⇦Chinese Physics Letters, 2016, Vol. 33, No. 10, Article code 107804⇨ Effects of MgO Thickness and Roughness on Perpendicular Magnetic Anisotropy in MgO/CoFeB/Ta Multilayers * Yi Liu(刘毅)1, Tao Yu(于涛)1, Zheng-Yong Zhu(朱正勇)2, Hui-Cai Zhong(钟汇才)3, Kai-Gui Zhu(朱开贵)1,4** Affiliations 1School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191 2Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029 3Integrated Circuit Advanced Process Center, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029 4Key Laboratory of Micro-nano Measurement-Manipulation and Physics (Ministry of Education), Beihang University, Beijing 100191 Received 15 July 2016 ^*Supported by the National Basic Research Program of China under Grant No 2011CB921804, and the Beijing Key Subject Foundation of Condensed Matter Physics under Grant No 0114023.
^**Corresponding author. Email: kgzhu@buaa.edu.cn Citation Text: Liu Y, Yu T, Zhu Z Y, Zhong H C and Zhu K G 2016 Chin. Phys. Lett. 33 107804 Abstract The dependence of perpendicular magnetic anisotropy (PMA) on the barrier layer MgO thickness in MgO/CoFeB /Ta multilayers is investigated. The results show that the strongest PMA occurs in a small window of about 2–4 nm with the increase of MgO thickness from 1–10 nm. The crystalline degree of MgO and the change of interatomic distance along the out-of-plane direction may be the main reasons for the change of PMA in these multilayers. Moreover, the roughnesses of 2- and 4-nm-thick MgO samples are 3.163 and 1.8 nm, respectively, and both the samples show PMA. These results could be used to tune the magnetic characteristic of the ultra thin CoFeB film for future applications in perpendicular magnetic devices. DOI:10.1088/0256-307X/33/10/107804 PACS:78.67.Pt, 75.70.Cn, 75.70.Rf, 75.60.Nt © 2016 Chinese Physics Society Article Text Interfacial perpendicular magnetic anisotropy (PMA) of CoFeB/MgO structure is of great interest since it can be used as a perpendicular magnetic electrode of high-density magnetic random access memories (MRAMs) with low critical current density of magnetization switching, high thermal stability factor $E/k_{_{\rm B}}T$ (where $E$ denotes energy barrier, $k_{_{\rm B}}$ is the Boltzmann constant, and $T$ is the absolute temperature) and high tunnel magnetoresistance (TMR) ratio compared with in-plane magnetic tunnel junctions (MTJs).[1-4] Among various structures, a strong PMA in an MTJ of CoFeB/MgO/CoFeB has been reported recently when the thickness of the CoFeB layer reduces below 1.5 nm.[5-7] These CoFeB/MgO p-MTJs have a high potential in meeting major requirements for integrating MTJs with CMOS.[8] Recently, different electrode materials and underlayers were tried out to improve the PMA in MTJs, including Mo, Al, Hf, Nb, Ru, Ti, Pt, Cu, Pd, etc.[9-16] Moreover, varying thicknesses of the CoFeB layer and effects of electrode materials on the PMA in MTJs of these electrode materials have also been investigated. The results show that the MTJs could maintain PMA when the thickness of CoFeB is within the range of 1–1.5 nm. However, there are few reports about the thickness of the MgO barrier layer affect on the PMA of MTJs. In addition, recent theoretical works also suggest that the origin of PMA at Fe(Co)/MgO interface is attributed to the overlap between O-2$p$ and transition metal 3$d$ orbitals.[17,18] Actually, the MgO/CoFeB interface and crystalline degree of MgO layer may affect the PMA in these MTJs, and such thickness dependence was less reported. In this work, the interfacial magnetic anisotropy of a series of MgO/CoFeB/Ta multilayers by varying the MgO thickness under a wide scope is deposited directly on a thermally oxidized Si substrate. Here we just investigate the effect of MgO thickness and roughness on the PMA in MgO/CoFeB/Ta multilayers, thus these multilayers just grew on the thermally oxidized Si substrate but not a virtual bottom ferromagnetic electrode. Multilayers of Ta(5)MgO(2)/Co$_{20}$Fe$_{60}$B$_{20 t}$ ($0.9\leq t\leq1.5$)/Ta(2) and MgO$_{t}$($t=1$, 1.5, 2, 4 and 10)/Co$_{20}$Fe$_{60}$B$_{20}$ (1.1)/Ta(2) (in nm) were grown on a thermally oxidized Si (100) substrate. The base pressure of the sputtering system was better than $4\times10^{-5}$ Pa and a working argon pressure was 0.5 Pa. Ta and CoFeB were deposited by dc sputtering, and MgO was deposited by rf sputtering. High purity of Ta (99.95%), MgO (99.99%), and Co$_{20}$Fe$_{60}$B$_{20}$ (99.9%) were used as the target materials. The annealing treatment was carried out in a vacuum furnace (vacuum better than $5\times10^{4}$ Pa) for 1 h at $T_{\rm a}=300^{\circ}\!$C without any external magnetic field. The in-plane and perpendicular magnetization curves ($M$–$H$ curves) of thin films were measured with alternating gradient magnetometer (AGM) at room temperature, and the surface characterization of varying the MgO film deposited on a thermally oxidized Si (100) substrate annealing at $T_{\rm a}=300^{\circ}\!$C from 0–10 nm is performed on a HITACHI S-4800 scanning electron microscope (SEM) and a WET-SPM-9700 environment controlled scanning probe microscope (ECSPM), respectively. To clarify the effect of CoFeB thickness on PMA and to obtain the strongest PMA in these top MTJs structures, magnetic properties of the Ta/MgO/CoFeB$_{t}$/Ta multilayers were studied. The representative $M$–$H$ curves of Ta(5 nm)/MgO(2 nm)CoFeB($t_{\rm CoFeB}$)/Ta(2 nm) samples annealing at $T_{\rm a}=300^{\circ}\!$C with external fields applied at perpendicular and in-plane directions are shown in Fig. 1. The samples show clearly PMA when the thicknesses of CoFeB are 0.9 and 1.1 nm, the perpendicular $M$–$H$ curves of these two samples show well squared shapes with squareness of 0.8 and 1, respectively. However, the PMA deteriorates obviously when the CoFeB thickness increases to about 1.3 nm, and the multilayers start to show in-plane magnetic anisotropy. Figure 1 also shows the $M$–$H$ curves of Ta/MgO/CoFeB(1.5 nm)/Ta multilayers at perpendicular and in-plane directions, the sample displays strong in-plane magnetic anisotropy, and the saturation field in the perpendicular direction is about 2000 Oe. We can estimate that the volume saturation magnetization is about 1418 emu/cm$^{3}$ from the slope of $M/A$ versus $t$. In the following we study the MgO thickness and roughness effect on the PMA in MgO/CoFeB/Ta multilayers. Here the thicknesses of CoFeB and Ta are fixed at 1.1 and 2 nm in these multilayers, respectively.

