junction spin-valve depending on top MgO barrier thickness

pinned perpendicular magnetic-tunnel- junction spin-valve depending on top MgO barrier thickness

Cite as: AIP Advances 10, 065126 (2020); https://doi.org/10.1063/5.0007064

Submitted: 15 March 2020 • Accepted: 26 May 2020 • Published Online: 18 June 2020 Han-Sol Jun, Jin-Young Choi, Kei Ashiba, et al.

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Multi-level resistance uniformity of double pinned perpendicular magnetic-tunnel-junction

spin-valve depending on top MgO barrier thickness

Cite as: AIP Advances10, 065126 (2020);doi: 10.1063/5.0007064 Submitted: 15 March 2020•Accepted: 26 May 2020•

Published Online: 18 June 2020

Han-Sol Jun,¹ Jin-Young Choi,² Kei Ashiba,^2,3 Sun-Hwa Jung,¹ Miri Park,² Jong-Ung Baek,¹ Tae-Hun Shim,² and Jea-Gun Park^1,2,a)

AFFILIATIONS

1MRAM Center, Department of Nanoscale Semiconductor Engineering, Hanyang University, Seoul 04763, South Korea

2MRAM Center, Department of Electronics and Computer Engineering, Hanyang University, Seoul 04763, South Korea

3Wafer Engineering Department, SUMCO Corporation, 1-52 Kubara, Imari, Saga 849-4256, Japan

a)Author to whom correspondence should be addressed:parkjgl@hanyang.ac.kr.Present address:17 Haengdang-dong, Seongdong-gu, Seoul 04763, South Korea.Tel.:(+82)-2-2220-0234.Fax:(+82)-2-2296-1179.

ABSTRACT

In order to utilize perpendicular spin-torque-transfer magnetic-random-access-memory (p-STT MRAM) as a storage class memory, the achievement of performing multi-level-cell (MLC) operation is important in increasing the integration density of p-STT MRAM. For a dou- ble pinned perpendicular magnetic tunneling junction spin-valve performing MLC (i.e., four-resistance level) operation, the uniformity in the resistance difference between four-level resistances was investigated theoretically and experimentally. The uniformity in the resistance difference between four-level resistances was strongly dependent on the top MgO tunneling-barrier thickness. Particularly, the most uniform resistance difference between four resistance states could be achieved at a critical top-MgO tunneling thickness (i.e.,∼1.15 nm).

© 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/10.1063/5.0007064., s

INTRODUCTION

Recently, perpendicular spin-transfer-torque magnetic-random access memory (p-STT MRAM) has been intensively researched as an embedded memory and a stand-alone memory to replace the current dynamic-random-access-memory (DRAM) because of many advantages such as non-volatile characteristics, low power consumption (<1 pJ/bit), fast write and read speed (∼10 ns), high data retention (>10 year), and write and erase endurance (>10¹⁶).^1–6In addition, storage class memory (SCM) has been pro- posed to cover a memory hierarchy between the current DRAM and NAND flash memory, called a three dimensional cross-point memory.⁷SCM has 10–100 times faster latency than NAND, while it provides real-time access to data like DRAM at a lower cost. In

particular, an SCM implemented with p-STT-MRAM has been sug- gested as a memory mapped SCM, having a similar write and read speed and 2 times higher latency than DRAM. However, an SCM implemented with p-STT-MRAM would be essentially necessary to achieve multi-level-cell (MLC) operation to overcome NAND flash memory, where MLC or triple-bit-cell (TLC) operation is essen- tially necessary to provide high integration density memory cells and reduce production cost per bit. Attempts have been previously made to demonstrate MLC operation with in-plane perpendicular mag- netic tunneling junction (p-MTJ) spin-valves.^8–10However, a p-MTJ has a lower critical switching current density than in-plane MTJ with high thermal stability theoretically and experimentally, and it is pos- sible to fabricate it under a 20 nm node; therefore, it has a higher possibility as a highly integrated memory.^11–14In our previous study,

