Effect of annealing conditions on the perpendicular magnetic anisotropy of tacofebmgo multilayers

Effect of annealing conditions on the perpendicular magnetic anisotropy of Ta/CoFeB/MgO multilayers Effect of annealing conditions on the perpendicular magnetic anisotropy of Ta/ CoFeB/MgO multilayers[.]

Effect of annealing conditions on the perpendicular magnetic anisotropy of Ta/ CoFeB/MgO multilayers

Yan Liu, Liang Hao, and Jiangwei Cao,

Citation: AIP Advances 6, 045008 (2016); doi: 10.1063/1.4947132

View online: http://dx.doi.org/10.1063/1.4947132

View Table of Contents: http://aip.scitation.org/toc/adv/6/4

Published by the American Institute of Physics

Effect of annealing conditions on the perpendicular

magnetic anisotropy of Ta/CoFeB/MgO multilayers

Yan Liu,1Liang Hao,2and Jiangwei Cao2,3, a

1School of Information Engineering, Lanzhou University of Finance and Economics,

Lanzhou 730020, PR China

2Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education,

Lanzhou University, Lanzhou 730000, PR China

3Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, Lanzhou 730000, PR China

(Received 27 December 2015; accepted 7 April 2016; published online 14 April 2016)

Films with a structure of Ta (5 nm)/Co20Fe60B20 (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm) were deposited on Corning glass substrates by magnetron sputtering The as-deposited films with CoFeB layer thickness from 0.8 to 1.3 nm show perpendicular magnetic anisotropy (PMA) After annealing at a proper temperature, the PMA of the films can be enhanced remarkably A maximum effective anisotropy field of

up to 9 kOe was obtained for 1.0- and 1.1-nm-thick CoFeB layers annealed at

an optimum temperature of 300 ◦C A 4-kOe magnetic field was applied during annealing to study its effect on the PMA of the CoFeB layers The results confirmed that applying a perpendicular magnetic field during annealing did not improve the maximum PMA of the films, but it did enhance the PMA of the thinner films at

a lower annealing temperature C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4947132]

I INTRODUCTION

In the drive to develop magnetic storage technology, magnetic thin films with perpendicular magnetic anisotropy (PMA) have been pursued for several decades.1 3Recently, the discovery of PMA in Ta/CoFeB/MgO structures has drawn much attention because CoFeB layers are widely used in magnetic tunnel junctions (MTJs).47 MTJs containing a CoFeB layer displaying PMA have been considered as the core structure of spin-transfer torque magnetoresistive random-access memory (STT-MRAM),8 which has the advantages of high thermal stability, low critical current density, and high tunneling magnetoresistance (TMR) ratio.9However, to further improve the stor-age density of STT-MRAM, one has to shrink the storstor-age cell It is widely acknowledged that the thermal stability factor KuV/KBT of the recording cell needs to exceed 40 for the cell to possess acceptable thermal stability Thus, enhancement of the PMA of CoFeB layers is critical to decrease the dimensions of STT-MRAM Although some groups reported that as-deposited CoFeB layers sandwiched by Ta and MgO show strong PMA, annealing is necessary to crystallize the CoFeB layer to obtain a large TMR ratio for MRAM applications In general, magnetic field annealing is used to enhance PMA and crystallize the CoFeBlayer.10 – 12However, the anisotropy induced by field annealing is not particularly strong, normally of the order of 103 erg/cc in permalloys, which is much lower than the required anisotropy to achieve PMA in thin films In addition, several groups recently reported that CoFeB with PMA can also be obtained by annealing without an applied mag-netic field.13 – 16Worledge et al.17applied an in-plane field during annealing of a Ta/CoFeB/MgO stack, and the effect of this in-plane field on the PMA of samples was negligible Therefore, the role

of an applied magnetic field during annealing to obtain PMA is not clear.A systematic study of the effect of annealing conditions on the PMA of CoFeB layers is necessary

a Corresponding author: caojw@lzu.edu.cn

045008-2 Liu, Hao, and Cao AIP Advances 6, 045008 (2016)

In this work, the effects of annealing temperature and applied magnetic field on the PMA of

