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Towards zero-energy computation with magnetic nanodots - Giovanni Carlotti - Mercredi 13 juillet 2016 à 16 h 30

INSP - 4 place Jussieu - 75252 PARIS Cedex 05 - Barre 22-23 - 3e étage, salle 317

Giovanni Carlotti - Professor of Condensed Matter Physics at the University of Perugia, Italy

Abstract

Magnetic dots, with lateral size as small as a few tens of nanometers, can be nowadays produced by state of the art nanofabrication tools and are exploited in a variety of running and forthcoming devices, such as magnetic memory switches, bit-patterned hard disks, nano-magnetic logic gates, spin-torque oscillators and read heads. The objective of this seminar is twofold. First of all, I will consider the irreversible switching protocols of magnetic dots with lateral dimensions ranging from the micrometric to the nanometric scale, comparing conventional and precessional switching mechanisms. Emphasis will be given to the calculation of the dissipated energy and to the characteristics of the spin wave eigenmodes excited during switching. In second place, I will consider energy-aware switching protocols in the above magnetic nanodots, when they are considered as the basic switches for a new generation of low-power, non-volatile, logic gates. The fundamental energy limits of adiabatic (slow) reversal or erasure of a magnetic nanodot and of performing logic operations in gates consisting of clusters of interacting nanodots will be discussed and the data of both micromagnetic simulations and real experiments will be presented. In particular, the energy dissipated during a “reset to 1” operation, also known as Landauer erasure, was accurately measured at room temperature by vectorial magneto-optical measurements in arrays of 10 nm thick Permalloy dots where the longer axis D of the elliptical dots was scaled down from about 900 to 80 nm. The experimental results show that the dissipated energy increases with the dot volume and is from one to three order of magnitude above the theoretical Landauer limit of kBT·ln(2). These experimental findings are corroborated by micromagnetic simulations that confirm significant deviations from the ideal macrospin behavior, when D exceeds about 200 nm, because of inhomogeneous magnetization distribution and edge effects. These phenomena lead to an average dissipated energy that is appreciably larger than the Landauer limit, in agreement with the experimental findings. This analysis is important to envisage novel computing paradigms with radically improved efficiency, where it will be possible to trade the minimum amount of energy dissipated with either the speed of switching or the computational precision of magnetic switches subject to thermal fluctuations.