The desire for multifunctional devices has driven significant research toward exploring multiferroics, where the coupling between electric, magnetic, optical, and structural order parameters can provide new functionality. While BiFeO3 is a well-studied multiferroic, recent research has shown that the addition of BaTiO3 can improve material properties.1 In this talk we focus on coherent phonon (CP) generation in BaTiO3-BiFeO3 (BTO-BFO) layered structure as well as nanorod arrays. Usually, CPs are used to provide detailed spectroscopy information and to characterize surfaces and buried interfaces. However, the ability to generate strain via ultrafast optics offers the intriguing possibility of dynamically manipulating the strain with ultrashort optical pulses and opens the possibility of creating a new class of devices, where the strain is manipulated in time to control the properties and operation of a device. In our nanorod arrays, we demonstrated several coherent modes, with possible signatures of the coexistence of CPs and magnons.2 While magnons, in general, are hard to manipulate and control, a strong magneto-elastic interaction between phonons and magnons can be important for a variety of reasons: (i) Coherent Acoustic Phonons generated with ultrafast optical pulses can propagate long distances from the surface, into the sample. With strong magneto-elastic coupling, they can carry the spin information along with them into the sample, perhaps between different regions of a chip. (ii) Strong interactions with phonons can enhance the excitation, manipulation, and detection of the magnons for possible applications in memory devices.
In this talk, I will present our observations in several BTO-BFO films and nanorod arrays with different interfaces to demonstrate the tunability of CPs and discuss the possibility of the co-existence of CPs and magnons. I will also discuss the possibility of controlling these coherent states using external magnetic fields which have been demonstrated to increase the sensitivity of the CPs’ detection in other systems.3
 S.-C. Yang, A. Kumar, V. Petkov, and S. Priya, J. of Appl. Phys., 113, 144101 (2013).
 R. R. H. H. Mudiyanselage, B. A. Magill, J. Burton, M. Miller, J. Spencer, K. McMillan, G. A. Khodaparast, H.-B. Kang, M.-G. Kang, D. Maurya, S. Priya, J. Holleman, S. McGill, and C. J. Stanton, J. Mater. Chem. C, 7, 14212 (2019).
 B. A. Magill, S. Thapa, J. Holleman, S. McGill, H. Munekata, C. J. Stanton, and G. A. Khodaparast, Phys. Rev. B 102, 045306 (2020).
Acknowledgment: This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-17-1-0341 and DURIP funding (FA9550-16-1-0358). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.