Session Y4: High TMR MgO Tunneling and Spin Momentum Transfer Materials, Physics, and Devices

Giant tunneling magnetoresistance and tunneling spin polarization in magnetic tunnel junctions with MgO (100) tunnel barriers

Recent advances in generating, manipulating and detecting spin-polarized electrons and electrical current make possible new classes of spin based sensor, memory and logic devices [1]. One key component of many such devices is the magnetic tunneling junction (MTJ) - a sandwich of thin layers of metallic ferromagnetic electrodes separated by a tunneling barrier, typically an oxide material only a few atoms thick. The magnitude of the tunneling current passing through the barrier can be adjusted by varying the relative magnetic orientation of the adjacent ferromagnetic layers. As a result, MTJs can be used to sense the magnitude of magnetic fields or to store information. The electronic structure of the ferromagnet together with that of the insulator determines the spin polarization of the current through an MTJ -- the ratio of 'up' to 'down' spin electrons. Using conventional amorphous alumina tunnel barriers tunneling spin polarization (TSP) values of up to $\sim $55{\%} are found for conventional 3d ferromagnets, such as CoFe, but using highly textured crystalline MgO tunnel barriers TSP values of more than 90{\%} can be achieved for otherwise the same ferromagnet [2]. Such TSP values rival those previously observed only with half-metallic ferromagnets. Corresponding giant values of tunneling magnetoresistance (TMR) are found, exceeding 350{\%} at room temperature and nearly 600{\%} at 3K. Perhaps surprisingly the MgO tunnel barrier can be quite rough: its thickness depends on the local crystalline texture of the barrier, which itself is influenced by structural defects in the underlayer. We show that the magnitude and the sign of the TMR is strongly influenced by defects in the tunnel barrier and by the detailed structure of the barrier/ferromagnet interfaces. The observation of Kondo-assisted tunneling phenomena will be discussed as well as the detailed dependence of TMR on chemical bonding at the interfaces [3]. [1] .S.S.P. Parkin, X. Jiang, C. Kaiser, et al., Proc. IEEE 91, 661 (2003). [2] S. S. P. Parkin, C. Kaiser, A. Panchula, et al., Nature Mater. 3, 862 (2004). [3] C. Kaiser, S. van Dijken, S.-H. Yang, H. Yang and S.S.P. Parkin, Phys. Rev. Lett. 94, 247203 (2005).   

Giant TMR effect and spin momentum transfer in MgO-based magnetic tunnel junctions

First-principle theories predicted an extremely high magnetoresistance (MR) ratio over 1000{\%} in fully epitaxial Fe(001)/MgO(001)/Fe(001) MTJs [1]. This giant tunneling magnetoresistance (TMR) effect originates from a coherent spin-dependent tunneling of highly spin-polarized $\Delta _{1}$ electronic states. We have fabricated fully epitaxial Fe$_{1-x}$Co$_{x}$(001)/MgO(001)/Fe(001) MTJs [2,3] and CoFeB/MgO(001)/CoFeB MTJs [4] and achieved giant MR ratios above 200{\%} at room temperature. A low resistance-area (\textit{RA}) product indispensable for magnetic sensor application has also been achieved in CoFeB/MgO(001)/CoFeB MTJs [5]. Because of the high spin polarization of tunneling electrons, the MgO-based MTJs have an advantage in spin transfer phenomena, too. Current-induced magnetization reversal due to spin transfer torque has been demonstrated using CoFeB/MgO(001)/CoFeB MTJs [6]. The MTJ was also found to act as a microwave detector [7]. When an ac current with a microwave frequency is applied to the MTJ, a dc offset voltage is generated. This phenomenon, named as spin-torque diode effect, originates from spin momentum transfer, ferromagnetic resonance and the giant TMR effect. The giant TMR effect and spin momentum transfer in MgO-based MTJs are the key for next-generation spintronic devices. References [1] W. H. Butler \textit{et al}., Phys. Rev. B\textbf{ 63}, 054416 (2001). [2] S. Yuasa \textit{et al}., Nature Mater. \textbf{3}, 868 (2004). [3] S. Yuasa \textit{et al}., Appl. Phys. Lett. \textbf{87}, 222508 (2005). [4] D. D. Djayaprawira \textit{et al}., Appl. Phys. Lett. \textbf{86}, 092502 (2005). [5] K. Tsunekawa et al., Appl. Phys. Lett. \textbf{87}, 072503 (2005). [6] H. Kubota \textit{et al}., Jpn. J. Appl. Phys. \textbf{44}, L1237 (2005). [7] A. A. Tulapurkar \textit{et al}., Nature \textbf{438}, 339 (2005).   

