Double Pinned Perpendicular-Magnetic-Tunnel-Junction Spin-Valve Providing Multi-level Resistance States

A new design for high density integration greater than gigabits of perpendicular-magnetic-tunnel-junction (p-MTJ) spin-valve, called the double pinned (i.e., bottom and top pinned structures) p-MTJ spin-valve achieved a multi-level memory-cell operation exhibiting four-level resistances. Three key magnetic properties, the anisotropy exchange field (Hex) of the bottom pinned structure, the coercivity (Hc) of the double free-layer, and the Hc of the top pinned structure mainly determined four-level resistances producing tunneling-magnetoresistance (TMR) ratios of 152.6%, 33.6%, and 166.5%. The three key-design concepts are: i) the bottom pinned structure with a sufficiently large Hex to avoid a write-error, ii) the Hc of the double free-layer (i.e., ~0.1 kOe) much less than the Hc of the top pinned structure (i.e., ~1.0 kOe), and iii) the top pinned structure providing different electron spin directions.

bottom single SyAF [Co/Pt] n multilayers (Fig. 1b) because the upper [Co/Pt] 3 SyAF multilayer anti-ferro-coupled with the lower [Co/Pt] 6 SyAF multilayers (Fig. 1a) via a Ru spacer produced considerably high surface roughness 18 . The design concept of the double pinned p-STT MTJ will be explained in more detail in the following section. In addition, we investigated static magnetic properties of the double pinned p-MTJ spin-valve, tested the achievement of four-level magnetic-resistance states, and analyzed the operation mechanism of four-level resistances.