Fig. 1. Representative $M$–$H$ curves of samples Ta(5 nm)/MgO(2 nm)CoFeB($t_{\rm CoFeB}$)/Ta(2 nm) with external field applied at perpendicular (a) and in-plane (b) directions.

Figures 2(a)–2(e) show the $M$–$H$ curves under in-plane and perpendicular magnetic fields for the multilayers of MgO$_{t}$/CoFeB/Ta by varying MgO thicknesses from 1–10 nm after annealing at $T_{\rm a}=300^{\circ}\!$C, respectively. The samples show strong in-plane magnetic anisotropy when the thicknesses of MgO are 1 and 1.5 nm. With the MgO thickness increases to 2 nm, the magnetic easy axis of the multilayer turns from in-plane to weak perpendicular. When the MgO thickness is 4 nm, the strength of PMA also increases dramatically, the in-plane direction saturation field increases from about 800 to 1400 Oe compared with the sample containing 2-nm-thick MgO, which also illustrates that the PMA is strongly depending on the MgO thickness. However, the multilayer shows obvious in-plane anisotropy when the MgO thickness is 10 nm. The magnetic anisotropy turns from in-plane to out-of-plane at the beginning and then turns back to in-plane, which could be due to the onset of crystalline MgO forming with the increase of MgO thickness. When the thickness of MgO is thinner, it is amorphous. However, with the increase of MgO thickness, it starts to be partially polycrystalline, such a PMA mechanism can qualitatively explain the recent experimental result on the exhibit significant reduction of PMA for the CoFeB deposited on the MgO(100) single crystal substrate.[19] The interatomic distance along the out-of-plane direction may increase with the partially polycrystalline of MgO film, which results in the reduction of PMA strength.[20] In the following we study the effective magnetic anisotropy energy density ($K_{\rm eff}$) in the MgO/CoFeB/Ta multilayer with and without the Ta buffer layer.

