MgO barrier-perpendicular magnetic tunnel junctions with CoFe/Pd multilayers and ferromagnetic
K. Mizunuma1, S. Ikeda1, a), J. H. Park1, H. Yamamoto2,1, H. Gan1, K. Miura2,1, H. Hasegawa1, J.
Hayakawa2, F. Matsukura1, and H. Ohno1, b)
1 Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication,
Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
2 Advanced Research Laboratory, Hitachi, Ltd., 1-280 Higashi-koigakubo, Kokubunji-shi, Tokyo
The authors studied an effect of ferromagnetic (Co20Fe60B20 or Fe) layer insertion on tunnel
magnetoresistance (TMR) properties of MgO-barrier magnetic tunnel junctions (MTJs) with
CoFe/Pd multilayer electrodes. TMR ratio in MTJs with CoFeB/MgO/Fe stack reached 67% at an-
nealing temperature (Ta) of 200oC and then decreased rapidly at Taover 250oC. The degradation of
the TMR ratio may be related to crystallization of CoFe(B) into fcc(111) or bcc(011) texture result-
ing from diffusion of B into Pd layers. MTJs which were in-situ annealed at 350oC just after depos-
iting bottom CoFe/Pd multilayer showed TMR ratio of 78% by post annealing at Ta =200°C.
a) Electronic mail (corresponding author): email@example.com
b) Electronic mail: firstname.lastname@example.org
Spin transfer torque magnetic tunnel junctions with perpendicular magnetic anisotropy electrodes
(perpendicular MTJs) attract much interest from the possibility of nonvolatile spin devices compati-
ble with the latest technology node (< 45 nm) in DRAMs having low critical switching current (Ic0)
as well as high thermal stability (E/kBT).1-4 Currently, perpendicular MTJs prepared using conven-
tional sputtering are actively being studied and have reached the TMR ratios at room temperature
(RT) of up to 64% with rare-earth transition metal alloys,5-7 up to 120% with L10-ordered (Co,
Fe)-Pt,4,8-10 and up to 15% with Co/(Pd, Pt) multilayers.11-13 For perpendicular MTJs, Ic0 and E/kBT
are in a trade-off relation.1 In order to reduce Ic0, it is necessary to moderately reduce E/kBT. From
this view point, multilayer electrodes have advantages; multilayer is relatively easy to control Ms and
Hk by changing the number of the layer stack and the thicknesses. In addition, multilayer films are
comparatively easy to realize perpendicular magnetic anisotropy, yet show high magnetic thermal
stability. However, it is not clear how one can achieve high tunnel magnetoresistance (TMR) in
MTJs based on multilayers. In our previous study,14 MgO barrier MTJs with Co90Fe10/Pd multilayer
electrodes showed TMR ratio of a few %. In order to establish the technology for high TMR ratio,
we investigated an effect of ferromagnetic (Co20Fe60B20 or Fe) layer insertion on TMR properties of
MgO-barrier MTJs with Co90Fe10/Pd multilayer electrodes.
A set of rf-sputtered MTJ films with a pseudo-spin-valve structure were first studied for electri-
cal measurements. They consist of from the substrate side,
(1.2)/cap-layer (in nm) where FM is Co20Fe60B20 or Fe. For comparison, we fabricated MTJs con-
sisting of buffer-layer/[Pd(1.2)/Co90Fe10(0.2)]3/MgO(2)/[Co90Fe10(0.2)/Pd(1.2)]10/cap-layer and
buffer-layer/Pd(3.6)/Co20Fe60B20(1.8)/MgO(2)/Co20Fe60B20(1.8)/Pd(12)/cap-layer. All junctions were
fabricated using a conventional photolithography technique with post-baking process of photoresist
at 120oC. The MTJs were annealed at 200 ~ 300oC for 1 h under an out-of-plane magnetic field of 4
kOe. The TMR ratio was measured at RT using a dc four probe method with out-of-plane field of up
to 8 kOe. The structures were investigated by high resolution transmission electron microscopy
(HRTEM) and by fast Fourier transform (FFT) of the digitized HRTEM image. The compositions
were analyzed by secondary ion mass spectrometer (SIMS) using Ar ion beam.
