- Email: [email protected]
Pt]4 structures with perpendicular magnetic anisotropy
Accepted Manuscript Strong antiferromagnetic interlayer exchange coupling in [Co/Pt]6/Ru/[Co/Pt]4 structures with perpendicular magnetic anisotropy Yi Liu, Jiangang Yu, Huicai Zhong PII: DOI: Reference:
S0304-8853(18)32244-3 https://doi.org/10.1016/j.jmmm.2018.10.090 MAGMA 64501
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
17 July 2018 11 October 2018 18 October 2018
Please cite this article as: Y. Liu, J. Yu, H. Zhong, Strong antiferromagnetic interlayer exchange coupling in [Co/ Pt]6/Ru/[Co/Pt]4 structures with perpendicular magnetic anisotropy, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.10.090
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strong antiferromagnetic interlayer exchange coupling in [Co/Pt]6/Ru/[Co/Pt]4 structures with perpendicular magnetic anisotropy Yi Liua, *, Jiangang Yub, Huicai Zhongc a
School of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, PR China b School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, PR China c Integrated Circuit Advanced Process Center, Institute of Microelectronics, Chinese Academy Of Sciences, Beijing, 100029, PR China
*Corresponding author: Yi Liu E-mail: [email protected].
Abstract: The interlayer exchange coupling properties of perpendicular synthetic antiferromagnetic(p-SAF) structure were investigated in this work by varying the Ru spacer thickness (tRu). We observed an oscillatory decay of interlayer exchange coupling field (Hex) and coupling strength (Jex) with a period of about 0.6 nm. The greatest values of Hex and Jex are 13.2 kOe and 1.77 erg/cm2 at the 1st peak, respectively. The Hex decreases significantly to about 6.2 kOe with the increase of tRu from 0.44 nm at the 1st peak to 1.03 nm at the 2nd peak. The effect of post-annealing temperature on the Hex was also investigated. The Hex decreases with the increase of annealing temperature. Furthermore, after annealing at 380℃, the Jex value remains as high as 0.94 erg/cm2 at the 1st peak. Our results represent a candidate for spin-transfer torque magnetic random-access memory(STT-MRAM).
Key words: Synthetic antiferromagnetic; Interlayer exchange coupling; Perpendicular magnetic anisotropy; Oscillation.
1. Introduction Perpendicular Magnetic tunnel junctions(p-MTJs) based on MgO with ferromagnetic electrodes are great interest due to its potentialities for realizing 1
next-generation high-density non-volatile memory with high thermal stability and low critical current [1-3]. However, there are still some key issues need to be solved in p-MTJ. The most important one is the effect of stray field on the free layer, which may affect the storage stability of the device. The stray field can be effectively compensated by a synthetic antiferromagnetic (SAF) structure [4-6]. The SAF structure is consisted by two ferromagnetic (FM) layers and a nonmagnetic spacer layer. Actually, SAF coupling is a feature of the interlayer exchange coupling (IEC) eff ect [7,8], and the exchange bias field (Hex) of the SAF structure is related to the PMA strength of FM [9]. Cu, Ru, and Rh are the most common spacer materials in SAF structures [10-12]. Due to the strong perpendicular magnetic anisotropy (PMA) in [Co/Pd]n, [Co/Pt]n, and [Co/Ni]n multilayers [13-16], they are also the most popular ferromagnetic layer materials in SAF structures. The Hex is the shift of reference layer (RL) minor magnetic hysteresis loop in a SAF structure. The Jex value can be evaluated by Jex=Hext*Ms[17], where the Jex is the coupling strength, Ms and t are the saturation magnetization and thickness of RL, respectively. For Ru space layer SAF structures, Parkin et al.[18] demonstrated that the highest Hex occurred at the “first peak” of the oscillation. However, very few studies utilized the highest Hex at the first peak in perpendicular SAF (p-SAF) systems, because it reduces significantly with a slight variation of the Ru thickness[19, 20]. Experimentally, Gan et al.[21] obtained an oscillation curve of Hex with varying Ru thickness from 0.4–1.1 nm, the Hex had two peaks, and the first and second oscillation peaks occurred at about 0.45 nm and 0.9 nm, respectively. Chae et al.[22] also reported the dependence of Hex on the thickness of Ru, what is more, two oscillation peaks were found in their work. However, the Jex of 1st peak was weaker than that of the 2nd peak, which can be due to the roughness and intermixing at the interfaces of the ultrathin Ru layer. So the 1st peak has a weaker antiferromagnetic coupling (AFC) than the 2nd peak. For this reason, the 2nd peak of the SAF structure was always applied in most p-MTJs[23-25]. Recently, many research groups have obtained the oscillation curves of Hex dependence of Ru thickness [26, 27], and a higher Hex (~6 kOe) was achieved at the 1st oscillation peak with a Ru thickness of about 0.5 nm. Yakushiji et al.[28] reported a higher Hex and Jex of about 10 kOe and 2
2.2 erg/cm2, respectively. Yun et al.[29] reported a greater Hex and Jex of 12.6 kOe and 2.55 erg/cm2 in [Pt/Co]6/Ru/[Co/Pt]3 structures at the 1st oscillation peak, respectively. Recently, Yakushiji et al.[30] reported a higher Hex and Jex of 12 kOe and 2.6 erg/cm2, respectively. The Jex value is also the highest so for in a similar structure. Actually, both large PMA and atomically smooth surface/interface are required in FM layers to achieve a higher AFC strength. In previous work, we have prepared strong PMA FM layers based on Co/Pt superlattices [31]. In this work, based on the higher PMA Co/Pt superlattices, we report a higher Hex and Jex values of about 13.2 kOe and 1.77 erg/cm2, respectively, 13.2 kOe is also the highest Hex value so for in a similar SAF structure.
