Determination of fine magnetic structure
of magnetic multilayer with quasi
antiferromagnetic layer by using polarized
neutron reflectivity analysis
Cite as: AIP Advances 10, 015323 (2020); https://doi.org/10.1063/1.5130445
Submitted: 03 October 2019 . Accepted: 27 December 2019 . Published Online: 16 January 2020
Yongshi Zhong, Yuichiro Kurokawa, Gen Nagashima, Shu Horiike, Takayasu Hanashima, Daniel Schönke, Pascal
Krautscheid, Robert M. Reeve , Mathias Kläui , and Hiromi Yuasa 
COLLECTIONS
Paper published as part of the special topic on 64th Annual Conference on Magnetism and Magnetic Materials,
Chemical Physics, Energy, Fluids and Plasmas, Materials Science and Mathematical Physics
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© 2020 Author(s).
AIP Advances ARTICLE scitation.org/journal/adv
Determination of fine magnetic structure
of magnetic multilayer with quasi
antiferromagnetic layer by using polarized
neutron reflectivity analysis
Cite as: AIP Advances 10, 015323 (2020); doi: 10.1063/1.5130445
Presented: 5 November 2019 • Submitted: 3 October 2019 •
Accepted: 27 December 2019 • Published Online: 16 January 2020
Yongshi Zhong,1 Yuichiro Kurokawa,1 Gen Nagashima,1 Shu Horiike,1 Takayasu Hanashima,2 Daniel Schönke,3
Pascal Krautscheid,3 Robert M. Reeve,3 Mathias Kläui,3 and Hiromi Yuasa1,a)
AFFILIATIONS
1Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka 819-0395, Japan
2Neutron Science and Technology Center, CROSS, Tokai 319-1106, Japan
3Institute of Physics, Johannes Gutenberg-University Mainz, 55099 Mainz, Germany
Note: This paper was presented at the 64th Annual Conference on Magnetism and Magnetic Materials.
a)Email: hiromi.yuasa@ed.kyushu-u.ac.jp
ABSTRACT
We carried out polarized neutron reflectivity (PNR) analysis to determine the fine magnetic structure of magnetic multilayers with quasi-
antiferromagnetic (quasi-AFM) layers realized by 90-deg coupling using two Co90Fe10 layers, and quantitatively evaluated the magnetization
of quasi-AFM layers. Two types of samples with different buffer layers, Ru buffer and a NiFeCr buffer, were investigated and the average
angles between the respective magnetization of the two Co90Fe10 layers were estimated to be +/− 39 degrees and +/− 53 degrees. In addition,
less roughness was found in the NiFeCr buffer sample resulting stronger 90-deg coupling. A perfect quasi-AFM is expected to be realized by
a flat interface of the magnetic multilayer.
© 2020 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/). https://doi.org/10.1063/1.5130445., s
Antiferromagnetic (AFM) spintronics had attracted increasing named a quasi-AFM layer, whose magnetic domains are alternating
attention in the past few years owing to the different properties and antiparallel.15 STO is expected to be observed in quasi-AFM layer
potentials compared to ferromagnetic (FM) spintronics such as high because it exhibits FM coupling within one domain and each mag-
resonance frequencies and a zero stray field.1–4 It is well known that netic moment can oscillate simultaneously. In addition, zero stray
spin torque oscillation (STO) can be observed in FM materials with fields can be realized owing to the alternating antiparallel magne-
ease, but there are also dipolar interactions resulting in leakage of the tization in the next magnetic domains. The magnetic behavior of
magnetic field which is disadvantageous for STO applications like quasi-AFM is considered an intermediate state between FM and
magnetoresistive random access memory (MRAM).5 AFM materi- AFM materials.
