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参考文献

1 Introduction▲
2 Experimental▲
3 Results and discussion▲
4 Conclusion▲
图表▲

Rare Metals2016年第10期

Enhancement of post-annealing stability in Co/Ni multilayers with perpendicular magnetic anisotropy by Au insertion layers

Yi Cao Ming-Hua Li Kang Yang Xi Chen Guang Yang Qian-Qian Liu Guang-Hua Yu

School of Materials Science and Engineering,University of Science and Technology Beijing

收稿日期：16 April 2016

基金：financially supported by the National Natural Science Foundation of China(Nos.51101012, 51271211,51331002,51371025,51371027,51471028 and 51571017);the National Key Scientific Research Projects of China(No. 2015CB921502);the Beijing Nova Program(No.Z141103001814039);the Fundamental Research Funds for the Central Universities(No. FRF-TP-14-002C1);

Enhancement of post-annealing stability in Co/Ni multilayers with perpendicular magnetic anisotropy by Au insertion layers

Yi Cao Ming-Hua Li Kang Yang Xi Chen Guang Yang Qian-Qian Liu Guang-Hua Yu

School of Materials Science and Engineering,University of Science and Technology Beijing

Abstract：

Enhancement of post-annealing stability in Co/Ni multilayers with perpendicular magnetic anisotropy(PMA) was obtained by inserting Au layers into Ni/Co interfaces.After annealing at 350℃,the effective magnetic anisotropy density(K_eff) for Ta(3)/Pt(2)/[Co(0.3)/Ni(0.6)/Au(0.3)]×₃/Co(0.3)/Pt(l)/Ta(3)(in nm) keeps at0.48 ×10⁵ J·m^-3.Scanning transmission electron microscopy-high-angle annular dark field(STEM-HAADF)analysis shows that the diffusion between Ni and Co layers is obstructed by the Au insertion layers among them,which is responsible for the post-annealing stability enhancement of the multilayers.Multilayers with Pt insertion layers were also investigated as reference samples in this work.Compared with Pt-layer-inserted Co/Ni multilayers,the Au insertion layers are found to bring seldom interfacial PMA to the multilayers,making it competitive in being employed to enhance the post-annealing stability of PMA Co/Ni multilayers which are used for magnetic random access memory devices(MRAM).

Keyword：

Co/Ni multilayers; Perpendicular magnetic anisotropy; Post-annealing stability; Au insertion layers;

Author： Ming-Hua Li,e-mail:mhli@ustb.edu.cn.;

Received： 16 April 2016

1 Introduction

Being fast and non-volatile,magnetic random access memory device (MRAM) consisting magnetic tunnel junctions (MTJs) has been considered as one of the most promising candidates for the next-generation memory technology [ 1] .Usually,the MTJs are sandwich structures with two perpendicular magnetic anisotropy (PMA) ferromagnetic layers and one insulating barrel layer between them.As to the materials,MgO is preferred for the middle insulating barrel layer nowadays [ 2, 3] .Meanwhile,the choice for the two PMA ferromagnetic layers is various,including Co,Fe,CoFeB,Heusler alloy,Co/Pt multilayers,Co/Pd multilayers,and Co/Ni multilayers [ 4, 5, 6, 7, 8, 9, 10, 11, 12] .Among these competitors,Co/Ni multilayers attract special interest from the scientific as well as technological community for the following two advantages.Firstly,the Gilbert damping constant of Co/Ni multilayers can be as low as 0.01 [ 13] .This makes it especially promising for use in the spin transfer torque magnetic random access memories (STT-MRAM),an energy-efficient solution for modern MRAM devices [ 14, 15, 16, 17] .Secondly,with a relatively larger magnetostrictive coefficient,its magnetocrystalline anisotropy(MCA) can be easily altered and thereby its PMA can be effectively controlled by applying strain [ 18] .This helps it become the preferred material for strain-as sis ted magnetization reversal technology,an advanced way to reduce the energy consumption of a MR AM [ 19] .Despite these advantages,however,the poor post-annealing stability of PMA Co/Ni multilayers has already become a real limitation for its MRAM applications [ 20] .Generally,the fabrication process of MR AM devices requires annealing at above 350℃in order to improve the crystallization of the MgO barrel layer so that a large tunnel magnetoresistance(TMR) value can be obtained [ 21] .But during the annealing process,Co and the Ni layers are likely to diffuse into each other [ 22] .Then the annealed Co/Ni multilayers would no longer be a PMA magnetic material and therefore could not be used in a MRAM.Hence,the key issue comes to how to enhance the post-annealing stability of Co/Ni multilayers,or in other words,how to maintain the PMA of the Co/Ni multilayers after annealing at 350℃(Table 1).

