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

2015/11/13
装置責任者 Name of responsible person

2014B0120

Kenichi OIKAWA

実験課題名 Title of experimentError rate evaluation of MRAM with perpendicular magnetic 装置名 Name of Instrument/（BL No.）NOBORU (BL No. 10)

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

実施日 Date of Experiment

Yuzuru NARITA
所属 Affiliation

2014/12/15～2014/12/16

Yamagata University

2015/ 3/20～2015/ 3/23

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, 436 p-MTJ (magnetic tunnel junctions with a perpendicular magnetic easy axis)
devices equipped with an MgO/CoFeB/Ta/CoFeB/MgO double-interface recording structure formed on a
40×40 mm2 (3-inch) SiO2/Si wafer (Fig. 1) were fabricated atTohoku University. Each p-MTJ has a stacked profile structurefrom the SiO2/Si substrate side: Ta (5)/Ru (10)/Ta (5)/Pt (5)/[Co(0.4)/Pt (0.4)]6/Co (0.4)/Ru (0.4)/[Co (0.4)/Pt (0.4)]2/Co (0.4)/Ta(0.3)/CoFeB(1.0)/MgO(1)/CoFeB(1.6)/Ta(0.4)/CoFeB(1.0)/MgO (1)/Ta (5)/Ru (5), where the numbers in parentheses
are the nominal thickness in nanometers, and the subscripts
indicate the number of Co/Pt multilayers. The stack is processed
into circular D = 40 nm MTJs using electron beam lithography
and Ar ion milling, followed by annealing at 300 °C undervacuum with a perpendicular magnetic field of 0.4 T for 1 h. TheFig. 1: Photograph of p-MTJ
device.

sample was the same as that used in the 2014A0102 experiment.
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 beamirradiation area was approximately 80×80 mm2. Low-energy neutrons were separated from the neutron
beam using 5-mm-thick B4C slits while gamma radiation was blocked using a 25-mm-thick Pb block.
During irradiation, gamma radiation was monitored using an alanine dosimeter set near the p-MTJs
within the beam irradiation area. The tunnel magnetoresistance (TMR) ratio for each p-MTJ wasdetermined from resistance-magnetic field (R-H) measurements.
12. 実験方法及び結果（つづき） Experimental method and results (continued)

In this experiment, fast neutron irradiation of p-MTJ devices with D = 40 nm was performed for 4days. The beam time was used in two separate periods (1 day + 3 days) to measure the influence of fastneutron irradiation. The beam powers of the first (2014/12/15 – 2014/12/16) and second (2015/3/20 –2015/3/23) periods were 0.3 and 0.4 MW, respectively. The total fast neutron fluence (1 MeV equivalent
neutrons/cm2) with BL-10 was calculated to be 2.1×1012 neutrons/cm2 in this study. The number ofneutrons irradiated on a junction with D = 40 nm was approximately 26.
In the 2014A0102 experiment, a single p-MTJ
junction with D = 40 nm was irradiated with a total of(a)approximately 5 fast neutrons. Figure 2(a) shows acomparison of the TMR ratios before and afterirradiation with 5 fast neutrons. 436 p-MTJ devices
with TMR ratios above 70% were used. After
irradiation with 5 fast neutrons, the TMR ratio of two
p-MTJ devices was significantly improved; however,
the TMR ratio of the other devices slightly deteriorated.
In this experiment, a single p-MTJ junction with D =40 nm was additionally irradiated with approximately
26 fast neutrons, where the same sample from the(b)2014A0102 experiment was used. Figure 2(b) shows acomparison of the TMR ratios before and afterirradiation with 31 (5+26) fast neutrons. The TMR
ratios before irradiation on the horizontal axis were thesame as those in Fig. 2(a). After irradiation with 31 fastneutrons, the TMR ratios for many of the p-MTJ
devices were changed. Although a slight degradation
was observed for devices with an original TMR ratio of>90%, a significant improvement was observed fordevices with an original TMR ratio of approximately

Fig. 2: Comparison of TMR ratio before and70%. While the main cause of this phenomenon has notafter fast neutron irradiation. One p-MTJ

yet been elucidated, the effect of fast neutrons

junction with D = 40 nm was irradiated withirradiated to the p-MTJ devices correlates with aapproximately (a) 5 and (b) 31 (5 + 26) fastchange in the TMR ratio. To obtain more reliable

neutrons.

results, we will continue this experiment in the 2015A
proposal round.

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