Os cientistas descobriram uma estratégia para mudar a magnetização em camadas finas de um ferromagneto, uma técnica que poderia eventualmente levar ao desenvolvimento de dispositivos de memória magnética mais eficientes em termos energéticos. (Conceito do artista.)
Os pesquisadores da Cornell identificaram uma abordagem para mudar a magnetização em camadas finas de um ferroímã, mantendo o material adequado no ângulo certo – uma técnica que poderia levar ao desenvolvimento de dispositivos de memória magnética mais eficientes em termos de energia.
O artigo da equipe de pesquisa, “Tilted Spin Current Generated by the Collinear Antiferromagnet Ruthenium Dioxide”, foi publicado hoje (5 de maio de 2022) na revista Eletrônicos da Natureza. Os autores co-líderes do artigo são o pesquisador de pós-doutorado Arnab Bose e os estudantes de doutorado Nathaniel Schreiber e Rakshit Jain.
Durante décadas, os físicos tentaram mudar a orientação dos spins dos elétrons em materiais magnéticos, manipulando-os com campos magnéticos. Mas cientistas, incluindo Dan Ralph, professor de física FR Newman na Faculdade de Artes e Ciências e autor sênior do artigo, procuraram usar correntes de spin transportadas por elétrons, que existem quando os elétrons têm spins predominantemente orientados em uma direção.
Quando essas correntes de spin interagem com uma fina camada magnética, elas transferem seu momento angular e geram torque suficiente para mudar a magnetização em 180 graus. (O processo de mudar essa orientação magnética é como se escreve informações em dispositivos de memória magnética.)
O grupo de Ralph se concentrou em encontrar maneiras de controlar a direção do spin nas correntes de spin, gerando-as com materiais antiferromagnéticos. Em antiferromagnetos, todos os outros spins de elétrons apontam na direção oposta, portanto, não há magnetização líquida.
“Essencialmente, a ordem antiferromagnética pode diminuir as simetrias das amostras o suficiente para permitir que existam orientações não convencionais de corrente de spin”, disse Ralph. “O mecanismo dos antiferromagnetos parece fornecer uma maneira de obter correntes de spin bastante fortes também.”
A equipe estava experimentando o dióxido de rutênio antiferromagneto e medindo as maneiras como suas correntes de spin inclinavam a magnetização em uma fina camada de níquel-ferro magnético.[{” attribute=””>alloy called Permalloy, which is a soft ferromagnet. In order to map out the different components of the torque, they measured its effects at a variety of magnetic field angles.
“We didn’t know what we were seeing at first. It was completely different from what we saw before, and it took us a lot of time to figure out what it is,” Jain said. “Also, these materials are tricky to integrate into memory devices, and our hope is to find other materials that will show similar behavior which can be integrated easily.”
The researchers eventually identified a mechanism called “momentum-dependent spin splitting” that is unique to ruthenium oxide and other antiferromagnets in the same class.
“For a long time, people assumed that in antiferromagnets spin up and spin down electrons always behave the same. This class of materials is really something new,” Ralph said. “The spin up and spin down electronic states essentially have different dependencies. Once you start applying electric fields, that immediately gives you a way of making strong spin currents because the spin up and spin down electrons react differently. So you can accelerate one of them more than the other and get a strong spin current that way.”
This mechanism had been hypothesized but never before documented. When the crystal structure in the antiferromagnet is oriented appropriately within devices, the mechanism allows the spin current to be tilted at an angle that can enable more efficient magnetic switching than other spin-orbit interactions.
Now, Ralph’s team is hoping to find ways to make antiferromagnets in which they can control the domain structure – i.e., the regions where the electrons’ magnetic moments align in the same direction – and study each domain individually, which is challenging because the domains are normally mixed.
Eventually, the researchers’ approach could lead to advances in technologies that incorporate magnetic random-access memory.
“The hope would be to make very efficient, very dense and nonvolatile magnetic memory devices that would improve upon the existing silicon memory devices,” Ralph said. “That would allow a real change in the way that memory is done in computers because you’d have something with essentially infinite endurance, very dense, very fast, and the information stays even if the power is turned off. There’s no memory that does that these days.”
Reference: “Tilted spin current generated by the collinear antiferromagnet ruthenium dioxide” by Arnab Bose, Nathaniel J. Schreiber, Rakshit Jain, Ding-Fu Shao, Hari P. Nair, Jiaxin Sun, Xiyue S. Zhang, David A. Muller, Evgeny Y. Tsymbal, Darrell G. Schlom and Daniel C. Ralph, 5 May 2022, Nature Electronics.
DOI: 10.1038/s41928-022-00744-8
Co-authors include former postdoctoral researcher Ding-Fu Shao; Hari Nair, assistant research professor of materials science and engineering; doctoral students Jiaxin Sun and Xiyue Zhang; David Muller, the Samuel B. Eckert Professor of Engineering; Evgeny Tsymbal of the University of Nebraska; and Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry.
The research was supported by the U.S. Department of Energy, the Cornell Center for Materials Research (CCMR), with funding from the National Science Foundation’s Materials Research Science and Engineering Center program, the NSF-supported Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM), the Gordon and Betty Moore Foundation’s EPiQS Initiative, and the NSF’s Major Instrument Research program.
The devices were fabricated using the shared facilities of the Cornell NanoScale Science and Technology Facility and CCMR.
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