Three-Terminal Spin Transfer Torque device Utilizing the giant Spin Hall effect of Tantalum CNF Project Number : 111-80

Here, we report the study of magnesium oxide (MgO)based magnetic tunneling junctions (MTJs) with symmetric electrodes FeCoB in both the as-grown and annealed (350°C) state. Analysis of the adjusted torkance indicates that the annealing reduces the inelastic tunneling component of the junction conductance, and creates or enhances a peak at about 0.15V above the Fermi level in the density of states of the minority band of one or both of the electrodes. This peak location is essentially the same as the peak of the minority band states that has previously observed on the surface of bcc (100) Fe [1-2]. Thus we conclude that the most likely source of the asymmetry of the bias dependent torkance is due to Fe minority band states that are localized at the electrode-tunnel barrier interface. Summary of Research: In this report, we study MgO-based MTJs of the composition (thickness in nanometers) bottom contact/IrMn 10/CoFeB 2/ MgO 1/CoFeB 1.5/ top contact. The top CoFeB was patterned by e-beam lithography and an ion beam etching technique to produce an elliptical or circular cross-section with a nominal size of 200 × 120 nm2 or 150 × 150 nm2 for current induced switching and spin torque ferromagnetic resonance (STFMR) measurement (Figure 1). The resistance-area (RA) product for the parallel configuration in our as-grown MTJs (annealed) is about 16Ω*μm2 (20Ω*μm2). The etching was stopped at the MgO barrier and thus the bottom layer is largely unpatterned to minimize the stray field from the bottom ferromagnetic electrode. After the patterning process, some of the MTJs were measured directly and others cut from the same wafer were annealed at 350°C for 1h under an easy-axis magnetic field of 2T in vacuum to enhance the tunneling magnetoresistance (TMR). Our results indicate that the tunneling mechanisms for two cases. For the as-grown sample (Figure 2), inelastic tunneling determines the bias dependence of in plane torque. Specifically, the distribution of inelastic tunneling centers is asymmetric, with more scattering on the left side. This asymmetry is probably due to the film growing process: the left interface (between the bottom electrode and the tunneling barrier) is further oxidized or damaged by oxygen plasma during the following MgO sputtering, which does not happen for the right interface. After annealing (Figure 3), the dominated tunneling mechanism is elastic tunneling with DOS dependent on energy. Especially there is a DOS peak (probably interface states of electrodes) in minority band located just above the Fermi level. Considering the fact that the main difference between those two samples is the crystalline structure of the FeCoB, our results indicate that: First, the inelastic tunneling is favored in the MTJs with amorphous electrodes, while annealed samples prefer elastic tunneling. Regarding B diffusion and its reaction with O at the interface during the annealing, the interfaces have been cleaned. With improved interfaces and the barrier, inelastic tunneling could be suppressed, leading to a much higher TMR. Second, samples with crystalline electrodes have much sharper feature in minority DOS close to Fermi level than those with amorphous FeCoB layers, which is reasonable because the disorder in crystalline structure usually acts to broaden the DOS peaks.