2. Experimental procedure Film deposition was performed by using an ULVAC Megnetron Sputtering on 4 inch SiOx substrates at room temperature. The work pressure was below 5×10 -7 Pa. All samples were post-annealed at various temperatures (Ta) of 300℃, 350℃, and 380℃ for 1 hour. The transmission electron microscopy (TEM) was used to analysis the total thickness and microstructure of the multilayer. The magnetic properties of the multilayers were measured by using the superconducting quantum interference device-vibration sample magnetometer (SQUID-VSM) and the Magneto-Optical Kerr effect mangnetometer. The saturation magnetizations (Ms) of the multilayers were calculated by including the total volume of Co and Pt sublayers
3. Results and Discussion In this work, we develop p-SAF multilayer films based on Co/Pt superlattices by using nearly monoatomic-layer alternation of Co and Pt. These stack structures are Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]4,
Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6
and
Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1 .0) (units are nm), where, tRu is ranged from 0.36 to 1.10 nm. These ultrathin films can be considered as an artificial superlattice rather than multilayer and they display high post-annealing stability. A systematic study of the dependence of Hex and Jex on the tRu ranging from 0.36 to 1.1 nm will be presented here. Since the strengths of both Jex and Hex in a SAF structure are strongly dependent on the appropriate deposition conditions. The thickness of Co/Pt and the number of 3
cycles effect on the PMA of [Co/Pt]n superlattices have been discussed in our previous
study
[31].
Fig.1
displays
the
M-H
loops
of
as-deposited
[Co(0.30)/Pt(0.27)]4 and [Co(0.30)/Pt(0.27)]6 superlattices under optimized sputtering conditions with magnetic fields applied in-plane or out-of-plane. The in-plane saturation field(Hs) values for [Co(0.30)/Pt(0.27)]4 and [Co(0.30)/Pt(0.27)]6 are about 39 kOe and 34 kOe, respectively, the corresponding Ms are 676 and 721 emu/cc, respectively. The effective PMA energy density (Keff) obtained by the area method for samples of [Co(0.30)/Pt(0.27)]4 and [Co(0.30)/Pt(0.27)]6 are about 1.32×107 and 1.23×107 erg/cc, respectively. So the high Hex values could be partly attributed to the higher Keff values in bottom and top multilayer of SAF structure. Fig.2(a)
is
a
schematic
of
the
stacked
structure:
Si/SiO2
substrate/Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0. 30)]3/Pt(1) (units are nm). Fig.2(b) to (d) show the perpendicular magnetic hysteresis loops for the as-deposited samples with tRu=0.44, 0.72, and 1.03 nm, respectively, where Hex denotes the exchange field. In Figs.2(b) and 2(d), with the magnetic field decreases from saturation, the magnetization reversal occurs before it reaches to zero, which indicates that the two ferromagnetic multilayers are AFC. At remanence state, the moments of the upper and lower multilayers are aligned antiparallel, and the net magnetic moments are equal to the difference of upper and lower multilayers moment. For tRu=0.72 nm sample, the Hex is about 0.56 kOe，which is also the lowest Hex value among all Hex. So the antiferromagnetic coupling in the tRu=0.72 nm sample is also the most weak one under the range of Ru thickness in this work. Fig.3 shows the dependence of Hex on the tRu. It is clear that Hex exhibits an oscillatory-like behavior although all samples show AFC in this work, and the curve exhibits two peaks: the 1st peak at tRu = 0.44 nm and a 2nd peak at tRu =1.03 nm. The value of the Hex at the 1st peak is about 13.2 kOe. The inset of Fig.3 shows the dependence of Jex on the tRu, the Jex also exhibits an oscillatory-like behavior. The value of the Jex at the 1st peak is about 1.77 erg/cm2. However, the values of both Hex and Jex reduce to about 6.2 kOe and 0.80 erg/cm2 at the 2nd peak. What is more, the Jex value at tRu= 0.44 nm is two times higher than that at tRu =1.03 nm. The curve shows an oscillatory behavior reminiscent to the Ruderman-Kittel-Kasuya-Yosida (RKKY) 4
with the varying thickness of Ru spacer, similar oscillatory-like behavior was obtained in other previous studies[18, 26, 32], and the Jex at the first peak has a much higher value than the second one. For the two peaks of Hex appearing in the RKKY coupling region, the strong hybridization happens at the Pt/Co interfaces and the density of state of Ru should be taken into account [32,33]. TEM was applied to investigate the total thickness and microstructure of Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30]3/Pt( 1.0). Fig.4 displays the TEM cross sectional image and selected area electron diffraction (SAED) patterns. The interfaces of Ta/Pt and Pt/bottom Co/Pt superlattice are clear. The higher Jex at the 1st peak is due to the flat interfaces at atomic scale flatness. Moreover, the bottom and top Co/Pt superlattice layers are relatively flat. The obtained d-spacing of (11-1)(d11-1) in the 3 nm Pt is 0.230 nm, and the d11-1 in the bottom and top Co/Pt superlattice are 0.219 and 0.224 nm, respectively. The d111 in the top superlattice is larger than that of bottom one, which means the strain in bottom superlattice is larger than top one. So the strong PMA in the top superlattice could be partly due to the increase of magnetoelastic anisotropy, which comes from the increase of strain[34]. Since it is important to study the temperature dependence on the coupling strength, the post-annealing stability of SAF structure has also be studied here. Fig.5(a) shows the perpendicular hysteresis loops of as-deposited and after annealing at 300℃ 350℃,
and
380℃
for
the
sample
of
[Co(0.30/Pt0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1.0). The Hex annealed at 300℃, 350℃, and 380℃ are 10 kOe, 9 kOe, and 7 kOe, respectively. Fig.5(b) presents the Hex of as-deposited and post-annealed samples changing with tRu thickness. The interdiff usion of Co/Ru/Co interfaces may result in the degradation of Jex and Hex values, so the values of Hex decreases with the increase of annealing temperature. For a certain annealing temperature, the strength of Hex at the 2nd peak is about half of the 1 st one. What’s more, the decreased rate of Hex value at 1st peak is more than the 2nd peak. A large value of Jex of 0.94erg/cm2 was obtained even after annealed at 380 C for one hour at 1st peak, which is greater than the Jex criteria (>0.70 erg/cm2) to ensure the read/write failure in p-STT-MRAM-cell array[35]. 5
4. Conclusion In summary, the effect of the Ru thickness and annealing temperature on the Hex and Jex in the SAF structure based on Co/Pt ultrathin superlattices have been investigated. The Jex shows an oscillatory decay with a period of about 0.6 nm, the greatest values of Hex and Jex are about 13.2 kOe and 1.77 erg/cm2 at the 1st peak, respectively. The larger Jex values could be attributed to the higher Keff values in bottom and top suerlattice and the atomic scale flatness of the SAF structure. A higher Jex value of 0.94erg/cm2 was also obtained at Ta=380℃. Our results represent a candidate for STT-MRAM.
