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

2017/5/18
装置責任者 Name of responsible person

2015A0134

Kenichi OIKAWA
装置名 Name of Instrument/（BL No.）

実験課題名 Title of experimentError 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

2015/11/7～2015/11/9

Yuzuru NARITA

2016/6/18～2016/6/20

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 used 461 p-MTJ (magnetic tunnel junction with a perpendicular magnetic
easy axis) devices equipped with a CoFeB-MgO recording structure. Each p-MTJ has a stacked
profile structure from the SiO2/Si substrate side: Ta (5)/Ru (10)/Ta (5)/Pt (5)/[Co (0.34)/Pt (0.4)]6/Co(0.34)/Ru (0.44)/[Co (0.34)/Pt (0.4)]2/Co (0.34)/Ta (0.3)/CoFeB/MgO/CoFeB/Ta (1)/Ru (7), where
the numbers in parentheses are the nominal thicknesses in nanometers, and the subscripts indicate thenumber of Co/Pt multilayers. The stack is processed into circular p-MTJs using electron beamlithography and Ar ion milling, followed by annealing at 300 °C under vacuum with a perpendicularmagnetic field of 0.4 T for 1 h. The junction diameter (D) of each p-MTJ was within the range of 46
to 64 nm. The p-MTJ devices having a tunnel magnetoresistance (TMR) ratio above 90% at room
temperature were used.

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 experiment was conducted using the BL10 beamline. The neutron
beam irradiation area was approximately 80×80 mm2. Low-energy neutrons were separated from theneutron beam using 5-mm-thick B4C slits while gamma radiation was blocked using a 25-mm-thickPb block. During irradiation, gamma radiation was monitored using an alanine dosimeter placed near
the p-MTJs within the beam irradiation area. The effect of neutron irradiation was evaluated bycomparing the resistance-magnetic field (R-H) curves of the p-MTJs before and after neutronirradiation.
12. 実験方法及び結果（つづき） Experimental method and results (continued)

In the 2015A0134 experiment, fast neutron irradiation of the p-MTJ devices was performed
over 4 days. The beam time was utilized in two separate periods (2 days + 2 days) to measure theinfluence of the fast neutron irradiation. The beam powers for the first (2015/11/7 – 2015/11/9) andsecond (2016/6/18 – 2016/6/20) periods were 0.5 and 0.2 MW, respectively. The total fast neutron
fluence (1 MeV equivalent neutrons/cm2) at BL-10 was calculated to be 1.32×1012 neutrons/cm2 inthe 2015A0134 experiment. The total fluence of fast neutrons irradiated to the 461 p-MTJ devices
was 3.79×1012 neutrons/cm2 (proposal numbers 2014A0102, 2014B0120, and 2015A0134), whichcorresponded to 1.90×1011 h (approximately 22 million years) irradiation in fast neutron
environments at ground level (20 cm−2h−1).
Figure 1(a) shows R-H curves for a p-MTJ with D = 48 nm and a TMR ratio of 108%
following neutron irradiation up to 3.79×1012 cm-2. In this figure, the dashed lines indicate theresistance values in the parallel and anti-parallel states prior to irradiation, RPbefore and RAPbefore,respectively. As can be seen, the resistance values and switching fields were unchanged from thoseprior to irradiation. Figure 1(b) shows a comparison of the TMR ratios of the 461 p-MTJs before andafter irradiation of 3.79×1012 neutrons/cm2. After the irradiation, the variation induced in the TMR
ratios was in the range of approximately ±1%. Therefore, the properties of the CoFeB-MgO p-MTJ
devices were not significantly affected by exposure to a neutron fluence of up to 3.79×1012
neutrons/cm2.
We summarized the data obtained from the neutron irradiation experiments using BL-10(proposal numbers 2014A0102, 2014B0120, and 2015A0134) and submitted an article to theJapanese Journal of Applied Physics.
(a)

(b)

Fig. 1 (a) R-H curves for a p-MTJ with D = 48 nm and a TMR ratio of 108% following neutronirradiation up to 3.79×1012 cm-2. (b) Comparison of TMR ratio before and after fast neutron irradiationof 3.79×1012 cm-2.

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