An-Cheng Sun

Epitaxial growth mechanism of L10 FePt thin films on Pt/Cr bilayer with amorphous glass substrate

Ordered L10 FePt films with magnetic perpendicular anisotropy were fabricated with a Pt∕Cr bilayer. The squareness of the L10 FePt film with a Cr underlayer and a Pt buffer was close to one when a magnetic field was applied perpendicular to the film’s plane, because a semicoherent epitaxial growth was initiated from the Cr (002) underlayer; continued through the Pt buffer layer, and extended into the L10 FePt (001) magnetic layer. Without the Pt buffer layer, the Cr atoms may diffuse directly into the FePt magnetic layer. Consequently, an epitaxial barrier of the Cr-rich FePtCr alloy formed between the Cr underlayer and the FePt magnetic layer, degrading the magnetic performance and epitaxial growth of the latter.

Interfacial effects on magnetostriction of CoFeB/AlOx/Co junction

Saturation magnetostriction (λs) of CoFeB∕AlOx∕Co magnetic tunnel junctions (MTJs) has been measured. There are three kinds of MTJs in this study, i.e., glass∕CoFeB(tA)∕AlOx(δto)∕Co(tB) with tA+tB=150A; tA=100, 75, and 50A; and δto=0–30A. When plotting λs as a function of δto, the curve is concave up. We also investigated the compositional distribution of Fe, Al, and O across the tunneling part of the CoFeB(75A)∕AlOx∕Co(75A) junction. Based on these results, we propose a model to describe how the total λs is affected by the CoFeB∕AlOx and AlOx∕Co interfaces.

Enhancement of perpendicular coercivity in L11 CoPt thin films by replacement of Co with Cu

Magnetic properties and microstructures of L11 (Co50−xCux)Pt50 films sputter-deposited at 350 °C on MgO(111) substrates are reported. The addition of Cu significantly improves the alignment of c-axis and chemical ordering. Perpendicular coercivity (Hc⊥) also increases markedly from 0.1 to 1.9 kOe while in-plane coercivity declines from 0.5 to 0.07 kOe for the 20 nm thick films at x=26. Similar phenomena are observed with larger effects for the 50 nm thick films. The coercive mechanism is attributed to domain-wall pinning produced by the compositional segregation of nanoscaled nonmagnetic Cu-rich and magnetic Co-rich regions within a coherent L11 crystal domain. Therefore, an intermediate value of Hc⊥ can be obtained from this hardening mechanism when further microstructure modifications are enforced, which largely increases the potential for the use in spintronic devices or patterned media.

Magnetic Reversal Behaviors of Perpendicular Exchange-Coupled Fe/FePt Bilayer Films

Fe top layers with a thickness t Fe between 1 and 15 nm were deposited on textured FePt(001)/Pt(001)/Cr(002) layers to study the associated exchange coupling effect and their switching behaviors. The deposition of a Fe layer does not change the crystalline structure of the FePt layer. Microstructural analysis reveals that the Fe(200) texture is formed epitaxially on the FePt(001) surface. When t Fe = 1 nm, the remanent coercivity (H cr) remains the same as that without a Fe top layer. The lack of reversible magnetization reveals that the Fe layer is thinner than the critical thickness; the strong coupling force throughout the whole Fe layer facilitates the cooperatively magnetic rotation of Fe and FePt layers. Significant exchange spring was observed in the sample with t Fe = 3 and 5 nm, reducing H cr by 14% and 43% (the maximum reduction), respectively. The results are consistent with the theoretical prediction. As t Fe was increased up to 8 and 12 nm, the exchange spring effect disappeared. Although the two films exhibited similarly reduced H cr values of about 1.5 kOe, the reduction of H cr is caused by the tilt of the magnetic moments rather than the exchange spring. This incline of the moments is caused by the weakened exchange coupling and the complex domain structure. The magnetic reversal was further examined by measuring the variation in longitudinal and transverse magnetizations during magnetic field sweeping. The results suggest that at t Fe = 1 and 3 nm, the reversal occurs cooperatively in a single direction. As t Fe is increased to 5 and 8 nm, the moments switch randomly, in a manner that may be related to the increase in the density of the domain wall, observed by MFM. This work demonstrates the feasibility of reducing coercivity through exchange-spring mechanism and provides information on detailed magnetic reversal mechanism in perpendicular exchange coupled FePt.