Fig. 2. Representative $M$–$H$ curves with external field applied at perpendicular direction and in-plane for MgO$_{t}$/CoFeB/Ta with MgO thicknesses of (a) 1 nm, (b) 1.5 nm, (c) 2 nm (d) 4 nm, and (e) 10 nm, respectively.

In this work, the values of area magnetization for varying MgO thickness samples are almost a constant ($\sim$1.4 $\times$ 10$^{-4}$ emu/cm$^{2}$). If we ignore the thickness of so-called magnetic dead layer (MDL), which is generally produced in the interface during deposition or intermixing upon annealing,[21,22] we can obtain that the magnetization $M_{\rm s}$ is about 1272 emu/cm$^{3}$, which is in agreement with the previous results for the MgO/CoFeB/Ta thin films.[23] Here $K_{\rm eff}$ is used to characterize the orientation of magnetization in a magnetic thin film, which is calculated from the enclosed area between the perpendicular and in-plane magnetization curves, and $K_{\rm eff}$ can be given by[24] $$\begin{align} K_{\rm eff} =\int_0^{M_{\rm s} } {(H_{//} -H_\bot )} dM,~~ \tag {1} \end{align} $$ where $M_{\rm s}$, $H_{//}$, and $H_\bot$ are the saturation magnetization and the in-plane and out-of-plane magnetic fields, respectively, and $K_{\rm eff}$ also includes higher order contributions of uniaxial anisotropy. We can see from Eq. (1) that PMA can be observed when $K_{\rm eff}>0$. Experimentally, if a perpendicular or in-plane easy axis of magnetization is strong enough, Eq. (1) can be calculated as follows: $$\begin{align} K_{\rm eff} =\frac{H_{\rm k} M_{\rm s} }{2},~~ \tag {2} \end{align} $$ where $H_{\rm k}$ is the anisotropy field measured along the hard axis, and the values of $K_{\rm eff}$ of samples containing 2- and 4-nm-thick MgO are calculated by Eq. (2), which are $5.10\times10^{5}$ and $8.9\times10^{5}$ erg/cm$^{3}$, respectively. We can also see that the PMA of the sample containing 4-nm MgO is stronger than the sample containing 2-nm MgO, and the PMA of the 2-nm-MgO sample is only about 57% of the 4-nm-MgO one. The results in this work show that the most optimal MgO thickness is about 4 nm for top MTJs structure without Ta buffer. However, for Si/SiO$_{2}$/Ta/MgO/CoFeB/Ta multilayer with 5 nm Ta buffer, the calculated $K_{\rm eff}$ is $1.53\times10^{6}$ erg/cm$^{3}$, which is significantly greater than the same multilayer without Ta buffer. This demonstrates that the buffer layer also plays an important role for the PMA in the top MgO/CoFeB/TaMTJs structure. Figures 3(a)–3(f) show the surface images of varying MgO thickness from 0–10 nm deposited on thermally oxidized Si wafers annealed at 300$^{\circ}\!$C. We can clearly see that the thin film shows amorphous-like behavior when the MgO thickness is thinner. However, when it is thicker than a certain value, it becomes partially polycrystalline. Ikeda et al.[25] demonstrated that the film was partially polycrystalline in the annealed 10 nm MgO directly on the substrate. In this work, when the thickness of MgO is 10 nm, the sample shows in-plane magnetic anisotropy while not PMA, which could be due to the partially polycrystalline of MgO. The interatomic distance along the out-of-plane direction may increase with the partially polycrystalline of the MgO film, resulting in the reduction in the PMA strength.[20] However, a certain crystallization is necessary in an actual MgO barrier MTJ. This may increase the TMR ratio of MTJ. Furthermore, MTJ with a thicker MgO barrier shows higher resistance area, which is not suitable for current induced magnetization switching.

