SUMMARY AND FUTURE WORK
In this dissertation, we systematically investigated the VAN architecture from the growth and interface control, to vertical strain tuning and interface coupling, as well as their effect on magnetotransport, magnetic anisotropy, exchange bias and FE properties. The VAN structure provides a versatile pathway to achieve enhanced and/or novel functionalities because of its simpler self-assembly, excellent structure compatibility in a variety of functional oxides, large vertical interfacial area and effective vertical strain tuning.
First, highly-textured LSMO:ZnO VAN films have been integrated on Si substrates with a bilayer STO/TiN buffer. The VAN films on Si have exhibited comparable LFMR results to those of films grown on single-crystal STO substrates, which provides an important step to enable the practical application of VAN films. Second, different types of magnetically coupled interfaces have been studied in BFO:CFO and BFO:LSMO VAN films. The enhanced perpendicular magnetic anisotropy has been obtained in BFO:CFO VAN films with significantly strain-relaxed interface. Detailed analysis indicates the spin-flop coupling at BFO-CFO interfaces introduces the observed enhanced anisotropy. In LSMO:BFO VAN films, the AFM-FM spin coupling at the vertical interface exhibits strong perpendicular exchange bias in micrometer-thick films. A systematic strain tuning has been achieved by varying the film composition, which affect the interface structure, AFM spin reorientation and magnetic
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phase transition. Third, by employing the exchange coupling at the vertical LSMO-NiO interfaces, a dynamic and reversible control of the out-of-plane magnetotranport has been achieved, which provides a different way to study the magnetotransport property in VAN films along the vertical interface. Last, using BFO:CFO VAN system as an example, both vertical and gradient interfaces have been achieved by adjusting the film deposition frequency. The room-temperature magnetic anisotropy, coercive field and FE switching behavior of VAN films with different interfaces are dominated by different mechanisms depending on the strain accommodation and interface coupling.
VAN is a novel thin film platform with unique vertical interface coupling and strain tuning that can be used to achieve novel/enhanced multifunctionalities. Further research efforts are suggested to explore the following aspects:
(1) Fundamental growth mechanism. For example, can we come up with a general algorithm that guides the material selection for the growth of VAN structures either by nucleation and growth or by (pseudo-)spinodal decomposition? Can we integrate non-oxide materials and their additional wide-range functionalities in the VAN structure?
(2) Self-assembled or directed structure control. Can we achieve the exact control of VAN pillar size and interspacing in a long-range order by self-assembled or directed growth, and what are the limits on their length scales? Can we achieve a desired level of control of vertical interface defects and roughness for specific applications?
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(3) Novel structure design. Can we integrate other nanostructure in the form of zero, one or two dimension to the VAN structure to further add functionality and tunability?
(4) Multifunctionality exploration. The coupling effect is not only limited in the ME and photomagnetic effects which have been demonstrated in VAN systems. Other parameters, including thermal and optical effect, can be explored for the mutually coupled multifunctionalities, such as thermoelectric, photochemical and so on.
(5) Interface effect. The structural discontinuity at the vertical semiconherent heterointerface of different phases in the VAN structure may generate a high concentration of structural disorders, such as misfit dislocations, grain and phases boundaries, which affects the electric and ionic transport, and in some cases the flux pinning properties in superconductors. Besides the transport performance, the interface effects on other related phenomena can be explored.
(6) Device integration. Can we incorporate the VAN structure in current devices or design new device structures based on the VAN architecture? This may require research efforts from the field of advanced device physics for the device application of the VAN structure.
171 REFERENCES
1. Bednorz, J. G.; Müller, K. A. Z. Phys. Con. Mat. 1986, 64, 189-193. 2. Eerenstein, W.; Mathur, N. D.; Scott, J. F. Nature 2006, 442, 759-765. 3. Ramirez, A. P. J. Phys.: Condens. Matter 1997, 9, 8171.
4. Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105-114.
5. Garcia-Barriocanal, J.; Rivera-Calzada, A.; Varela, M.; Sefrioui, Z.; Iborra, E.; Leon, C.; Pennycook, S. J.; Santamaria, J. Science 2008, 321, 676-680.
6. Catalan, G.; Scott, J. F. Adv. Mater. 2009, 21, 2463-2485. 7. Ohtomo, A.; Hwang, H. Y. Nature 2004, 427, 423-426.
8. Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Science 2003, 299, 1719-1722.
9. Martin, L. W.; Chu, Y. H.; Ramesh, R. Mater. Sci. Eng., R 2010, 68, 89-133. 10. Benedek, N. A.; Rondinelli, J. M.; Djani, H.; Ghosez, P.; Lightfoot, P. Dalton
Trans. 2015, 44, 10543-10558.
