Research on the ultra-precision manufacturing process of a large aperture silicon aspheric optics using MRF

Ji Fang

Supervisor：Xu Min

MRF (Magnetorheological finishing) is a high-deterministic and flexible polishing technique developed in more than a decade. Compared with the traditional computer numerically controlled polishing techniques, MRF has some significant advantages such as extremely high precision, convergence rates and less surface defects, which is capable of polishing large aperture and aspheric optics. The large aperture silicon aspheric optics is widely applied in many fields, but it is difficult to be manufactured due to the large size, extremely low removal rate and asphericity. This report studied the ultra-precision manufacturing of large aperture silicon aspheric optics based on a self-developed MRF machine.

The initial figure error of the silicon aspheric surface is PV 5.9μm, with large mid-spatial frequency error exists on the surface and a peak error at the center. Therefore, we took four steps to improve the low, middle, and high frequency errors, and the high-gradient center error was corrected. Firstly, we controlled the low frequency errors based on the dwell time principle and concentric circle tool path. A dwell time algorithm according to the matrix equation and optimization theory was used. Four experiments had been performed to correct the low frequency figure error with this algorithm. After every experiment the low frequency had been improved by PV 5.9μm to 2μm, indicating the algorithm has excellent figure correction capability. Owing to the area of central error is small and the gradient is so large, so we use a smaller spot to polish the center at a certain short time, and finally the central high-gradient error was removed. We combined the removal function, polishing path optimization, and little removal amount to control the mid-spatial frequency error, and it was improved by PV 2.3μm to 0.7μm. The roughness was effectively controlled by using large span and polishing bidirectionally with diamond magnetorheological fluids. Finally, after the magnetorheological finishing, figure accuracy was improved by PV 5.9μm to 0.56μm＜1λ and roughness Rq ＜10nm, which meet the requirements.

This report studied the magnetorheological finishing for aspheric surface manufacturing, and the figure error and roughness were successfully controlled. One hand this showed the polishing ability on manufacturing the large aperture, special material, and aspheric surface. At other hand, this demonstrated MRF was excellent technology in polishing large aperture silicon aspheric optics.

References

[1] S. D. Jacobs, S. A. Arrasmith, I. A. Kozhinova, et al, An overview of magnetorheological finishing (MRF) for precision optics manufacturing. Annual Meeting of the American Ceramic Society, 1999. 102.

[2] D. Golini, W. I. Kordonski, P. Dumas, et al. Magnetorheological finishing (MRF) in commercial precision optics manufacturing. Proc SPIE, 3782:80–91(1999).

[3] ZHU Haibo. Study on computer controlled polishing of the large size plane optical components[D]. Chengdu: Sichuang University, 2005.

[4] Aric Shorey. Magnetorheological Finishing of large and lightweight optics, Proc SPIE, Optical Science and Technology, Denver, CO, 2004.

[5] P. Dumas, D. Golini, P. Murphy, et al. Improve Figure and Finish of Diamond Turned Surfaces with Magneto-Rheological Finishing (MRF). Proc. ASPE, 2004.

[6] SHI Feng, DAI Yifeng, PENG Xiaoqiang. Magnetorheological finishing for high-precision optical surface [J]. Optics and Precision Engineering, 2009, 17(8): 1859-1864.

[7] ZHANG Yunfei, WANG Yang, WANG Yajun, et al. Dwell time algorithm based on the optimization theory for magnetorheological finishing [J]. Applied Optics, 2010, 31(4): 657-662.

Role of TbFe on Perpendicular Magnetic Anisotropy and Giant Magnetoresistance Effect in [Co/Ni]_N-Based Spin Valves

