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Two layers made of ferromagnetic (FM) materials exhibit an antiferromagnetic (AFM) interlayer exchange coupling (IEC) when separated by a nonmagnetic (NM) material [1]. In addition, the IEC oscillates between the AFM and the FM coupling for a variable width of the NM spacer [2]. The oscillating nature of the IEC with the width of the spacer can be explained by the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange coupling [3]. This phenomenon allows one to tune the interaction from FM to AFM by changing the thickness of NM material between two magnetic layers, and optimize the various magnetic properties. The tunability of magnetic multilayers makes them attractive for magnetic sensors, magnetic random access memory (MRAM), spin-torque oscillator, magnetic racetrack memory, etc. [4]. The IEC can also be modified by chemically altering the composition of nonmagnetic material by introducing magnetic impurities. The presence of magnetic impurities results in the renormalization of IEC by changing the oscillatory period and amplitude via Fermi surface shifts of the nonmagnetic layer. In Co/Ru multilayers, the Ru spacer alloyed with Fe develops a vortex-like spine texture at intermediate Fe concentrations. Fe spins tend to couple antiferromagnetically, resulting in a noncollinear alignment of Co layers [5]. The noncollinear alignment of FM layers is favored by the biquadratic coupling constant. Alloying NM Ru with FM Fe can be used to control the noncollinear alignment of magnetic moments in FM layers, opening the perspective of using Co/Ru multilayers for spin-transfer torque MRAM devices [6].
In our previous work, we had shown that the experimentally measured values of the bilinear coupling for different concentrations x of magnetic impurities Y = (Co,Fe) for Co/Ru(1−x)Y(x) multilayers are well reproduced by Density Functional Theory (DFT) calculations [7]. In this work, we used the same computational framework to quantify the effect of alloying Ru with all known magnetic elements Y=(Cr,Mn,Co,Fe,Ni) on the AFM bilinear exchange coupling. Our analysis reveals a correlation between the distribution of spin moments and the changes in the IEC value for different magnetic impurities. We quantify this analysis within a simple Heisenberg model considering both direct exchange and RKKY-type interactions.
[1] P. Grünberg et al., Phys. Rev. Lett. 57, 2442 (1986), URL https://link.aps.org/doi/10.1103/PhysRevLett.57.2442.
[2] S. S. P. Parkin, Phys. Rev. Lett. 67, 3598 (1991), URL https://link.aps.org/doi/10.1103/PhysRevLett.67.3598.
[3] P. Bruno, Phys. Rev. B 52, 411 (1995), URL https://link.aps.org/doi/10.1103/PhysRevB.52.411.
[4] S. Parkin and S.-H. Yang, Nature Nanotechnology 10, 195 (2015), ISSN 1748-3395, URL https://doi.org/10.1038/nnano.2015.41.
[5] Z. R. Nunn et al., Science Advances 6, eabd8861 (2020), URL https://www.science.org/doi/abs/10.1126/sciadv.abd8861.
[6] A. D. Kent and D. C. Worledge, Nature Nanotechnology 10, 187 (2015), ISSN 1748-3395, URL https://doi.org/10.1038/nnano.2015.24.
[7] K. Winther et al., Phys. Rev. Appl. 22, 024058 (2024), URL https://link.aps.org/doi/10.1103/PhysRevApplied.22.024058.