Wittig James E, Bentley James, Allard Lawrence F
Interdisciplinary Materials Science, Vanderbilt University, PMB 351683, 2301 Vanderbilt Place, Nashville, TN 37232, USA.
Materials Science and Technology Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6376, USA.
Ultramicroscopy. 2017 May;176:218-232. doi: 10.1016/j.ultramic.2016.11.025. Epub 2016 Dec 11.
In situ high-resolution electron microscopy was used to reveal information at the atomic level for the disordered-to-ordered phase transformation of equiatomic FePt nanoparticles that can exhibit outstanding magnetic properties after transforming from disordered face-centered-cubic into the tetragonal L1 ordered structure. High-angle annular dark-field imaging in the scanning transmission electron microscope provided sufficient contrast between the Fe and Pt atoms to readily monitor the ordering of the atoms during in situ heating experiments. However, during continuous high-magnification imaging the electron beam influenced the kinetics of the transformation so annealing had to be performed with the electron beam blanked. At 500°C where the reaction rate was relatively slow, observation of the transformation mechanisms using this sequential imaging protocol revealed that ordering proceeded from (002) surface facets but was incomplete and multiple-domain particles were formed that contained anti-phase domain boundaries and anti-site defects. At 600 and 700°C, the limitations of sequential imaging were revealed as a consequence of increased transformation kinetics. Annealing for only 5min at 700°C produced complete single-domain L1 order; such single-domain particles were more spherical in shape with (002) facets. The in situ experiments also provided information concerning nanoparticle sintering, coalescence, and consolidation. Although there was resistance to complete sintering due to the crystallography of L1 order, the driving force from the large surface-area-to-volume ratio resulted in considerable nanoparticle coalescence, which would render such FePt nanoparticles unsuitable for use as magnetic recording media. Comparison of the in situ data acquired using the protocol described above with parallel ex situ annealing experiments showed that identical behavior resulted in all cases.
原位高分辨率电子显微镜被用于揭示等原子比FePt纳米颗粒从无序到有序相变过程中原子水平的信息，这些纳米颗粒从无序的面心立方结构转变为四方L1有序结构后可展现出优异的磁性能。扫描透射电子显微镜中的高角度环形暗场成像在Fe和Pt原子之间提供了足够的对比度，以便在原位加热实验中轻松监测原子的有序化过程。然而，在连续的高倍率成像过程中，电子束影响了相变动力学，因此必须在电子束熄灭的情况下进行退火。在500°C时反应速率相对较慢，使用这种顺序成像方案观察相变机制发现，有序化从(002)表面晶面开始，但并不完全，形成了包含反相畴界和反位缺陷的多畴颗粒。在600和700°C时，由于相变动力学增加，揭示了顺序成像的局限性。在700°C仅退火5分钟就产生了完全的单畴L1有序；这种单畴颗粒形状更接近球形，具有(002)晶面。原位实验还提供了有关纳米颗粒烧结、聚结和固结的信息。尽管由于L1有序的晶体结构对完全烧结有阻力，但大的表面积与体积比产生的驱动力导致了大量的纳米颗粒聚结，这将使这种FePt纳米颗粒不适用于磁记录介质。将使用上述方案获得的原位数据与平行的非原位退火实验进行比较表明，在所有情况下都产生了相同的行为。
