MRAM is a promising candidate to be a universal memory'. In a conventional MKAM design, information is stored in the free layer in MTJ, usually pattemed into ellipse-like shape. The data integrity and switching field between 1 and 0 is mainly determined by the shape anisotropy. However, the switching field can be grcatly varied by the free layer initial magnetization states ("C" or "S" state)2 and sometimes leads in to metastable vortex state3, increasing switching field and its variation, causing write failure. In this work, we present results of experimental and micromagnetic modeling study of intermediate magnetization states, their effects on the switching behavior and designs to eliminate them.Elliptical MRAM cell with varies aspect ratio (c/a=1.0-5.0) and minor axis length (a=2.0 -0.2 pm) was built using synthetic pinned layer-AlOx-free layer structure. The process is optimized to obtain good film properties. Various magnetic materials were investigated. Resistance-area production is in the range of 1000-5000 k R p * with magneto-resistance ratio 43% for the NiFe to 73.4% for the CoFeB. Very high -0.79 V was consistently achieved.At-field and remnant R-H curves were collected for various design and process parameters: free layer thickness, cell aspect ratio, different free layer material NiFe, CoFe, CoFeB and combinations of them. For not well optimized designs, the at-field R-H curves are not desirable squares with significant amount of irregularities (kinks), similar as reported before'-5. Those kinks can be separated into two types: type "V" as shown by Figure la, and type "H" by Fig. Ib. For the type V, the slope of kinks extrapolates through mid level of high and low resistance. Four sequential major to minor loops were performed on this device starting from R-normalized= -1, as shown by Fig. 2. It shows that when the minor loop field is higher than the kink nucleation field while smaller than the exit field H,, then a metastable magnetization state can be locked in. Micro-magnetic modeling utilizing Landau-Lifshitz-Gilbert equation is performed and both the at-field and minor loop magnetization CUWCS calculated were identical to thc experimental results. From micro-magnetic modeling, the magnetization of this kink state (type "V") is identified as vortex configuration. Once vortex is formed, its center rotates along the direction perpendicular to the applied field directions, break out near edges when the applied field exceeds H,. Hence the €4, has an angular dependence with smallest value for the applied field along the long axis of ellipse. Fig. 3 shows the remnant R-H curves with various hard axis field of the same device. It clearly shows a metastable mid-resistance vortex state for hard axis field Hy=O and 4 Oe. The vortex state disappears for higher Ily>SOc but reappears momentarily at Hy=25Oe. It is very troublesome that the vortex state cannot be completely eliminated by applying high hard axis field and has to be measured extensively to catch it, note that measurement #3 in Fig.la shows perfe...
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