als, however, do not have magnetic field leakage as they exhibit alter- The quasi-AFM layer is realized by 90-deg coupling, an inter-
nating antiparallel magnetic moments. This characteristic is useful layer exchange interaction between two FM layers separated by a
in device design because the stray field can be neglected. Although thin layer.15–37 The magnetic coupling energy E of the magnetic mul-
theoretical spin transfer torque (STT) in AFM materials has been tilayer can be explained by the following equation if the quadratic
reported1,6,7 and experimentally observed,6–14 obvious and practi- term is considered:15–36
cal STO has not yet been directly obtained because of the strong
exchange coupling between adjacent atoms. To realize STO without
a stray field, we successfully created a new type of magnetic material, E = −A12(M1 ⋅M2) − B12(M 21 ⋅M2) , (1)
AIP Advances 10, 015323 (2020); doi: 10.1063/1.5130445 10, 015323-1
© Author(s) 2020
AIP Advances ARTICLE scitation.org/journal/adv
where M1 and M2 are the unit magnetization in the first and sec- Materials and Life Science Experimental Facility (MLF), Japan Pro-
ond FM layers respectively, and A12 and B12 are the bilinear and ton Accelerator Research Complex (J-PARC).42,43 PNR depends on
biquadratic coupling coefficients, respectively. Equation (1) can be the nuclear coherent scattering length and the magnetic-scattering
converted to: length density profile, which is useful for analyzing crystal struc-
A12 ture and magnetic structure simultaneously. The reflectivity wasE ∝ −B12cos θ(cos θ + ), (2)B measured as a function of wave-vector transfer.38,3912
where θ is the angle between the magnetization of the two FM lay- Q = 4π sin(Θ)/λ (3)
ers, as shown in Fig. 1(a). When |A12| < 2|B12| and B12 < 0, M1 and
M2 should take up an intermediate orientation in an angle between where Θ and λ are the angles of incidence and wavelength of neu-
0○ and 180○, i.e., 90-deg coupling. When the magnetization in the tron, respectively. PNR consists of two types of measurements, the
first FM layer is fixed in one direction, the magnetization in the sec- reflected neutrons with the same polarization, i.e., non-spin-flip,
ond FM layer should be alternating antiparallel because of the 90-deg (R++ and R−−), and the reflected neutrons with polarizations oppo-
coupling energy and the magnetostatic energy. Thus, we successfully site the incident polarization, i.e., spin-flip, (R+− and R−+). R++
fabricated a quasi-AFM layer. To utilize the unique quasi-AFM layer and R−− refer to the parallel and antiparallel between neutron spin
as a tunable material between FM and AFM, it is important to under- polarization and the sample polarization, respectively. R+− and R−+
stand the fine magnetic structure corresponding to the magnetic refer to the verticality of neutron spin polarization and the sam-
process. Therefore, in this study, we carried out polarized neutron ple polarization, respectively. In our experiment, the sample size
reflectivity (PNR) analysis of the multilayer with quasi-AFM layers was 20 mm × 20 mm in the x-y plane and the neutron beam was
because PNR is suitable for analyzing the magnetization quantitively irradiated along the x-axis with changing incident angle Θ at room
and obtaining 3D information of multilayers.38,39 Additionally, PNR temperature (RT), as shown in Fig. 1(d). The wave-vector transfer Q
makes it possible to understand the magnetization dependence on ranged from 0.07 to 3.5nm−1. The applied magnetic fields along the
the external fields. x-axis were +1000 Oe, +29 Oe, and −28 Oe for Ru buffer sample;
Two types of multilayer films with different buffer layers, Ru and +1000 Oe, +32 Oe, and −45 Oe for the NiFeCr buffer sam-
buffer and NiFeCr buffer, were grown on thermally oxidized Si ple. The correction for the polarization efficiencies was applied for
wafers by sputtering, as shown in Figs. 1(b) and (c), respectively. the non-spin-flip reflectivity and the spin-flip reflectivity. As there
The magnetization of Co90Fe10 (A) was fixed in the +x-direction are too many fitting parameters to fit correctively, X-ray reflectivity
by IrMn, and there was 90-deg coupling between Co90Fe10 (A) and (XRR) analysis was carried out to decide the structure parameters,
Co90Fe10 (B) through Fe–O. As a result, Co90Fe10 (B) became the i.e., thickness, atomic density and roughness of each layer. The data
quasi-AFM layer, which was macroscopically confirmed by mag- reduction software Utsusemi44 in BL17 was applied for conversion
netic hysteresis (MH) curves Direct observation using scanning from the data to the polarized reflectivity, and GenX was used for
electron microscopy with polarization analysis (SEMPA)40,41 indi- the PNR analysis.45 In GenX, the figure of merit (FOM) is the indi-
cated that Co90Fe10 (B) had imperfect 90 degrees magnetization with cation of the consistency of the fitting curve and experimental data.
