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Enhancing room-temperature ferromagnetism in Bi0.5Na0.5TiO3 via FeTiO3 solid solution D. D. Dung 1 & M. M. Hue 1 & N. Q. Dung 2 & N. H. Lam 1 & L. T. K. Phuong 3 & L. H. Bac 1 & N. N. Trung 1 & N. V. Duc 4 & D. Odkhuu 5 Received: 11 June 2018 / Accepted: 8 March 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020

Abstract A solid solution of FeTiO3 in host Bi0.5Na0.5TiO3 materials was fabricated by sol–gel method. The X-ray diffraction and Raman scattering analyses indicated that the Fe cations were randomly incorporated into the host lattice of Bi0.5Na0.5TiO3 materials, thereby resulting in the distortion of the crystal structural. The absorbance spectra of FeTiO3-modified Bi0.5Na0.5TiO3 samples revealed that the optical band gap decreased from 3.11 eV for pure Bi0.5Na0.5TiO3 materials to 2.41 eV with the addition of increasing amounts of FeTiO3 solid solution into the host Bi0.5Na0.5TiO3 materials (up to 9 mol% FeTiO3). The tunable magnetic properties of host Bi0.5Na0.5TiO3 materials were obtained as the concentration of FeTiO3 as solid solution was controlled. The weak ferromagnetism as compensation of diamagnetism of pure Bi0.5Na0.5TiO3 materials at room temperature was changed to ferromagnetism and end-off ferromagnetism combine with paramagnetism and/or antiferromagnetism with increasing FeTiO3 concentration during the formation of a solid solution. This work proposed an alternative method for obtaining ferromagnetism at room temperature for lead-free ferroelectric materials. Keywords FeTiO3 . Bi0.5Na0.5TiO3 . Ferromagnetism . Sol-gel

1 Introduction Inducing ferromagnetism in oxide ferroelectric materials exhibits potential for fabricating next-generation smart devices [1, 2]. Traditional ferroelectric materials, such as PbTiO3, possess room-temperature ferromagnetism at nanoscale because of self-defects [3–6]. Zhang et al. pointed out that the ferromagnetism of PbTiO3 nanocrystalline was strongly enhanced

* D. D. Dung [email protected] * D. Odkhuu [email protected] 1

School of Engineering Physics, Ha Noi University of Science and Technology, 1 Dai Co Viet road, Ha Noi, Vietnam

2

Department of Chemistry, Thai Nguyen University of Education, 20 Luong Ngoc Quyen street, Thai Nguyen, Vietnam

3

School of Electronics and Telecommunications, Ha Noi University of Science and Technology, 1 Dai Co Viet road, Ha Noi, Vietnam

4

Department of Technology and Engineering, Hong Duc University, 556 Quang Trung street, Dong Ve ward, Thanh Hoa, Vietnam

5

Department of Physics, Incheon National University, Incheon 22012, Republic of Korea

by annealing in vacuum environment than annealing in air because of the presence of oxygen vacancies in the former [3]. In addition, low sintering temperature for fabrication of PbTiO3 materials enhanced ferromagnetism due to surface defect and/or promotion of oxygen vacancies [3–5]. Theoretical calculation predicted that O and Ti vacancies induced ferromagnetism by different mechanisms, such as oxygen vacancy driving the spin-polarized Eg state of the