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−푥Mn푥O3/Ni81Fe19 Bilayers

YUAN Xue-Yong(袁学勇)1, XUE Xiao-Bo(薛晓波)2, SI Li-Fang(司丽芳)1, DU Jun(杜军)2, XU Qing-Yu(徐庆宇)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−푥Mn푥O3 films with 푥 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−푥Mn푥O3 layer, which decreases drastically with increasing Mn doping concentration and finally to zero when 푥 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−푥Mn푥O3 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] 푇N∼643 K and Curie temperature 푇C∼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−푥Mn푥O3/Ni81Fe19 magnetic domains.[3] At room temperature, it has a (NiFe) bilayers is systematically investigated. The rhombohedral 푅3푐 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−푥Mn푥O3 films, which transforms toa wavelength (∼62 nm) cycloidal modulation.[5] How- collinear antiferromagnetic spin structure when 푥 is ever, BiFeO3 thin films might show rather different above 0.20. properties from those of bulk samples. Many studies The BiFe1−푥Mn푥O3 (푥 = 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−푥Mn푥O3/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−푥Mn푥O3, 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 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−푥Mn푥O3 port on bulk BiFe1−푥Mn푥O3 ceramics. The Mn films was controlled by the number of laser pulses and 2푝 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- 2푝3/2 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 2푝3/2 at 641.7 eV indicates and calibrated by the C 1푠 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 (푀–퐻) 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 2푝 XPS

spectrum of Fe. The binding energy of Fe 2푝3/2 is at

=0.30 709.9 eV, suggesting the existence of Fe2+.[27] How- units)

ever, the decomposition of the Fe 2푝 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−푥Mn푥O3 (푥 = 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−푥Mn푥O3 films on SiO2/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. 2푝 2푝 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 푅3푐 talline BiFe1−푥Mn푥O3 films. Except for the strong structure with 푥 increasing from 0 to 0.30. With peak at 520 cm−1 corresponding to the Si substrate,[31] further increase of 푥 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 푅3푐 structure. remaining peaks can still be indexed to the 푅3푐 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. Their peak intensities decrease report.[25] Figure 1(b) shows the magnified patterns with increasing Mn concentration, and nearly disap- around 2휃 = 32∘. The (104) and (110) peaks both pear with 푥 above 0.20. This indicates that a phase shift to higher angles with increasing Mn doping con- transition may occur when 푥 is above 0.20,[33] which centration, which is consistent with the previous re- is consistent with the XRD result. Compared with 097701-2 CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097701

[17] the pure BiFeO3 film, two strong and wide bands al. have suggested that the uncompensated spins can be clearly observed, i.e., one at 620 cm−1 and the at the interface is due to the local spin canting in −1 −1 other in the range from 450 cm to 550 cm in the BiFeO3. It has been further demonstrated by Heron BiFe1−푥Mn푥O3 films. These two distinct bands have et al. in the electrical field manipulation of the magne- 3+ been attributed to the distortion of [(Mn, Fe) O6] tization of a CoFe layer on BiFeO3 film that the spin octahedral.[34] of the ferromagnetic layer lies parallel to the net spin

of the canted spins.[35] The neutron diffraction study shows that Mn doping results in a transformation

=0.3 from a long-range spiral spin modulation of BiFeO3 to a collinear antiferromagnetic spin structure with increasing Mn concentration beyond 0.20,[36] which

=0.2 would lead to the decrease of net spins at the interface. units) Based on the above discussions, we can conclude that the Mn doping will suppress the local spin canting.

=0.1 Therefore, the net spin at the ferromagnet/BiFeO3 interface will decrease with increasing Mn doping con- centration, leading to a decrease of the exchange bias

=0.05 Intensity (arb. field and coercivity.

A -1

1

Si A -2

1

A -3

1

NiFe =0.1 1.0 =0 =0.05

=0

0.60 Oe 0.5 =0.5 =0.3 =0.2

0.0

1.0

100 200 300 400 500 600 700 800

-0.5 0.55 Oe

-1

-1.0

Raman shift (cm )

-3-2-1 0 1 2 3 40

H 0.5 c

Fig. 3. Raman spectra of LaNiO3/BiFe1−푥Mn푥O3 (푥 = 30

H

E 0, 0.05, 0.10, 0.20, 0.30) bilayers. (O e ) E

20

0.0 , ,

10 In order to obtain a sizeable exchange bias, 3.6 nm c

0 Normalized

NiFe thin films were deposited on the BiFe1−푥Mn푥O3 0.0 0.1 0.2 0.3 0.4 0.5

-0.5 films. The corresponding 푀–퐻 loops are shown in Mn concentration Fig. 4. It can be clearly seen that the central position