Design of Double pinned p-MtJ Spin-valve. The double pinned p-MTJ spin-valve was vertically stacked
with the bottom electrode, bottom Co 2 Fe 6 B 2 ferromagnetic pinned structure (called bottom pinned structure), double MgO based Co 2 Fe 6 B 2 ferromagnetic free layer (called the double free-layer), top Co 2 Fe 6 B 2 ferromagnetic pinned structure (called top pinned structure), and top electrode, as shown in Fig. 1b. The magnetic layers of the double pinned p-MTJ spin valve can be divided largely into five groups (M 1 , M 2 , M 3 , M 4 , and M 5 layers). The bottom electrode was made by sputtering tungsten (W) and titanium nitride (TiN) layers on a thermally oxidized 300-mm-diameter Si wafer, followed by the chemical-mechanical-planarization (CMP). For the bottom pinned structure, the Ta buffer layer was used to intersect the f.c.c. crystalline texturing of the TiN electrode since the Ta layer had an amorphous structure. The face-centered-cubic crystalline Pt-seed-layer was used to form the L10 crystalline structure of the bottom-lower SyAF [Co (0.47 nm)/Pt(0.23 nm)] 3 multilayer (M 1 layer) so that its spin direction was perpendicularly upward, as shown in Fig. 1c [24][25][26] . These layers were anti-ferro-coupled with the single Co (0.51 nm)/ Pt(0.23 nm)/Co(0.47 nm) buffer layer via Ru spacer, called the single SyAF [Co/Pt] n layer. Simultaneously, the single Co/Pt/Co buffer-layer was ferro-coupled to the bottom Co 2 Fe 6 B 2 ferromagnetic pinned layer (0.95 nm) via the W bridge layer, defined as M 2 layer. The spin direction of the M 2 layer was perpendicularly downward, as shown Fig. 1c. Thus, the spin directions of the M 1 and M 2 layers always face vertically inward towards each other. In particular, the anisotropy exchange field (H ex ) of the bottom pinned structure should be sufficiently higher than the coercivity (H c ) of the top pinned structure to fix the spin direction of both the M 1 and M 2 layers. Then, the double Co 2 Fe 6 B 2 free-layer (M 3 layer) was stacked on the M 2 layer, where the thicknesses of the bottom MgO tunneling-barrier, Fe insertion layer, lower Co 2 Fe 6 B 2 ferromagnetic free layer, W spacer, upper Co 2 Fe 6 B 2 ferromagnetic free layer, and top MgO tunneling-barrier layer were 1.15, 0.3, 1-0.05, 0.2, 1.05, and 1.0 nm, respectively. The spin direction of M 3 layer was dependent of the polarity of the applied perpendicular-magnetic-field; i.e. vertically downward for a negative field and vertically upward for a positive field, as shown in Fig. 1c. In particular, the H c of M 3 layer should be considerably smaller than the H c of the top pinned structure to make four different spin direction states between the M 3 and M 4 layers [i.e., anti-parallel (AP) state 1, AP state 2, parallel (P) state, and AP state 3]. For the top pinned structure, the Fe insertion layer (0.3 nm) and top Co 2 Fe 6 B 2 ferromagnetic pinned layer (0.75 nm) were stacked on the M 3 layer. The top-lower [Co(0.47 nm)/Pt(0.23 nm)] 3 SyAF multilayer was ferro-coupled with the top Co 2 Fe 6 B 2 ferro-magnetic pinned-layer (0.75 nm) via a W bridge layer and Co/Pt seed layer, which is defined as M 4 layer. The spin direction of the M 4 layer was dependent of the polarity of the applied magnetic-field. Simultaneously, M 4 layer was always anti-ferro-coupled with the top-upper [Co(0.47 nm)/Pt(0.23 nm)] 3 SyAF multilayers (M 5 layer), via the Ru spacer. Thus, the spin direction of the M 5 layer was always in the opposite of the M 4 layers, as shown in Fig. 1(c). In particular, the number (m) of the top-lower [Co(0.47 nm)/Pt(0.23 nm)] m SyAF multilayers should be higher than that (n) of the top-upper [Co(0.47 nm)/Pt(0.23 nm)] n multilayers to produce four-different spin direction states between the M 3 and M 4 layers. If m is lower than n, only two different spin direction states would be generated between the M 3 and M 4 layers. Finally, a top Ta/Ru electrode was stacked on the M 5 layer.
In summary, the design of the double pinned p-MTJ spin-valve could produce four-different spin direction states between the M 3 and M 4 layers: AP state 1 (perpendicularly upward spin direction for both M 3 and M 4 layers), AP state 2 (spin direction facing outward between M 3 and M 4 layers), P state (downward spin direction for both M 3 and M 4 layers), and AP state 3 (spin direction facing inward between the M 3 and M 4 layers, as shown in Fig. 1c). To form four-different spin direction states, we essentially need three key-design concepts: 1) for designing the bottom pinned structure, the spin direction of the M 1 and M 2 layers should face always inward toward each other, 2) for designing M 3 layer, its H c should be remarkably smaller than the H c of the M 4 layer to assure to produce four-different spin direction states between the M 3 and M 4 layers, and 3) for designing the top pinned structure, the H ex of the M 4 layers should be sufficiently higher than the H c of the the M 3 