Liu Ying, Zhou Peng, Regmi Sudhir, Bidthanapally Rao, Popov Maksym, Zhang Jitao, Zhang Wei, Page Michael R, Zhang Tianjin, Gupta Arunava, Srinivasan Gopalan
Department of Physics, Oakland University, Rochester, MI, 48309, USA.
Department of Materials Science and Engineering, Hubei University, Wuhan, 430062, China.
Sci Rep. 2022 Apr 29;12(1):7052. doi: 10.1038/s41598-022-10814-8.
This work focuses on the nature of magnetic anisotropy in 2.5-16 micron thick films of nickel ferrite (NFO) grown by liquid phase epitaxy (LPE). The technique, ideal for rapid growth of epitaxial oxide films, was utilized for films on (100) and (110) substrates of magnesium gallate (MGO). The motivation was to investigate the dependence of the growth induced anisotropy field on film thickness since submicron films of NFO were reported to show a very high anisotropy. The films grown at 850-875 C and subsequently annealed at 1000 C were found to be epitaxial, with the out-of-plane lattice constant showing unanticipated decrease with increasing film thickness and the estimated in-plane lattice constant increasing with the film thickness. The uniaxial anisotropy field H, estimated from X-ray diffraction data, ranged from 2.8-7.7 kOe with the films on (100) MGO having a higher H value than for the films on (110) MGO. Ferromagnetic resonance (FMR) measurements for in-plane and out-of-plane static magnetic field were utilized to determine both the magnetocrystalline the anisotropy field H and the uniaxial anisotropy field H. Values of H range from -0.24 to -0.86 kOe. The uniaxial anisotropy field H was an order of magnitude smaller than H and it decreased with increasing film thickness for NFO films on (100) MGO, but H increased with film thickness for films on (110) MGO substrates. These observations indicate that the origin of the induced anisotropy could be attributed to several factors including (i) strain due to mismatch in the film-substrate lattice constants, (ii) possible variations in the bond lengths and bond angles in NFO during the growth process, and (iii) the strain arising from mismatch in the thermal expansion coefficients of the film and the substrate due to the high growth and annealing temperatures involved in the LPE technique. The LPE films of NFO on MGO substrates studied in this work are of interest for use in high frequency devices.
这项工作聚焦于通过液相外延（LPE）生长的2.5 - 16微米厚的镍铁氧体（NFO）薄膜中的磁各向异性本质。这种技术对于外延氧化物薄膜的快速生长而言是理想的，被用于在镓酸镁（MGO）的（100）和（110）衬底上生长薄膜。其动机是研究生长诱导各向异性场对薄膜厚度的依赖性，因为据报道NFO的亚微米薄膜显示出非常高的各向异性。发现在850 - 875℃生长并随后在1000℃退火的薄膜是外延的，其面外晶格常数随着薄膜厚度增加呈现出意外的减小，而估计的面内晶格常数随着薄膜厚度增加。根据X射线衍射数据估计的单轴各向异性场H范围为2.8 - 7.7 kOe，（100）MGO上的薄膜的H值高于（110）MGO上的薄膜。利用面内和面外静磁场的铁磁共振（FMR）测量来确定磁晶各向异性场H和单轴各向异性场H。H的值范围为 - 0.24至 - 0.86 kOe。单轴各向异性场H比H小一个数量级，对于（100）MGO上的NFO薄膜，它随着薄膜厚度增加而减小，但对于（110）MGO衬底上的薄膜，H随着薄膜厚度增加。这些观察结果表明，诱导各向异性的起源可归因于几个因素，包括（i）由于薄膜 - 衬底晶格常数不匹配引起的应变，（ii）生长过程中NFO中键长和键角可能的变化，以及（iii）由于LPE技术中涉及的高生长和退火温度导致的薄膜和衬底热膨胀系数不匹配引起的应变。这项工作中研究的MGO衬底上的NFO的LPE薄膜对于高频器件的应用很有意义。