-1.0 of 푀–퐻 loops exhibit a shift (exchange bias field, 퐻퐸) towards a negative field with the increasing Mn dop- -100 -50 0 50 100 (Oe) ing concentration up to 0.20. When the substitution of Mn increases up to 0.30 or even larger, the exchange Fig. 4. 푀–퐻 curves of BiFe1−푥Mn푥O3/NiFe (푥 = 0, 0.05, 0.10, 0.20, 0.30, 0.50) bilayers at room temperature. bias field vanishes. The inset of Fig. 4 shows the de- The left top inset shows the 푀–퐻 curve of a single NiFe pendences of the exchange bias field and coercivity layer (3.6 nm thick), confirming that the residual field of on Mn doping concentration. It shows that the ex- the magnet is nearly zero (<0.1 Oe). The right bottom in- change bias field decreases drastically even with only set shows the dependences of the exchange bias field and coercivity on Mn doping concentration. 5% Mn doping. The coercivity also shows a decrease with increasing Mn doping concentration. A similar In conclusion, we have systematically studied the phenomenon has been reported previously by Allibe exchange bias in polycrystalline BiFe1−푥Mn푥O3/NiFe bilayers. The XPS results have confirmed that the et al. on BiFeO3/CoFeB bilayers, i.e., the exchange bias field decreases from 51 Oe to 25 Oe and the coer- doped Mn ions are partially in a +4 valence state. The civity decreases from 42 Oe to 17 Oe with only 5% Mn exchange bias field decreases drastically with increas- doping.[18] ing Mn doping concentration and finally to zero when Generally, the exchange bias was attributed to the 푥 is above 0.20. These results clearly demonstrate exchange interaction between the pinned uncompen- that the interface exchange bias field comes from the sated spins in the antiferromagnet and the magnetic interfacial net spins due to the canted antiferromag- moments in the ferromagnet, whereas the increase in netic spin structure in polycrystalline BiFe1−푥Mn푥O3 the coercivity of the ferromagnet has been related films. The drastic decrease of exchange bias isdue to some coupling between unpinned uncompensated to the transformation to a collinear antiferromagnetic spins and ferromagnetic moments.[18] The decrease of spin structure with increasing Mn doping concentra- the exchange bias field and coercivity might be at- tion. tributed to the decrease of the uncompensated spins at the ferromagnet/antiferromagent interface. BiFeO3 References basically has a G-type antiferromagnetic spin arrange- ment with canted neighboring spins. Lebeugle et [1] Spaldin N A and Fiebig M 2005 Science 309 391 097701-3 CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 097701

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097701-4 Chinese Physics Letters Volume 29 Number 9 September 2012

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CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES 096101 Dislocation Behavior in AlGaN/GaN Multiple Quantum-Well Films Grown with Different Interlayers SUN He-Hui, GUO Feng-Yun, LI Deng-Yue, WANG Lu, ZHAO De-Gang, ZHAO Lian-Cheng 096102 The Energy State and Phase Transition of Cu Clusters in bcc-Fe Studied by a Molecular Dynamics Simulation GAO Ning, WEI Kong-Fang, ZHANG Shi-Xu, WANG Zhi-Guang 096103 Effect of Minor Co Substitution for Ni on the Glass Forming Ability and Magnetic Properties of Gd55Al20Ni25 Bulk Metallic Glass WANG Peng, CHAN Kang-Cheung, LU Shuang, TANG Mei-Bo, XIA Lei 096201 Plasmonic Nanostructured Electromagnetic Materials H. Sadeghi, H. Khalili, M. Goodarzi

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES 097101 Optical and Electrical Properties of Single-Crystal Si Supersaturated with Se by Ion Implantation MAO Xue, HAN Pei-De, HU Shao-Xu, GAO Li-Peng, LI Xin-Yi, MI Yan-Hong, LIANG Peng

097102 Optoelectronic Response of GeZn2O4 through the Modified Becke–Johnson Potential Iftikhar Ahmad, B. Amin, M. Maqbool, S. Muhammad, G. Murtaza, S. Ali, N. A. Noor 097201 Experimental Research on Carrier Redistribution in InAs/GaAs Quantum Dots LI Chuan-Feng, CHEN Geng, GONG Ming, LI Hai-Qiao, NIU Zhi-Chuan 097202 Suppression of the Drift Field in the p-Type Quasineutral Region of a Semiconductor p–n Junction CAI Xue-Yuan, YANG Jian-Hong, WEI Ying 097203 Theoretical Studies on Ultrasound Induced Hall Voltage and Its Application in Hall Effect Imaging CHEN Xuan-Ze, MA Qing-Yu, ZHANG Feng, SUN Xiao-Dong, CUI Hao-Chuan 097204 Spin Dynamics in (111) GaAs/AlGaAs Undoped Asymmetric Quantum Wells WANG Gang, YE Hui-Qi, SHI Zhen-Wu, WANG Wen-Xin, MARIE Xavier, BALOCCHI Andrea, AMAND Thierry, LIU Bao-Li 097301 A Drain Current Model Based on the Temperature Effect of a-Si:H Thin-Film Transistors QIANG Lei, YAO Ruo-He 097302 High Quantum Efficiency Back-Illuminated AlGaN-Based Solar-Blind Ultraviolet p–i–n Photodetectors WANG Guo-Sheng, LU Hai, XIE Feng, CHEN Dun-Jun, REN Fang-Fang, ZHANG Rong, ZHENG You-Dou 097303 Confined Mie Plasmons in Monolayer Hexagonal-Close-Packed Metallic Nanoshells CHEN Jing, DONG Wen, WANG Qiu-Gu, TANG Chao-Jun, CHEN Zhuo, WANG Zhen-Lin 097304 Improved Efficiency Droop in a GaN-Based Light-Emitting Diode with an AlInN Electron-Blocking Layer WEN Xiao-Xia, YANG Xiao-Dong, HE Miao, LI Yang, WANG Geng, LU Ping-Yuan, QIAN Wei-Ning, , ZHANG Wei-Wei, WU Wen-Bo, CHEN Fang-Sheng, DING Li-Zhen