Minghong Tang

Supervisor：Zongzhi Zhang

       Giant magnetoresistive (GMR) devices consisting of two magnetic layers with perpendicular magnetic anisotropy (PMA) separated by a nonmagnetic spacer have attracted much interest for their potential applications in high density spin-transfer-torque magnetic random access memories (STT-MRAMs), where spin polarized current could be used to reverse the magnetization orientation [1–4]. In order to achieve high GMR signal and low switching current, various perpendicular magnetic films have been investigated [4-6]. Among those structures, the ferromagnetic [Co/Ni]_N is considered as one of the most promising free or/and reference layer materials due to its relatively high spin polarization and small Gilbert damping factor [2, 7, 8]. However, although the perpendicular coercivity (H_c^) of [Co/Ni]_N can be increased by tuning the structural parameters such as the repetition number N, or the layer thicknesses of seed layer and magnetic sublayers, the enhancement is very limited. In order to prevent simultaneous switching of the Co/Ni free and reference MLs, the reference layer switching field should be enlarged. An appropriate approach is to use a rare earth-transition metal (RE-TM) layer coupled with [Co/Ni]_N because the RE-TM alloy film in a proper composition ratio can display strong PMA and tunable net magnetization. In our previous work, we have fabricated spin valves (SVs) with a perpendicular [Co/Ni]_N/TbCo composite reference layer structure, which displayed prominent features including significant switching field difference between the free and reference layers, stable GMR ratio, and negligible offset in the minor GMR curves[10]. In order to take full advantage of such composite reference layer structure, it is very necessary to examine the role of other kinds of RE-TM materials on the perpendicular magnetic properties and GMR signal of Co/Ni based SVs.

       In this work, we present our study on perpendicular magnetic [Co/Ni]_N/TbFe multilayers both in Tb-rich and Fe-rich conditions. High perpendicular coercivity H_c^ of the composite film were obtained by adjusting the TbFe thickness and the repetition number N. Surprisingly, the application of TbFe layer in Co/Ni-based SVs shows considerable influence on the free layer PMA and GMR signal, and both of them decrease strongly with increasing TbFe thickness or Tb content, which are quite different from the experimental results for SVs containing a [Co/Ni]_N/TbCo reference layer [6]. We attribute this phenomenon to the large magnetostriction effect of TbFe which probably induces strong stress acting on the free layer and hence reduces its interfacial PMA.

References

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5. K. Yakushiji, T. Saruya, H. Kubota, A. Fukushima, T. Nagahama, S. Yuasa, K. Ando. Appl. Phys. Lett. 97(23), 232508 (2010).

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Optical properties of epitaxial BiFeO₃ thin film grown on SrRuO₃-buffered SrTiO₃ substrate

Ji-Ping Xu

Supervisor: Rong-Jun Zhang

BFO thin film is paid much attention to due to its large spontaneous polarization, which is an order higher than its bulk counterpart, and then BFO thin film combined with nanostructures could be a promising candidate in applications in electronics, data storage, and spintronics. In addition to its structural and electronic properties, optical properties of BFO thin films are focused on. However, in the published literatures on optical studies BFO thin film is usually directly deposited on perovskite oxide SrTiO₃ (STO) and DyScO₃ (DSO) substrate for epitaxial growth. So far, there is no report on optical properties of BFO thin film with an electrode structure in spite of the fact that the lower electrode is necessary for the study on electronic and ferroelectric properties of BFO thin film as well as for its applications including nonvolatile memory devices.

In this work, we have investigated the optical properties of BFO thin film grown on SrRuO₃ (SRO)-buffered STO substrate where SRO is often chosen as the lower electrode for BFO thin film as well as buffer layer to control its nanoscale domain architecture. The epitaxial BFO thin film was deposited by PLD on SRO-buffered (111) STO single-crystal substrate. XRD and AFM are employed to investigate the crystal phases and surface morphologies of the film sample. Spectroscopic ellipsometry (SE) measurements were taken to investigate the optical properties of the BFO film. The ellipsometric spectra (Ψ and Δ) were collected for the STO substrate, the SRO buffer layer and the BFO film, respectively.

The dielectric functions of STO, SRO, and BFO are obtained by fitting their spectra data to different models in which BFO corresponds to a five-medium optical model consisting of a semi-infinite STO substrate/SRO film/BFO film/surface roughness/air ambient structure. The BFO film and surface roughness thickness are identified as 99.19 and 0.71 nm, respectively. The optical constants of the BFO film are determined through the Lorentz model describing the optical response, and a direct bandgap at 2.68 eV is obtained which near-bandgap transitions could contribute to. Moreover, the gap value is compared to the BFO thin film with similar thickness deposited on various substrate prepared by PLD, indicating the dependence of the bandgap for the epitaxial BFO thin film on the in-plane compressive strain. In addition, the transition at 3.08 eV disclosed by the Lorentz model in our work suggests that the bandgap of BFO single crystals is less than 3 eV as previously reported. The results given in this work are helpful in understanding the optical properties of the BFO thin film and developing its application in optical field.

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Time:  6:30 pm, Thursday, 2013.10.23

Location:Optical Building. Room 5