Magnetization Reversal in Antiferromagnetically Coupled [Pt/CoFeB]_N/Ru/[CoFeB/Pt]_N Structures with Perpendicular Anisotropy

Yili Xiao

Abstract

Synthetic antiferromagnetic (SAF) structures which employ CoFeB as the ferromagnetic layer have in recent years been extensively studied experimentally[1-4]. The structure, which consists of a nonmagnetic spacer layer sandwiched by two ferromagnetic layers, can be tuned via varying the thickness of the spacer layer to achieve antiferromagnetic coupling between the two ferromagnetic layers. When used as the free layer in a spin torque driven magnetic tunnel junction (STT-MTJ), the SAF structure has the advantage of reducing the effective magnetic moment per unit area, hence achieving lower Jc, while maintaining sufficiently high volume for thermal stability[5]. This is essential for the development of high density, low power consumption data storage devices. On the other hand, it has been established that giant magnetoresistive (GMR) pillars with a perpendicular easy axis is also advantageous for magnetic stability, high information density, and switching reliability[6-8]. However, most of the research on SAFs with CoFeB has been done on structures which have an in-plane magnetic anisotropy[9 10]. It is thus important for future applications to study the magnetic coupling, switching behavior and thermal stability of CoFeB based SAF structures with perpendicular anisotropy.

     In this presentation, I will report on our detailed study of perpendicularly magnetized SAF structures that consist of two [CoFeB/Pt]_N multilayers separated by a Ru spacer. An oscillatory behavior of the interlayer coupling between the two FM layers is observed by varying the Ru layer thickness. The layer repetition number N is found to significantly influence the switching behavior of the structure under an external magnetic field. A series of measurements under different temperatures is also carried out, and a method is devised to enhance the antiferromagnetic coupling strength by substituting for CoFeB a thin layer of CoFe near the interface with Ru.

References

[1] N. Wiese, T. Dimopoulos, M. Ruhrig, et al., “Magnetic properties of antiferromagnetically coupled CoFeB/Ru/CoFeB”, J. Magn. Magn. Mater., vol. 290-291, p. 1427, 2005.

[2] J. C. A. Huang, C. Y. Hsu, S. F. Chen, et al., “Enhanced antiferromagnetic saturation in amorphous CoFeB-Ru-CoFeB synthetic antiferromagnets by ion-beam assisted deposition”, J. Appl. Phys., vol. 101, p. 123923, 2007.

[3] R. Lavrijsen, A. Fernandez-Pacheco, D. Petit, et al., “Tuning the interlayer exchange coupling between single perpendicularly magnetized CoFeB layers”, vol. 100, p. 052411, 2012.

[4] N. Wiese, T. Dimopoulos, M. Ruhrig, et al., “Strong temperature dependence of antiferromagnetic coupling in CoFeB/Ru/CoFeB”, Europhysics Letters, vol. 78, no. 6, 2007.

[5] J. Hayakawa, S. Ikeda, K. Miura, et al., “Current-induced magnetization switching in MgO barrier magnetic tunnel junctions with CoFeB-based synthetic ferrimagnetic free layers”, IEEE Trans. Magn., vol. 44, no. 7, 2008.

[6] S. Mangin, D. Ravelosona, J. A. Katine, et al., “Current-induced magnetization reversal in nanopillars with perpendicular anisotropy”, Nat. Mater., vol. 5, pp. 210–215, 2006.

[7] H. Meng and J. P. Wang, “Spin transfer in nanomagnetic devices with perpendicular anisotropy,” Appl. Phys. Lett., vol. 88, p. 172506, 2006.

[8] X. Li, Z. Zhang, Q. Y. Jin, and Y. Liu, “Domain nucleation mediated spin-transfer switching in magnetic nanopillars with perpendicular anisotropy,” Appl. Phys. Lett., vol. 92, p. 122502, 2008.

[9] Y. Gong, Z. Cevher, M. Ebrahim, et al., “Determination of magnetic anisotropies, interlayer coupling, and magnetization relaxation in FeCoB/Cr/FeCoB”, J. Appl. Phys., vol. 106, p. 063916, 2009.

[10] N. Wiese, T. Dimopoulos, M. Ruhrig, et al., “Antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers”, Appl. Phys. Lett., vol. 85, no. 11, 2004.

Research on Edge Control in the Process of Polishing Using Ultra Precise Bonnet on Optical Elements

Wei Wang, Min Xu

School of Information and Engineering, Fudan University, Shanghai, 200433;

Abstract

High-quality optical elements are very important in modern technology; in particular top-quality aspheric optical elements. The concept of an extremely large ground based telescope is a significant technical challenge particularly for the manufacture of the optical components necessary to realize the very demanding performance. Extremely large ground based telescopes require many high-quality, large-diameter optical elements for their construction. The method of optical polishing, using an ultra precise bonnet, is based upon the use of computer controlled fabrication of an optical surface. A bonnet filled with air is used as a precise polishing tool which is flexible and can adapt itself well to the shape of the part, compared with other polishing methods. As with other polishing techniques the edge quality is a key factor affecting the performance of the optical element. In this paper, the effects of edge performance are analyzed, and three compensating techniques are discussed. It is demonstrated that, good edge control can be achieved by using a special removal function applied to the polishing process. Some experimental results are shown and a consecutive polishing process is described.