respect to the magnetization of Co Fe (A).1590 10 We could obtain the reasonable results after XRR and PNR fittings
To obtain more accurate magnetization information for the two were repeated alternately until the FOM was less than 2e−1.
types of multilayer films, we carried out PNR measurements using a Figures 2(a) and (b) show the PNR profiles and fitting curves
neutron wavelength of 0.24–0.88 nm at SHARAKU (BL17) in the for the Ru and NiFeCr buffer samples, respectively, when the applied
FIG. 1. Angle θ between magnetiza-
tion of the two FM layers (a). Sample
structures with Ru buffer sample (b) and
NiFeCr buffer sample (c). PNR measure-
ment layout and Cartesian coordination
definition (d).
AIP Advances 10, 015323 (2020); doi: 10.1063/1.5130445 10, 015323-2
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FIG. 2. PNR results and fitting curves; (a) Ru buffer sam-
ple in +1000 Oe, +29 Oe, −28 Oe and, (b) NiFeCr buffer
sample in +1000 Oe, +32 Oe, −45 Oe.
fields are changed. The dots indicate the experimental data and the of FeO, Fe3O4, and Fe2O3. Finally, the calculated curve of the
lines show the calculated results of fitting. After repeating the XRR FeO/Fe2O3 bilayer showed the best agreement with the experimen-
and PNR fitting, we obtain sufficiently good agreement between the tal data. The structure parameters did not change when the external
experimental data and the calculated data with FOM less than 2e−1. fields were changed, indicating that the fitting results were reason-
The fitting results of the parameters of each layer are summarized in able. In addition, the magnetic structure parameters were widely
Table I. Regarding the Fe–O layer, we tried various combinations changed by the applied field. To clarify the magnetic structure, we
TABLE I. The structure parameters and the magnetic structure parameters derived from PNR analysis for the Ru and NiFeCr buffer samples.
Ru buffer sample NiFeCr buffer sample
Magnetization Magnetization
Thickness Density Roughness Thickness Density Roughness
/nm /g cm-3 /nm +1000 Oe +29 Oe -28 Oe /nm /g cm-3 /nm +1000 Oe +32 Oe -45 Oe
Ta2O5 2.69 9.17 0.81 3.33 8.88 0.30
Ta 2.50 15.92 0.60 2.43 16.53 0.02
Cu 0.80 9.45 0.40 0.70 8.02 0.49
Co Fe (C) 2.20 9.54 1.10 1.90 μB 1.90 μB 1.90 μB 2.80 9.64 0.50 1.80 μB 1.90 μB 1.83 μB90 10 90 deg 85 deg -84 deg 89 deg 88 deg -90 deg
Cu 5.49 8.97 0.50 5.56 8.02 0.50
Co Fe (B) 1.98 7.79 0.40 1.78 μB 1.42 μB 1.41 μB 1.78 μB 1.10 μB 1.12μB90 10 91 deg 1 deg 0 deg 2.00 9.63 0.25 92 deg 1 deg -19 deg
Fe2O3 1.00 2.61 0.50 1.71 2.65 0.01
FeO 0.95 4.18 0.70 0.10 5.17 0.02
Co Fe (A) 1.97 9.54 0.30 1.90 μB 1.90 μB 1.90 μB 1.91 9.64 0.33 1.90 μB 1.78 μB 1.75 μB90 10 51 deg 1 deg 0 deg 63 deg 3 deg -28 deg
Ir22Mn78 5.56 11.59 0.50 5.19 11.58 0.13
Co90Fe10 1.18 9.54 0.15
Ru or
Ni Fe Cr 2.10 14.10 0.60 4.71 7.21 0.4248 22 40
Ta 5.20 14.72 0.27 4.77 15.02 0.14
SiO2 200 2.10 0.32 204 1.80 0.01
AIP Advances 10, 015323 (2020); doi: 10.1063/1.5130445 10, 015323-3
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FIG. 3. Correspondence between MH curves and schematic images of magnetization for Co90Fe10 (A), Co90Fe10 (B) and Co90Fe10 (C) estimated from PNR analysis for (a)
Ru and (b) NiFeCr buffer samples.