In the most recent work,large enhancement of postannealing stability in PMA Co/Ni multilayers was realized by inserting Pt layers into the Ni/Co interfaces [ 23] .On one side,the diffusion between Ni and Co layers was physically obstructed by the Pt interlayers.On the other side,the PMA increased because an additional interfacial PMA was brought by the introduced Co/Pt interfaces.Although these enabled Co/Ni multilayers to keep its PMA after annealing process,the introduction of Pt could diminish the aforementioned two inherent advantages at the same time.Particularly,the strong Pt-induced spin-orbit interactions(SOI) would increase the damping factor of the system [ 24] .Moreover,the contribution of MCA to the total PMA of the sample would be cut down by the additional interfacial PMA,which could decrease the effectiveness of strain-mediated PMA control.

Nevertheless,the method of inserting interlayers is still applicable.In this research,another noble metal Au,with reliable stability under high temperature and insignificant SOI (hence insignificant contribution to the damping factor),was adopted as the interlayers and inserted into Co/Ni multilayers [ 25] .The observed effective magnetic anisotropy density (K_eff)of 0.48×10⁵ J·m^-3 after annealing at350℃as well as its comparison with Pt insertion layers shows a significant enhancement of the post-annealing stability in PMA Co/Ni multilayers without bringing additional interfacial PMA,making this system competitive for using in a MRAM.

2 Experimental

The multilayers used in this study had the following four structures:Ta(3)/Pt(2)/[Co(0.3)/Ni(0.6)/Au(t_Au)]×₃/Co(0.3)/Pt(1)/Ta(3)(Sample 1);Ta(3)/Pt(2)/[Co(0.3)/Ni(0.6)/Pt(t_Pt)]×₃/Co(0.3)/Pt(1)/Ta(3)(Sample 2);Ta(3)/Pt(2)/[Co(0.3)/Au(t_Au)/Ni(0.6)]×₃/Co(0.3)/Pt(1)/Ta(3)(Sample 3);Ta(3)/Pt(2)/[Co(0.3)/Pt(t_Pt)/Ni(0.6)]×₃/Co(0.3)/Pt(1)/Ta(3)(Sample 4).The numbers in the parentheses are nominal layer thickness in nm,and t is the thickness of the insertion layers.When t_Au (or t_Pt)=0 nm,the sample turns into a pure Co/Ni multilayers without insertion layers,i.e.,a sample with the structure of Ta(3)/Pt(2)/[Co(0.3)/Ni(0.6)]×₃/Co(0.3)/Pt(1)/Ta(3).Despite their complicated forms,the core function parts of the structures of the four samples are the Co/Ni three period layers within the square bracket (Fig.1),while the Ta(3)/Pt(2) and Pt(1)/Ta(3) are the buffer and capping layers,respectively.All samples were deposited on glass substrates by direct current (DC)magnetron sputtering.The base pressure of the sputtering system was approximately 2×10^-5 Pa,and the argon working pressure was 0.27 Pa.Each element was sputtered for 2000 s,and then the thicknesses of the films were measured,the deposition rates for Ta,Pt,Co,Ni,and Au were obtained as 0.105,0.075,0.027,0.027,and0.079 nm·s^-1,respectively.The nominal thickness of the inpidual layers was controlled by varying the deposition time.Annealing treatment at 350℃was performed in a high vacuum furnace for 30 min in the absence of an external magnetic field.Magnetization hysteresis (M-H) loops were measured using the vibrating sample magnetometer (VSM) option of a physical property measurement system (PPMS-9).The value of the magnetization(M) was obtained by piding the magnetic moments (m) by the total volume of the ferromagnetic layers (not including the nonmagnetic layers).The effective magnetic anisotropy density (K_eff) was determined from the integration area difference between the in-plane and the out-of-plane M-H loops when the magnetic field (H)>0.Besides,the highangle annular dark field (HAADF) image was taken with a scanning transmission electron microscopy (STEM)attached on a high-resolution transmission electron microscopy (HRTEM,Tecnai F20).

3 Results and discussion

M-H loops of Sample 1 (containing Au insertion layers at Ni/Co interfaces) with t_Au=0.15,0.30,and 0.60 nm after annealing at 350℃are shown in Fig.2a-f.The out-ofplane M-H loops exhibit a square profile with a 100%remanence ratio,while the in-plane M-H loops exhibit hard-axis behavior with no hysteresis,indicating wellestablished PMA in the Co/Ni multilayers with Au insertion layers after annealing at 350℃.However,the out-ofplane M-H loops of pure Co/Ni multilayers after annealing at 350℃in Fig.2g exhibit no square profile,but instead a much slimmer hysteresis shape;correspondingly,its inplane M-H loops in Fig.2h also exhibit no hard-axis behaviors,with a much higher sloop around the zero magnetic field.This indicates a fractional PMA in the pure Co/Ni multilayers after annealing at 350℃.To make it clear,the evolution of K_eff value as a function of t_Au for Sample 1 after annealing at 350℃is shown in Fig.2i.The K_eff value of Sample 1 with t_Au=0 nm,i.e.,pure Co/Ni multilayers is nearly zero.Meanwhile,the K_eff value of Sample 1 with t_Au=0.15,0.30 and 0.60 nm are0.32×10⁵,0.48×10⁵ and 0.46×10⁵ J.m^-3,respectively.Namely,large enhancement of PMA in Co/Nimultilayers after annealing at 350℃is obtained,or in other words,large enhancement of post-annealing stability in Co/Ni multilayers with PM A is obtained by inserting Au layers into their Ni/Co interfaces.