Acknowledgments This work was supported by National fundamental research Program of China (973 program) (Grantno. 2011CB921804), the funding of the doctorate in Chongqing University of Posts and Telecommunications (Grantno. A2017-120).
6
References [1] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.H. Yang, Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers, Nat.Mater. 3(2004)862-867. [2] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H.D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, H. Ohno, A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction, Nat.Mater. 9(9) (2010)721-724. [3] H. Meng, J.P. Wang, Spin transfer in nanomagnetic devices with perpendicular anisotropy, Appl.Phys.Lett. 88(2006)172506. [4] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions, Nat.Mater. 3(12)(2004)868-871. [5] D.D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, K. Ando, 230% room-temperature magnetoresistance in CoFeB∕MgO∕CoFeB magnetic tunnel junctions, Appl.Phys.Lett. 86(2005)092502. [6] S. Yuasa, D.D. Djayaprawira, Giant tunnel magnetoresistance in magnetic tunnel junctions
with
a
crystalline
MgO
(001)
barrier,
J.Phys.D:Appl.Phys.
40(2)(2007)R337-R354. [7] P. Bruno, C. Chappert, Oscillatory coupling between ferromagnetic layers separated by a nonmagnetic metal spacer, Phys.Rev.Lett. 67(12)(1991)1602-1605. [8] P. Bruno, Theory of interlayer magnetic coupling, Phys.Rev.B 52(1)(1995) 411-439. [9] Y. Xiao, S. Chen, Z. Zhang, B. Ma, Q.Y. Jin, Magnetization reversal in antiferromagnetically
coupled
[Pt/CoFeB] N1/Ru/[CoFeB/Pt]N2
structures
with
perpendicular anisotropy, J.Appl.Phys. 113(2013)17A325. [10] M. Kowalewski, B. Heinrich, J.F. Cochran, P. Schurer, Studies of interlayer exchange
coupling
in
Fe/Cu/Fe
ultrathin
heterostructures,
J.Appl.Phys.
81(1998)3904-3906. [11] S. Zoll, A. Dinia, M. Gester, D. Stoeffier, H.A.M.V.D. Berg, A. Herr, R. Poinsot, H. Rakoto, Giant antiferromagnetic exchange coupling in ultrahigh-vacuum grown (111) Co/Rh sandwiches, J. Magn.Magn.Mater. 165(1)(1997)442-445. 7
[12] N. Wiese, T. Dimopoulos, M. Rührig, J. Wecker, H. Brückl, G. Reiss, Antiferromagnetically
coupled
CoFeB∕Ru∕CoFeB
trilayers,
Appl.Phys.Lett.
85(2004)2020-2022. [13] J.H. Park, C. Park, T. Jeong, M.T. Moneck, N.T. Nufer, J.G. Zhu, Co/Pt multilayer based magnetic tunnel junctions using perpendicular magnetic anisotropy, J.Appl.Phys. 103(2008)07A917. [14] M.T. Rahman, A. Lyle, G. Hu, W.J. Gallagher, J.P. Wang, High temperature annealing stability of magnetic properties in MgO-based perpendicular magnetic tunnel junction stacks with CoFeB polarizing layer, J.Appl.Phys. 109(2011)07C709. [15] L.E. Nistor, B. Rodmacq, S. Auffret, B. Dieny, Pt/Co/oxide and oxide/Co/Pt electrodes
for
perpendicular
magnetic
tunnel
junctions,
Appl.Phys.Lett.