represent the magnetization images of Co90Fe10(A), (B), and (C) in Co90Fe10 (A) recovered in the x-direction as expected. As the applied
Fig. 3. field swept to the negative value, the magnetization of the Co90Fe10
Figures 3(a) and (b) show the correspondence between the MH (A) layer for the NiFeCr buffer sample tilted toward −28○, whereas
curves and the magnetic configurations of Co90Fe10 (A), Co90Fe10 while that for the Ru buffer sample stayed the same, which corre-
(B), and Co90Fe10 (C) estimated from PNR fitting for the Ru buffer sponds to the fact that the exchange bias from IrMn for the NiFeCr
sample and NiFeCr buffer samples, respectively. The magnetic field buffer sample was weaker than that of the Ru buffer sample.15
was applied in the PNR measurement along the y-axis which is nor- Finally, let us consider the PNR results of the Co90Fe10 (B)
mal to the biasing field of Co90Fe10 (A) from IrMn along the x-axis. layer, which is expected to be the quasi-AFM layer. At +1000 Oe,
Here, if the Co90Fe10 (A) magnetization is perfectly fixed in the +x- the magnetization was 1.78μB, 91○ and 1.78μ , 92○B for the Ru and
direction by IrMn, the +x-directions corresponds to θ = 0○, and the NiFeCr buffer samples, respectively, meaning that the magnetiza-
+y and−y direction correspond to θ = 90○ and θ =−90○, respectively. tion saturated in the applied field direction. At +29 Oe and +32 Oe,
First, let us focus on the PNR results of the Co90Fe ○ ○10 (C) layer the magnetization was 1.42μB, 1 and 1.10μB, 1 for the Ru and
in the two samples, in which the magnetization is expected to be free NiFeCr buffer samples, respectively. At −28 Oe and −45 Oe, the
to reverse. At +1000 Oe, the magnetization for the Ru and NiFeCr magnetization was 1.41μB, 0○ and 1.12μB, −19○, respectively. From
buffer samples was 1.90μ ○B, 90 and 1.90μ , 89○B , respectively, which these results, we derived the images of magnetization of Co90Fe10 (B)
means that the magnetization saturated in the applied field direction, quantitively as explained below.
the +y-direction. When the applied field Hy for the Ru and NiFeCr We consider the Ru buffer sample first. The magnetization of
buffer samples was changed from +29 Oe to −28 Oe and from +32 1.42μB is evidently lower than the saturated moment 1.82μB, which
Oe to −45 Oe, respectively, the magnetization changed from 1.90μB, means a part of the magnetization of Co90Fe10 (B) in the low field
85○ and 1.90μB, 88○ to 1.90μB, −84○ and 1.83μB, −90○, respectively. was canceled out due to the antiparallel magnetization originat-
These results mean that the magnetization reversed as expected. ing from the 90-deg coupling with Co90Fe10 (A). From the energy
Second, let us see the PNR results of the Co90Fe10 (A) layer in Eq. (3), the magnetization of Co90Fe10 (A) and Co90Fe10 (B) should
both samples, in which the magnetization is expected to be fixed in be at angle θ. In the +29 Oe field, because the magnetization of
the +x-direction in a low applied field. At +1000 Oe, the magnetiza- Co ○90Fe10 (A) was 1.90μB, 1 , the magnetization angle of Co90Fe10
tion was 1.90μ ○B, 51 and 1.90μ , 63○B for the Ru and NiFeCr buffer (B) should be 1○ + θ or 1○ − θ to satisfy the smallest magnetic static
samples, respectively. These results are attributed to the competi- energy. Because the magnetization with an angle of 1○ + θ can be
tion of the exchange bias in the direction of the x-axis by the IrMn decomposed into the +y-direction and the +x-direction, whereas the
layer and the external field Hy in the direction of the y-axis. As the magnetization with an angle of 1○ − θ can be decomposed into the
applied field decreased to +29 Oe and +32 Oe, the magnetization of −y-direction and the +x-direction, the components of magnetization
AIP Advances 10, 015323 (2020); doi: 10.1063/1.5130445 10, 015323-4
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in the +y-direction and the −y-direction will be canceled out. As a 5R. Sbiaa, H. Meng, and S. N. Piramanayagam, Phys. Status Solidi RRL 7, 332
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12
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