Realization of Ferromagnetic Resonance in 1nm Co ultrathin film

Abstract

Physics in nanometer-thick metallic films has been attracting many researchers because of abundant spintronics nature. In ultrathin films, some physical properties, such as the spin-charge conversion [1] and ferromagnetism [2], are gate-tunable. To extend further potential of ultrathin metallic films in spintronics, we demonstrate ferromagnetic resonance (FMR) in a 1 nm-thick ultrathin Co film [3], which exhibits sufficiently small FMR resonance field and narrow FMR linewidth simultaneously, for future realization of tunable magnon excitation. We prepared SiO2 (capping layer)/Co (tCo )/SiO2 (Sample A), where tCo was set to be 1, 2, 3 and 5 nm, and SiO2 (capping layer)/Co (1 nm)/Ta (3 nm)/SiO2 (Sample B). The Co was grown by using magnetron sputtering. The sample was placed in the center of an electron spin resonance (ESR) cavity (TE011) and FMR spectra were measured. The external dc magnetic field (Hex) and ac magnetic field (hrf) were applied along the in-plane direction of the film as shown in Fig. 1. Figure 2 shows comparison of the FMR spectra between Sample A (tCo = 1 nm) and Sample B. Whereas the thickness of the Co of both samples are the same, the clear FMR signal was observed only from Sample B, where the half-width at half-maximum and the resonance field were comparable to that of the bulk Co system. The results directly indicate that the insertion of 3 nm-thick Ta plays a dominant role for generating FMR in the ultrathin Co film. Furthermore, the narrow linewidth and the low resonance field indicate that the ultrathin Co film simultaneously possesses uniform and strong magnetization, which has not been achieved previously [4,5]. The successful detection of FMR from ultrathin Co enables efficient magnon excitation that is gate-tunable. More detailed discussion will be given in the presentation.