ISSN: 0256-307X 中国物理快报 Chinese Physics Letters Volume 29 Number 9 September 2012 A Series Journal of the Chinese Physical Society Distributed by IOP Publishing Online: http://iopscience.iop.org/cpl http://cpl.iphy.ac.cn C HINESE P HYSICAL S OCIETY CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097701 * Exchange Bias in Polycrystalline BiFe1−xMnxO3/Ni81Fe19 Bilayers YUAN Xue-Yong(袁Æ])1, XUE Xiao-Bo(Å¡Å)2, SI Li-Fang(iw芳)1, DU Jun(杜军)2, XU Qing-Yu(M庆宇)1;3** 1Department of Physics, Southeast University, Nanjing 211189 2National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093 3Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096 (Received 4 July 2012) Polycrystalline BiFe1−xMnxO3 films with x up to 0.50 are prepared on LaNiO3 buffered surface oxidized Si substrates. The doped Mn is confirmed to be partially in a +4 valence state. A clear exchange bias effectis observed with a 3.6 nm Ni81Fe19 layer deposited on the top BiFe1−xMnxO3 layer, which decreases drastically with increasing Mn doping concentration and finally to zero when x is above 0.20. These results clearly demonstrate that the exchange bias field comes from the net spins due to the canted antiferromagnetic spin structure in polycrystalline BiFe1−xMnxO3 films, which transforms to a collinear antiferromagnetic spin structure whenthe Mn doping concentration is larger than 0.20. PACS: 77.55.Nv, 75.30.Et, 75.50.Ee DOI: 10.1088/0256-307X/29/9/097701 Multiferroic materials which present simultane- pling in BiFeO3, the mechanism of the exchange bias is ously ferroelectric and magnetic orderings have at- still under debate. The surface roughness,[11] 109∘ fer- tracted extensive interest due to their abundant roelectric domain walls,[12] antiferromagnetic domain [1] [13] physics and potential applications in novel devices. size, canted magnetic moment of BiFeO3 near the However, room-temperature multiferroic materials are interface due to the interface exchange coupling,[15;16] [17] very rare. BiFeO3 is an antiferromagnetic-ferroelectric spin canting of BiFeO3, etc. have been proposed to compound at room temperature (Neel temperature explain the exchange bias. Therefore, further studies [2] TN∼643 K and Curie temperature TC∼1103 K). The are still needed to clarify the mechanism. coupling between the antiferromagnetic and ferroelec- Furthermore, studies on polycrystalline BiFeO3 tric orderings has been confirmed experimentally by are still rare.[19;20] In this Letter, the exchange the observation of coupled ferroelectric and antiferro- bias effect in polycrystalline BiFe1−xMnxO3/Ni81Fe19 magnetic domains.[3] At room temperature, it has a (NiFe) bilayers is systematically investigated. The rhombohedral R3c perovskite structure with a large drastic decrease of exchange bias field with increas- electric polarization (60 µC/cm2) pointing along the ing Mn concentration indicates that the exchange elongated [111] direction.[4] bias field comes from the spin canting due tothe 3+ In bulk BiFeO3, the Fe spins order in a G-type canted antiferromagnetic spin structure in polycrys- antiferromagnetic structure with a superimposed long- talline BiFe1−xMnxO3 films, which transforms toa wavelength (∼62 nm) cycloidal modulation.[5] How- collinear antiferromagnetic spin structure when x is ever, BiFeO3 thin films might show rather different above 0.20. properties from those of bulk samples. Many studies The BiFe1−xMnxO3 (x = 0, 0.05, 0.1, 0.2, have been devoted to epitaxially grown single crys- 0.3, 0.5) targets were prepared by the tartaric [6] [21] talline BiFeO3 films. The cycloidal spin structure acid modified sol-gel method. The bilayer of was destroyed due to the epitaxial strains in single BiFe1−xMnxO3/NiFe (∼80 nm and 3.6 nm in thick- [7] crystalline BiFeO3 films. Due to the antiferromag- ness, respectively) magnetic heterostructures were de- netic nature of BiFeO3, the most plausible application posited on surface oxidized Si (100) substrates by in spintronics is suggested to be an antiferromagnetic pulsed laser deposition (PLD) for the oxide lay- pinning layer.