Nanomagnetics

Theme IV: Nanomagnetics

The goal of the Nanomagnetics team is to use current-driven magnetic elements for electrical communication and switching at sub-femtojoule energies, and with fast switching speeds as low as <10 picoseconds. The primary approach is to take advantage of newly discovered ultra-sensitive, current-driven switches employing actuated spin-orbit torque (Spin Hall Effect) to switch a magnet. Such a component can have current in/current out gain, as well as fanout.

Since the constituents tend to be metallic, the voltage requirement is low, compatible with the goal of low dynamic power as the digital circuits switch. At the same time, these magnetic switches can operate at ultra-high speed, employing a-thermal, non-equilibrium, magnetic phase transitions. This opens the possibility to generate ultrafast switches for memory and logic gated by a single stimulus, without the need of polarization modulation. In addition, a key detractor with all currently known magnetic materials is the low ON/OFF ratio of present magneto-resistors—a drawback for which solutions are currently explored in the Center from a circuit design and architectural perspective.

Challenges

Understanding and exploiting the fundamental physics and dynamics of the spintronic phenomena that underlie magnetic switching has been a key focus of Theme IV research. For all its desirable properties, magnetics-based switching is currently hampered by low switching speeds and low ON/OFF ratio of state-of-the-art magneto-resistors. The Nanomagnetics team has been addressing these issues and has made a significant breakthrough in magnetic switching speed, reaching sub-10-picosecond levels.

Current Projects

Picosecond Magnetic Switching

An interesting phenomenon has emerged in ferrimagnetic GdFeCo samples: when hit by femtosecond laser pulses, magnetic “toggling” has been observed, indicating that magnetization reversal can occur on picosecond time scales. Nanomagnetics researcher at E3S demonstrated this helicity-independent, ultrafast magnetic switching phenomenon  even when hit with laser pulses as long as 10 picosecond. Since this is a range that can be reached also by electrical pulses, the discovery might pave the path to ultrafast magnetic switching using short electrical current pulses in place of laser pulses, enabling picosecond magnetic switching in monolithic magnetic memory and logic devices.

Selected Recent Publications

• Ultrafast Magnetic Switching of GdFeCo with Electronic Heat Currents, Phys. Rev. B, vol. 95, no. 18, pp. 180409, May 2017.
• Model for Multishot All-Thermal All-Optical Switching in Ferromagnets, Phys. Rev. B, vol. 94, no.2, pp. 020409, Jul 2016.

Spin-Orbit Torque and Spin-Transfer Torque Magnetic Switching

E3S researchers have experimentally demonstrated a new way of switching perpendicular nanomagnets with an in-plane current without the need of a symmetry-breaking magnetic field. These results are significant for the field of spintronics by providing new insight into the physics of spin-orbit torque. Moreover, switching without a magnetic field could lead to significant impact in high-density storage applications. In addition, the ultra-low sub-micro-ampere switching currents observed in sub-10-nm spin transfer torque magnetic tunnel junctions is explored for magnetic switching. Current work focuses on understanding the underlying physics and demonstration of a functional three-terminal device.

Selected Recent Publications

• Switching of Perpendicularly Polarized Nanomagnets with Spin Orbit Torque Without an External Magnetic Field by Engineering a Tilted Anisotropy, Proc. Natl. Acad. Sci. U.S.A., vol. 112, no. 33, pp. 10310-10315, Aug 2015.
• Deterministic Domain Wall Motion Orthogonal to Current Flow due to Spin Orbit Torque, Sci. Rep., vol. 5, pp. 11823, July 2015.
• The Physics of Spin-Transfer Torque Switching in Magnetic Tunneling Junctions in Sub-10-nm Size Range, IEEE Trans. Magn., vol. 52, no. 7, pp. 1-4, Jul 2016.

Circuit Architectures with Nanomagnetic Switches

While magneto-electrical switching now benefits from very low switching current, the challenge in magnetic devices is the low current ON/OFF ratio of present magneto-resistors, causing static leakage. However, nanomagnetic switches operate at extremely low supply voltages. This non-volatility of magnets can be used to reduce the static power losses. Theme IV researchers work with E3S System Integration team to take advantage of these characteristics by combining the good ON/OFF ratio of typical CMOS transistors with the sensitivity and non-volatility of magnetic switches. The goal is to reduce the power consumption of computers by turning off large portions of the computer that are not in use. The use of nonvolatile memory can make normally-off computing viable, since the shut-down part of the computer can be restored into its original state (once it is needed) without needing to write to or read from external storage.

Selected Recent Publications

• Design Requirements for a Spintronic MTJ Logic Device for Pipelined Logic Applications, IEEE Trans. Electron Dev., vol. 63, no. 4, pp. 1754-1761, Apr 2016.
• Energy-Efficient Abundant-Data Computing: The N3XT 1,000x, Computer, vol. 48, no. 12, pp. 24-33, Dec 2015.