Dimitar V. Dimitrov

Spintronic Memristor Through Spin-Torque-Induced Magnetization Motion

Existence of spintronic memristor in nanoscale is demonstrated based upon spin-torque-induced magnetization switching and magnetic-domain-wall motion. Our examples show that memristive effects are quite universal for spin-torque spintronic device at the time scale that explicitly involves the interactions between magnetization dynamics and electronic charge transport. We also proved that the spintronic device can be designed to explore and memorize the continuum state of current and voltage based on interactions of electron and spin transport.

Demonstration of multilevel cell spin transfer switching in MgO magnetic tunnel junctions

Multilevel cell is an important concept to improve the density of spin transfer torque memory. We demonstrated two-bit multilevel cell spin transfer switching using MgO-based magnetic tunnel junctions. Two types of cell structural design are discussed. Multiple resistance levels are depicted in a current-driven spin transfer switching loop, which shows a switching current density of ∼2×106A∕cm2. Reversible transitions between high and low states of the soft bit are achieved using minor-loop spin transfer switching. The influence of external magnetic field to spin transfer switching is also discussed.

Thermal fluctuation effects on spin torque induced switching: Mean and variations

Thermal fluctuation effects on mean and variation of spin torque induced magnetic element switching are analyzed. Asymptotic forms of the switching time distribution from the stochastic Landau–Lifshitz–Gilbert equation, and numerical solutions of the first and second moments of switching time from the corresponding Fokker–Planck equation, are used to characterize switching time and switching current density for the whole time range, from the second thermal reversal region to the nanosecond dynamic reversal region. It is shown that as time scales become shorter, switching time distributions become narrower, whereas switching current distributions may become broader. This paper provides a physical understanding of these different scaling behaviors.

Spin Torque Random Access Memory Down to 22 nm Technology

Spin torque random access memory (ST-MRAM) design spaces down to CMOS 22 nm technology node are explored using a dynamic magnetic tunneling junction (MTJ)-CMOS model. The coupled dynamics of MTJ and CMOS is modeled by a combination of MTJ micromagnetic simulation and CMOS SPICE circuit simulation. The paper analyzes trade-offs between MTJ current threshold, MTJ thermal stability and CMOS driving strength. The analysis provides information on physics requirements and technology bottlenecks for MTJ to achieve maximum capacity supported by CMOS 22 nm technology node. Magnetic solutions for MTJ to fully achieve CMOS 22 nm potential capacities are reviewed.

Dielectric breakdown of MgO magnetic tunnel junctions

We have investigated high-quality MgO tunnel junctions with a range of barrier thickness in order to identify the underlying physical mechanism responsible for dielectric breakdown. Two types of dielectric breakdown (“soft” and “hard”) were observed. Soft breakdown was observed in a few percent of the devices. This breakdown mode is not intrinsic and is attributed to tunnel junction imperfections. The hard breakdown occurs because a critical electric field is reached across the tunnel barrier. Other possible breakdown mechanisms, such as thermally driven mass diffusion or charge trapping, were not consistent with the hard dielectric breakdown data and were ruled out.

Spin Torque Induced Magnetization Switching Variations

Spin torque induced magnetization switching variations are studied using experimental measurement and theoretical modeling. The study covers a wide time range, from the short time dynamic switching (10 ns) to the long time thermal switching (0.1 s). The modeling results agree well with the measurement data on switching mean and variation. Both data and model show different scaling behaviors of switching current variation and switching time variation as time scales down from the second to the nanosecond region. This study not only provides understanding of the physics involved, but also tools for making design tradeoffs between current switching magnitude, switching variation, switching speed, and thermal stability.

2

We review the 2 Tbit/in2 reader design landscape based on existing knowledge and projection. We found that the reader signal-to-noise ratio (SNR) requirement will be highly challenging due to the rapid increase in noise and the additional requirements from assisted writing. An acceptable level of channel bit density can be achieved in spite of a slow head-to-media spacing (HMS) reduction provided that both the shield-to-shield (SS) spacing and the ¿a¿ parameter scale with the bit length. We expect the side reading control for high ktpi to be difficult, and potentially a reader side shield will be required. The reader will likely use a higher quality MgO tunneling giant magnetoresistance (TGMR) stack with improved permanent-magnet coercivity. Certain new structures such as the differential reader or the trilayer will likely be part of the solution.

{\hbox{Tbit/in}}^{2}

We proposed a combined magnetic and circuit level technique to explore the design methodology of Spin-Torque Transfer RAM (SPRAM). A dynamic magnetic model of magnetic tunneling junction (MTJ), which is based upon measured spin torque induced magnetization switching behavior, is also proposed. The response of CMOS circuitry is characterized by SPICE and used as the input of our MTJ model to simulate the dynamic behavior of SPRAM cell. By using this technique, we explored the design margin of SPRAM cell with one-transistor-one-MTJ (ITU) structure. Simulation results show that our technique can significantly reduce the design pessimism, compared to conventional SRPAM cell model.

Reader Design Outlook

Narrow-track current-perpendicular-to-the-plane giant magnetoresistive heads containing Heusler alloy layer have been fabricated utilizing an abutted junction hard bias design. The head performance has been tested quasistatically and dynamically under high density recording conditions using a perpendicular magnetic recording media.

Design Margin Exploration of Spin-Torque Transfer RAM (SPRAM)

Magnetic tunnel junctions with the magnetization perpendicular to the plane of the layers can be switched between configurations with parallel or antiparallel magnetization of the layers using spin-polarized currents. Here we use finite-temperature micromagnetic modeling to study the switching behavior of such structures. In particular, we examine the effect of materials properties, such as magnetization density and magnetic anisotropy, on the current and time needed for the magnetization to switch from the parallel configuration to the antiparallel configuration. In addition, we study the possibility of lowering the required current density through a so-called exchange-spring effect in one of the magnetic layers. For circular devices of 70 nm diameter, we find a range of materials properties for which a current density of 2.6 MA/cm2 can switch the magnetization configuration within 20 ns. However, for the parameters studied here, only small further reductions in the switching current density, to about 2....