J. Langer

Switching current reduction using perpendicular anisotropy in CoFeB–MgO magnetic tunnel junctions

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.

Ultrafast switching in magnetic tunnel junction based orthogonal spin transfer devices

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to a free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. Reliable switching is observed at room temperature with 0.7 V amplitude pulses of 500 ps duration. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy of less than 450 fJ.

Tunneling magnetothermopower in magnetic tunnel junction nanopillars.

We study tunneling magnetothermopower (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by an electric heater line. Thermopower voltages up to a few tens of μV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence the thermal gradient. The thermopower signal varies by up to 10  μV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100  μV/K and a large TMTP change of the tunnel junction of up to 90%.

Deep subnanosecond spin torque switching in magnetic tunnel junctions with combined in-plane and perpendicular polarizers

We show that adding a perpendicular polarizer to a conventional spin torque memory element with an in-plane free layer and an in-plane polarizer can significantly increase the write speed and decrease the write energy of the element. We demonstrate the operation of such spin torque memory elements with write energies of 0.4 pJ and write times of 0.12 ns.

Ultra-low switching energy and scaling in electric-field-controlled nanoscale magnetic tunnel junctions with high resistance-area product

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 voltage-induced switching. The measured performance enables integration with same-size CMOS transistors in compact memory and logic integrated circuits.

1∕f noise in linearized low resistance MgO magnetic tunnel junctions

Low RA MgO magnetic tunnel junctions prepared at Singulus (Ta3∕CuN30∕Ta5∕PtMn20∕CoFe2.5∕Ru0.7∕CoFeB3∕MgO1.2∕CoFeB3∕Ta5 (thickness in nanometers) were microfabricated at INESC-MN. The junctions were patterned into micron-sized sensors (5–20μm2) with controlled shape anisotropy (aspect ratio ranging from 2 to 20). A small external longitudinal bias field (15–30 Oe) was further used to improve sensor linearity. The MgO junctions have a resistance-area product of 150Ωμm2 and a maximum tunnel magnetoresistance of 130%. Noise measurements were done in linearized sensors, from dc to 500 kHz. The magnetic and nonmagnetic contributions to the 1∕f noise were determined. From the data fitting, Hooge parameters of ∼2.20×10−9μm2 were obtained for the nonmagnetic 1∕f noise. Analysis of direct experimental data revealed the possibility to detect variations of magnetic fields in the order of 10−10T∕Hz0.5 with these MgO junctions, demonstrating their potential for ultralow-field detection.

Voltage-induced switching of nanoscale magnetic tunnel junctions

We demonstrate voltage-induced (non-STT) switching of nanoscale, high resistance voltage-controlled magnetic tunnel junctions (VMTJs) with pulses down to 10 ns. We show ~10x reduction in switching energies (compared to STT) with leakage currents <; 105 A/cm2. Switching dynamics, from quasi-static to the nanosecond regime, are studied in detail. Finally, a strategy for eliminating the need for external magnetic-fields, where switching is performed by set/reset voltages of different amplitudes but same polarity, is proposed and verified experimentally.

Low Write-Energy Magnetic Tunnel Junctions for High-Speed Spin-Transfer-Torque MRAM

This letter presents energy-efficient MgO based magnetic tunnel junction (MTJ) bits for high-speed spin transfer torque magnetoresistive random access memory (STT-MRAM). We present experimental data illustrating the effect of device shape, area, and tunnel-barrier thickness of the MTJ on its switching voltage, thermal stability, and energy per write operation in the nanosecond switching regime. Finite-temperature micromagnetic simulations show that the write energy changes with operating temperature. The temperature sensitivity increases with increasing write pulsewidth and decreasing write voltage. We demonstrate STT-MRAM cells with switching energies of <;1 pJ for write times of 1-5 ns.

Electric-Field-Controlled Magnetoelectric RAM: Progress, Challenges, and Scaling

We review the recent progress in the development of magnetoelectric RAM (MeRAM) based on electric-field-controlled writing in magnetic tunnel junctions (MTJs). MeRAM uses the tunneling magnetoresistance effect for readout in a two-terminal memory element, similar to other types of magnetic RAM. However, the writing of information is performed by voltage control of 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 or spin-orbit torque) 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 operations 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 kbit 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.

Effect of resistance-area product on spin-transfer switching in MgO-based magnetic tunnel junction memory cells

We use ultrafast current-induced switching measurements to study spin-transfer switching performance metrics, such as write energy per bit (EW) and switching current density (Jc), as a function of resistance-area product (RA) (hence MgO thickness) in magnetic tunnel junction cells used for magnetoresistive random access memory (MRAM). EW increases with RA, while Jc decreases with increasing RA for both switching directions. The results are discussed in terms of RA optimization for low write energy and current drive capability (hence density) of the MRAM cells. Switching times <2 ns and write energies <0.3 pJ are demonstrated for 135 nm×65 nm CoFeB/MgO/CoFeB devices.