Research Digest 13to Spin-triplet supercurrent can be generated by making magnetization configuration noncollinear between two FM layers in FM/NM/FM/SC multilayered structure. For highly efficient generation of spin-triplet supercurrent in FMs, it is important to choose materials to realize stable noncollinear magnetization configuration. To get the stable noncollinear magnetization configuration, I have focused on Cr as a material for the NM layer because it can yield orthogonal magnetization configuration due to the 90° magnetic coupling4). Fully epitaxial Co-based Heusler alloy (10 nm)/Cr (2 nm)/CoFe (7 nm)/Nb (20 nm) structures on Cr(20 nm)/Ag(80 nm) buffer layers were grown on cleaned MgO(001) substrates as illustrated in Fig.1(a). All layers were deposited using either DC or RF sputtering at room temperature and in-situ post-deposition annealing in an ultrahigh vacuum sputtering system with base pressures (Pbase) of ~10-6 Pa. I measured magnetic field dependence of magnetization (M-H curve) using vibrating sample magnetometer to observe the magnetization configuration of the fabricated structure. Figure 1(b) shows the observed M-H curve. The magnetization is normalized by saturation magnetization (Ms). The ratio of remanent magnetization (Mr) to Ms was estimated to be ~0.7, suggesting the presence of 90° magnetic coupling, namely, noncollinear magnetization configuration between the Co-based Heulser alloy and CoFe layers. M-H curve can generally described as M/Ms = {tHeuslercos(α) + tCoFecos(β)}/(tHeusler + tCoFe), where tHeusler (α) and tCoFe (β) are thickness (angle of magnetization direction with respect to the direction of magnetic field) of the Co-based Heulser alloy and CoFe, respectively. I analyzed the observed M-H curve based on the above model. As a result, the relative angle (α − β) was estimated to be about ~90°, indicating the realization of noncollinear magnetization configuration of the fabricated magnetic multilayer. Fig. 1. (a) Illustration of the fabricated Co-based Heusler alloy (Co2MnSi)/Cr/CoFe/Nb epitaxial multilayered structure. (b) M-H curve for noncollinear magnetization configuration.References1) T. Kontos et al., Phys. Rev. Lett. 89, 137007 (2002). 2) R. S. Keiser et al., Nature 439, 825 (2006).3) K. Elphick et al., Sci. Technol. Adv. Mater. 22, 235 (2021).4) H. Wang et al., Appl. Phys. Lett. 90, 142510 (2007).1. Outline of ResearchToday’s progress in science and technology has strongly been supported by large-scale computing. As a promising technology for next-generation large-scale computing, superconductor (SC)-based electronics has attracted great attention because it is expected realize high-performance classical/quantum electronic devices, such as rapid single flux quantum (RSFQ) circuit and quantum bit (qubit). Ferromagnetic Josephson junctions (FJJs), including middle layers of ferromagnets (FMs) sandwiched by SC layers, have widely been explored because they can induce 0-π superconducting phase shift between the SC layers. The 0-π phase shift is expected to enable us to construct ultralow-power-consumption RSFQ circuits and highly scalable qubits. To date, the 0-π phase shift has been demonstrated by fabricating basic SC/FM/SC structured FJJs1) with a variety of FMs. However, since their critical current is strongly sensitive to the thickness of FM layers, they can easily induce the large dispersion of critical current. For real device application, new structure and materials of FM/SC hybrid systems should be explored to suppress the dispersion of critical current of FJJs. As one of the potential solutions for this issue, spin-triplet correlation can be promising because its long decay length in FMs is possible to make the critical current less sensitive to thickness of FM layers compared to conventional spin-singlet correlation. For robust generation and propagation of the spin-triplet supercurrent, half-metallic FMs (HMFs) which have extremely high spin polarization (P) should be a promising material because relaxation of spin-triplet correlation is expected to be suppressed in HMFs2). I focus on the half-metallic Co-based Heusler alloy which is the most used half-metallic material in the field of spintronics3). In this digest, I introduce my and collaborators’ work on the development of a process to fabricate fully epitaxial magnetic multilayers with a half-metallic Co-based Heusler alloy toward highly efficient generation of spin-triplet supercurrent.2. Research ActivitiesMagnetic Multilayer for Noncollinear Magnetization ConfigurationYuichi FUJITA
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