Lu Qinwen, Liu Zhiwei, Yang Qun, Cao Hui, Balakrishnan Purnima, Wang Qing, Cheng Long, Lu Yalin, Zuo Jian-Min, Zhou Hua, Quarterman Patrick, Muramoto Shin, Grutter Alexander J, Chen Hanghui, Zhai Xiaofang
Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China.
School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China.
ACS Nano. 2022 May 24;16(5):7580-7588. doi: 10.1021/acsnano.1c11065. Epub 2022 Apr 21.
The combination of small coercive fields and weak magnetic anisotropy makes soft ferromagnetic films extremely useful for nanoscale devices that need to easily switch spin directions. However, soft ferromagnets are relatively rare, particularly in ultrathin films with thicknesses of a few nanometers or less. We have synthesized large-area, high-quality, ultrathin freestanding LaMnO films on Si and found unexpected soft ferromagnetism along both the in-plane and out-of-plane directions when the film thickness was reduced to 4 nm. We argue that the vanishing magnetic anisotropy between the two directions is a consequence of two coexisting magnetic easy axes in different atomic layers of the LaMnO film. Spectroscopy measurements reveal a change in Mn valence from 3+ in the film interior to approximately 2+ at the surfaces where considerable hydrogen infiltration occurs due to the water dissolving process. First-principles calculations show that protonation of LaMnO decreases the Mn valence and switches the magnetic easy axis from in-plane to out-of-plane as the Mn valence approaches 2+ from its 3+ bulk value. Our work demonstrates that ultrathin freestanding films can exhibit functional properties that are absent in homogeneous materials, concomitant with their convenient compatibility with Si-based devices.
小矫顽场和弱磁各向异性的结合使得软铁磁薄膜对于需要轻松切换自旋方向的纳米级器件极为有用。然而，软铁磁体相对较少，特别是在厚度为几纳米或更小的超薄膜中。我们在硅上合成了大面积、高质量的超薄自支撑LaMnO薄膜，并发现当薄膜厚度减小到4纳米时，在面内和面外方向均出现了意想不到的软铁磁性。我们认为，两个方向之间磁各向异性的消失是LaMnO薄膜不同原子层中两个共存的易磁轴的结果。光谱测量表明，薄膜内部的Mn价态为3+，而在表面由于水溶解过程发生大量氢渗透，Mn价态约为2+。第一性原理计算表明，随着Mn价态从其体相值3+接近2+，LaMnO的质子化降低了Mn价态，并将易磁轴从面内切换到面外。我们的工作表明，超薄自支撑薄膜可以展现出均质材料中不存在的功能特性，同时它们与硅基器件具有良好的兼容性。
