IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 4, APRIL 2014 2900104 Influence of the Change in Oxidation Number of Mn on Magnetic Properties of BaTi1−x Mn x O3 T L Phan1 , T D Thanh1,2, T A Ho1 , P D Thang3, and S C Yu1 Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea of Materials Science, Vietnam Academy of Science and Technology, Hanoi, Vietnam Department of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi 10000, Vietnam Institute We prepared BaTi1−x Mn x O3 samples (x = and 0.05) by standard solid-state reaction at 700 °C and 900 °C in an Ar ambient to change oxidation number of Mn dopants The fabricated samples were then studied structural characterization and electronic structures by means of X-ray diffraction and absorption, and Raman scattering and electron-spin-resonance spectrometers We found oxidation numbers 2+ and 3+ of Mn ions coexisting in BaTi1−x Mn x O3 with x = 0.05, where the Mn2+ /Mn3+ ratio is about one for the sample annealed at 700 °C These Mn ions prefer locating at the Ti site in the tetragonal BaTiO3 host lattice In particular, there is a change in the oxidation number of Mn2+ → Mn3+ when the annealing temperature changes from 700 °C to 900 °C Their magnetic properties are accordingly changed Meanwhile, annealing pure BaTiO3 in an Ar ambient at 700 °C and 900 °C does not lead to ferromagnetic (FM) order; they are almost diamagnetic With the obtained results, we believe that FM order in Mn-doped BaTiO3 annealed in an Ar ambient is associated with exchange interactions of Mn2+ ions mediated by oxygen vacancies rather than associated with Mn3+ ions Index Terms— BaTiO3 -based multiferroics, local structure, magnetic properties I I NTRODUCTION I N RECENT years, BaTiO3 -based multiferroics [defined as materials exhibiting simultaneously either two of ferroelectric, ferromagnetic (FM), and ferroelastic properties] have attracted much interest of the solid-state physics community because doping a transition metal (TM, with TM = Mn, Co, Fe, Cr, or Ni, etc.) can lead to FM order at room temperature [1]–[8] This makes TM-doped BaTiO3 compounds become promising candidates for applications of multifunctional electronic and spintronic devices Among these, BaTi1−x Mn x O3 materials have been of special interest because it is realized that the Mn doping can give a large value of magnetic moment [1], [6], [7], and enhances the positive temperature coefficient of resistance [9] The application range of BaTiO3 -based materials in electronic and spintronic devices is thus extended to high-speed nonvolatile and holographic memories, capacitors with tunable capacitance, actuators, dielectric resonators, electromagnetic-interference filters, and so forth [10], [11] Previous studies revealed that the magnetic properties of BaTi1−x Mn x O3 depend on many factors, such as Mn-doping concentration (x), fabrication conditions (annealing temperature, pressure, and ambient), and sample types (i.e., bulk single-crystal and polycrystalline samples, thin films or nanostructures) [3]–[5], [7], [8], [12]–[14] For bulk ceramic samples, it has been found that magnetic changes as varying x are related directly to the crystalline fraction of tetragonal and hexagonal phases, the strength of double-exchange Manuscript received August 2, 2013; revised October 7, 2013; accepted October 24, 2013 Date of current version April 4, 2014 Corresponding author: S C Yu (e-mail: scyu@chungbuk.ac.kr) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2013.2288293 interactions between Mn3+ and Mn4+ ions, and the concentration of Mn3+,4+ ions [1] Their ferromagnetism at low temperatures (