FIG. 1. Schemes and the design con- cept of the double pinned p-MTJ spin- valve: (a) the double pinned p-MTJ spin- valve structure, (b) the spin direction of four resistance states, and (c) the esti- mated resistance of the double pinned p-MTJ spin-valve depending on top MgO tunneling barrier thickness.

we have demonstrated a p-STT MRAM with a double pinned p- MTJ spin-valve that exhibits 4-level resistance, but the resistance difference was not uniform. Hence, in this study, we suggested a p-STT MRAM fabricated with a double pinned p-MTJ spin-valve to perform the MLC operation, as shown in Fig. 1(a). In partic- ular, we investigated the dependency of the top MgO tunneling- barrier thickness on the MLC (4-level resistance states) operation mechanism change and resistance difference uniformity between 4- level resistance states, which is closely related to the resistance area [RA(Ω/μm²)]and tunneling magnetic ratio (TMR) of the top MTJ (ΔR=RP⋅TMR).

METHODS

12-in.-diameter Si/SiO2/W/TiN bottom electrodes were made by chemical mechanical planarization to achieve a surface roughness of less than 0.5 nm using a CMP polisher (EBARA) and were verti- cally stacked by a double pinned p-MTJ spin-valve multi-chamber sputtering system under a high vacuum pressure of less than 1

×10⁻⁸Torr, as shown inFig. 1(a). The double pinned p-MTJ spin- valve was divided largely into five magnetic moment layers depend- ing on the magnetic layers:M1,M2,M3,M4, and M5. The dou- ble pinned p-MTJ spin-valve was vertically stacked with TiN elec- trode/seed/synthetic anti-ferro layer (SyAF) (M1)/spacer/Co buffer + spacer + lower Co2Fe6B2 pinned (1.1 nm) layer (M2)/bottom MgO tunneling barrier (1.0 nm)/lower free Co2Fe6B2(0.9 nm) layer + spacer + upper free Co2Fe6B2(1.0 nm) layer (M3)/upper MgO tunneling barrier (1.05 nm)/top Co2Fe6B2pinned (1.0 nm) +spacer

+ SyAF layer (M4)/spacer/SyAF layer (M5)/capping layer/top Ru electrode tunneling barrier. Then, the p-MTJ spin-valves were ex- situ annealed at 350^○C under a vacuum of less than 10⁻⁶Torr and a perpendicular magnetic field of 3 T for 30 min. The TMR ratios of the double pinned p-MTJ spin-valves were estimated by using current-in-plane-technique (CIPT) measurement at room temper- ature. TheR–Hcurve was measured with a patterned p-MTJ spin- valve with a cell diameter of 1-μm via photolithography and ion- milling. The patterned p-MTJ spin-valves were wire-bonded to the sample holder and were installed into a home-made electrical prob- ing system with a∼1 T electromagnet using a Keithley 236 source measure unit and an Agilent B2902A semiconductor parameter analyzer.

RESULTS AND DISCUSSION

The magnetization direction of theM1andM2(bottom pinned layer) ferro-magnetic layers was designed to be in the opposite direc- tion (M1is along the−z-axis whileM2is along the +z-axis direction) to the anti-ferro coupling via a Ru spacer, as seen inFig. 1(a). In par- ticular, the magnetization direction of the Co buffer and Co2Fe6B2

pinned layer ofM2layers was aligned in the same direction normal to the sample plane either in the +z axis or−z axis direction by RKKY (Ruderman-Kittel-Kasuya-Yosida) through the W spacer. The −z axis direction of theM2layers, shown inFig. 1(b), is formed when the saturation magnetization direction is along the +z axis direc- tion. Similarly, the Co2Fe6B2free layers ofM3were ferro-coupled to each other via a W spacer (called a double free layer), and the

magnetization direction of theM3ferro-magnetic layers depended on the direction of the applied perpendicular-magnetic-field (Happ).