Figure 1 shows the annealing temperature (Ta) dependence of the TMR ratio for the MTJs
with [Pd(1.2)/Co90Fe10(0.2)]3/MgO(2)/[Co90Fe10(0.2)/Pd(1.2)]10 and
layer stack structures. By inserting the CoFeB layers between CoFe/Pd multilayers and MgO barrier,
the TMR ratio at RT increased from 1.5% to 43%. The TMR ratio of the MTJ with CoFeB insertion
increased after annealing at 200oC and then rapidly decreased at Ta over 250oC. In this MTJ system,
no high TMR ratios of several hundred % shown in the previous reports of CoFeB/MgO/CoFeB
MTJs with in-plane magnetic anisotropy15,16 were obtained.
To understand the reasons for the low TMR ratio in the MTJs with CoFeB insertion layers,
HRTEM was employed for structural characterization. Figure 2(a) shows cross-sectional HRTEM
image for the MTJ with Co20Fe60B20 insertion annealed at 300oC which showed low TMR ratio of
less than 1%. The MgO barrier have (001) oriented texture, whereas the top and bottom CoFeB elec-
trodes consist of fcc(111) or bcc(011) oriented texture with lattice fringe spacing of 0.210-0.218 nm
according to the FFT images (not shown). In the partial area in which the lattice fringe in the
HRTEM image was clearly observed, we could confirm the epitaxy of
fcc-Pd(111)//bcc-CoFe(B)(110), as indicated by the lattice fringes with intersection
angles of 71o+55o and 60o.17,18 The fcc(111) or bcc(011) oriented crystallization of the inserted,
initially amorphous, CoFeB layers is likely to be one of the reasons for the low TMR ratios, because
high TMR ratios require bcc(001) oriented ferromagnetic electrodes and MgO (001) barrier. It is
known that NiFe19,20or CoFe21,22adjacent to CoFeB acts as a template for crystallization of initially
amorphous CoFeB into fcc(111) or bcc(011) oriented texture through annealing over crystallization
temperatures which strongly depend on the thermal diffusion of B into the adjacent layers. We inves-
tigated the B diffusion by SIMS analysis. A simplified structure of
Pd(3.6)/Co20Fe60B20(1.8)/MgO(2)/Co20Fe60B20(1.8)/Pd(12) was used for this analysis. Figures 2(b)
and 2(c) show the composition depth profiles of B, Pd and Co in the samples before and after an-
nealing at Ta = 300oC, respectively. In as-deposited state, B is located in CoFe, whereas after an-
nealing B diffuses into Pd. These observations suggest a scenario that the diffusion of B into Pd lay-
ers reduces the crystallization temperature of CoFeB at the CoFeB/Pd interfaces, and crystallization
of CoFe(B) into fcc(111) or bcc(011) oriented texture starts from the fcc(111) Pd seed layers. In the
SIMS depth profiles before and after annealing, the high intensity of B is detected in MgO barrier.
However, electron energy-loss spectroscopy analysis showed a large amount of B existed in the
metal layers (buffer and cap layers) adjacent to the CoFe(B), and the concentration of B in the MgO
barrier was low.18,23 The difference in the B detection may be caused by the matrix effect in SIMS.
The above results strongly suggest that even higher TMR ratio may be realized once the
Fe(Co) electrodes adjacent to the MgO barrier can be made bcc(001). We thus examined the anneal-
ing dependence of TMR ratio for the MTJs with Fe layers; note that Fe becomes bcc(001) when de-
posited on highly oriented (001) MgO. Figure 3 (a) shows the Ta dependence of the TMR ratio for
the MTJs with [Co90Fe10(0.2)/Pd(1.2)]3/FM(1.8)/MgO(2)/FM(1.8)/[Pd(1.2)/Co90Fe10(0.2)]10
(FM=Co20Fe60B20 or Fe) stack structures along with the one shown in Fig. 1 for reference. The MTJs
with Fe/MgO/Fe stack show lower TMR ratio (= 9.3 %) than those of the MTJs with
Co20Fe60B20/MgO/Co20Fe60B20 stack. On the other hand, the TMR ratio of the Co20Fe60B20/MgO/Fe
stack increased and reached 67%. In the former, Fe is deposited on fcc(111) Pd and thus it is likely
that Fe is in a crystal structure or orientation other than bcc(001), resulting in low TMR. On the other
hand, in the case of Co20Fe60B20/MgO/Fe stack, the top Fe layer deposited on MgO has a bcc(001)
oriented texture as observed in the TEM images (not shown). Thus the enhancement of TMR ratio
for the Co20Fe60B20/MgO/Fe MTJs as annealing proceeds can be attributed to the (001) oriented
top-Fe layer on MgO barrier; TMR increases as the bottom electrode/MgO interface becomes crys-
talline upon annealing.
