In situ UHVEM observation of atomic ordering in magnetic nanoparticles using a direct detection camera

Abstract

Recent demands for ultrahigh density magnetic storage technology require the development of recording media with higher magnetocrystalline anisotropy energy in order to ensure thermal stability of magnetization as well as ultrahigh recording density. For such a purpose, equiatomic CoPt alloy nanoparticles (NPs) are one of the candidate materials [1]. The hard magnetic property of this alloy is attributed to the tetragonal L1 0 ‐type ordered structure; the anisotropy energy is dependent on the degree of order of the ordered phase. Therefore, formation of the L1 0 ordered phase is the key issue for practical applications. In this study, we hence intend to observe atomic ordering of CoPt NPs by ultra‐high voltage electron microscopy (UHVEM) equipped with a direct detection camera. Thin films of disordered CoPt alloy NPs were synthesized by co‐deposition of Co and Pt targets using rf‐magnetron sputtering onto NaCl(001) substrates kept at 620 K. After the deposition of Co and Pt, surface of the NPs were coated by an amorphous carbon (a‐C) thin film. The specimen films were removed from the NaCl substrate by immersing the substrate in distilled water, and then floating films were mounted onto microgrids for electron transparency. Structure and composition of the NPs were characterized using a JEOL JEM‐ARM200F TEM (200 kV). Electron irradiation experiments and the simultaneous in situ heating observation were carried out using a JEOL JEM‐1000EES UHVEM (1 MV) equipped with a Gatan K2‐IS electron direct detection CMOS camera newly installed at the Research Center for UHVEM, Osaka University [2]. Figure 1 shows a selected area electron diffraction (SAED) pattern and a TEM image of as‐sputtered CoPt alloy NPs with disordered fcc structure. As seen, (100)‐oriented CoPt alloy NPs with sizes of ~15 nm are dispersed. Average alloy composition was Co‐45at%Pt as determined by EDX analysis (Fig. 1(c)). Figure 2 shows two successive TEM images of a CoPt NP acquired in situ with a frame rate of 1/400 s. The observation was made at 583 K under electron irradiation at 1 MV with a dose rate of 1.5 × 10 25 e/m 2 s. In Fig. 2(a), disordered phase with the fcc structure is seen. After 2.5 ms, the ordered fringes suddenly appeared as shown in the middle of the NP surrounded by the square (Fig. 2(b)). The atomic ordering can also be confirmed by appearance of weak 001 superlattice reflection in the attached Fast Fourier Transform (FFT) pattern. The observed kinetic ordering temperature of 583 K for binary CoPt NPs is lower than that reported in the previous study (> 800 K) [3]. The low temperature atomic ordering can be attributed to the enhancement of atom migration by high‐energy electron irradiation [4].