本研究探討的系統為反尖晶石結構(AB2O4)的磁性薄膜，包含鈷鐵氧體、鎳鐵氧體和鐵三氧四，之前的文獻已顯示出鎳鐵氧體薄膜的磁性特性會受到晶格形變的影響而改變。為了討論鎳鐵氧體與鐵三氧四跟應變相關的機制，本研究中將薄膜沉積在可撓式的白雲母基板上，藉由控制白雲母基板的曲率半徑來改變施加在薄膜上的應力大小，藉以觀察在不同應力下反尖晶石結構薄膜的物性變化。利用拉曼光譜來判定各種反尖晶石薄膜的結構對稱性，並觀察沿著不同晶軸方向施加不同的應力情況下，從聲子的振動行為來推估各種反尖晶石薄膜的晶格變化。之後利用變溫拉曼實驗觀察薄膜振動峰頻率在不同應力下的變化，並討論應力對鎳鐵氧體發生陽離子躍遷效應的影響，與對鐵三氧四生Verwey相變的影響。

Ferrimagnetic spinels (AB2O4), such as CoFe2O4 (CFO), NiFe2O4 (NFO), or Fe3O4 are receiving great attention due to their potential used in spintronics. In this study, the responses of Fe3O4 and NFO thin films under different strained states with two kinds of bending directions are investigated by Raman spectroscopy. We determined the symmetry of Fe3O4 and NFO thin films by polarized Raman spectroscopy. We mainly observed the shift of normal mode’s frequency under different strains and the abnormal change of these phonon frequencies with temperature. Thin films were deposited on a flexible muscovite (mica) substrate, thus we can exert a stress on thin films by curving muscovite substrate.
The Raman modes of Fe3O4 bending along [1 -1 0] show an increase in frequency when increasing an in-plane compressive strain. In contrast, it shows a decrease in frequency when increasing an in-plane tensile strain. Except Fe3O4 bending along [1 -1 0], the Raman modes show an increase in frequency when increasing an in-plane compressive strain and tensile strain. The slope of A1 mode’s frequency shift of NFO versus temperature was changed at 600 K under the strained state. We thought that the slope of frequency changed was attributed to cation migration effect. The Temperature of Verwey transition with Fe3O4 did not change under the strained state.

[1] Michael Foerster et.al, “Distinct magnetism in ultrathin epitaxial NiFe2O4 films on MgAl2O4 and SrTiO3 single crystalline substrates”, Phys. Rev. B, 84, 144422, (2011).
[2] C. Y. Tsai et.al, “Anisotropic strain, magnetic properties, and lattice dynamics in self-assembled multiferroic CoFe2O4-PbTiO3 nanostructures”, J. Appl. Phys., 115, 134317, (2014).
[3] Venkataiah Gorige et al., “Strain mediated magnetoelectric coupling in a NiFe2O4–BaTiO3 multiferroic composite”, J. Phys. D: Appl. Phys., 49, 405001, (2016).
[4] A. Lisfi et.al, “Reorientation of magnetic anisotropy in epitaxial cobalt ferrite thin films”, Phys. Rev. B, 76, 054405, (2007).
[5] Ulrike Lüders et.al, “Enhanced magnetic moment and conductive behavior in NiFe2O4 spinel ultrathin films”, Phys. Rev. B, 71, 134419, (2005).
[6] F. Rigato et al., “Strain-induced stabilization of new magnetic spinel structures in epitaxial oxide heterostructures”, Materials Science and Engineering B, 144, 43–48, (2007).
[7] Daniel Fritsch and Claude Ederer, “Strain effects in spinel ferrite thin films from first principles calculations”, J. Phys. C, 292, 012014, (2011).
[8] V. G. Ivanov et al., “Short-range B-site ordering in the inverse spinel ferrite NiFe2O4”, Phys. Rev. B, 82, 024104, (2010).
[9] M. N. Iliev et al., “Monitoring B-site ordering and strain relaxation in NiFe2O4 epitaxial films by polarized Raman spectroscopy”, Phys. Rev. B, 83, 014108, (2011).
[10] J. M. Hastings and L. M. Corliss, “Neutron Diffraction Studies of Zinc Ferrite and Nickel Ferrite”, Reviews of Modern Physics, 25, 1, (1953).
[11] K. N. Subramanyam, “Neutron and X-ray diffraction studies of certain doped nickel ferrites”, J. Phys. C, 4, 2266, (1971).
[12] C. Haas, “Phase transitions in crystals with the spinel structure”, J. Phys. Chem. Solids, 26, 1225, (1965).
[13] O. Chaix-Pluchery et al., “Strain analysis of multiferroic BiFeO3-CoFe2O4 nanostructures by Raman scattering”, Appl. Phys. LETTERS, 99, 072901, (2011).
[14] D. M. Phase, S. Tiwari, R. Prakash, A. Dubey, V. G. Sathe and R. J. Choudhary, J. Appl. Phys. 100 (12), 5, (2006).
[15] M. Iizumi, T.F. Koetzle, G. Shirane, S. Chikazumi, M.Matsui, S. Todo, Acta Crystallogr. B 38 (1982) 2121.
[16] Andrzej Koslowski,“The impact of orbital ordering on the Verwey transition in magnetite”,Krakow, (2014).
[17] G. Perversi, J. Cumby, E. Pachoud, J. P. Wright and J. P. Attfield, Chem. Commun. 52 (27), 4864-4867 (2016).
[18] O. N. Shebanova and P. Lazor, J. Raman Spectrosc. 34 (11), 845-852 (2003).
[19] 吳昆鴻, “鈷鐵氧體薄膜在可撓式基板上的應變調製拉曼研究”, 成功大學, 碩士論文, (2016).
[20] John R. Ferraro, Kazuo Nakamoto and Chris W. Brown, “Introductory Raman Spectroscopy”, Second edition, (2003).
[21] 簡永順, “釕酸鍶/鈷鐵氧體系統複合結構中的應力調製藕合研究”, 成功大學, 碩士論文, (2012).
[22] Heng-Jui Liu et al., “Flexible Heteroepitaxy of CoFe2O4/Muscovite Bimorph with Large Magnetostriction”, Applied Materials and Interfaces, 9, 7297−7304, (2017).
[23] Essentials of Paleomagnetism: Second Web Edition.
[24] Ping-Chun Wu et al., “Heteroepitaxy of Fe3O4/Muscovite: A New Perspective for Flexible Spintronics”, Appl. Mater. Interfaces, 8, 33794−33801, (2016).