Key words: Aspheric Optics, Bonnet Polishing, Removal Function, Edge Control

Reference

1. R. G. Bingham, D. D. Walker, D-H. Kim, etc. A novel automated process for aspheric surfaces[C]. Proc. SPIE, 2000, 4093: 445-450.

2. D. D. Walker, A. Beaucamp, D. Brooks, etc. Novel CNC Polishing Process for Control of Form and Texture on Aspheric Surfaces[C]. Proc. SPIE, 2002, 4767: 99-106

3. D. D. Walker, A. Beaucamp, D. Brooks, etc. New Results from the Precessions Polishing Process Scaled to Larger Sizes[C]. Proc. SPIE, 2004, 5494: 71-81

4. D. D. Walker, A. Beaucamp, V. Doubrovski, etc. Automated Optical Fabrication-First results From the New Precessions 1.2m CNC polishing machine[C]. Proc. SPIE, 2006, 6273: 91-98

5. A. Cordero-Davila, J. Gonzalez-Garcia, M. Pedrayes-Lopez, etc. Edge Effects with the Preston Equation for a Circular Tool and Workpiece[J], Appl. Opt. Vol. 43, 1250-1254 (204).

Relative research on perpendicular magnetic anisotropy(PMA) of multilayer

Shaohai Chen

Supervisor: Zongzhi Zhang

Abstracts:

Recently, with the rapid development of spintronics, Giant magneto resistive (GMR) devices with perpendicular magnetic anisotropy (PMA) are of great interest due to their potential applications for data storage. In particular, they are as promising candidates for spin-transfer magnetic random access memory (MRA), because of reduced magnetic noise and their potential for high density devices.^1–4 Compared with in-plane devices, perpendicularly magnetized devices exhibit coherent switching with small switching field distribution as well as stable and uniform magnetization even in submicron cells with a low aspect ratio of 1. For successful application of spin-transfer switching to high density MRAM, the perpendicular magnetic GMR structures should be optimized to have smaller free layer coercivity (H_c) in order to lower the critical switching current, whereas the magnetization of the reference layer should be fixed by using an antiferromagnetic pinning layer or by employing a high Hc material in order to realize independent magnetization switching. Moreover, materials with high spin polarization are preferred for high GMR signal and low spin-transfer switching current.

The perpendicular [Co/Ni]_N-based GMR structure is of great importance in the practical application of spin-transfer MRAM because of its high spin torque efficiency.³ Therefore, in this work we have carried out a detailed investigation of Co/Ni MLs and three types [Co/Ni]_N-based SVs: the pseudo- ,FeMn-biased and SAF spin valves. The observed GMR ratio for the pseudo-spin-valve is as high as 7.7%, but it rapidly decreases below 1.0% after annealing in a perpendicular field at 200 °C. Such poor temperature stability is ascribed to simultaneous switching of the free and reference multilayer caused by loss of their coercivity difference. In contrast, an FeMn-biased sample with a similar structure has a slightly lower GMR signal of 6.5% but exhibits much better thermal stability, with the GMR reduction occurring at an elevated anneal temperature of over 300 °C. This GMR reduction is due to Mn diffusion and a reduction in perpendicular anisotropy. ⁵Compared with the rapid drop of GMR signal for the normal [Co/Ni]-based pseudo spin valves after annealing at low temperature (Ta) of 150⁰, the spin valve with SAF reference layer exhibits much stable thermal stability due to the large switching field difference between the free and reference layers which avoids the simultaneous magnetization rotation. The GMR signal of the SAF spin valve sample is 6.0% at room temperature, it decreases very gradually with the increase of Ta. We attribute the slow GMR reduction observed in the SAF spin valve to the effects of domain formation and perpendicular anisotropy deterioration caused by high temperature anneals.⁶

Reference:

[1] N. Nishimura, T. Hirai, A. Koganei, T. Ikeda, K. Okano, Y. Sekiguchi, and Y. Osada, J. Appl. Phys. 91, 5246 (2002).

[2] X. Li, Z. Zhang, Q. Y. Jin, and Y. Liu, Appl. Phys. Lett. 92, 122502 (2008).

[3] H. Meng and J. P. Wang, Appl. Phys. Lett. 88, 172506 (2006).

[4] S. Mangin, D. Ravelosona, J. A. Katine, M. J. Carey, B. D. Terris, and E.E. Fullerton, Nature Mater. 5, 210 (2006).

[5] Zhenya Li, Zongzhi Zhang, Hui Zhao, Bin Ma, and Q. Y. Jin, J. Appl. Phys. 106, 013907 (2009)

[6] He He, Zongzhi Zhang, Bin Ma, and Qingyuan Jin, IEEE Trans. Magn. 46, 6( 2010)