Ta/CoFeB/MgO multilayers are investigated We report that with a proper CoFeB thickness, strong PMA can be obtained in as-deposited Ta/CoFeB/MgO layers After annealing at 250–300◦C, the effective anisotropy field (Hk) is enhanced up to 9 kOe We also find that applying a magnetic field during annealing does not improve the maximum PMA of the CoFeB layer, but may lower the annealing temperature needed to reach the maximum PMA

II EXPERIMENTAL

Films with a structure of Ta (5 nm)/Co20Fe60B20 (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm)were deposited on Corning glass substrates using a magnetron sputtering system(CMS-18) with a base pressure less than 1.7×10−7Torr Ta and CoFeB layers were deposited by DC magnetron sputtering, while the MgO layer was deposited by radio frequency sputtering Argon gas pressure was kept

at 5 mTorr during sputtering of all the layers After deposition, the samples were annealed at 200–400 ◦C for an hour in a vacuum chamber with a base pressure less than 2×10−5Torr with or without a 4-kOe applied magnetic field The magnetic properties of the films were measured using a MicroSenseEV9 vibrating sample magnetometer

III RESULTS AND DISCUSSION

Figure1shows the in-plane and out-of-plane hysteresis loops of as-deposited Ta (5 nm)/CoFeB (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm) films The film with a 1.5-nm-thick CoFeB layer exhibits obvious in-plane easy magnetization With decreasing CoFeB thickness, the easy axis gradually turns to the normal direction of the films The films with CoFeB layer thickness ranging from 0.9 to 1.2 nm exhibit large PMA with an in-plane saturation field of up to 5 kOe for 1.0- and 1.1-nm-thick CoFeB layers When CoFeB layer thickness is decreased to 0.8 nm, the PMA deteriorates and the films show slight superparamagnetic behavior, suggesting that the films become discontinuous

It should be noted that the saturation magnetization (Ms) of the films is much lower than the corresponding bulk values because of the presence of a thick dead layer at the Ta/CoFeB interface.18

The PMA of the films can be enhanced obviously after annealing under suitable conditions Figure2(a)displays typical hysteresis loops of a Ta (5 nm)/CoFeB (1.1 nm)/MgO (1 nm)/Ta (1 nm) film annealed at 300 ◦C, which exhibited a large Hk of up to 9 kOe The Hk values for all the annealed films are provided in Fig.2(b) With increasing annealing temperature, Hkincreased until

300 ◦C, and a maximum Hk value of around 9 kOe was obtained for the 1.0- and 1.1-nm-thick CoFeB films At the optimized annealing temperature (300◦C), even the 1.5-nm-thick CoFeB films exhibit obvious PMA After annealing at 400◦C, the PMA started to deteriorate and the easy axis turned to the in-plane direction for all the films

The effective perpendicular anisotropy energy density can be calculated from Hk and Ms ac-cording to the relationship Ke ff= MsHk/2 In the calculation, the real Ms values of the CoFeB layers were used after subtracting the thickness of the deadlayer For perpendicular (in-plane) anisotropy,Hk is positive (negative) and determined from the in-plane (perpendicular) saturation field In general, Ke ff is determined by three intrinsic factors: magnetocrystalline anisotropy (Ku), shape anisotropy (2π M2), and interface anisotropy (Ki), where

Ke ff= Ku− 2π Ms2+ Ki

tCoFeB

Here, Ku is negligible and shape anisotropy −2π M2is negative, so the positive Ke ffmostly comes from the interface anisotropy Figure3shows the product of PMA energy density (Ke ff) and CoFeB layer thickness (t) for the films with different CoFeBlayer thickness after annealing under different conditions The PMA of the films is enhanced after annealing below 300 ◦C The reason for the degradation of PMA after annealing above 350 ◦C should be the diffusion of the Ta buffer layer, which deteriorates the CoFeB/MgO interface

FIG 1 Hysteresis loops of as-deposited Ta (5 nm)/CoFeB (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm) films (black squares: in plane, red circles: out of plane).

FIG 2 Magnetic properties of Ta (5 nm) /CoFeB (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm) after annealing (a) Typical hysteresis loops of Ta (5 nm) /CoFeB (1.1 nm)/MgO (1 nm)/Ta (1 nm) after annealing at 300 ◦ C; (b) the contour of e ffective anisotropy field (H ) for films with di fferent thickness and annealed at various temperatures.