Fig. 3. The SEM images of MgO deposited on thermally oxidized Si wafers annealed at 300$^{\circ}\!$C with varying MgO thicknesses of (a) 0 nm, (b) 1 nm, (c) 1.5 nm, (d) 2 nm, (e) 4 nm, and (f) 10 nm, respectively.

Fig. 4. (a) The dependence of roughness on MgO thickness deposited on a substrate. (b) AFM surface topography of the films containing 2-nm-thick MgO deposited on thermally oxidized Si wafers annealed at 300$^{\circ}\!$C.

To better observe the surface morphologies of the films with varying MgO thickness from 1 to 10 nm deposited on thermally oxidized Si wafers, the AFM surface topographies of the films with varying MgO thickness from 1 to 10 nm deposited on substrates from 0 to 10 nm has been measured. Figure 4(a) shows the roughness dependence on the MgO thickness deposited on the substrate annealed at 300$^{\circ}\!$C, the roughnesses of 2 and 4 nm MgO samples are 3.163 and 1.8 nm, respectively. They are also the greatest two roughnesses in all five samples. As shown in Fig. 2, the multilayers with 2- and 4-nm MgO show the PMA. The toughness may affect the interatomic distance along the out-of-plane direction, which may result in the changing strength of PMA. However, all these need to be further confirmed. Figure 4(b) shows the AFM surface topography of the Si/SiO$_{2}$/MgO(2 nm) sample annealed at 300$^{\circ}\!$C, these results also illustrate that the hybridization of MgO/CoFeB interface may play an important role for the formation of PMA in the MgO/CoFeB/Ta multilayer. In summary, a detailed study of the varying MgO thicknesses effect on the PMA in MgO/CoFeB/Ta top structure is presented. A strong dependence of PMA on the MgO barrier layer thickness is observed, and the PMA of 1.1 nm CoFeB only exists in the sample containing about 2–4 nm MgO with a 2 nm Ta cap layer. The interatomic distance along the out-of-plane direction may increase with the partially polycrystalline MgO film, which results in the reduction strength of the PMA. These results could be used to tune the magnetic characteristic of the ultra thin CoFeB film for future applications in perpendicular magnetic devices. References A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junctionUltrathin Co/Pt and Co/Pd superlattice films for MgO-based perpendicular magnetic tunnel junctionsReduction in critical current for spin transfer switching in perpendicular anisotropy spin valves using an in-plane spin polarizerReduction of switching current by spin transfer torque effect in perpendicular anisotropy magnetoresistive devices (invited)Influence of perpendicular magnetic anisotropy on spin-transfer switching current in CoFeB∕MgO∕CoFeB magnetic tunnel junctionsElectric-field effects on thickness dependent magnetic anisotropy of sputtered MgO/Co[sub 40]Fe[sub 40]B[sub 20]/Ta structuresDependence of magnetic anisotropy on MgO thickness and buffer layer in Co20Fe60B20-MgO structureJunction size effect on switching current and thermal stability in CoFeB/MgO perpendicular magnetic tunnel junctionsInfluence of capping layers on CoFeB anisotropy and dampingThermally robust Mo/CoFeB/MgO trilayers with strong perpendicular magnetic anisotropyHigh thermal stability and low Gilbert damping constant of CoFeB/MgO bilayer with perpendicular magnetic anisotropy by Al capping and rapid thermal annealingPerpendicular magnetic anisotropy induced by a cap layer in ultrathin MgO/CoFeB/NbInfluence of Magnetic Field Annealing on Magnetic Properties for Nanocrystalline $({\rm Fe}_{0.5}{\rm Co}_{0.5})_{73.5}{\rm Cu}_{1}{\rm Mo}_{3}{\rm Si}_{13.5}{\rm B}_{9}$ AlloyInterfacial perpendicular magnetic anisotropy in CoFeB/MgO structure with various underlayersPerpendicular magnetic anisotropy in CoFeB/X (X=MgO, Ta, W, Ti, and Pt) multilayersLarge enhanced perpendicular magnetic anisotropy in CoFeB∕MgO system with the typical Ta buffer replaced by an Hf layerFirst-principles study of perpendicular magnetic anisotropy in CoFe/MgO and CoFe/Mg

_{3}

_{2}

_{6}

interfacesFirst-principles investigation of the very large perpendicular magnetic anisotropy at Fe