TheM4(top pinned layer) andM5layers were anti-ferro coupled by the Ru spacer layer. The magnetization direction of theM4and M5layers was always opposite to each other, but the magnetization directions of the two layers were either facing toward or against each other along the z-axis, depending onHapp. Note that the required Happto change the magnetization direction ofM4was larger than that ofM3, where the coercivity ofM3was 17 Oe while that ofM4

was 611 Oe.

In order for the double pinned p-MTJ spin-valve to have uniform resistance difference between four-level resistances, two parameters should be considered: (1) the resistance ratio between the top and bottom MgO tunneling barrier (Rb,p/Rt,p) and (2) the TMR ratio of the top and bottom p-MTJ. For simple understanding, we assumed a double p-MTJ perpendicular-magnetic-anisotropy (PMA) structure, as shown inFig. 1(b), where the bottom p-MTJ1

producedR1andTMR1while the top bottom p-MTJ2generatedR2

andTMR2. The four different resistances were defined by the combi- nation of the magnetization direction (i.e., parallel and anti-parallel) of p-MTJ1or p-MTJ2, as shown in the equation below:

RPstate=R1,P+R2,P, (1a)

RAP state1=R1,AP+R2,P, (1b)

RAP state2=R1,P+R2,AP, (1c)

RAP state3=R1,AP+R2,AP. (1d)

The resistance of the anti-parallel states was calculated from the TMR ratio by the following equations:

R1,AP= (1 +TMR1

100 )R1,P, (2a)

R2,AP= (1 +TMR2

100 )R2,P. (2b)

The resistances of Eq.(1)(RP,total,RAP1,RAP2, andRAP3) corre- spond to the P state, AP state 1, AP state 2, and AP state 3, respec- tively. The magnetization directions required for the four resistance states are shown inFig. 1(b), which can be explained in more detail throughFig. 2. The resistances were calculated, as shown inFig. 1(c), using the experimental data of a conventional double MgO based p-MTJ spin-valve, shown insupplementary materialFig. S1. It was assumed that the RA of p-MTJ2would be about 70% lower than that of p-MTJ1although the MgO tunneling thickness of p-MTJ2was the same as that of p-MTJ1since the crystallinity degradation of the top MgO tunneling barrier would be induced from the roughness of the under layers (M1,M2, andM3).^15–22Using Eqs.(1)and(2)and RAs fromsupplementary materialFig. S1, four resistance states (Rp,RAP1, RAP2, andRAP3) were simulated, as shown inFig. 1(c). The P state (Rp) and AP state 3 (RAP3) represented the lowest and highest resis- tance states of the double pinned p-MTJ spin-valve, respectively, where both p-MTJ1and p-MTJ2were parallel or anti-parallel states.

However, both AP state 1 (RAP1) and AP state 2 (RAP2) strongly depended on the top tunneling-barrier thickness, i.e., AP state 1 (RAP1) decreased with increasing top MgO tunneling barrier thick- ness, while AP state 2 (RAP2) increased with top MgO tunneling- barrier thickness. Thus, a uniform resistance difference among four

FIG. 2. Static magnetic behavior of dou- ble pinned p-MTJ spin-valves and the spin magnetization direction configura- tion depending on external magnetic field: (a) theM–Hcurve at a wide exter- nal magnetic field range of 6.5 kOe, (b) the spin configuration of the double pinned p-MTJ spin-valve for magnetic field sweep from +6.5 to−6.5 kOe, (c) theM–Hcurve at a narrow external mag- netic field of 1.0 kOe, and (d) the spin configuration of the double pinned p-MTJ spin-valve for a magnetic field sweep of 1.0 kOe.

resistance states (Rp,RAP1,RAP2, andRAP3) could be obtained at spe- cific thickness of the top MgO tunneling barrier, i.e., at 1.1 nm. The detailed mechanism by which four resistance states (Rp,RAP1,RAP2, and RAP3) varied as a function of the top-MgO tunneling-barrier thickness will be explained later.

