Unusual magnetic hysteresis and transition between vortex and double pole states arising from interlayer coupling in diamond-shaped nanostructures

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Parente Campos, Ana
Navarro, H.
Vargas, N. M.
Lapa, P.
Basaran, Ali C.
Redondo, C.
Morales, R.
Schuller, Ivan K.
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Amer Chemical Soc.
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Controlling the magnetic ground states at the nanoscale is a longstanding basic research problem and an important issue in magnetic storage technologies. Here, we designed a nanostructured material that exhibits very unusual hysteresis loops due to a transition between vortex and double pole states. Arrays of 700 nm diamond-shaped nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30 nm) (Py, permalloy (Ni_(80)Fe_(20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (t_(Ru)) governs the interaction between the Py layers. We found this interaction mainly mediated by two mechanisms: magnetostatic interaction that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers and exchange interaction that oscillates between ferromagnetic (FM) and AFM couplings. For a certain range of Ru thicknesses, FM coupling dominates and forms magnetic vortices in the upper and lower Py layers. For Ru thicknesses at which AFM coupling dominates, the magnetic state in remanence is a double pole structure. Our results showed that the interlayer exchange coupling interaction remains finite even at 4 nm Ru thickness. The magnetic states in remanence, observed by magnetic force microscopy (MFM), are in good agreement with corresponding hysteresis loops obtained by the magneto-optic Kerr effect (MOKE) and micromagnetic simulations.
©2022 American Chemical Society The authors acknowledge Prof. J. M. Alameda and Prof. G. Armelles for fruitful discussion. This work was supported by Comunidad de Madrid and Universidad Complutense de Madrid under the project 2018-T1/IND-10360 granted by “Atracción de Talento” program, the European Union under the H2020 research and innovation Marie Sklodowska-Curie Grant Agreement MAGNAMED 734801 (H2020-MSCA-RISE-2016), the Spanish MINECO grants FIS2016-76058 (AEI/FEDER, UE), PID2019- 104604RB/AEI/ 10.13039/501100011033, PID2021-122980OB-C52 and Basque Government grant IT1491-22. The magnetic measurements and interpretation were funded by the Department of Energy’s Office of Basic Energy Science, under grant # DE-FG02-87ER45332. IMDEA Nanociencia acknowledges support from the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (MICINN, Grant CEX2020-001039-S). The design of the experiments and the multiple versions of the manuscript were discussed and written with extensive contributions from all the authors.