94(2009)012512. [16] H. Kurt, M. Venkatesan, J.M.D. Coey, Enhanced perpendicular magnetic anisotropy in Co/Ni multilayers with a thin seed layer, J.Appl.Phys. 108(2010) 073916. [17] W. Folkerts, Calculated magnetic phase diagrams and magnetoresistance curves for an antiferromagnetically coupled multilayer system. J.Magn.Magn.Mater. 94(1991)302-310. [18] S.S.P. Parkin, Systematic variation of the strength and oscillation period of indirect magnetic exchange coupling through the 3d, 4d, and 5d transition metals, Phys.Rev.Lett. 67(25)(1991)3598-3601. [19] K. Ounadjela, D. Muller, A. Dinia, A. Arbaoui, P. Panissod, G. Suran, Perpendicular anisotropy and antiferromagnetic coupling in Co/Ru strained superlattices, Phys.Rev.B 45(14)(1992)7768. [20] H. Sato, S. Ikeda, S. Fukami, H. Honjo, S. Ishikawa, M. Yamanouchi, K. Mizunuma, F. Matsukura, H. Ohno, Co/Pt multilayer based reference layers in magnetic tunnel junctions for nonvolatile spintronics VLSIs, Jpn.J.Appl.Phys. 53(4S)(2014)04EM02. [21] H. Gan, R. Malmhall, Z. Wang, B.K. Yen, J. Zhang, X. Wang, Y. Zhou, X. Hao, D. Jung, K. Satoh, Y. Huai, Perpendicular magnetic tunnel junction with thin CoFeB/Ta/Co/Pd/Co reference layer, Appl.Phys.Lett. 105(2014)192403. 8
[22] K.S. Chae, T.H. Shim, J.G. Park, Dependency of anti-ferro-magnetic coupling strength on Ru spacer thickness of [Co/Pd] n-synthetic-anti-ferro-magnetic layer in perpendicular magnetic-tunnel-junctions fabricated on 12-inch TiN electrode wafer, J.Appl.Phys. 116(2014)033904. [23] K. Yakushiji, A. Fukushima, H. Kubota, M. Konoto, S. Yuasa, Ultralow-voltage spin-transfer switching in perpendicularly magnetized magnetic tunnel junctions with synthetic antiferromagnetic reference layer, Appl. Phys.Express 6(2013)113006. [24] G. Hu, T. Topuria, P.M. Rice, J.S. Jean, D.C. Worledge, Optimization of tunneling magnetoresistance in perpendicular magnetic tunnel junctions with Co|Pd reference layers, IEEE Magn.Lett. 4(4)(2013)3000104. [25] G.M. Choi, I.J. Shin, B.C. Min, K.H. Shin, Perpendicular magnetic tunnel junctions with synthetic ferrimagnetic pinned layer, J.Appl.Phys. 108(2010)073913. [26] S. Chen, Y. Xiao, W.H. Xie, Z. Zhang, B. Ma, Q.Y. Jin, Interfacial effects on magnetic interlayer coupling between perpendicular Co/Ni multilayers across Ru spacer. Appl.Phys.Lett. 105(2014)262403. [27] J.H. Jung, S.H. Lim, S.R. Lee, Interlayer exchange coupling between [Pd/Co] multilayers and CoFeB/MgO layers with perpendicular magnetic anisotropy, Appl.Phys.Lett. 101(2012)242403. [28] K. Yakushiji, H. Kubota, A. Fukushima, S. Yuasa, Perpendicular magnetic tunnel junctions with strong antiferromagnetic interlayer exchange coupling at first oscillation peak, Appl.Phys.Express 8(2015)083003. [29] S.J. Yun, H.L. Sang, S.R. Lee, Strong interlayer exchange coupling and high post-annealing stability in perpendicularly magnetized [Pt/Co]/Ru/[Co/Pt] structures, Aip Adv. 6(2)(2016)3000204. [30] K. Yakushiji, A. Sugihara, A. Fukushima, H. Kubota, S. Yuasa, Very strong antiferromagnetic interlayer exchange coupling with iridium spacer layer for perpendicular magnetic tunnel junctions, Appl.Phys.Lett. 110(2017)092406. [31] Y. Liu, J. Qiu, S. Ter Lim, S.L. Toh, Z. Zhu, G. Han, Strong perpendicular magnetic
anisotropy
in
[Co/Pt]n
ultrathin
superlattices. A
Appl.Phys.Express, 10(2017)013005. [32] J. Zhao, Y.J. Wang, Y.Z. Liu, X.F. Han, Z. Zhang, Perpendicular anisotropy 9
dependence of oscillatory interlayer coupling behavior in [Pt/Co]5/Ru/[Co/Pt]5 multilayers, J.Appl.Phys.104(2008)023911. [33] F.J. Himpsel, K. Christmann, P. Heimann, D.E. Eastman, Experimental energy-band
dispersions
and
lifetimes
for
ruthenium,
Phys.Rev.B
23(23)(1981)2548-2552. [34] S. Mohanan, U. Herr, Optimization of magnetic properties of Co/Pd multilayers by applying a large persistent biaxial stress. J.Appl.Phys. 102(2007)093903. [35] Samsung Global MRAM Innovation Program, see http://www.samsung. com/global/business/semiconductor/news-events/mram (2013).
Figure Captions Figure 1. The M-H loops of as-deposited(a) [Co(0.30)/Pt(0.27)]4 and (b) [Co(0.30)/Pt(0.27)]6 with out-of-plane and in-plane magnetic fields applied, respectively. Figure 2. (a) Schematic of p-SAF structure based on [Co/Pt] superlattice. The out-of-plane magnetic hysteresis loops for the as-deposited Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) samples of tRu = (b) 0.44 nm, (c) 0.72 nm, and (d) 1.03 nm, respectively. Figure 3. The Hex in as-deposited Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) (units are nm) SAF structure as a function of tRu thickness from 0.36 to 1.10 nm. Inset: the Jex in as-deposited Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) (units are nm) SAF structure samples as a function of tRu ranging from 0.36 to 1.10 nm. Figure 4. Cross section TEM image and SAED patterns of Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30)] 3/Pt (1). Figure 5. (a) The out-of-plane magnetic hysteresis loops for the Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt (1) samples at as-deposited and annealing temperature of 300℃ to 380℃. (b) The Hex for the Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) samples as a function of tRu ranking from 0.36–1.10 nm for as-deposited and annealed at various temperature (Ta) from 300℃ to 380℃.
10
Figure 1 11
12
Figure 2
13
Figure 3
14
Figure 4
15
Figure 5
16
Graphical abstract
17
Highlights
The maximum of Hex and Jex achieved at the 1st peak are about 13.2 kOe and 1.77 erg/cm2, respectively.
The Jex shows an oscillatory decay with a period of about 0.6 nm.
The Hex decreased rapidly to about 6.2 kOe with the increase of tRu from 0.44 nm at the 1st peak to about 1.03 nm at the 2nd peak.