[8] The exchange bias has been mostly ers and magnetron sputtering for the metallic lay- [20] reported in epitaxial single crystalline BiFeO3 with ers, as described previously. Before the growth of various ferromagnetic layers, such as NiFe, CoFeB, BiFe1−xMnxO3, a LaNiO3 buffer layer∼ ( 30 nm thick) [9−18] CoFe, La0:7Sr0:3MnO3, and Fe3O4. Due to the was first deposited by PLD. Finally, Ta as the capping complicated spin structure and magnetoelectric cou- layer for preventing the NiFe layer from oxidization *Supported by the National Basic Research Program of China under Grant Nos 2010CB923404 and 2010CB923401, the Na- tional Natural Science Foundation of China (51172044, 11074112, 11174131), the National Science Foundation of Jiangsu Province (BK2011617, BK2010421), the New Century Excellent Talent Project of the Ministry of Education of China under Grant No NCET-09-0296, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Southeast University (the Excellent Young Teachers Program and Seujq201106). **Corresponding author. Email: [email protected] © 2012 Chinese Physical Society and IOP Publishing Ltd 097701-1 CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097701 [26] was deposited. The thickness of the BiFe1−xMnxO3 port on bulk BiFe1−xMnxO3 ceramics. The Mn films was controlled by the number of laser pulses and 2p XPS spectrum of the BiFe0:95Mn0:05O3 film was calibrated by a transmission electron microscope. The taken to study the valence state of the doped Mn crystal structure of the films was examined by x-ray ions, as shown in Fig. 2(a). The binding energy of Mn diffraction (XRD) with CuK훼 radiation. X-ray pho- 2p3=2 in MnO, Mn2O3 and Mn3O4 are between 641 [27] toelectron spectroscopy (XPS, ThermoFisher SCIEN- and 641.5 eV, while that of MnO2 is around 642 eV. TIFIC) with an Al K훼 x-ray source (ℎ휈=1486.6 eV), The binding energy of Mn 2p3=2 at 641.7 eV indicates and calibrated by the C 1s line (284.8 eV) binding that the doped Mn ions are partially in a +4 valence energy.[22] Raman measurements were carried out on state. The radius of Mn+4 (0.67 Å) is smaller than a Horiba Jobin Yvon LabRAM HR 800 micro-Raman that of Fe+3 (0.69 Å),[28] leading to decrease of the spectrometer with 785 nm excitation under air ambi- lattice constant. This result indicates that the lat- ent conditions at room temperature. The magnetic tice parameter is changed by Mn substitution and a hysteresis (M–H) loops were measured by a vibrat- gradual phase transition from the rhombohedral dis- ing sample magnetometer (VSM, Microsense EV7) at tortion to orthorhombic or tetragonal structure with room temperature with an applied field parallel to the the increase of Mn doping content, as reported by film plane. Singh et al.[29] Furthermore, the substituted Mn ions in a +4 valence state will suppress the O vacancies (b) (a) Bi O 2 3 due to charge compensation, leading to the effec- tive suppression of the leakage current and improved [18] =0.50 ferroelectricity. Figure 2(b) shows the Fe 2p XPS spectrum of Fe. The binding energy of Fe 2p3=2 is at =0.30 709.9 eV, suggesting the existence of Fe2+.[27] How- units) ever, the decomposition of the Fe 2p spectrum into =0.20 3=2 a superposition of symmetric components is ques- =0.10 tionable, thus it is complicated to obtain the exact concentration of Fe2+.[30] The clear observation of the *( 012) =0.05 satellite peaks and the similar curve shape to that Intensity (arb. of Fe2O3 indicate that Fe ions are mainly in the +3 ( 110) ( 110) ( 104) ( 104) [30] ( 012) valence state. *( 110) ( 024) *(110) =0 ( 116) *( 024) (a) Mn 31 32 33 34 20 30 40 50 2 641.7 eV 3/2 2 (deg) 2 (deg) 2 653.0 eV 1/2 Fig. 1. XRD patterns of (a) LaNiO3/BiFe1−xMnxO3 (x = 0, 0.05, 0.10, 0.20, 0.30, 0.50) bilayers. The as- terisks denote the diffraction peaks from LaNiO3, the im- purity peak has been indexed to Bi2O3, and the rest are 635 640 645 650 655 from BiFeO3. (b) The magnified view in the vicinity of (b) Fe 2 709.9 eV 2휃 = 32∘. 3/2 2 723.7 eV 1/2 Intensity (arb. units) Figure 1 shows the XRD patterns of BiFe1−xMnxO3 films on 2SiO /Si (100) substrates with LaNiO3 as the buffer layer. The pseudocubic [23] 705 710 715 720 725 730 735 lattice constant is 3.84 Å for LaNiO3 and 3.96 Å [24] Binding energy (eV) for BiFeO3. Thus the polycrystalline BiFeO3 might Fig. 2. 2p 2p be epitaxially grown on the LaNiO3 grains, as indi- The Mn (a) and Fe (b) XPS spectra for the cated in our previous report.[20] Besides the diffrac- BiFe0:95Mn0:05O3 film. tion peaks corresponding to LaNiO3, all the other Figure3 shows the Raman spectra of polycrys- peaks can be indexed to the BiFeO3 of a pure R3c talline BiFe1−xMnxO3 films. Except for the strong structure with x increasing from 0 to 0.30. With peak at 520 cm−1 corresponding to the Si substrate,[31] further increase of x up to 0.50, a strong impurity the clearly resolved Raman modes can all be indexed [32] peak of Bi2O3 can be clearly observed, though the to the modes of BiFeO3 with the R3c structure. remaining peaks can still be indexed to the R3c struc- The A1-1, A1-2 and A1-3 modes are associated with ture. These results are consistent with the previous the Bi-O vibrations.