To understand by which mechanism the double pinned p- MTJ spin-valve can perform the MLC operation, the static mag- netic switching behavior of the double pinned p-MTJ spin-valve with a top MgO tunneling barrier thickness of 1.15 nm was investi- gated with the magnetic moment vs external applied magnetic field (M–H) curves, as shown inFig. 2. Be reminded that the double pinned p-MTJ spin-valve was composed of bottom p-MTJ1and top p-MTJ2layers. The resistances of the bottom p-MTJ1and the top p-MTJ2layers wereR1andR2, respectively. First, the magnetization direction of all ferro-magnetic layers was aligned along the +z-axis at an Happ of +6.5 kOe, resulting in a total magnetic moment of

∼1040μemu. First, whenHappdecreased to +4 kOe, the perpendic- ular magnetic moment decreased by 730μemu, as shown in (1) of Figs. 2(a)and2(b). This corresponded to the switching ofM2and M5 due to anti-ferro coupling with theM1 andM4, respectively.

The magnetization direction ofM2andM5was along the−z axis direction, while that ofM1,M3, andM4was along the +z axis direc- tion. Thus,M2andM3were under an anti-parallel state, whileM3

and M4were under a parallel state, producing AP state 1 (RAP1).

Second, whenHappdecreased from +4.0 to∼−0.5 kOe, the perpen- dicular magnetic moment decreased by∼405μemu, as shown in (2) ofFigs. 2(a)and2(b), which occurs by the switching ofM3. As a result, the magnetization direction ofM2,M3, andM5was along the−z-axis direction, while that ofM1andM4was along the +z-axis direction. Thus,M2andM3were under a parallel state, whileM3and M4were under an anti-parallel state, generating AP state 2 (RAP2).

Third, whenHappdecreased from−0.5 to∼ −1.0 kOe, the perpen- dicular magnetic moment decreased by∼265μemu, as shown in (3) ofFigs. 2(a)and2(b). This implies the consecutive switching of both M4andM5, where the magnetization direction ofM4was switched from the +z-axis to−z-axis direction, while the magnetization direc- tion ofM5was switched from the−z-axis to + z-axis direction. As a result, the magnetization direction ofM2,M3, andM4was facing the

−z-axis direction, while that of theM1andM5layers was facing the +z-axis direction. Thus,M2,M3, andM4were under a parallel state, inducing the P state (RP). Fourth, whenHappdecreased from−1.0 to

−4.2 kOe, the perpendicular magnetic moment decreased a further

∼219μemu, as shown in (4) ofFigs. 2(a)and2(b), which indicates the switching ofM5. As a result, the magnetization direction ofM2, M3,M4, andM5was along the−z-axis direction, while only that of M1was along in the +z-axis direction. However,M2,M3, andM4

were all in a parallel state, which would have the same resistance as the P state. Finally, whenHappdecreased to−6.5 kOe, the perpendic- ular magnetic moment decreased by a further∼448μemu, as shown in (5) ofFigs. 2(a)and2(b). In addition, the magnetization of CoFeB and Co/Pt SyAF is calculated using theM–Hcurves ofFig. 2(a)and supplementary material2, which is similar to that of other stud- ies.^23–25However, the magnetization value decreases for Co/Pt SyAF of M1, M2, M4, and M5, which can be accounted for by the decrease in the PMA of the upper magnetic layers of M4 and M5 due to inter- facial roughness.²⁶The magnetization direction of all ferro-magnetic layers was aligned along the−z-axis direction, which has a resistance value same as that of the P state.Happis scanned from−6.5 kOe to

+6.5 kOe, and the magnetization direction of the five magnetic layers are switched exactly in the same order whenHappis scanned from +6.5 kOe to−6.5 kOe (seesupplementary material3). As a result, only three resistance states (P state, AP state 1, and AP state 2) can be confirmed as the switching of the magnetization direction of the bottom pinned structure (M1andM2) occurs with theHapp scan from +6.5 kOe to−6.5 kOe.

To clarify four resistance states,Happwas only scanned from

−1000 Oe to +1000 Oe, which was the operation range to prevent the switching ofM1andM2. The initial resistance began from AP state 1 (RAP1), as shown inFigs. 2(c)and2(d). First of all, when Happwas scanned from +1000 to−500 Oe, the magnetization direc- tion ofM3was only rotated from the +z-axis to the−z-axis where the perpendicular magnetic moment decreased by∼405μemu, pro- ducing AP state 2 (RAP2), as shown in (1) ofFigs. 2(c)and 2(d).