Magnetic Properties of Percolated Perpendicular FePt–MgO Films

Percolated perpendicular FePt-MgO films with a (Fe48 Pt 52)100-x-(MgO)x/Pt(001)/Cr(002) structure were prepared by conventional dc magnetron sputtering (x=0-6.13). Magnetic measurements demonstrate that the coercivity of the magnetic film drastically increases from 169 to 285 kA/m as the MgO content is increased from 0 to 0.15 vol.%. However, the grain sizes of the FePt phase do not significantly varying upon doping with MgO. MgO does not appear at the grain boundaries of the FePt phase, but is present as crystalline dots that are uniformly precipitated in the FePt matrix. The MFM images revealed that the domain structure transformed from extending to isolate when the MgO dots precipitated into the FePt grains. Consequently, the MgO dots serve as pinning sites of the domain wall and enhance perpendicular coercivity. Percolated perpendicular magnetic recording is thus regarded as a solution to the problem of thermal instability in ultrasmall grains

Coercivity enhancement in L11 Co50-xCuxPt50 thin films

This study investigates structural and magnetic properties of the (Co50-xCux)Pt50 films deposited on the MgO(111) substrate. Experimental results indicate that adding Cu significantly improves the alignment of L11[111] and decreases the structural defects of the epitaxial films. Additionally, a large increment of an order of magnitude in out-of-plane coercivity (Hc⊥) from 0.2 to 2.2 kOe is achieved when x is increased from 0 to 23. Such an increase accompanies a reduction of in-plane coercivity (Hc||) from 1.5 to less than 0.1 kOe. The enhancement in Hc⊥ is attributed to the domain wall pinning effect induced by compositional segregation of Cu and Co. This study provides an effective approach to optimize the perpendicular magnetic properties of L11 CoPt.

Low temperature growth of FePt and CoPt films on MgO(111) substrate

Equiatomic CoPt and FePt thin films grown on MgO(111) substrates at temperatures (Ta) from room temperature (RT) to 400 °C were studied. Distinct phase evolution was observed. In CoPt films, a metastable phase of L11 appears at Ta = 250 °C prior to the formation of thermodynamic equilibrium L10 phase; whereas no intermediate structure is found in the FePt films. Good epitaxial growth of CoPt films can be obtained at low Ta before ordering. However, in FePt films, high quality epitaxy appears after the occurrence of L10 ordering. Either L11 or L10 ordering induces magnetic hardening. Perpendicular magnetic anisotropy was obtained in the L11 films, yet the L10 sample exhibited isotropic magnetism. Corresponding evolution in magnetic domain structure was also reported.

Effect of Ge on the magnetic properties and crystal structure of melt spun SmCo7–xGex ribbons

The effect of Ge on the magnetic properties and crystal structure of melt spun SmCo7–xGex (x = 0–0.8) ribbons have been investigated. SmCo7–xGex ribbons (x = 0–0.6) spun at a high wheel speed of 40 m/s crystallize in the TbCu7-type structure. The x-ray diffraction results analyzed by the Rietveld refinement method show that Ge prefers to occupy the 2e sites in the TbCu7-type structure. The grain size of the ribbons is only slightly decreased with the proper amount of Ge substitution (x < 0.7). When increasing x from 0–0.6, the coercivity of the SmCo7–xGex ribbons is enhanced from 1.9 kOe for binary SmCo7 to 7.5 kOe for SmCo6.5Ge0.5 which is mainly due to the enhancement of the magnetic anisotropy field by the Ge substitution for Co in 2e sites. Furthermore, a slight addition of carbon in the alloy is effective in refining the grain size and further improving the coercivity of the ribbons. Coercivity as high as 11.4 kOe could be obtained for SmCo6.7Ge0.3C0.1 ribbons.

Fabrication of L11 Co-Pt-Cu perpendicular anisotropic films with enhanced coercivity on glass substrate

Structure and magnetic properties of L11 CoPtCu thin films sputter-deposited on glass substrates were studied. Perpendicular coercivity (Hc⊥) was largely enhanced by the replacement of Co with Cu with x = 23 and 26, the magnetic properties obtained was better than the film grown on MgO(111) substrates. The enhancement of Hc⊥ existed in a wide range of film thickness from 2 to 40 nm. A maximum value of Hc⊥ of 2.7 kOe appeared in the films with thickness of 4 nm. The results of surface morphology indicated that the random distribution of the in-plane orientation of L11 grains induced by Pt(111) underlayer may create magnetic inhomogeneities acting as pinning site to domain walls, providing additional impedance for domain wall motion.

Magnetic properties and microstructure of bulk Nd–Fe–B magnets solidified in magnetic field

The Nd–Fe–B bulk magnets with a slab shape of 0.9 × 4 × 15 mm3 were prepared by injection casting into a copper mold. The effects of applying a magnetic field during the casting process on the magnetic properties and microstructure of Nd9.5Fe71.5Ti2.5Zr0.5Cr1B14.5C0.5 alloy have been studied. The results show that the sample cast with magnetic field has a stronger (00L) texture of Nd2Fe14B phase with the c-axis perpendicular to the slab plane than the sample cast without magnetic field. The intensity of the texture weakens from surface to inner region of the bulk magnets. Applying a magnetic field during the casting process is helpful to refine the grain size effectively. As a result, the magnetic properties are improved from Br = 5.8 kG, iHc = 6.5 kOe, and (BH)max = 5.9 MGOe for thesample cast without magnetic field to Br = 6.1 kG, iHc = 10.3 kOe, and (BH)max = 7.3 MGOe for the sample cast with a 3.7 kOe magnetic field.