orientation is developed for films with high x = 0.15. The grain size and surface roughness are reduced with increasing x. All studied BBFO films show desired ferroelectric and ferromagnetic properties. The good ferroelectric properties with the remanent polarization (2Pr) of 36-70 μC/cm2 and electrical coercive field (Ec) of 318-570 kV/cm are attained. On the other hand, the substitution of Ba2+ for Bi3+ in the A site of the BFO crystal structure can effectively enhance the ferromagnetic properties with magnetization (Ms) of 9.4-13.9 emu/cm3 and coercivity (Hc) of 1216-1380 Oe. The ferromagnetic and ferroelectric properties and leakage behavior as functions of Ba content x are discussed.

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.

Structural and magnetic properties of Fe–Pt–Nb sputtered films

Abstract (FePt) 100− x Nb x thin films were prepared by high-vacuum sputtering. In the studied composition range of x =0.0–2.1, remanence and squareness ratio were found to increase with x , whereas coercivity was found to decrease with x . The ordering parameter, K , measured from X-ray diffraction data indicates that the addition of Nb significantly decreases the K value. The K values from 0.20 to 0.30 can provide a two-phase (ordered and disordered) structure with strong intergranular coupling and a high-energy product as the thin films are annealed at 500–600°C.