Consequently, whenHappwas scanned from−500 to−1000 Oe, the magnetization direction ofM4 was only rotated from the +z-axis to the−z-axis by changing the perpendicular magnetic moment to

∼265μemu, forming the P state (RP), as shown in (2) ofFigs. 2(c) and2(d). The P state (RP) showed the lowest resistance state since M3was in a parallel state with bothM2andM4. This shows the same switching trend whenHappwas scanned from +1000 to−1000, as shown inFigs. 2(a)and2(b). Then, whenHappwas scanned back- wards from−1000 to +300 Oe, the magnetization direction ofM3

was only rotated from the−z-axis to the + z-axis direction with a perpendicular magnetic moment change of∼386μemu, generating AP state 3 (RAP3), as shown in (3) ofFigs. 2(c)and2(d). AP state 3 (RAP3) presented the highest resistance state sinceM3produced anti-parallel states with bothM2 andM4. Finally, when Happ was scanned from +300 to +1000 Oe, the magnetization direction ofM4

was only rotated from the−z-axis to the + z-axis direction with a per- pendicular magnetic moment change of∼284μemu, returning back to AP state 1 (RAP1), as shown in (4) ofFigs. 2(c)and2(d). Thus, the scanning of the applied magnetic-field from−1000 to +1000 Oe clearly demonstrated four resistance states, when the coercivity of M4was larger than that ofM3and the magnetization direction of M1andM2should not be changed, being able to reduce write-errors.

The resistance of the four states depending on the configuration of the magnetization direction is summarized inTable I. These results indicate that AP state 3 (RAP3) would have the highest resistance, while the P state (RP) would have the lowest resistance. However, the resistance level of AP state 1 (RAP1) and AP state 2 (RAP2) would be determined by the top MgO tunneling-barrier thickness, as shown in Fig. 1(c), which will be examined by experiments later. In addition, the static magnetic switching behaviors of the double pinned p-MTJ spin-valve depending on the top MgO tunneling barrier thickness were not different from those shown inFigs. 2(a)and2(b), and it can be confirmed throughsupplementary materialFig. S4.

In order to examine the resistance change depending on the top MgO tunneling barrier, the R–H curves were measured with 1.0-μm-cell-size cylindrical patterned double-pinned-p-MTJ spin- valves, as shown in Fig. 1(a). The resistance changes accordingly with the switching mechanism, as shown in Figs. 2(c) and 2(d), which cycles around four resistance states, AP state 1 (red), AP state 2 (blue), the P state (purple), and AP state 3 (green), as shown inFig. 3(a). When the top MgO tunneling-barrier thick was thin (0.90 nm), the resistance of AP state 2 and AP state 1 was similar to that of the P state and AP state 3, respectively, where

TABLE I. Resistance of each 4-level depending on the configuration of the magnetization direction.

Resistance configuration MTJ1 MTJ2 Total resistance

AP state 1 Anti-parallel (Rhigh) Parallel (Rlow) Intermediate AP state 2 Parallel (Rlow) Anti-parallel (Rhigh) Intermediate

P state Parallel (Rlow) Parallel (Rlow) Low

AP state 3 Anti-parallel (Rhigh) Anti-parallel (Rhigh) High

the resistance differencesΔRAP2–P,ΔRAP1–AP2, andΔRAP3–AP1were 1.2 Ω, 18.6 Ω, and 1.5 Ω, respectively, as shown inFig. 3(a). This is due to the small resistance change in the top p-MTJ junction when the MgO tunneling-barrier thickness is very thin. The resis- tance differences |ΔRAP2–P|, |ΔRAP1–AP2|, and |ΔRAP3–AP1| is summa- rized inTable II. Second, when the MgO tunneling-barrier thickness increased from 0.90 nm to 1.15 nm, the resistance of AP state 1 decreased, while that of AP state 2 increased, resulting in an increase in bothΔRAP2–PandΔRAP3–AP1whereas a decrease inΔRAP1–AP2, as shown inFig. 3(b).^27–29As a result, ΔRAP2–P (10.2 Ω), ΔRAP1–AP2