045008-4 Liu, Hao, and Cao AIP Advances 6, 045008 (2016)

FIG 3 Product of PMA energy density (k e ff ) and CoFeB layer thickness (t) as a function of t for samples prepared at di fferent annealing temperatures.

The effect of annealing can depend on the texture of the seed layer To investigate this effect, θ-2θ X-ray diffraction measurements were conducted on as-deposited and annealed Ta(5nm)/Co20Fe60

B20/MgO/Ta structures No obvious diffraction peaks originating from Ta were found for both as-deposited and annealed samples This suggests that the thin Ta layer may be amorphous, which is consistent with previous reports.19 , 20Another possibility for the lack of diffraction peaks is because they are beyond the limit of sensitivity of the x-ray diffraction instruments The small critical switching current in the current-induced magnetization switching by spin-orbit torque of the Ta layer in these structures suggests that the β-Ta phase may exist in both as-deposited and annealed samples

Next, we investigated the effect of magneticfield annealing on PMA of CoFeB layers Figure4

shows the M-H loops of Ta (5 nm)/CoFeB (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm) films annealed

at 250 ◦C without and with a 4-kOe applied magnetic field Compared with the films annealed without a magnetic field, no obvious enhancement of PMA is found for the films annealed with a 4-kOe applied field, except for those with 0.8-nm-thick CoFeB layers This suggests that application

of a magnetic field is not necessary during annealing of MTJs, which was previously believed

to be indispensable for the enhancement of PMA in ultra-thin CoFeB layers These results are also important for the fabrication of STT-MRAM with PMA from the viewpoint of application Further systematic investigation of the effect of annealing under an applied magnetic fieldon the PMA of films with different thickness was conducted; the results are presented in Fig.5 Similar

to the case without an applied magnetic field, the PMA is enhanced after annealing at a proper temperature and deteriorates after annealing at a higher temperature An obvious difference is that the Hkof the thinner CoFeBlayers(0.8 and 0.9nm) after field annealing is larger than that of samples annealed without a field In addition, the difference of Hkbetween the films annealed at

250, 300, and 350 ◦C with a 4-kOe applied field is much larger than that of the films annealed without an applied magnetic field The fieldannealing temperature that gives the maximum PMA is

250 ◦C, which is slightly lower than that of the films annealed without an applied magnetic field These results suggest that field annealing helps to enhance the PMA of thinner films at a lower annealing temperature Although the mechanism of this phenomenon is unknown at present, it may

be related to the enhancement of the kinetics of Fe-Co pairs induced by the external magnetic field

The above results reveal that applying a 4-kOe magnetic field during annealing does not improve the maximum PMA in Ta/CoFeB/MgO film stacks To confirm this further, we compared PMA for the films annealed at the same temperature under with different external magnetic field

FIG 4 Comparison of the hysteresis loops of Ta (5 nm) /CoFeB (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm) annealed at 250 ◦ C without (left column) and with (right column) a 4-kOe applied magnetic field.

FIG 5 E ffective anisotropy field (H ) values of the films with di fferent thickness annealed under different conditions.

045008-6 Liu, Hao, and Cao AIP Advances 6, 045008 (2016)

FIG 6 Hysterisis loops of Ta (5 nm) /CoFeB (1.1 nm)/MgO (1 nm)/Ta (1 nm) films annealed at 300 ◦ C with a 4-kOe magnetic field applied along the (a) normal and (b) in-plane directions.

directions (in-plane or out-of-plane) No obvious difference of magnetic properties was found for the films annealed with an external field along the in-plane or out-of-plane direction, as shown

in Fig 6 for the Ta (5 nm)/CoFeB (1.3 nm)/MgO (1 nm)/Ta (1 nm) film structure annealed at