The Hex was found to decrease with the increase of annealing temperature.
18
S0304-8853(18)32244-3 https://doi.org/10.1016/j.jmmm.2018.10.090 MAGMA 64501
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
17 July 2018 11 October 2018 18 October 2018
Please cite this article as: Y. Liu, J. Yu, H. Zhong, Strong antiferromagnetic interlayer exchange coupling in [Co/ Pt]6/Ru/[Co/Pt]4 structures with perpendicular magnetic anisotropy, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.10.090
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strong antiferromagnetic interlayer exchange coupling in [Co/Pt]6/Ru/[Co/Pt]4 structures with perpendicular magnetic anisotropy Yi Liua, *, Jiangang Yub, Huicai Zhongc a
School of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, PR China b School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, PR China c Integrated Circuit Advanced Process Center, Institute of Microelectronics, Chinese Academy Of Sciences, Beijing, 100029, PR China
*Corresponding author: Yi Liu E-mail: [email protected].
Abstract: The interlayer exchange coupling properties of perpendicular synthetic antiferromagnetic(p-SAF) structure were investigated in this work by varying the Ru spacer thickness (tRu). We observed an oscillatory decay of interlayer exchange coupling field (Hex) and coupling strength (Jex) with a period of about 0.6 nm. The greatest values of Hex and Jex are 13.2 kOe and 1.77 erg/cm2 at the 1st peak, respectively. The Hex decreases significantly to about 6.2 kOe with the increase of tRu from 0.44 nm at the 1st peak to 1.03 nm at the 2nd peak. The effect of post-annealing temperature on the Hex was also investigated. The Hex decreases with the increase of annealing temperature. Furthermore, after annealing at 380℃, the Jex value remains as high as 0.94 erg/cm2 at the 1st peak. Our results represent a candidate for spin-transfer torque magnetic random-access memory(STT-MRAM).
Key words: Synthetic antiferromagnetic; Interlayer exchange coupling; Perpendicular magnetic anisotropy; Oscillation.
1. Introduction Perpendicular Magnetic tunnel junctions(p-MTJs) based on MgO with ferromagnetic electrodes are great interest due to its potentialities for realizing 1
next-generation high-density non-volatile memory with high thermal stability and low critical current [1-3]. However, there are still some key issues need to be solved in p-MTJ. The most important one is the effect of stray field on the free layer, which may affect the storage stability of the device. The stray field can be effectively compensated by a synthetic antiferromagnetic (SAF) structure [4-6]. The SAF structure is consisted by two ferromagnetic (FM) layers and a nonmagnetic spacer layer. Actually, SAF coupling is a feature of the interlayer exchange coupling (IEC) eff ect [7,8], and the exchange bias field (Hex) of the SAF structure is related to the PMA strength of FM [9]. Cu, Ru, and Rh are the most common spacer materials in SAF structures [10-12]. Due to the strong perpendicular magnetic anisotropy (PMA) in [Co/Pd]n, [Co/Pt]n, and [Co/Ni]n multilayers [13-16], they are also the most popular ferromagnetic layer materials in SAF structures. The Hex is the shift of reference layer (RL) minor magnetic hysteresis loop in a SAF structure. The Jex value can be evaluated by Jex=Hext*Ms[17], where the Jex is the coupling strength, Ms and t are the saturation magnetization and thickness of RL, respectively. For Ru space layer SAF structures, Parkin et al.[18] demonstrated that the highest Hex occurred at the “first peak” of the oscillation. However, very few studies utilized the highest Hex at the first peak in perpendicular SAF (p-SAF) systems, because it reduces significantly with a slight variation of the Ru thickness[19, 20]. Experimentally, Gan et al.[21] obtained an oscillation curve of Hex with varying Ru thickness from 0.4–1.1 nm, the Hex had two peaks, and the first and second oscillation peaks occurred at about 0.45 nm and 0.9 nm, respectively. Chae et al.[22] also reported the dependence of Hex on the thickness of Ru, what is more, two oscillation peaks were found in their work. However, the Jex of 1st peak was weaker than that of the 2nd peak, which can be due to the roughness and intermixing at the interfaces of the ultrathin Ru layer. So the 1st peak has a weaker antiferromagnetic coupling (AFC) than the 2nd peak. For this reason, the 2nd peak of the SAF structure was always applied in most p-MTJs[23-25]. Recently, many research groups have obtained the oscillation curves of Hex dependence of Ru thickness [26, 27], and a higher Hex (~6 kOe) was achieved at the 1st oscillation peak with a Ru thickness of about 0.5 nm. Yakushiji et al.[28] reported a higher Hex and Jex of about 10 kOe and 2
2.2 erg/cm2, respectively. Yun et al.[29] reported a greater Hex and Jex of 12.6 kOe and 2.55 erg/cm2 in [Pt/Co]6/Ru/[Co/Pt]3 structures at the 1st oscillation peak, respectively. Recently, Yakushiji et al.[30] reported a higher Hex and Jex of 12 kOe and 2.6 erg/cm2, respectively. The Jex value is also the highest so for in a similar structure. Actually, both large PMA and atomically smooth surface/interface are required in FM layers to achieve a higher AFC strength. In previous work, we have prepared strong PMA FM layers based on Co/Pt superlattices [31]. In this work, based on the higher PMA Co/Pt superlattices, we report a higher Hex and Jex values of about 13.2 kOe and 1.77 erg/cm2, respectively, 13.2 kOe is also the highest Hex value so for in a similar SAF structure.