(7.4 Ω), and ΔRAP3–AP1 (9.5 Ω) became uniform. Third, when the MgO tunneling-barrier thickness increased from 1.15 nm to 1.19 nm, the resistance of AP state 1 decreased, while that of AP state 2 increased, increasingΔRAP2–PandΔRAP3–AP1while decreas- ingΔRAP1–AP2, as shown inFig. 3(c). In particular, the uniformity of ΔRAP2–P(16.8 Ω),ΔRAP1–AP2(3.0 Ω), andΔRAP3–AP1(13.5 Ω) became worse, but the resistance of AP state 1 sustained longer than that of AP state 2. Fourth, when the MgO tunneling-barrier thickness increased from 1.19 nm to 1.25 nm, the resistance of AP state 1

decreased further, while that of the AP state 2 increased further, producingΔRAP1–P,ΔRAP2–AP1andΔRAP3–AP2of 15.0 Ω, 3.7 Ω, and 15.6 Ω, respectively, where the resistance of AP state 1 was lower than that of AP state 2, as shown inFig. 3(d). Thus, the unifor- mity ofΔRAP2–P,ΔRAP1–AP2, andΔRAP3–AP1at 1.25 nm was similar to that at 1.19 nm. Finally, when the MgO tunneling-barrier thick- ness increased from 1.25 nm to 1.35 nm, the resistance of AP state 1 further decreased, while that of AP state 2 further increased, causing a decrease inΔRAP1–PandΔRAP3–AP2but an increase inΔRAP2–AP1, as shown in Fig. 3(e). As a result,ΔRAP1–P (23.4 Ω), ΔRAP2–AP1

(16.6 Ω), andΔRAP3–AP2(24.3 Ω) became uniform again, similar to those at 1.15 nm. Note that the resistances of all states exponen- tially increased with the top MgO tunneling-barrier, as shown in Fig. 3(f). Thus, a thick top MgO tunneling-barrier thickness such as 1.35 nm would cause a breakdown of the bottom MgO tunnel- ing barrier since the switching current of the double pinned p-MTJ spin-valve was enhanced. In addition, the TMR ratio between AP state 1 and the P state decreases from 167.8% to 41.5%, as shown in thesupplementary material(Fig. S4). Therefore, the dependency of

FIG. 3. Dependency of top MgO tunneling barrier on resistance properties of the double pinned p-MTJ spin-valve in a narrow scanning magnetic-field range (−1 kOe to +1 kOe): (a) a pie chart to illustrate the change in the resistance level by external field, theR–Hloop of the double pinned p-MTJ spin-valve with a cell size of 1μm where the top MgO thickness was (b) 0.9 nm, (c) 1.15 nm, (d) 1.19 nm, (e) 1.25 nm, and (f) 1.35 nm, and (g) the resistance trend of the double pinned p-MTJ spin-valve depending on MgO thickness.

TABLE II. The difference between AP state three, the highest resistance level, and AP state1, the difference between AP state1 and AP state 2, and the difference between AP state1 and the P state, the was lowest resistance level. The parentheses represent the resistance difference percentage of the given top MgO tunneling barrier thickness.