097701 Exchange Bias in Polycrystalline BiFe1−xMnxO3/Ni81Fe19 Bilayers YUAN Xue-Yong, XUE Xiao-Bo, SI Li-Fang, DU Jun, XU Qing-Yu 097801 Electrical and Optical Characterization of n-GaN by High Energy Electron Irradiation LIANG Li-Min, XIE Xin-Jian, HAO Qiu-Yan, TIAN Yuan, LIU Cai-Chi

097802 F4TCNQ-Induced Exciton Quenching Studied by Using in-situ Photoluminescence Measurements ZHU Jian, LU Min, WU Bo, HOU Xiao-Yuan 097803 Preparation and Characterization of Bimodal Magnetofluorescent Nanoprobes for Biomedical Application LEI Jie-Mei, XU Xiao-Liang, LIU Ling, YIN Nai-Qiang, ZHU Li-Xin 097804 A GaN p–i–p–i–n Ultraviolet Avalanche Photodiode ZHENG Ji-Yuan, WANG Lai, HAO Zhi-Biao, LUO Yi, WANG Lan-Xi, CHEN Xue-Kang 097805 White Hybrid Light-Emitting Devices Based on the Emission of Thermal Annealed Ternary CdSe/ZnS Quantum Dots QU Da-Long, ZHANG Zhen-Song, YUE Shou-Zhen, WU Qing-Yang, YAN Ping-Rui, ZHAO Yi, LIU Shi-Yong

CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY 098101 A New Grating Fabrication Technique on Metal Films Using UV-Nanoimprint Lithography TANG Min-Jin, XIE Hui-Min, LI Yan-Jie, LI Xiao-Jun, WU Dan 098102 A Self-Aligned Process to Fabricate a Metal Electrode-Quantum Dot/Nanowire-Metal Electrode Structure with 100% Yield FU Ying-Chun, WANG Xiao-Feng, FAN Zhong-Chao, YANG Xiang, BAI Yun-Xia, ZHANG Jia-Yong, MA Hui-Li, JI An, YANG Fu-Hua 098401 A Repairable Linear m-Consecutive-k-Out-of-n:F System TANG Sheng-Dao, HOU Wei-Gen 098402 Effect of Aluminium Nanoparticles on the Performance of Bulk Heterojunction Organic Solar Cells YANG Shao-Peng, YAO Ming, JIANG Tao, LI Na, ZHANG Ye, LI Guang, LI Xiao-Wei, FU Guang-Sheng 098501 Performance Improved by Incorporating of Ru Atoms into Zr-Si Diffusion Barrier for Cu Metallization , SONG Zhong-Xiao, ZHANG Mi-Lin 098502 Enhanced Light Output of InGaN-Based Light Emitting Diodes with Roughed p-Type GaN Surface by Using Ni Nanoporous Template YU Zhi-Guo, CHEN Peng YANG Guo-Feng, LIU Bin, XIE Zi-Li, XIU Xiang-Qian, WU Zhen-Long, XU Feng, XU Zhou, HUA Xue-Mei, HAN Ping, SHI Yi ZHANG Rong, ZHENG You-Dou 098503 Performance Improvement of Ambipolar Organic Field Effect Transistors by Inserting a MoO3 Ultrathin Layer TIAN Hai-Jun, CHENG Xiao-Man, ZHAO Geng, LIANG Xiao-Yu, DU Bo-Qun, WU Feng 098801 Effects of the Molybdenum Oxide/Metal Anode Interfaces on Inverted Polymer Solar Cells WU Jiang, GUO Xiao-Yang, XIE Zhi-Yuan 098901 A New Definition of Modularity for Community Detection in Complex Networks YE Zhen-Qing, ZHANG Ke, HU Song-Nian, YU Jun 098902 Modeling and Simulation of Pedestrian Counter Flow on a Crosswalk LI Xiang, DONG Li-Yun 098903 A Multilane Traffic Flow Model with Lane Width and the Number of Lanes TANG Tie-Qiao, YANG Xiao-Bao, -Hong, CACCETTA Lou, HUANG Hai-Jun

GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS 099401 Field-Aligned Electrons in Polar Region Observed by Cluster on 30 September 2001 ZHANG Zi-Ying, SHI Jian-Kui, CHENG Zheng-Wei, Andrew Fazakerley