In order to fully evaluate the TMR properties, it is necessary to stably realize parallel and an-
tiparallel magnetization configurations. We tried to obtain a clear difference in coercivity between
top and bottom ferromagnetic electrodes, i.e., the stable antiparallel state, by increasing the perpen-
dicular magnetic anisotropy of the bottom CoFe/Pd multilayer electrode with in-situ annealing (see
Fig. 4(a)). In-situ annealing at 350oC for 1 h in vacuum chamber was applied right after depositing
the bottom [CoFe/Pd]3 multilayer, and then the CoFeB/MgO/Fe/[Pd/CoFe]10 top stack structure was
deposited without air exposure. Figure 4(b) shows the TMR ratio as a function of resistance-area
product (RA) in MTJs with/without 350oC in-situ annealing, which were annealed at Ta=200oC. The
MTJs without in-situ annealing have a wide distribution of TMR ratio. In contrast, the TMR ratio for
the MTJs with in-situ annealing was enhanced to 78% together with reduction of its distribution. A
typical TMR loop is shown in Fig. 4(c). The improved distribution of TMR ratio is consistent with
the above-mentioned scenario regarding stabilization of the antiparallel magnetization configuration
by in-situ annealing.
In summary, we investigated the TMR properties and film structures of perpendicular MTJs
with CoFe/?Pd multilayer and different insertion layers such as CoFeB and Fe. The insertion of
CoFeB layers between CoFe/Pd multilayers and MgO barrier resulted in an increase of TMR ratio
from a few % to up to 43%. By applying combination of bottom CoFeB and top Fe insertion layers,
the TMR ratio reached 67%. For the MTJs with in-situ annealing The TMR ratio of 78% was ob-
served. However, the TMR ratio rapidly decreased at Ta over 250oC. The degradation of the TMR
ratio for the MTJs annealed at high Ta may be related to the crystallization of CoFe(B) into fcc(111)
or bcc(011) texture resulting from the diffusion of B into Pd layers. Even higher TMR ratio may be
expected in MTJs with perpendicular magnetic CoFe/Pd multilayer electrodes, if one can realize
bcc(001) orientation of CoFeB, for example, by suppression of B diffusion between the CoFe/Pd
multilayer and the CoFeB layer.
This work was supported in part by the “High-Performance Low-Power Consumption Spin
Devices and Storage Systems” program under Research and Development for Next-Generation In-
formation Technology of MEXT. The authors wish to thank Y. Ohno for discussion and I. Morita
and T. Hirata for their technical support in MTJ fabrication and discussion.
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FIG. 1. TMR ratio as a function of annealing temperature (Ta) for the MTJs with
[Pd(1.2)/Co90Fe10(0.2)]3/MgO(2)/[Co90Fe10(0.2)/Pd(1.2)]10 and [Co90Fe10(0.2)/Pd(1.2)]3/
Co20Fe60B20(1.8)/MgO(2)/Co20Fe60B20(1.8)/[Pd(1.2)/Co90Fe10(0.2)]10 multilayer stacks. TMR ratio at
Ta = 120oC was obtained in as-made MTJ with post-baking process of photoresist at 120oC.
FIG. 2. (a) Cross-sectional HRTEM image for a
o90Fe10(0.2)]10/Pd(1.2)/cap-layer stack after annealing at 300oC. Composition depth profiles of B, Pd,
and Co in the samples with
buffer-layer/Pd(3.6)/Co20Fe60B20(1.8)/MgO(2)/Co20Fe60B20(1.8)/Pd(12)/cap-layer stack (b) before
and (c) after annealing at 300oC analyzed by SIMS.
FIG. 3. TMR ratio as a function of annealing temperature (Ta) for the MTJs with buffer-layer/
er stack where FM is Co20Fe60B20 or Fe insertion.
FIG. 4. (a) M-H loops for the samples consisting of buffer-layer/Pd(1.2)/ [Co90Fe10(0.2)/Pd(1.2)]3
(with/without in-situ annealing at 350oC)/MgO(2)/Ta(5)/Ru(5). (b) TMR ratios as functions of resis-
tance-area product (RA) for the MTJs with (filled circles) and without (open circles) in-situ anneal at
350oC just after depositing bottom CoFe/Pd multilayer. (c) R-H curve at RT of the MTJ with in-situ
TMR ratio (%)
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MR ratio (%)
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