We report electric-field-induced switching with write energies down to 6 fJ/bit for switching times of 0.5 ns, in nanoscale perpendicular magnetic tunnel junctions (MTJs) with high resistance-area product and diameters down to 50 nm. The ultra-low switching energy is made possible by a thick MgO barrier that ensures negligible spin-transfer torque contributions, along with a reduction of the Ohmic dissipation. We find that the switching voltage and time are insensitive to the junction diameter for high-resistance MTJs, a result accounted for by a macrospin model of purely voltageinduced switching. The measured performance enables integration with same-size CMOS transistors in compact memory and logic integrated circuits. V
Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of particular interest for magnetic random-access memories because of their excellent thermal stability, scaling potential, and power dissipation. However, the major challenge of current-induced switching in the nanopillars with both a large tunnel magnetoresistance ratio and a low junction resistance is still to be met. Here, we report spin transfer torque switching in nano-scale perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to 249% and a resistance area product as low as 7.0 Ω µm2, which consists of atom-thick W layers and double MgO/CoFeB interfaces. The efficient resonant tunnelling transmission induced by the atom-thick W layers could contribute to the larger magnetoresistance ratio than conventional structures with Ta layers, in addition to the robustness of W layers against high-temperature diffusion during annealing. The critical switching current density could be lower than 3.0 MA cm−2 for devices with a 45-nm radius.
We review the recent progress in the development of magnetoelectric random access memory (MeRAM), based on electric-fieldcontrolled writing in magnetic tunnel junctions (MTJs). MeRAM uses the tunneling magnetoresistance (TMR) effect for readout in a two-terminal memory element, similar to other types of magnetic random access memory (MRAM). However, the writing of information is performed by voltage control of the magnetic anisotropy (VCMA) at the interface of an MgO tunnel barrier and the CoFeB-based free layer, as opposed to current-controlled (e.g. spin-transfer torque, STT or spin-orbit torque, SOT) mechanisms. We present results on voltage-induced switching of MTJs in both resonant (precessional) and thermally activated regimes, which demonstrate fast (< 1 ns) and ultralow-power (< 40 fJ/bit) write operation at voltages ~ 1.5 -2 V. We also discuss the implications of the VCMA-based write mechanism on memory array design, highlighting the possibility of crossbar implementation for high bit density. Results are presented from a 1 Kb MeRAM test array. Endurance and voltage scaling data are presented. The scaling behavior is analyzed, and material-level requirements are discussed for the translation of MeRAM into mainstream memory applications.Index Terms-Nonvolatile memory, MeRAM, MRAM, voltage control of magnetic anisotropy, spin transfer torque. † These authors contributed equally to this work.
We present in-plane CoFeB–MgO magnetic tunnel junctions with perpendicular magnetic anisotropy in the free layer to reduce the spin transfer induced switching current. The tunneling magnetoresistance ratio, resistance-area product, and switching current densities are compared in magnetic tunnel junctions with different CoFeB compositions. The effects of CoFeB free layer thickness on its magnetic anisotropy and current-induced switching characteristics are studied by vibrating sample magnetometry and electrical transport measurements on patterned elliptical nanopillar devices. Switching current densities ∼4 MA/cm2 are obtained at 10 ns write times.
Recent studies have shown that material structures, which lack structural inversion symmetry and have high spin-orbit coupling can exhibit chiral magnetic textures and skyrmions which could be a key component for next generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that stabilizes skyrmions is an anti-symmetric exchange interaction favoring non-collinear orientation of neighboring spins. It has been shown that material systems with high DMI can lead to very efficient domain wall and skyrmion motion by spin-orbit torques. To engineer such devices, it is important to quantify the DMI for a given material system. Here we extract the DMI at the Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary measurement schemes namely asymmetric domain wall motion and the magnetic stripe annihilation. By using the two different measurement schemes, we find for W(5 nm)/Co 20 Fe 60 B 20 (0.6 nm)/MgO(2 nm) the DMI to be 0.68 ± 0.05 mJ/m 2 and 0.73 ± 0.5 mJ/m 2 , respectively. Furthermore, we show that this DMI stabilizes skyrmions at room temperature and that there is a strong dependence of the DMI on the relative composition of the CoFeB alloy. Finally we optimize the layers and the interfaces using different growth conditions and demonstrate that a higher deposition rate leads to a more uniform film with reduced pinning and skyrmions that can be manipulated by Spin-Orbit Torques.Recent advances in thin film fabrication processes have led to the accelerated development of magnetic storage devices. This has opened exciting areas of research due to the effects occurring at the interface between a heavy metal (HM) and a ferromagnet (FM). This interface is the building block for next generation memory devices such as the Spin-Orbit Torque (SOT) MRAM 1-4 . There are a number of important phenomena associated with the interface 5 : interfacial contributions to the SOTs 6 , interfacial perpendicular anisotropy 7,8 , and interfacial Dzyaloshinskii-Moriya interaction (DMI) 9-12 . DMI is an anti-symmetric exchange interaction which favours non-collinear alignment of neighbouring spins S 1
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