2. Experimental procedure Film deposition was performed by using an ULVAC Megnetron Sputtering on 4 inch SiOx substrates at room temperature. The work pressure was below 5×10 -7 Pa. All samples were post-annealed at various temperatures (Ta) of 300℃, 350℃, and 380℃ for 1 hour. The transmission electron microscopy (TEM) was used to analysis the total thickness and microstructure of the multilayer. The magnetic properties of the multilayers were measured by using the superconducting quantum interference device-vibration sample magnetometer (SQUID-VSM) and the Magneto-Optical Kerr effect mangnetometer. The saturation magnetizations (Ms) of the multilayers were calculated by including the total volume of Co and Pt sublayers
3. Results and Discussion In this work, we develop p-SAF multilayer films based on Co/Pt superlattices by using nearly monoatomic-layer alternation of Co and Pt. These stack structures are Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]4,
Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6
and
Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1 .0) (units are nm), where, tRu is ranged from 0.36 to 1.10 nm. These ultrathin films can be considered as an artificial superlattice rather than multilayer and they display high post-annealing stability. A systematic study of the dependence of Hex and Jex on the tRu ranging from 0.36 to 1.1 nm will be presented here. Since the strengths of both Jex and Hex in a SAF structure are strongly dependent on the appropriate deposition conditions. The thickness of Co/Pt and the number of 3
cycles effect on the PMA of [Co/Pt]n superlattices have been discussed in our previous
study
[31].
Fig.1
displays
the
M-H
loops
of
as-deposited
[Co(0.30)/Pt(0.27)]4 and [Co(0.30)/Pt(0.27)]6 superlattices under optimized sputtering conditions with magnetic fields applied in-plane or out-of-plane. The in-plane saturation field(Hs) values for [Co(0.30)/Pt(0.27)]4 and [Co(0.30)/Pt(0.27)]6 are about 39 kOe and 34 kOe, respectively, the corresponding Ms are 676 and 721 emu/cc, respectively. The effective PMA energy density (Keff) obtained by the area method for samples of [Co(0.30)/Pt(0.27)]4 and [Co(0.30)/Pt(0.27)]6 are about 1.32×107 and 1.23×107 erg/cc, respectively. So the high Hex values could be partly attributed to the higher Keff values in bottom and top multilayer of SAF structure. Fig.2(a)
is
a
schematic
of
the
stacked
structure:
Si/SiO2
substrate/Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0. 30)]3/Pt(1) (units are nm). Fig.2(b) to (d) show the perpendicular magnetic hysteresis loops for the as-deposited samples with tRu=0.44, 0.72, and 1.03 nm, respectively, where Hex denotes the exchange field. In Figs.2(b) and 2(d), with the magnetic field decreases from saturation, the magnetization reversal occurs before it reaches to zero, which indicates that the two ferromagnetic multilayers are AFC. At remanence state, the moments of the upper and lower multilayers are aligned antiparallel, and the net magnetic moments are equal to the difference of upper and lower multilayers moment. For tRu=0.72 nm sample, the Hex is about 0.56 kOe，which is also the lowest Hex value among all Hex. So the antiferromagnetic coupling in the tRu=0.72 nm sample is also the most weak one under the range of Ru thickness in this work. Fig.3 shows the dependence of Hex on the tRu. It is clear that Hex exhibits an oscillatory-like behavior although all samples show AFC in this work, and the curve exhibits two peaks: the 1st peak at tRu = 0.44 nm and a 2nd peak at tRu =1.03 nm. The value of the Hex at the 1st peak is about 13.2 kOe. The inset of Fig.3 shows the dependence of Jex on the tRu, the Jex also exhibits an oscillatory-like behavior. The value of the Jex at the 1st peak is about 1.77 erg/cm2. However, the values of both Hex and Jex reduce to about 6.2 kOe and 0.80 erg/cm2 at the 2nd peak. What is more, the Jex value at tRu= 0.44 nm is two times higher than that at tRu =1.03 nm. The curve shows an oscillatory behavior reminiscent to the Ruderman-Kittel-Kasuya-Yosida (RKKY) 4
with the varying thickness of Ru spacer, similar oscillatory-like behavior was obtained in other previous studies[18, 26, 32], and the Jex at the first peak has a much higher value than the second one. For the two peaks of Hex appearing in the RKKY coupling region, the strong hybridization happens at the Pt/Co interfaces and the density of state of Ru should be taken into account [32,33]. TEM was applied to investigate the total thickness and microstructure of Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30]3/Pt( 1.0). Fig.4 displays the TEM cross sectional image and selected area electron diffraction (SAED) patterns. The interfaces of Ta/Pt and Pt/bottom Co/Pt superlattice are clear. The higher Jex at the 1st peak is due to the flat interfaces at atomic scale flatness. Moreover, the bottom and top Co/Pt superlattice layers are relatively flat. The obtained d-spacing of (11-1)(d11-1) in the 3 nm Pt is 0.230 nm, and the d11-1 in the bottom and top Co/Pt superlattice are 0.219 and 0.224 nm, respectively. The d111 in the top superlattice is larger than that of bottom one, which means the strain in bottom superlattice is larger than top one. So the strong PMA in the top superlattice could be partly due to the increase of magnetoelastic anisotropy, which comes from the increase of strain[34]. Since it is important to study the temperature dependence on the coupling strength, the post-annealing stability of SAF structure has also be studied here. Fig.5(a) shows the perpendicular hysteresis loops of as-deposited and after annealing at 300℃ 350℃,
and
380℃
for
the
sample
of
[Co(0.30/Pt0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1.0). The Hex annealed at 300℃, 350℃, and 380℃ are 10 kOe, 9 kOe, and 7 kOe, respectively. Fig.5(b) presents the Hex of as-deposited and post-annealed samples changing with tRu thickness. The interdiff usion of Co/Ru/Co interfaces may result in the degradation of Jex and Hex values, so the values of Hex decreases with the increase of annealing temperature. For a certain annealing temperature, the strength of Hex at the 2nd peak is about half of the 1 st one. What’s more, the decreased rate of Hex value at 1st peak is more than the 2nd peak. A large value of Jex of 0.94erg/cm2 was obtained even after annealed at 380 C for one hour at 1st peak, which is greater than the Jex criteria (>0.70 erg/cm2) to ensure the read/write failure in p-STT-MRAM-cell array[35]. 5
4. Conclusion In summary, the effect of the Ru thickness and annealing temperature on the Hex and Jex in the SAF structure based on Co/Pt ultrathin superlattices have been investigated. The Jex shows an oscillatory decay with a period of about 0.6 nm, the greatest values of Hex and Jex are about 13.2 kOe and 1.77 erg/cm2 at the 1st peak, respectively. The larger Jex values could be attributed to the higher Keff values in bottom and top suerlattice and the atomic scale flatness of the SAF structure. A higher Jex value of 0.94erg/cm2 was also obtained at Ta=380℃. Our results represent a candidate for STT-MRAM.
