実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report
課題番号 Project No.

提出日 Date of Report

2017/6/27
装置責任者 Name of responsible person

2016A0012

Kenichi OIKAWA

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Error rate evaluation of MRAM with perpendicular magnetic

NOBORU (BL No. 10)
実施日 Date of Experiment

tunnel junction under fast neutron irradiation
実験責任者名 Name of principal investigator

2016/11/5～2016/11/14

Yuzuru NARITA
所属 Affiliation

Yamagata University
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.

In this study, we employed 24 magnetic tunnel junction devices having a perpendicularmagnetic easy axis (p-MTJ), each equipped with an MgO/CoFeB/Ta/CoFeB/MgO double-interface
recording structure. Each p-MTJ also had a stacked profile structure on the SiO2/Si substrate side
consisting of Ta/Ru/Ta/Pt/[Co/Pt]6/Co/Ru/[Co/Pt]2/Co/Ta/CoFeB/MgO/CoFeB/Ta/CoFeB/MgO/Ta/Ru, where the subscripts indicate the number of Co/Pt multilayers. These stacks were processed into
circular p-MTJs using electron beam lithography and Ar ion milling, followed by annealing at
300 °C under vacuum while applying a perpendicular magnetic field of 0.4 T for 1 h. The junction
diameter (D) of each p-MTJ was less than 20 nm, and these devices exhibited a tunnel
magnetoresistance (TMR) ratio above 50% at room temperature.

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

The neutron irradiation experiments were performed using the BL-10 beamline, applying aneutron beam irradiation area of approximately 80 × 80 mm. Low-energy neutrons were separated
from the neutron beam using 5-mm-thick B4C slits while gamma radiation was blocked by a
25-mm-thick Pb block. The effects of neutron irradiation were evaluated by comparing theresistance-magnetic field (R-H) curves of the p-MTJs before and after neutron irradiation.

12. 実験方法及び結果（つづき） Experimental method and results (continued)

In the 2016A0012 experiment, fast neutron irradiation of the p-MTJ devices was performed
over nine days. Since the beam power was 0.15 MW, the fast neutron fluence (1 MeV equivalent
neutrons/cm2) generated by the BL-10 beamline was calculated to be 1.89 × 1012 cm-2. Since the fastneutron irradiation to the 24 p-MTJ devices was conducted in the 2014B0120 and 2015A0134
experiments before this experiment, the total fast neutron fluence irradiated to the 24 p-MTJ devices
was 5.21 × 1012 cm-2, corresponding to exposure to a fast neutron environment at ground level at 20
cm−2h−1 over a time span of 2.60 × 1011 h (approximately 30 million years).
Figure 1(a) shows R-H curves obtained from a p-MTJ with D = 19 nm and a TMR ratio of
100% before and after a neutron irradiation up to 3.05 × 1012 cm-2. As can be seen, the resistancevalue and switching field were unchanged from those prior to irradiation. Figure 1(b) plots thechanges in the TMR ratios of 24 p-MTJs as a function of D following the same neutron irradiation.
After irradiation, the variation induced in the TMR ratios was in the range of approximately ±3%.
These data demonstrate that the properties of p-MTJ devices having an MgO/CoFeB/Ta/
CoFeB/MgO double-interface recording structure and a D value below 20 nm were not significantly
affected by exposure to a neutron fluence of up to 3.05 × 1012 cm-2. In our previous study (Y. Narita
et al., JJAP 56, 0802B3 (2017)), we demonstrated that the performance of CoFeB-MgO p-MTJs
having a single-interface recording structure and D values between 44 and 64 nm was not degradedfollowing irradiation with fast neutrons at a fluence up to 3.79 × 1012 cm-2. These results together
with our present data show that p-MTJ devices are unaffected by fast neutron irradiation at suchlevels, regardless of the junction size.
At present, our group is acquiring and analyzing the R-H curves of 24 p-MTJs following
neutron irradiation up to 5.21 × 1012 cm-2 and the resulting data will be submitted for publication in a
future paper.
(b)

(a)

Fig. 1 (a) R-H curves for a p-MTJ with D = 19 nm and a TMR ratio of 100% before and after neutron
irradiation of 3.05 × 1012 cm-2. (b) Changes in the TMR ratios of 24 p-MTJs versus D following fastneutron irradiation at a fluence of 3.05 × 1012 cm-2.

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