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Research of LATTE Graduate Students Gen Yin and Yizhou Liu Leads to a $300K NSF Grant on Skyrmions for Information Processing and Storage

skyrmionThis suc­cess­ful three year NSF pro­posal was based on the research results of LATTE grad­u­ate stu­dents Gen Yin and Yizhou Liu work­ing in close col­lab­o­ra­tion with Dr. Jiadong Zang at Johns Hop­kins. A mag­netic skyrmion is a topo­log­i­cally pro­tected, cir­cu­lar, swirling spin tex­ture in which the spins on the periph­ery are polar­ized ver­ti­cally, the cen­tral spin is polar­ized in the oppo­site direc­tion, and, in between, the spins smoothly tran­si­tion between the two oppo­site polar­iza­tions. Skyrmions exist in cer­tain heli­mag­netic mate­ri­als such as the B20 mag­nets in which bro­ken inver­sion sym­me­try gives rise to the Dzyaloshinskii-Moriya inter­ac­tion. The radius of a skyrmion ranges from 3 nm to 100 nm depend­ing on the ratio of the Dzyaloshinskii-Moriya inter­ac­tion and the sym­met­ric Heisen­berg inter­ac­tion. Skyrmion lat­tices and iso­lated skyrmions have been observed in bulk and in thin films, and the elec­tri­cal cur­rent required to move a skyrmion is 4 to 5 orders of mag­ni­tude less than that required to move a more con­ven­tional mag­netic domain wall. Because of the their small size, their sta­bil­ity, the demon­stra­tion of their indi­vid­ual cre­ation and anni­hi­la­tion, and their facile move­ment with low cur­rent, skyrmions are being inves­ti­gated for mag­netic infor­ma­tion stor­age appli­ca­tions. For such appli­ca­tions, an under­stand­ing of the process of skyrmion cre­ation, anni­hi­la­tion, decay, and read­out is required. This project will inves­ti­gate these processes, and it will opti­mize a new method for low-energy skyrmion cre­ation and anni­hi­la­tion using nanosec­ond cur­rent pulses. Read-out using the topo­log­i­cal Hall effect will be ana­lyzed. Four dif­fer­ent archi­tec­tures that exploit a skyrmion state vari­able for infor­ma­tion pro­cess­ing are pro­posed, and open ques­tions con­cern­ing the under­ly­ing phys­i­cal mech­a­nisms and fun­da­men­tal lim­its of the skyrmion sys­tems are iden­ti­fied. These ques­tions will be answered using the­o­ret­i­cal and com­pu­ta­tional meth­ods includ­ing the Landau-Lifshitz-Gilbert equa­tion, ab ini­tio den­sity func­tional the­ory, and the non-equilibrium Green func­tion for­mal­ism. For the Landau-Lifshitz-Gilbert dynam­i­cal sim­u­la­tions of the spin sys­tem, a sto­chas­tic field will be applied to include the effect of ther­mal fluc­tu­a­tions at finite tem­per­a­ture. A new method for low-energy cre­ation and anni­hi­la­tion of skyrmions using a nanosec­ond cur­rent pulse will be opti­mized. The lat­tice ver­sion of the topo­log­i­cal charge will be used to ana­lyze the micro­scopic dynam­i­cal processes. It pro­vides a clear pic­ture of the spin tra­jec­to­ries and ori­en­ta­tions that locally trig­ger a topo­log­i­cal tran­si­tion, it reveals the topo­log­i­cal ori­gin of a skyrmion’s sta­bil­ity at finite tem­per­a­tures, and it sig­nals the exact moment at which a skyrmion is cre­ated or destroyed. A cou­pled Landau-Lifshitz-Gilbert / non-equilibrium Green func­tion approach will be used to inves­ti­gate the use of the topo­log­i­cal Hall volt­age for skyrmion read­out and the inter­ac­tion of spin waves with skyrmions for non-boolean, holo­graphic infor­ma­tion pro­cess­ing. A suc­cess­ful project could lead to new approaches to spin and topo­log­i­cal based com­put­ing. The exploita­tion of low-energy, sta­ble, high-temperature, topo­log­i­cally pro­tected states can result in high-payoff in the tech­nolo­gies of mem­ory and com­pu­ta­tion.
Fig­ure cre­ated by UCR grad­u­ate stu­dent Gen Yin from the sub­mit­ted man­u­script, G. Yin, Y. Li, L. Kong, R. K. Lake, C. L. Chien, and J. Zang, ‘Topo­log­i­cal Charge Analy­sis of Sin­gle Skyrmion Cre­ation with a Nanosec­ond Cur­rent Pulse.’