下载原图

Table 1 Comparison of mechanisms between Au and Pt insertion layers to enhance post-annealing stability in Co/Ni multilayers with PMA

Fig.1 Schematic diagrams of core function structures of a Sample 1,b Sample 2,c Sample 3 and d Sample 4

To reveal the underlying reason for this result,it is necessary to analyze the microstructure information of the sample.Hence,the HRTEM-STEM technology was employed to take an HAADF image of the sample after annealing at 350℃(Fig.3),on which the different ultrathin layers can be well distinguished by their contrasts [ 26] .Ta,Pt,and Au with higher atomic numbers are brighter than the lighter elements of Co and Ni in a HAADF image.Thus,the bright thin stripes in the middle of the sample from Fig.3 should be the Au insertion layers,while the dark thin stripes around them are Co/Ni layers.This clear distribution of Au and Co/Ni layers shows a perfect layered structure within the sample even after annealing at 350℃,which in turn proves the effectiveness of inserting an Au layer for obstructing the diffusion between its adjacent Ni and Co layers on each side during the annealing process.This microstructural result is identical with that of previous work in which Pt was used as the insertion layer [ 23] .

As mentioned in the introduction section,Au insertion layers are expected not to bring the additional interfacial PM A that Pt insertion layers do.Thus,the comparison between Au and Pt insertion layers about their contributions to interfacial PMA is essential to this study.In Fig.4a,the evolutions of K_eff value as a function of insertion layer thicknesses (t,particularly t_Au or t_Pt) for Sample 1 (with Au insertion layers at Ni/Co interfaces) and Sample 2 (with Pt insertion layers at Ni/Co interfaces) in the as-deposited state are shown in Fig.4a.The K_eff value of Sample 1 decreases from 1.39×10⁵ J·m^-3 at t_Au=0 nm to 0.52×10⁵ J·m^-3 at t_Au=1.00 nm in an oscillating way;conversely,the K_eff value of Sample 2increases monotonically to 3.5×10⁵ J·m^-3 as t_Pt increases to 1.00 nm,which should be attributed to an additional interfacial PMA brought by the Pt/Co interfaces [ 23] .Here,it can be seen that the Au insertion layers,on the other side,have not shown any contribution to the PMA of Sample 1,indicating no additional interfacial PMA at Au/Co interfaces.Additionally,the evolutions of the K_eff values as a function of t for Sample 3 (with Au insertion layers at Co/Ni interfaces) and Sample 4 (with Pt insertion layers at Co/Ni interfaces) in the as-deposited state are shown in Fig.4b.Similar to Sample 1,both Samples 3 and 4 show an oscillating decrease of K_eff value to below 0.5×10⁵J·m^-3 as t increases to 1.00 nm,indicating no additional interfacial PMA at the Co/Au and the Co/Pt interfaces,either.According to Refs. [ 27, 28, 29] ,Pt provides additional interfacial PMA only when it is beneath the magnetic layer.Hence,the results in Fig.4b further demonstrate the above discussions about Fig.4a that Pt insertion layers bring additional interfacial PMA to Co/Ni multilayers (Sample2),while Au insertion layers do not.

Fig.2 Out-of-plane (⊥) and in-plane (∥) M-H loops of Sample 1 annealed at 350℃with different Au thicknesses:a,b t_Au=0.15 nm,c,d t_Au=0.30 nm,e,f t_Au=0.60 nm,and g,h t_Au=0 nm;and i K_eff plot as a function of t_Au

Fig.3 HAADF image of Sample 1 annealed at 350℃with t_Au=0.60 nm.All numbers in parentheses being nominal layer thickness in nm

4 Conclusion

In summary,post-annealing stability of Co/Ni multilayers with perpendicular magnetic anisotropy is enhanced by inserting Au layers into the Ni/Co interfaces.This is ascribed to the obstructing effect of Au against the diffusion between its adjacent Ni and Co layers during the annealing process.Unlike Pt insertion layers,the inserting of Au layers brings no additional interfacial PMA to the system,making itself a competitive method to enhance the post-annealing stability of PMA Co/Ni multilayers that are used for a MRAM.