300 ◦C These results are consistent with those reported by Worledgeand colleagues.17Therefore,

we may conclude that the enhancement of PMA induced by field annealing mostly originates from the temperature effect, not the magnetic field This behavior can be easily understood because it

is widely acknowledged that the PMA in Ta/CoFeB/MgO structures originates from interfacial symmetry breaking and hybridization of Fe 3d and O 2p orbitals at the CoFeB/MgO interface,21

which is unaffected by the external magnetic field during annealing

IV CONCLUSIONS

The effect of annealing conditions on the PMA of CoFeB films in Ta (5 nm)/Co20Fe60B20 (0.8–1.5 nm)/MgO (1 nm)/Ta (1 nm) structures was investigated The as-deposited films with a CoFeB layer thickness of 0.8–1.3 nm displayed PMA Annealing at 250–300◦C can enhance the PMA up to 9 kOe for the 1.0- and 1.1-nm-thick CoFeB films Applying a 4-kOe magnetic field during annealing does not improve the maximum PMA, but it does enhance the PMA of thinner films at a lower annealing temperature

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Nos 61102002,

61272076, and 51371101) and SRF for ROCS, SEM

1 P F Carcia, A D Meinhaldt, and A Suna, Appl Phys Lett 47, 178 (1985).

2 F J A den Broeder, D Kuiper, H C Donkersloot, and W Hoving, Appl Phys A 49, 507 (1989).

3 B N Engel, C D England, R A Van Leeuwen, M H Wiedmann, and C M Falco, Phys Rev Lett 67, 1910 (1991).

4 S Ikeda, K Miura, H Yamamoto, K Mizunuma, H D Gan, M Endo, S Kanai, J Hayakawa, F Matsukura, and H Ohno, Nature Materials 9, 721 (2010).

5 S Ikeda, J Hayakawa, Y Ashizawa, Y M Lee, K Miura, H Hasegawa, M Tsunoda, F Matsukura, and H Ohno, Appl Phys Lett 93, 082508 (2008).

6 L Hao, J Cao, M Liu, H Dang, T Jin, Y Wang, F Wei, and D Wei, J Appl Phys 115, 17C122 (2014).

7 T Zhu, Y Yang, R C Yu, H Ambaye, V Lauter, and J Q Xiao, Appl Phys Lett 100, 202406 (2012).

8 S Ikeda, J Hayakawa, Young Min Lee, F Matsukura, Yuzo Ohno, T Hanyu, and H Ohno, IEEE Transactions on Electron Devices 54, 991 (2007).

9 H Meng, W H Lum, R Sbiaa, S Y H Lua, and H K Tan, J Appl Phys 110, 033904 (2011).

10 V Sokalski, M T Moneck, E Yang, and J -G Zhu, Appl Phys Lett 101, 072411 (2012).

11 M Yamanouchi, R Koizumi, S Ikeda, H Sato, K Mizunuma, K Miura, H D Gan, F Matsukura, and H Ohno, J Appl Phys 109, 07C712 (2011).

12 V Sokalski, D M Bromberg, M T Moneck, E Yang, and J -G Zhu, IEEE Trans Magn 49, 4383–4385 (2013).

13 J Sinha, M Hayashi, A J Kellock, S Fukami, M Yamanouchi, H Sato, S Ikeda, S Mitani, S -h Yang, S S P Parkin, and H Ohno, Appl Phys Lett 102, 242405 (2013).

14 H Meng, R Sbiaa, C C Wang, S Y H Lua, and M A K Akhtar, J Appl Phys 110, 103915 (2011).

15 L Cuchet, B Rodmacq, S Au ffret, R.C Sousa, and B Dieny, Appl Phys Lett 105, 052408 (2014).

16 V B Naik, H Meng, and R Sbiaa, AIP Advances 2, 042182 (2012).

17 D C Worledge, G Hu, D W Abraham, J Z Sun, P L Trouilloud, J Nowak, S Brown, M C Gaidis, E J O’Sullivan, and R P Robertazzi, Appl Phys Lett 98, 022501 (2011).

18 S Y Jang, S H Lim, and S R Lee, J Appl Phys 107, 09C707 (2010).

19 T Liu, J W Cai, and Li Sun, AIP Advances 2, 032151 (2012).

20 J Cao, J Kanak, T Stobiecki, P Wisniowski, and P P Freitas, IEEE Trans Magn 45(10), 3464 (2009).

21 K H Khoo, G Wu, M H Jhon, M Tran, F Ernult, K Eason, H J Choi, and C K Gan, Phys.Rev B 87, 174403 (2013).