11. Zener, C. Phys. Rev. 1951, 82, 403-405.
12. Rao, C. N. R.; Raveau, B., Transition metal oxides: structure, properties, and
synthesis of ceramic oxides. New York: Wiley-VCH, 2nd ed. 1998.
13. Thomson, W. Proc. Royal Soc. London 1856, 8, 546-550.
14. Baibich, M. N.; Broto, J. M.; Fert, A.; Van Dau, F. N.; Petroff, F.; Etienne, P.; Creuzet, G.; Friederich, A.; Chazelas, J. Phys. Rev. Lett. 1988, 61, 2472-2475.
172
15. Binasch, G.; Grünberg, P.; Saurenbach, F.; Zinn, W. Phys. Rev. B 1989, 39, 4828-4830.
16. Alex-engraver, Spin vale with giant mangetoresistance. URL http://upload.wikimedia.org/wikipedia/commons/9/93/Spin-valve_GMR.svg. Accessed at September, 2015.
17. Nesbet, R. K. J. Phys.: Condens. Matter 1994, 6, L449. 18. Julliere, M. Phys. Lett. A 1975, 54, 225-226.
19. Miyazaki, T.; Tezuka, N. J. Magn. Magn. Mater. 1995, 139, L231-L234. 20. Chappert, C.; Fert, A.; Van Dau, F. N. Nat. Mater. 2007, 6, 813-823.
21. Dexin, W.; Nordman, C.; Daughton, J. M.; Qian, Z.; Fink, J. IEEE Trans. Magn. 2004, 40, 2269-2271.
22. Lee, Y. M.; Hayakawa, J.; Ikeda, S.; Matsukura, F.; Ohno, H. Appl. Phys. Lett. 2007, 90, 212507.
23. Ikeda, S.; Hayakawa, J.; Ashizawa, Y.; Lee, Y. M.; Miura, K.; Hasegawa, H.; Tsunoda, M.; Matsukura, F.; Ohno, H. Appl. Phys. Lett. 2008, 93, 082508.
24. Mao, S.; Yonghua, C.; Liu, F.; Xingfu, C.; Bin, X.; Puling, L.; Patwari, M.; Haiwen, X.; Clif, C.; Miller, B.; Menard, D.; Pant, B.; Loven, J.; Duxstad, K.; Li, S.; Zhengyong, Z.; Johnston, A.; Lamberton, R.; Gubbins, M.; McLaughlin, T.; Gadbois, J.; Ding, J.; Cross, B.; Song, X.; Ryan, P. IEEE Trans. Magn. 2006, 42, 97-102.
25. Mangin, S.; Ravelosona, D.; Katine, J. A.; Carey, M. J.; Terris, B. D.; Fullerton, E. E. Nat. Mater. 2006, 5, 210-215.
173
26. Ikeda, S.; Miura, K.; Yamamoto, H.; Mizunuma, K.; Gan, H. D.; Endo, M.; Kanai, S.; Hayakawa, J.; Matsukura, F.; Ohno, H. Nat. Mater. 2010, 9, 721-724. 27. Gould, C.; Rüster, C.; Jungwirth, T.; Girgis, E.; Schott, G. M.; Giraud, R.;
Brunner, K.; Schmidt, G.; Molenkamp, L. W. Phys. Rev. Lett. 2004, 93, 117203. 28. Park, B. G.; Wunderlich, J.; Martí, X.; Holý, V.; Kurosaki, Y.; Yamada, M.;
Yamamoto, H.; Nishide, A.; Hayakawa, J.; Takahashi, H.; Shick, A. B.; Jungwirth, T. Nat. Mater. 2011, 10, 347-351.
29. Jonker, G. H.; Van Santen, J. H. Physica 1950, 16, 337-349.
30. Haghiri-Gosnet, A. M.; Renard, J. P. J. Phys. D: Appl. Phys. 2003, 36, R127. 31. Jahn, H. A.; Teller, E. Proc. R. Soc. Lond. A: Math. Phys. Sci. 1937, 161, 220-
235.
32. Ian , M. R.; Enrico , L. C.; Nava, S. Jpn. J. Appl. Phys. 1994, 33, 3984. 33. Goodenough, J. B. Phys. Rev. 1955, 100, 564-573.
34. Siwach, P. K.; Singh, H. K.; Srivastava, O. N. J. Phys.: Condens. Matter 2008, 20, 273201.
35. de Gennes, P. G. Phys. Rev. 1960, 118, 141-154.
36. Dagotto, E.; Hotta, T.; Moreo, A. Phys. Rep. 2001, 344, 1-153.
37. Jin, S.; Tiefel, T. H.; McCormack, M.; Fastnacht, R. A.; Ramesh, R.; Chen, L. H.
Science 1994, 264, 413-415.
38. Subramanian, M. A.; Toby, B. H.; Ramirez, A. P.; Marshall, W. J.; Sleight, A. W.; Kwei, G. H. Science 1996, 273, 81-84.