Suzuki Ippei, Kubo Shoichi, Sepehri-Amin Hosein, Takahashi Yukiko K
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan.
Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan.
ACS Appl Mater Interfaces. 2021 Apr 14;13(14):16620-16627. doi: 10.1021/acsami.0c22510. Epub 2021 Mar 31.
Epitaxial thin films of L1-ordered FePt alloys are one of the most important materials in magnetic recording and spintronics applications due to their large perpendicular magnetic anisotropy (PMA). The key to the production of these required superior properties lies in the control of the growth mode of the films. Further, it is necessary to distinguish between the effect of lattice mismatch and surface free energy on the growth mode because of their strong correlation. In this study, the effect of surface free energy on the growth mode of FePt epitaxial films was investigated using MgO, NiO, and MgON surfaces with almost the same lattice constant to exclude the effect of lattice mismatch. It was found that the growth mode can be tuned from a three-dimensional (3D) island mode on MgO to a more two-dimensional (2D)-like mode on MgON and NiO. Contact angle measurements revealed that MgON and NiO show larger surface free energy than MgO, indicating that the difference in the growth mode is due to their larger surface free energy. In addition, MgON was found to induce not only a flat surface as FePt grown on SrTiO (STO), which has a small lattice mismatch, but also a larger PMA than that of STO/FePt. As larger lattice mismatch is favored to induce a higher PMA into the FePt films, MgO substrates are exclusively used, but 3D island growth is indispensable. This work demonstrates that tuning the surface free energy enables us to achieve a large PMA and flat film surface in FePt epitaxial films on MgO. The results also indicate that modifying the surface free energy is pertinent for the flexible functional design of thin films.
L1有序FePt合金的外延薄膜因其具有大的垂直磁各向异性（PMA），是磁记录和自旋电子学应用中最重要的材料之一。制备这些所需优异性能的关键在于控制薄膜的生长模式。此外，由于晶格失配和表面自由能之间的强相关性，有必要区分它们对生长模式的影响。在本研究中，使用晶格常数几乎相同的MgO、NiO和MgON表面来研究表面自由能对FePt外延薄膜生长模式的影响，以排除晶格失配的影响。结果发现，生长模式可以从MgO上的三维（3D）岛状模式调整为MgON和NiO上更类似二维（2D）的模式。接触角测量表明，MgON和NiO的表面自由能比MgO大，这表明生长模式的差异是由于它们较大的表面自由能。此外，发现MgON不仅能使FePt在晶格失配较小的SrTiO（STO）上生长时形成平整表面，还能诱导出比STO / FePt更大的PMA。由于较大的晶格失配有利于在FePt薄膜中诱导出更高的PMA，因此专门使用MgO衬底，但3D岛状生长是必不可少的。这项工作表明，调节表面自由能能够使我们在MgO上的FePt外延薄膜中实现大的PMA和平整的薄膜表面。结果还表明，改变表面自由能对于薄膜的灵活功能设计至关重要。