Acknowledgments This work was supported by National fundamental research Program of China (973 program) (Grantno. 2011CB921804), the funding of the doctorate in Chongqing University of Posts and Telecommunications (Grantno. A2017-120).
6
References [1] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.H. Yang, Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers, Nat.Mater. 3(2004)862-867. [2] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H.D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, H. Ohno, A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction, Nat.Mater. 9(9) (2010)721-724. [3] H. Meng, J.P. Wang, Spin transfer in nanomagnetic devices with perpendicular anisotropy, Appl.Phys.Lett. 88(2006)172506. [4] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions, Nat.Mater. 3(12)(2004)868-871. [5] D.D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, K. Ando, 230% room-temperature magnetoresistance in CoFeB∕MgO∕CoFeB magnetic tunnel junctions, Appl.Phys.Lett. 86(2005)092502. [6] S. Yuasa, D.D. Djayaprawira, Giant tunnel magnetoresistance in magnetic tunnel junctions
with
a
crystalline
MgO
(001)
barrier,
J.Phys.D:Appl.Phys.
40(2)(2007)R337-R354. [7] P. Bruno, C. Chappert, Oscillatory coupling between ferromagnetic layers separated by a nonmagnetic metal spacer, Phys.Rev.Lett. 67(12)(1991)1602-1605. [8] P. Bruno, Theory of interlayer magnetic coupling, Phys.Rev.B 52(1)(1995) 411-439. [9] Y. Xiao, S. Chen, Z. Zhang, B. Ma, Q.Y. Jin, Magnetization reversal in antiferromagnetically
coupled
[Pt/CoFeB] N1/Ru/[CoFeB/Pt]N2
structures
with
perpendicular anisotropy, J.Appl.Phys. 113(2013)17A325. [10] M. Kowalewski, B. Heinrich, J.F. Cochran, P. Schurer, Studies of interlayer exchange
coupling
in
Fe/Cu/Fe
ultrathin
heterostructures,
J.Appl.Phys.
81(1998)3904-3906. [11] S. Zoll, A. Dinia, M. Gester, D. Stoeffier, H.A.M.V.D. Berg, A. Herr, R. Poinsot, H. Rakoto, Giant antiferromagnetic exchange coupling in ultrahigh-vacuum grown (111) Co/Rh sandwiches, J. Magn.Magn.Mater. 165(1)(1997)442-445. 7
[12] N. Wiese, T. Dimopoulos, M. Rührig, J. Wecker, H. Brückl, G. Reiss, Antiferromagnetically
coupled
CoFeB∕Ru∕CoFeB
trilayers,
Appl.Phys.Lett.
85(2004)2020-2022. [13] J.H. Park, C. Park, T. Jeong, M.T. Moneck, N.T. Nufer, J.G. Zhu, Co/Pt multilayer based magnetic tunnel junctions using perpendicular magnetic anisotropy, J.Appl.Phys. 103(2008)07A917. [14] M.T. Rahman, A. Lyle, G. Hu, W.J. Gallagher, J.P. Wang, High temperature annealing stability of magnetic properties in MgO-based perpendicular magnetic tunnel junction stacks with CoFeB polarizing layer, J.Appl.Phys. 109(2011)07C709. [15] L.E. Nistor, B. Rodmacq, S. Auffret, B. Dieny, Pt/Co/oxide and oxide/Co/Pt electrodes
for
perpendicular
magnetic
tunnel
junctions,
Appl.Phys.Lett.
