Lattice distortion and thermal stability of [Fe2CoSi/Pt]n multi-layers with high perpendicular magnetic anisotropy

Abstract

Multilayers with perpendicular magnetic anisotropy have attracted extensive attention due to their promising applications on magnetic random access memory (MRAM). Higher perpendicular magnetic anisotropy (PMA) can increase the thermal stability and reduce the critical switching current [1], [2]. Due to the orbital hybridization of Co or Fe and Pt or Pd atoms, the multilayers exhibit PMA such as [Co/Pt] n [3], [Co/Pd] n [4], [CoFeB/Pt] n [5], [Co 2 FeAl(Si)/Pt] n [6–8] and [Co 2 MnSi] n [9]. However, the PMA of Pt/ Fe-based Heusler multilayers hasn't been reported. Besides, the analysis of the ferromagnetic layer is lacking. Our group used Fe-based Heusler in the multilayers. Because the Fe 2 CoSi has smaller lattice mismatch with Pt than that of Co 2 FeAl. Hence it is reasonable to expect a higher PMA in [Fe 2 CoSi/Pt] n multilayers. In this work, we observed high PMA in [Fe 2 CoSi/Pt] n structure. The thermal stability and lattice distortion of Fe 2 CoSi is also discussed. The [Fe 2 CoSi (FCS)/Pt] n multilayers are deposited on amorphous oxidized Si with Pt buffer by ultra-high vacuum magnetron sputtering system at room temperature. The thickness of Pt, FeCoSi and the number of period are optimized and the highest $K_{u}$ of 2.64 Merg/cc is achieved in the stack Ta(3)/Pt(10)/[FCS(0.6)/Pt(2)]2/Pt(3). Optimal $\mathrm {t}_{Pt}$ of multilayers is found as 2 nm. Furthermore, PMA is only observed when $\mathrm {t}_{FCS} =0.6$ nm or 0.8 nm and the films gradually tune to in-plane anisotropy with FCS thickness larger than 0.8 nm. We confirmed that PMA originates from the orbital hybridization of Co and Pt according to nonexistence of PMA in [Fe 2 CrSi/Pt] multilayers. Since $L 2 _{1}$ or $B2$ phase is very crucial for Heusler alloys to exhibit good magnetization and also distortion would affect the magnetic anisotropy energy, so the phase information and lattice distortion is studied using X-ray diffraction (XRD) and Transmission electron microscope (TEM). We found that Fe 2 CoSi is in $B2$ phase with Pt thickness of 2 nm, as shown in Fig.1(a). The analysis of TEM result shown in Fig.2 suggests that the interface of Pt/ Fe 2 CoSi is smooth. Besides, the analysis of FFT suggests that the unit of Fe 2 CoSi is laterally compressed in only one direction and the vertical lattice parameter increases accordingly. The vertical tensile strain and the small lattice mismatch can account for the high PMA of [Fe 2 CoSi/Pt] n structure. Finally, the thermal stability is studied. Our results show that the optimized stack can maintain PMA after annealing process at 350C for 30 mins. Our work paves the way for application of Fe-based Heusler alloys in MRAM.