Hybrid memory layer with low Curie temperature CoPd/Pd multilayer for high-density magnetic random access memory cells.

Introduction Spin transfer torque (STT) switching is considered as a promising writing scheme to realize a Gbit-class magnetic random access memory (MRAM). To create a high density MRAM with more than 10 Gbit capacity, further improvements of the memory cells with high thermal stability Δ and low switching current density $J_{\mathrm{sw}}$ are required. We have designed bi-multilayers (MLs) stack with high Curie temperature $(T_{C})$ Co/Pd and low $T_{C}$ CoPd/Pd MLs for the memory layer to achieve efficient spin transfer torque (STT) switching [1, 2]. Previously, we have reported that the switching of only the high $T_{\mathrm{C}}$ Co/Pd MLs at 170°C promoted the full switching of bi-MLs hybrid stack due to the exchange coupling between high and low $T_{\mathrm{C}}$ MLs [1]. In this work, we fabricated a tri-MLs hybrid stack with Co/Pd, CoPd/ Pd, and Co/Pt MLs, since the tri-MLs stack is calculated to be more efficient than bi-MLs stack, and we report temperature dependence of the hysteresis of the tri-MLs and STT switching of the memory layer with low $T_{\mathrm{C}}$ CoPd/Pd. Experimental method The tri-MLs hybrid stacks were fabricated by magnetron sputtering on thermally oxidized silicon substrates. The stack is substrate / Ta (10 nm) / Pt (5nm) / [Pt (1.2 nm) / $\mathrm{Co} (0.4\,\mathrm{nm})]_{6}$ ML / [Pd (1.2 nm) / $\mathrm{Co}_{48}\mathrm{Pd}_{52}(0.3\,\mathrm{nm})]_{3}$ ML / [Pd (1.2 nm) / $\mathrm{Co} (0.4\,\mathrm{nm})]_{3}$ ML / SiN (5 nm). The intermediate Pd/CoPd ML exhibits low $T_{C}$ of ∽ 130°C, and we also fabricated the sample replacing the low $T_{C}$ Pd/CoPd by Pd (4.5 nm) as a control sample. We refer to the former stack as stack A and the latter stack B. Hysteresis loops were checked by magneto-optical Kerr spectrum measurement system at various sample temperatures. Experimental result Figure 1 (a) shows hysteresis loop of stack A at room temperature. The loop exhibits square shape indicating that the magnetizations of tri-MLs switch simultaneously due to the exchange coupling through intermediate CoPd/ Pd MLs. On the other hand, at 172°C, the loop exhibited two-step feature suggesting the Co/Pd and Co/Pt MLs switch independently as shown in Fig. 1 (b). The $H_{c}$ of Co/Pd and Co/Pt MLs were estimated to be 0.42 kOe and 1.8 kOe, respectively. The $H_{c}$ of the two MLs were similar to those of sample B with intermediate Pt 4.5 nm layer (not shown in the figure). Figure 2 shows temperature dependence of Kerr rotation and coercivities of the two MLs in the stack A estimated from the hysteresis shown in Fig. 1 (a) and (b). The Kerr rotation gradually decreased with increasing the temperature. The single step hysteresis, indicating the two MLs switch simultaneously, was observed up to 120°C, and then the two-step feature with different $H_{c}$ of Co/ Pd and Co/Pt MLs was obvious above 130°C, which is consistent with $T_{C}$ of the intermediate CoPd/Pd ML. The $H_{c}$ of Co/Pd (Co/Pt) ML decreased (increased) with increasing the temperature, which may indicate the gradual decrease of the exchange coupling through the CoPd/Pd ML by elevating the temperature. In the presentation, STT switching of the bi-MLs stack with CoPd/Pd and Co/Pd MLs is also discussed.