layer to avoid a write-error. These three key-design concepts will be treated later in detail. Using this concept, we redesigned the bottom pinned p-MTJ spin-valve ( Fig. 2a,b) with double and single SyAF [Co/Pt] n layers to match that of the double pinned p-MTJ spin-valve and investigated its magnetic properties. The arrow magnitude and direction corresponded to the relative magnetic-moment and spin direction of the magnetic layers when the applied magnetic field is changed from +6.5 kOe to −6.5 kOe, as shown in Fig. 2c,d. The inset of Fig. 2c,d shows the magnetic properties of the Co 2 Fe 6 B 2 free layers (M 3 layers) of the p-MTJ spin-valve with double and single SyAF [Co/Pt] n layers. The spin direction of both structures are aligned along the external field direction when external field is high in the upward direction (H > +5 kOe). When the external field becomes smaller than + 1.5 kOe, the spin direction of the M 2 layers are switched opposite the external field as the field is not strong enough to overcome the anti-ferro coupling between M 1 and M 2 layers. The M 2 layer squareness of the bottom p-MTJ spin-valve with single SyAF [Co/Pt] n layers (red box of Fig. 2d) is degraded compared to that of the spin-valve with double SyAF [Co/Pt] n layers (blue box of Fig. 2c). However, the H ex is increased from 2.35 kOe to 3.44 kOe which would mean that the M 1 and M 2 layers of the p-MTJ spin-valve with single SyAF [Co/Pt] n layers is more unsusceptible to switching. In addition, the peak-to-valley (Δ P-V ) of the MgO tunneling barrier decreased from 2.03 nm to 1.75 nm when the thickness of the SyAF layers is reduced from 8.87 to 4.65 nm as shown in Supplementary 2. Note that the TMR ratio of the double pinned p-MTJ spin-valve is expected to increase by reducing the roughness of the MgO tunneling barrier [27][28][29][30][31][32][33][34][35][36][37][38] . Thus, the magnetic property of the double free-layer was not degraded; i.e., a good squareness and the magnetic moment of ~ 0.2 memu, as shown in the inset of Fig. 2d. This result indicates that a single SyAF [Co/Pt] 3 multi-layers would be very suitable as a bottom pinned structure (M 1 and M 2 layers in Fig. 1b), since it could provide a sufficiently high H ex of 3.44 kOe and would increase the TMR ratio because of a lower surface roughness of the MgO tunneling-barrier. The spin direction schematic in Fig. 2c,d represents the change only when the magnetic field is changed from +6.5k Oe to −6.5 kOe. A detailed magnetic switching behavior of the p-MTJ spin-valve with double and single SyAF [Co/Pt] n layer under an external magnetic field sweep from −6.5 kOe to +6.5 kOe is shown in see  Fig. 3b. At the applied magnetic-field of +15 kOe, the spin direction of both the M 4 and M 5 layers faced perpendicularly upward. As the magnetic-field decreased from +15 to +4 kOe, the magnetic moment decreased from +0.6 to +0.3 memu, corresponding to the magnetic moment of the M 5 layer (i.e., 0.3 memu) rotating the spin direction of the M 5 layer from upward to downward. This occurred at the H ex of ~4.9 kOe arising from the anti-ferro coupling across the Ru spacer layer. As the magnetic-field decreased from +4 to −4 kOe, the magnetic moment changed from +0.3 to −0.3memu, responding to the magnetic moment of the M 4 and M 5 layers (i.e., 0.6 memu), rotating the spin direction of the M 4 layer from upward to downward. Simultaneously, the spin direction of the M 5 layers rotated from downward to upward to hold the anti-ferro coupling via the Ru spacer stably. As a result, the spin directions of the M 4 and M 5 layers facing perpendicularly inward changed to facing perpendicularly outward. As the magnetic-field increased over −4 kOe, the spin direction of the M 5 layer rotated from upward to downward so that the spin directions of both the M 4 and M 5 layers were perpendicularly downward. In contrast, as the magnetic-field changed from negative to positive direction, the change of the spin directions of the M 4 and M 5 layers followed the same order as the magnetic-field changed from positive to negative direction. In particular, the spin directions of the M 4 and M 5 layers facing perpendicularly outward changed to facing perpendicularly inward. Thus, this top pinned structure could produce two spin directions between the M 4 and M 5 layers when the magnetic-field is greater than the H c of the top pinned structure; facing perpendicularly outward for the negative magnetic-field and facing perpendicularly inward for the positive magnetic-field.   (Fig. 1c) since the H c of the top pinned structure (i.e., ~0.3 kOe) was not sufficiently higher than the H c of the double free-layer (Fig. 2d: i.e., ~0.2 kOe) to avoid the write error. Thus, we observed the dependency of the H c of the top pinned structure on the m:n ratio, as shown in Fig. 3d (Fig. 1c) since it is considerably larger than the H c of the double free-layer (~0.1 kOe) (Fig. 2d).