Top MgO thickness (nm) ΔRAP3–AP1(Ω) ΔRAP1–AP2(Ω) ΔRAP2–P(Ω)

0.9 1.5 (7.1%) 18.6 (87.3%) 1.2 (5.6%)

1.15 9.6 (35.3%) 7.4 (27.2%) 10.2 (37.5%)

1.19 13.5 (40.5%) 3 (9.0%) 16.8 (50.5%)

1.25 15.6 (45.4%) 3.7 (10.8%) 15 (43.7%)

1.35 24.3 (37.8%) 16.6 (25.8%) 23.4 (36.4%)

the uniformity between four resistances on the top MgO tunneling- barrier thickness evidently demonstrated that the best uniformity between four resistances could be achieved at a specific top MgO tunneling-barrier thickness (i.e.,∼1.15). Partial switching of the dou- ble pinned p-MTJ spin-valve with a top MgO thickness of 0.9 nm was achieved by STT switching at a pattern size of 1.0-μm-cell-size cylindrical patterned double-pinned-p-MTJ spin-valves, as shown insupplementary materialFig. S5. When the electron moves from the top to the bottom of the MTJ in the initial P state, the elec- tron with the down-spin is parallel to the magnetization direction of theM4,M3, andM2layers, making it easy to pass through the top and bottom tunnel barriers, so no switching occurs from the down-spin electrons. On the other hand, electrons with up-spins are reflected at both the top and bottom MgO tunneling barriers. The reflected up-spin electrons accumulate in both theM4andM3lay- ers, and the magnetization direction of theM3layer changes first due to its smaller Hc thanM4, and the resistance increases to AP state 3 at−0.88 V. Subsequently, theM4layer also switches to a higher voltage of−1.0V, changing the resistance state to AP state 1. The double pinned p-MTJ spin-valve has high overall RA due to two MgO tunneling barriers. In order to supply sufficient current for STT switching, reducing the RA of the tunneling barriers is needed.

Also, face-centered-cubic (fcc) crystallinity of the MgO tunneling barriers must be maintained in order to have high TMR ratios and STT efficiency to have a sufficient sensing margin between the four resistance states. Therefore, it is essential to reduce the overall RA of the two MgO tunneling barriers while maintaining their crystallinity to maximize the TMR ratio, thereby increasing the STT efficiency to switch the double pinned p-MTJ spin-valve.³⁰

CONCLUSION

In summary, the double pinned p-MTJ spin-valve exhibited an MLC (i.e., four level) operation theoretically and experimen- tally. In particular, the achievement of a uniform resistance dif- ference between four-level resistance states would be key design research of the double pinned p-MTJ spin-valve to avoid a write and read disturbance in integrated memory-cells. It was found that the uniformity of the resistance difference between four-level resistance states strongly depended on the top MgO tunneling- barrier thickness of the double pinned p-MTJ spin-valve, i.e., a specific top MgO tunneling-barrier thickness (i.e.,∼1.15 nm, 10%

thicker than bottom MgO tunneling barrier) would result in the best uniformity (ΔRAP3–AP1 was 35.3%, ΔRAP1–AP2 was 27.2%, and

ΔRAP2–Pwas 37.5%) of the resistance difference between four-level resistance states. Fundamentally, in a double pinned p-MTJ spin- valve, the TMR ratio of the top p-MTJ would be lower than that of the bottom p-MTJ spin-valve since the face-centered-cubic (fcc) crystallinity of the top p-MTJ could be worse than that of the bot- tom p-MTJ spin-valve. Thus, precise fcc crystallinity control of the interface between the top MgO tunneling barrier and the top Co2Fe6B2free layer and a design of a nano-scale buffer layer prevent- ing Pt atom diffusion would be essentially necessary for the TMR ratio of the double pinned p-MTJ spin-valve.^31–38In addition, the achievement of an MLC operation of the double pinned p-MTJ spin- valve with a larger memory density would give one an open path to develop 3D cross-point memory as an SCM and a stand-alone memory to replace the current DRAM.

SUPPLEMENTARY MATERIAL

See thesupplementary materialfor the TMR ratio of a single- level-cell p-MTJ spin-valve, static magnetic properties (M–Hcurve) depending on top MgO tunneling barrier thickness, and the TMR ratio between AP state 1 and the P state of a double pinned p-MTJ spin-valve.

ACKNOWLEDGMENTS

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Grant No.

2017R1A2A1A05001285) and the Brain Korea 21 PLUS Program in 2014.

DATA AVAILABILITY

The data that support the findings of this study are available within the article (and itssupplementary material).

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