NDnano Seminar: Spin wave eigenmodes of magnetic nanodots and fundamental limits of minimum energy dissipation during switching

Presented by Giovanni Carlotti, Associate Professor of Physics of Matter at the Engineering Faculty
University of Perugia, Italy

Single- or multi-layered planar magnetic dots, with lateral dimensions ranging from tens to hundreds of nanometers, are used as elemental switches in current and forthcoming devices for information and communication technology (ICT), including magnetic memories, spin-torque oscillators and nano-magnetic logic gates.  In this review article, we start from the analysis of the characteristics of the spin-wave eigenmodes of such nanodots, occurring in the GHz range of frequencies, with emphasis given to the peculiar properties that emerge when the lateral dimensions are squeezed from the micrometric to the nanometric size.  It is found that the inhomogeneity of the internal field and the enhanced role of exchange-energy cause qualitative and quantitative differences in the spin wave spectrum when the dot lateral dimensions are reduced below about 150 nm.  The spatial localization of the fundamental mode changes from the central region to the edges of the dot and, moreover, this mode softens for applied switching fields close to the coercive field, determining the initial steps of magnetization reversal. Therefore, a detailed knowledge of the magnetic eigenmodes spectrum of nanodots is also crucial to understand the reversal process and the dissipation of energy during switching.  To this respect, given the current challenge of reducing the heat production in ICT devices, a careful analysis of the fundamental limits of minimum energy consumption, shows that dissipationless operation is achievable, provided that both dynamic reversibility (arbitrarily slow application of external fields) and entropic reversibility (no free entropy increase) are insured.  However, recent theoretical and experimental tests of magnetic-dot erasure reveal that intrinsic defects related to materials imperfections such as roughness or polycrystallinity, may cause an excess of dissipation if compared to the minimum theoretical limit.

I will conclude providing an outlook on the most promising strategies to achieve a new generation of power-saving nanomagnetic logic devices based on clusters of interacting dots and on  straintronics. Dr. Carlotti's research interests are, among others, in spin waves and Brillouin scattering, magnonic crystals, and spintronics.  He has been national scientific coordinator of the project PRIN-2007 about magnonic crystals (2007-2009), in collaboration with groups from CNR-IFN (Rome) and Ferrara University.  Recently, he has been the scientific lead of the research unit of Perugia, participating in several national and international research projects, such as the European project "Landauer" (2012-2015), in collaboration with other four european groups, and PRIN 2011 Project "DyNaNoMag"  (2013-2016), in collaboration with four other Italian groups (Univ. Napoli "Federico II", Politecnico di Torino, Univ. Messina, INRIM).  He is the author of more than 200 publications in international journals on Condensed Matter Physics, and over 100 presentations at national and international conferences.