Design and Static perpendicular-Magnetic
Recall that at the m:n ratio of 3:3 the static magnetic moment of the M 4 layer was slightly larger than that of the M 5 layer so that the spin direction of the top pinned structure (M 4 and M 5 layers) faced vertically outward for the negative applied magnetic-field and vertically inward for the positive applied magnetic-field. If n is larger than m, the spin directions of the M 4 and M 5 layers could not be variable. Therefore, the design of choosing a proper H c and H ex of the top pinned structure would be a key research to stably produce four different spin direction states between M 3 and M 4 layers.

Static perpendicular-Magnetic Behaviour and Multi-level tMR ratio for Double pinned p-MtJ
Spin-Valve. By combining the top (3:3 of m:n in Fig. 3d) and bottom p-MTJ structures with the double free-layer (Fig. 2d), we fabricated the double pinned p-MTJ spin-valve shown in Fig. 1b. The M-H loop of the double pinned p-MTJ spin-valve showed the H ex of 4.9 kOe when the applied magnetic-field was scanned from www.nature.com/scientificreports www.nature.com/scientificreports/ −6.5 kOe to +6.5 kOe, as shown in Supplementary 5a. In addition, the resistance-vs.-magnetic-field (R-H) loop presented only three resistance states, when the applied magnetic-field was scanned from −H ex to +H ex kOe, as shown in Supplementary 5b,c. In order to produce four different resistance states, thus, the maximum scanning range of the applied magnetic-field should be sufficiently less than ±H ex (i.e., ~4.2 kOe) of the bottom pinned p-MTJ spin-valve, but greater than ±H ex (i.e., ~1 kOe) of the top pinned structure (M 4 and M 5 layers); i.e., ± 2 kOe. Thus, four different spin directions between the M 3 and M 4 layers could be stably produced when the applied magnetic-field was scanned from −2 kOe to +2kOe, as shown in the M-H loop of Fig. 4a. First, the AP state 1 was produced when the applied magnetic-field was scanned from +2 kOe to +0.5 kOe, where the spin directions of both the M 3 and M 4 layers were vertically upward and parallel while the spin directions of the M 4 and M 5 layers faced vertically inward toward each other via an anti-ferro-coupling, as shown Fig. 4a,b. Recall that this result corresponds to the combination of the top pinned structure (M 4 and M 5 layers) in Fig. 3d and the bottom pinned structure (M 1 and M 2 layers) with the double free-layer (M 3 layer) in Fig. 2d at the positive applied magnetic-field. Then, when the applied magnetic-field was scanned from +0.5 kOe to −0.5 kOe, the spin direction of only the M 3 layer was rotated from upward to downward while the spin directions of both the M 4 and M 5 layer did not change, generating the AP state 2, where the spin direction of the M 3 layer faced vertically outward against that of the M 4 layer, as shown in i in Fig. 4a,b. Furthermore, when the applied magnetic-field was scanned from −0.5 kOe to −2.0 kOe, the spin directions between the M 4 and M 5 layers facing vertically inward were rotated to face vertically outward while the spin direction of the double free-layer was sustained downward, forming the P state, where the spin directions of both M 3 and M 4 layers were vertically downward and in parallel, as shown in ii in Fig. 4a

Discussion
Our proposed double pinned p-MTJ spin-valve well demonstrated four-level resistance as a multi-level p-STT MRAM-cell, resulting in the TMR ratios of 152.6, 33.6, and 166.5%. The maximum TMR ratio of the double pinned p-MTJ spin-valve (166.5%) was slightly less than that of a single pinned p-MTJ spin-valve (i.e., 180%), since the Pt atoms in the top pinned structure diffused into both top and bottom MgO tunneling barrier so that the coherent tunneling of the spin-electron would be decreased as shown Supplementary 6. Thus, research on how to avoid the Pt diffusion from the top pinned structure is necessary; i.e., research on the design of a nano-scale buffer layer preventing Pt atom diffusion. In addition, to minimize a write-error originated between four-level resistances, the differences between four-level resistances should be as constant as possible. Thus, research on choosing a proper thickness between the top and bottom MgO tunneling barriers is also necessary. Success in the above-mentioned research will enable us to fabricate a terabit-level p-STT MRAM for embedded, stand-alone, and neuromorphic devices.

Methods
The p-MTJ spin-valves were fabricated using a 12-inch-wafer multi-chamber cluster-magnetron sputtering-system under a high vacuum (less than 1 × 10 −8 torr). In particular, the conventional p-MTJ spin-valve with the top double free-layer and double SyAF [Co/Pt] n layers in Fig. 1a Fig. 1b. In addition, a Co/Pt/Co buffer layer was used to bridge instead of the top-upper SyAF [Co(0.47 nm)/Pt(0.23 nm)] 3 layer (compare Fig. 1a,b). The MgO capping layer of the conventional double MgO-based p-MTJ spin-valve structure was used as the top MgO tunneling barrier followed by an Fe insertion layer (0. torr and a perpendicular magnetic-field of 3 tesla. The TMR ratios of the double pinned p-MTJ spin-valves fabricated on 12-inch Si wafers were estimated by using CIPT at room temperature. The wafers were cut into 1 × 1 cm 2 pieces. The magnetic properties of the double pinned spin-valves were characterized by using vibrating-sample magnetometer (VSM) at room temperature. The R-H curve was measured with a p-MTJ spin-valve with the cell size of 2-μm × 2-μm. The 2-μm-scale p-MTJ spin-valves were wire-bonded to the sample holder and were installed into a home-made electrical probing system with a ~1 Tesla electromagnet using a Keithley 236 source measure unit and Agilent B2902A semiconductor parameter analyzer.