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MgO and Co

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MgO interfacesPerpendicular magnetization of CoFeB on single-crystal MgOMagnetic dead layer in amorphous CoFeB layers with various top and bottom structuresAnnealing effects on CoFeB-MgO magnetic tunnel junctions with perpendicular anisotropyEffect of cap layer thickness on the perpendicular magnetic anisotropy in top MgO/CoFeB/Ta structuresPerpendicularly magnetized spin filtering Cu/Ni multilayersTunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB∕MgO∕CoFeB pseudo-spin-valves annealed at high temperature

[1]	Ikeda S, Miura K, Yamamoto H, Mizunuma K, Gan H D, Endo M, Kanai S, Hayakawa J, Matsukura F and Ohno H 2010 Nat. Mater. 9 721
[2]	Yakushiji K, Saruya T, Kubota H, Fukushima A, Nagahama T, Yuasa S and Ando K 2010 Appl. Phys. Lett. 97 232508
[3]	Law R, Tan E L, Sbiaa R, Liew T and Chong T C 2009 Appl. Phys. Lett. 94 062516
[4]	Sbiaa R, Lua S, Law R, Meng H, Lye R and Tan H K 2011 J. Appl. Phys. 109 07C707
[5]	Yakata S, Kubota H, Suzuki Y, Yakushiji K, Fukushima A, Yuasa S and Ando K 2009 J. Appl. Phys. 105 07D131
[6]	Endo M, Kanai S, Ikeda S, Matsukura F and Ohno H 2010 Appl. Phys. Lett. 96 212503
[7]	Yamanouchi M, Koizumi R, Ikeda S, Sato H, Mizunuma K, Miura K, Gan H D, Matsukura F and Ohno H 2011 J. Appl. Phys. 109 07C712
[8]	Sato H, Yamanouchi M, Miura K, Ikeda S, Gan H D, Mizunuma K Koizumi R, Matsukura F and Ohno H 2011 Appl. Phys. Lett. 99 042501
[9]	Natarajarathinam A, Tadisina Z R, Mewes T, Watts S, Chen E and Gupta S 2012 J. Appl. Phys. 112 053909
[10]	Liu T, Zhang Y, Cai J W and Pan H Y 2014 Sci. Rep. 4 5895
[11]	Wang D S, Lai S Y, Lin T Y, Chien C W, Ellsworth D, Wang L W, Liao J W, Lu L, Wang Y H, Wu M and Lai C H 2014 Appl. Phys. Lett. 104 142402
[12]	Cheng T I, Cheng C W and Chern G 2012 J. Appl. Phys. 112 033910
[13]	Lee D S, Chang H T, Cheng C W and Cher G 2014 IEEE Trans. Magn. 50 7
[14]	Oh Y W, Lee K D, Jeong J R and Park B G 2014 J. Appl. Phys. 115 17C724
[15]	Cui B, Song C, Wang G Y, Wang Y Y, Zeng F and Pan F 2013 J. Alloys Compd. 559 112
[16]	Liu T, Cai J W and Sun L 2012 AIP Adv. 2 032151
[17]	Khoo K H, Wu G, Jhon M H, Tran M, Ernult F, Eason K, Choi H J and Gan C K 2013 Phys. Rev. B 87 174403
[18]	Yang H X, Chshiev M, Dieny B, Lee J H, Manchon A and Shin K H 2011 Phys. Rev. B 84 054401
[19]	Lee K, Sapan J J, Kang S H and Fullerton E E 2011 J. Appl. Phys. 109 123910
[20]	Zhu T, Zhang Q and Yu R 2015 IEEE Magn. Conf. (INTERMAG) 1
[21]	Jang S Y, Lim S H and Lee S R 2010 J. Appl. Phys. 107 09C707
[22]	Meng H, Lum W H, Sbiaa R, Lua S Y H and Tan H K 2011 J. Appl. Phys. 110 033904
[23]	Cheng C W, Feng W, Chern G, Lee C M and Wu T 2011 J. Appl. Phys. 110 033916
[24]	Shirahata Y, Wada E, Itoh M and Taniyama T 2014 Appl. Phys. Lett. 104 032404
[25]	Ikeda S, Hayakawa J, Ashizawa Y, Lee Y M, Miura K, Hasegawa H Tsunoda M, Matsukura F and Ohno H 2008 Appl. Phys. Lett. 93 082508