Spin torque, tunnel-current spin polarization and magnetoresistance in MgO magnetic tunnel junctions

The ability of electron currents to transfer spin angular momentum, as well as charge, from one ferromagnetic electrode to another, and hence to exert a significant spin-torque on the electrodes, provides a powerful new tool for the study of spin transport in electronic structures, in addition to establishing new opportunities for future applications. The closely related issue of spin-dependent electron transport in magnetic tunnel junctions (MTJs) is of wide-spread interest, both fundamentally and because the importance this phenomena has for information storage. A critical aspect of MTJs is the bias dependence of the tunnel magnetoresistance (TMR), which in general, decreases as the voltage bias ($V)$ increases. Currently, there is no consensus as to a microscopic model that accounts for this behavior. In this study, we employ the spin torque response of MTJs with ultra-thin MgO tunnel barrier layers to investigate the relationship between spin transfer and TMR under finite bias, and find that the spin torque per unit current exerted on the free layer decreases by $<$ 10{\%} over a bias range where the TMR decreases by $>$ 40{\%}.~ This behavior is inconsistent with a decrease in the tunnel polarization factors calculated with the Julliere formula extended to finite bias, and as predicted by free-electron tunneling models, or by surface-magnon emission models that substantially decrease the surface magnetization with increasing bias. We find, however, that magnetic-state-dependent tunneling decay lengths (effective masses) as theoretically predicted for MgO tunnel barriers, are consistent with our results. Since these results also have significant implications for spin-torque driven magnetic random access memory, we will consider these effects in addition to our work with MTJs having two polarizing magnetic layers in order to boost spin-torque as well as allow us to determine the extent of the considerable self-heating for MTJs under bias.   

Structure Engineering And Shape Optimization To Decrease Switching Current For Spin Transfer MRAM Application

We present a systematic study of spin transfer switching in magnetic tunneling junctions (MTJs) to decrease the switching current density through material and structural engineering and MTJ element shape optimization. Data are presented for switching on MgO-based MTJs with high TMR of 170 {\%} and low intrinsic switching current density $J_{c0} \quad \le $ 1x10$^{6}$ A/cm$^{2}$. Micromagnetic modeling is used to study the spin transfer switching mechanism in nanosecond regime for elliptical shape of the MTJ element. The results suggest that the elliptical shape provides faster switching (lower switching current) and more reproducible switching than the conventional shapes optimized for magnetic field switched MRAM (Magnetic Random Access Memory).   

Thermal activation and switching dynamics in spin-torque-induced magnetic reversal in magnetic tunnel junctions with MgO barriers

Spin-torque induced magnetic reversal has been unambiguously demonstrated in magnetic tunnel junctions with MgO barriers. During a quasi-static measurement, the reversal is dominated by events determined by spin-current amplified thermal activation, resulting in a measured average switching current below that of the zero-temperature dynamic threshold. Such sub-threshold switching current generally shows stronger and non-linear magnetic field dependence, following a shape determined by the magnetic field dependence of the thermal barrier height. Time-resolved measurements are usually required for adequately assessing the dynamic switching threshold current for fast (nano-second-level) deterministic switching, and for revealing the magnetic field dependence of the threshold current. The later would give direct experimental verification of the role a large easy-plane demagnetization field plays as it determines the value of the dynamic switching current threshold.