Positive and Negative Magnetodielectric Effects in A-Site Ordered (BiMn 3 )Mn 4 O 12 Perovskite Naoki Imamura, † Maarit Karppinen,* ,†,‡ Teruki Motohashi, † Desheng Fu, † Mitsuru Itoh, † and Hisao Yamauchi †,‡ Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan, and Department of Chemistry, Helsinki UniVersity of Technology, FI-02015 TKK, Finland Received August 20, 2008; E-mail: maarit.karppinen@tkk.fi Materials with mutually coupled magnetic and (di)electric properties are the very focus of current interest as they are expected to provide us with novel phenomena of highly scientific and technological impact. 1,2 The so-called magnetodielectric or mag- netocapacitance (MC) effect, seen for some exotic magnetic insulators/semiconductors, such as multiferroic BiMnO 3 3 and magnetoresistive La 2 NiMnO 6 , 4 is expected to lead to unprecedented device applications, in which the magnitude of dielectric constant may be controlled by the magnetic field. The challenge is to expand the yet limited number of MC compounds to gain a deeper understanding of the magneto(di)electric phenomena and also to open up ways to put them to practical use. Here we present a novel bismuth manganese perovskite oxide, BiMn 7 O 12 or (BiMn 3 )Mn 4 O 12 , that exhibits either a positive or a negative dielectric constant response of appreciable magnitude in an external magnetic field depending on the temperature range/ type of magnetic ordering. The BiMn 7 O 12 sample was prepared in an essentially single-phase form through a high-pressure synthesis route at 5 GPa and 1100 °C with only minute amounts of BiMnO 3 and Bi 2 O 2 CO 3 as impurity phases. The new compound BiMn 7 O 12 is isostructural with the reported (LaMn 3 )Mn 4 O 12 , 5 possessing (at room temperature) a strongly distorted monoclinic perovskite structure with space group I2/m and lattice parameters a ) 7.548(3) Å, b ) 7.393(3) Å, c ) 7.556(3) Å,  ) 91.229(2)°, based on X-ray powder diffraction data. Rietveld refinement confirmed that the Bi 3+ and Mn 3+ ions are ordered in a 1:3 ratio at the perovskite A-site, i.e. (BiMn 3 )Mn 4 O 12 (see Figure 1). Resistivity showed an insulating/semiconducting-type dependence on temperature, being ∼10 4 Ω cm at room temperature. Figure 2a shows the dependence of magnetization on temperature for (BiMn 3 )Mn 4 O 12 under various magnetic fields. Parallel to the case of (LaMn 3 )Mn 4 O 12 , 6 two magnetic transitions are observed with decreasing temperature: an FM-like transition appears at T 2 ≈ 59 K and an AFM-like transition at T 1 ≈ 28 K. Taking analogy to the (LaMn 3 )Mn 4 O 12 phase, we tentatively assign the lower-temperature AFM-type transition and the higher- temperature FM-type transition to Mn 3+ -ion spin ordering at the A- and B-cation site, respectively. [For (LaMn 3 )Mn 4 O 12 both transitions at 20 and 76 K are of the AFM type. 6 ] A Curie-Weiss fit to the paramagnetic susceptibility data resulted in a Weiss temperature of -259 K and a paramagnetic moment of 4.96 µ B per one Mn 3+ ion which is close to the theoretical value of 4.9 µ B . An FM hysteresis can be clearly observed at temperatures below T 2 [see the inset of Figure 2a]. The resultant spontaneous magne- tization (∼1 µ B /formula unit with seven Mn 3+ ions) is rather small indicating an AFM coupling between the Mn 3+ ions at the B site. Therefore we attribute the FM feature to spin canting as has been done for many BiMO 3 perovskites, 7-9 except for the orbital-ordered BiMnO 3 ferromagnet. 10 Below T 1 a large difference is seen between the ZFC and FC curves when a small magnetic field is applied, indicating a frustrated AFM state. A somewhat unusual feature of the present material (BiMn 3 )Mn 4 O 12 is that the weak-FM regime in terms of temper- † Tokyo Institute of Technology. ‡ Helsinki University of Technology. Figure 1. Room-temperature crystal structure of (BiMn 3 )Mn 4 O 12 . Figure 2. Dependence on temperature (T) of (a) magnetization (M), (b) specific heat (expressed as C p /T), and (c) real part of relative dielectric constant (ε′; at 100 kHz) for (BiMn 3 )Mn 4 O 12 under various magnetic fields (H). The inset in (a) shows M/H curves at selected temperatures. Published on Web 10/17/2008 10.1021/ja806487d CCC: $40.75  2008 American Chemical Society 14948 9 J. AM. CHEM. SOC. 2008, 130, 14948–14949