Brian Lambson

Experimental test of Landauer’s principle in single-bit operations on nanomagnetic memory bits

The minimum energy dissipated in switching a magnetic bit measured to be consistent with the Landauer limit of kBT ln(2). Minimizing energy dissipation has emerged as the key challenge in continuing to scale the performance of digital computers. The question of whether there exists a fundamental lower limit to the energy required for digital operations is therefore of great interest. A well-known theoretical result put forward by Landauer states that any irreversible single-bit operation on a physical memory element in contact with a heat bath at a temperature T requires at least kBT ln(2) of heat be dissipated from the memory into the environment, where kB is the Boltzmann constant. We report an experimental investigation of the intrinsic energy loss of an adiabatic single-bit reset operation using nanoscale magnetic memory bits, by far the most ubiquitous digital storage technology in use today. Through sensitive, high-precision magnetometry measurements, we observed that the amount of dissipated energy in this process is consistent (within 2 SDs of experimental uncertainty) with the Landauer limit. This result reinforces the connection between “information thermodynamics” and physical systems and also provides a foundation for the development of practical information processing technologies that approach the fundamental limit of energy dissipation. The significance of the result includes insightful direction for future development of information technology.

Investigation of Defects and Errors in Nanomagnetic Logic Circuits

Nanomagnetic logic circuits have recently gained interest as a possible post CMOS ultralow-power computing platform. In these circuits, single-domain nanomagnets communicate and perform logical computations through nearest neighbor dipole interactions. The state variable is magnetization direction and computations can take place without passing an electric current. Both experiment and theory have shown, however, that errors in circuit operation can sometimes occur. In this paper, we investigate the reasons for this, develop a simple model to explain imperfections in 1-D chains of nanomagnets, and show that it agrees with experiment. Finally, we discuss possible improvements in nanomagnet design suggested by the model to improve error rates.

Cascade-like signal propagation in chains of concave nanomagnets

We lithographically control the anisotropy properties of single-domain nananomagnets for use in emerging nanomagnetic logic applications. By defining concave-shaped nanomagnets to enhance the effect of configurational anisotropy, we induce the property of dual-axis remanence needed for high-speed and reliable operation of nanomagnetic logic circuits. Magneto-optical measurements verify the anisotropy properties of isolated concave nanomagnets, and photoelectron emission microscopy measurements verify signal propagation in chains of concave nanomagnets.

Computing in Thermal Equilibrium With Dipole-Coupled Nanomagnets

In the 1970s, work at IBM by Charles Bennett suggested the possibility of a computer operating near thermal equilibrium and dissipating energy near the thermodynamic limits. Here, we demonstrate experimentally that a computing architecture based on dipole-coupled nanomagnets can operate near thermal equilibrium without the assistance of externally applied magnetic fields. The dynamics of digital signal propagation is demonstrated with micromagnetic simulation and then verified experimentally using time-lapse photoemission electron microscopy. A logic gate that computes using energy from the thermal bath without external fields is also demonstrated. Nanomagnetic logic circuits operating under these conditions are expected to dissipate energy near the fundamental thermodynamic limits of computation.

Sub-nanosecond signal propagation in anisotropy-engineered nanomagnetic logic chains

Energy efficient nanomagnetic logic (NML) computing architectures propagate binary information by relying on dipolar field coupling to reorient closely spaced nanoscale magnets. Signal propagation in nanomagnet chains has been previously characterized by static magnetic imaging experiments; however, the mechanisms that determine the final state and their reproducibility over millions of cycles in high-speed operation have yet to be experimentally investigated. Here we present a study of NML operation in a high-speed regime. We perform direct imaging of digital signal propagation in permalloy nanomagnet chains with varying degrees of shape-engineered biaxial anisotropy using full-field magnetic X-ray transmission microscopy and time-resolved photoemission electron microscopy after applying nanosecond magnetic field pulses. An intrinsic switching time of 100 ps per magnet is observed. These experiments, and accompanying macrospin and micromagnetic simulations, reveal the underlying physics of NML architectures repetitively operated on nanosecond timescales and identify relevant engineering parameters to optimize performance and reliability.

Signal propagation in dipole coupled nanomagnets for logic applications

As conventional Silicon-based transistors reach their scaling limits, novel devices for performing computations have emerged as alternatives to continue the improvements in information technology that have benefited society over the past 40 years. One candidate that has shown great promise recently is a device that performs logical computations using dipole coupled nanomagnets. In this paper, we discuss recent advances that have led to a greater understanding of signal propagation in nanomagnet arrays. In particular, we highlight recent experimental work towards the imaging of a propagating magnetic cascade.

Temperature dependence of heat dissipation during Landauer erasure of nanomagnets

In order to adhere to the well-known kT ln(2) limit, heat dissipation during Landauer erasure of a memory bit must scale linearly with temperature. We demonstrate a technique for measuring the temperature dependent energy dissipation of a nanomagnet during the Landauer erasure operation. A magneto-optical Kerr effect magnetometer configured to measure the in-plane magnetization of arrays of magnetic nanostructures produces hysteresis curves, the areas of which are proportional to heat dissipation. This result experimentally verifies a key aspect of the fundamental thermodynamic limits of computation in an integrated system.

Time-Resolved Photo-Emission Electron Microscopy of Nanomagnetic Logic Chains

We report a time-resolved study of precessional timescale nanomagnetic logic dynamics. We find both the desired cascade-like signal transmission behavior as well as various logical defects. For cascade-like behavior, we observe an average settling time of ~100 picoseconds per nanomagnet.

Error immunity techniques for nanomagnetic logic

Nanomagnetic logic (NML) is an alternative to electron charge-based information processing for energy efficient computing applications. However, experiments indicate that nanomagnets are susceptible to thermal and lithographic noise, resulting in logical errors during signal transmission and computation. Here, we study the origins of errors in nanomagnetic logic and present a technique for reducing error rates based on anisotropy engineering. Using photoelectron emission microscopy (PEEM), we verify the functionality and error-immunity properties of anisotropy-engineered nanomagnets in NML applications.