94(2009)012512. [16] H. Kurt, M. Venkatesan, J.M.D. Coey, Enhanced perpendicular magnetic anisotropy in Co/Ni multilayers with a thin seed layer, J.Appl.Phys. 108(2010) 073916. [17] W. Folkerts, Calculated magnetic phase diagrams and magnetoresistance curves for an antiferromagnetically coupled multilayer system. J.Magn.Magn.Mater. 94(1991)302-310. [18] S.S.P. Parkin, Systematic variation of the strength and oscillation period of indirect magnetic exchange coupling through the 3d, 4d, and 5d transition metals, Phys.Rev.Lett. 67(25)(1991)3598-3601. [19] K. Ounadjela, D. Muller, A. Dinia, A. Arbaoui, P. Panissod, G. Suran, Perpendicular anisotropy and antiferromagnetic coupling in Co/Ru strained superlattices, Phys.Rev.B 45(14)(1992)7768. [20] H. Sato, S. Ikeda, S. Fukami, H. Honjo, S. Ishikawa, M. Yamanouchi, K. Mizunuma, F. Matsukura, H. Ohno, Co/Pt multilayer based reference layers in magnetic tunnel junctions for nonvolatile spintronics VLSIs, Jpn.J.Appl.Phys. 53(4S)(2014)04EM02. [21] H. Gan, R. Malmhall, Z. Wang, B.K. Yen, J. Zhang, X. Wang, Y. Zhou, X. Hao, D. Jung, K. Satoh, Y. Huai, Perpendicular magnetic tunnel junction with thin CoFeB/Ta/Co/Pd/Co reference layer, Appl.Phys.Lett. 105(2014)192403. 8
[22] K.S. Chae, T.H. Shim, J.G. Park, Dependency of anti-ferro-magnetic coupling strength on Ru spacer thickness of [Co/Pd] n-synthetic-anti-ferro-magnetic layer in perpendicular magnetic-tunnel-junctions fabricated on 12-inch TiN electrode wafer, J.Appl.Phys. 116(2014)033904. [23] K. Yakushiji, A. Fukushima, H. Kubota, M. Konoto, S. Yuasa, Ultralow-voltage spin-transfer switching in perpendicularly magnetized magnetic tunnel junctions with synthetic antiferromagnetic reference layer, Appl. Phys.Express 6(2013)113006. [24] G. Hu, T. Topuria, P.M. Rice, J.S. Jean, D.C. Worledge, Optimization of tunneling magnetoresistance in perpendicular magnetic tunnel junctions with Co|Pd reference layers, IEEE Magn.Lett. 4(4)(2013)3000104. [25] G.M. Choi, I.J. Shin, B.C. Min, K.H. Shin, Perpendicular magnetic tunnel junctions with synthetic ferrimagnetic pinned layer, J.Appl.Phys. 108(2010)073913. [26] S. Chen, Y. Xiao, W.H. Xie, Z. Zhang, B. Ma, Q.Y. Jin, Interfacial effects on magnetic interlayer coupling between perpendicular Co/Ni multilayers across Ru spacer. Appl.Phys.Lett. 105(2014)262403. [27] J.H. Jung, S.H. Lim, S.R. Lee, Interlayer exchange coupling between [Pd/Co] multilayers and CoFeB/MgO layers with perpendicular magnetic anisotropy, Appl.Phys.Lett. 101(2012)242403. [28] K. Yakushiji, H. Kubota, A. Fukushima, S. Yuasa, Perpendicular magnetic tunnel junctions with strong antiferromagnetic interlayer exchange coupling at first oscillation peak, Appl.Phys.Express 8(2015)083003. [29] S.J. Yun, H.L. Sang, S.R. Lee, Strong interlayer exchange coupling and high post-annealing stability in perpendicularly magnetized [Pt/Co]/Ru/[Co/Pt] structures, Aip Adv. 6(2)(2016)3000204. [30] K. Yakushiji, A. Sugihara, A. Fukushima, H. Kubota, S. Yuasa, Very strong antiferromagnetic interlayer exchange coupling with iridium spacer layer for perpendicular magnetic tunnel junctions, Appl.Phys.Lett. 110(2017)092406. [31] Y. Liu, J. Qiu, S. Ter Lim, S.L. Toh, Z. Zhu, G. Han, Strong perpendicular magnetic
anisotropy
in
[Co/Pt]n
ultrathin
superlattices. A
Appl.Phys.Express, 10(2017)013005. [32] J. Zhao, Y.J. Wang, Y.Z. Liu, X.F. Han, Z. Zhang, Perpendicular anisotropy 9
dependence of oscillatory interlayer coupling behavior in [Pt/Co]5/Ru/[Co/Pt]5 multilayers, J.Appl.Phys.104(2008)023911. [33] F.J. Himpsel, K. Christmann, P. Heimann, D.E. Eastman, Experimental energy-band
dispersions
and
lifetimes
for
ruthenium,
Phys.Rev.B
23(23)(1981)2548-2552. [34] S. Mohanan, U. Herr, Optimization of magnetic properties of Co/Pd multilayers by applying a large persistent biaxial stress. J.Appl.Phys. 102(2007)093903. [35] Samsung Global MRAM Innovation Program, see http://www.samsung. com/global/business/semiconductor/news-events/mram (2013).
Figure Captions Figure 1. The M-H loops of as-deposited(a) [Co(0.30)/Pt(0.27)]4 and (b) [Co(0.30)/Pt(0.27)]6 with out-of-plane and in-plane magnetic fields applied, respectively. Figure 2. (a) Schematic of p-SAF structure based on [Co/Pt] superlattice. The out-of-plane magnetic hysteresis loops for the as-deposited Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) samples of tRu = (b) 0.44 nm, (c) 0.72 nm, and (d) 1.03 nm, respectively. Figure 3. The Hex in as-deposited Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) (units are nm) SAF structure as a function of tRu thickness from 0.36 to 1.10 nm. Inset: the Jex in as-deposited Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) (units are nm) SAF structure samples as a function of tRu ranging from 0.36 to 1.10 nm. Figure 4. Cross section TEM image and SAED patterns of Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30)] 3/Pt (1). Figure 5. (a) The out-of-plane magnetic hysteresis loops for the Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(0.44)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt (1) samples at as-deposited and annealing temperature of 300℃ to 380℃. (b) The Hex for the Ta(3.2)/Pt(3)/[Co(0.30)/Pt(0.27)]6/Co(0.43)/Ru(tRu)/Co(0.43)[Pt(0.27)/Co(0.30)]3/Pt(1) samples as a function of tRu ranking from 0.36–1.10 nm for as-deposited and annealed at various temperature (Ta) from 300℃ to 380℃.
10
Figure 1 11
12
Figure 2
13
Figure 3
14
Figure 4
15
Figure 5
16
Graphical abstract
17
Highlights
The maximum of Hex and Jex achieved at the 1st peak are about 13.2 kOe and 1.77 erg/cm2, respectively.
The Jex shows an oscillatory decay with a period of about 0.6 nm.
The Hex decreased rapidly to about 6.2 kOe with the increase of tRu from 0.44 nm at the 1st peak to about 1.03 nm at the 2nd peak.
The Hex was found to decrease with the increase of annealing temperature.
18