Exchange bias in sputtered FM/BiFeO3 thin films

Exchange bias in sputtered FM/BiFeO3 thin films (FM=Fe and Co) H. W. Chang, F. T. Yuan, C. W. Shih, W. L. Li, P. H. Chen et al. Citation: J. Appl. Phys. 111, 07B105 (2012); doi: 10.1063/1.3677801 View online: http://dx.doi.org/10.1063/1.3677801 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i7 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 111, 07B105 (2012)

Exchange bias in sputtered FM/BiFeO3 thin films (FM 5 Fe and Co) H. W. Chang,1,a) F. T. Yuan,2 C. W. Shih,3 W. L. Li,1 P. H. Chen,1 C. R. Wang,1 W. C. Chang,3 and S. U. Jen4 1

Department of Physics, Tunghai University, Taichung 407, Taiwan Department of Physics, National Taiwan University, Taipei 106, Taiwan 3 Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan 4 Institute of Physics, Academia Sinica, Taipei 115, Taiwan 2

(Presented 31 October 2011; received 21 September 2011; accepted 21 November 2011; published online 8 March 2012) Magnetic properties of sputter-deposited ferromagnetic (FM)/BiFeO3 (BFO) films on Pt/Ti/SiO2/ Si(100) substrate (FM ¼ Co and Fe) have been investigated. Isotropic perovskite BFO single phase is obtained for 200-nm-thick BFO films deposited at 300–450 C and BFO films at 400 C with thickness of 50–400 nm. Large exchange bias field (HEB) of 308–400 Oe and coercivity (Hc) of 1201–3632 Oe at RT are obtained for polycrystalline Co/BFO bilayers. The roughened surface induced by high deposition temperature and increasing thickness of BFO layer enhances localized shape anisotropy of FM layer, resulting in the increase of Hc the improved crystallinity and roughened surface of BFO/Co interface might be responsible for the HEB enhancement. Additionally, comparison on the HEB in polycrystalline Co/BFO and Fe/BFO systems is also C 2012 American Institute of Physics. [doi:10.1063/1.3677801] discussed. V Perovskite BiFeO3 (BFO), showing outstanding multiferroic (MF) properties of ferroelectricity (FE) (TC 810 C) and antiferromagnetism (AFM) (TN 370 C), has recently received considerable attention.1–15 High-transition temperatures permit the feasibility of uses for the advanced spintronic devices based on the magnetoelectric coupling.4–7 Although the net magnetic moment in BFO film is too small (0.01 lB/Fe)3,8 to be used as a ferromagnet (FM) in practical devices, the switching of electric polarization in BFO alters the AFM order, providing a mechanism to control the magnetic polarization of a ferromagnete (FM) through exchange bias (EB).3–7 Theoretically, the origin of the exchange bias phenomenon in FM/G-type AFM perovskite oxides interface has been explained by using both the Dzyaloshinskii-Moriya interaction and the standard superexchange, proposed by Dong et al.,9 and the exchange bias field (HEB) depends on domainwall density.10 Experimentally, it was reported that HEB is inversely proportional to the FE and AFM domain size in epitaxial CoFeB/BFO films,11 and dominated by the configuration and size of ferroelectric domain in Co0.9Fe0.1/BFO(001) films.12 Besides, Dho and Blamire13 reported that HEB mainly depends on the roughness at the interface in NiFe/BFO (001) films because BFO (001) has a compensated surface. However, EB in polycrystalline FM/BFO systems is rarely reported. In addition, FM/BFO bilayers have been prepared by the pulsed laser deposition (PLD)11–14 and chemical solution deposition (CSD),15 but no report is demonstrated by using sputtering method till now. In this work, we adopt the rf sputtering method to fabricate the BFO films on Pt/Ti/ SiO2/Si(100) substrate, and systematically study effect of BFO thickness (t) and different deposition temperature (Td) on the phase structure and surface morphology of BFO films a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Fax: 886-4-23594643.

0021-8979/2012/111(7)/07B105/3/$30.00

on Pt/Ti/SiO2/Si(100) substrate as well as magnetic properties of Co layer grown on the various BFO/Pt/Ti/SiO2/ Si(100) substrates. HEB of different FM layer on the FM/ BFO bilayer films (FM ¼ Co and Fe) is also reported. BFO films with 50–400 nm in thickness (t) were deposited on the Pt/Ti/SiO2/Si(100) substrates at various substrate temperatures (Td) in the range 350–500 C by radio frequency sputtering method from commercial Bi1.1FeO3 target. The base pressure was better than 5 107 Torr, and the deposition pressure and a rf power were maintained at 10 mTorr with Ar and O2 at the ratio of 4:1 and 60 W, respectively. The composition of the film was identified by an energy dispersive x-ray analysis (EDX) and double checked by x-ray fluorescence (XRF) analysis. Crystallographic structure was identified by conventional x-ray diffractometry (XRD) with Cu Ka radiation. Surface morphology was observed by scanning electron microscopy (SEM). Furthermore, FM Fe or Co layer with 5 nm in thickness was then sputtered onto BFO at RT and Ar pressure of 5 mTorr, followed by a 10-nm-thick Ta capping layer to prevent oxidation of the FM layer. A fieldcooling process at high vacuum of 5 107 Torr from 370 C to room temperature (RT) within the external magnetic field of 2 kOe was applied to align the spin of BFO prior to magnetic measurement. The magnetic properties at RT were measured by a vibrating sample magnetometer (VSM). Figure 1 shows XRD patterns of 200-nm BFO films deposited on Pt/Ti/SiO2/Si(100) substrates at various deposition temperature. BFO films deposited at 300–450 C exhibit pure perovskite polycrystalline structure with random orientation, and the diffraction peaks are consistent with the pseudocubic lattice.2 With the increment of deposition temperature from 300 C to 450 C, the preferred orientation is transformed from (110) plane for Td ¼ 350–400 C to (001) plane for Td ¼ 450 C. Lower deposition temperature for pure BFO phase in this study as compared to PLD11–14 and CSD15 methods might be a result of the enhanced mobility of surface

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FIG. 1. (Color online) XRD patterns of 200-nm BFO films grown on Pt/Ti/ SiO2/Si(100) substrates at various deposition temperatures.

FIG. 3. (Color online) XRD patterns of BFO films grown on Pt/Ti/SiO2/ Si(100) substrates at Td ¼ 400 C with different thicknesses.

atoms for sputtering process, and it is in agreement with the result reported by Lee et al.16 Furthermore, the part of BFO phase is decomposed into Bi2O3 and Fe2O3 phases at higher Td ¼ 500 C, which might be resulted from the vaporization of Bi at higher temperature. To further examine the quality of crystallization, we compare the size of coherent scattering domain (dcsd) in which the unit cells are perfectly aligned without defects and strain. In the present case, the value of dcsd can be determined by Scherrer’s formula because the main contributions to the diffraction linewidth are from grain and substructure boundaries. Larger dcsd corresponds to better crystallinity and vice versa. The values of dcsd measured from the full width at half-maximum (FWHM) of the (001) and (110) diffraction peaks are 20.2 nm and 20.6 nm for Td ¼ 300 C and 41.2 nm and 31.8 nm for Td ¼ 450 C, respectively. The increase of dcsd with Td indicates the enhancement of crystallinity. Figure 2 shows SEM images of 200 nm BFO films grown on Pt/Ti/SiO2/Si(100) substrates at various substrate temperatures. It is observed that BFO films displays the dense morphology, except for the films with Td ¼ 500 C. With the increase of Td, the mean grain size increases from 117 6 24 nm for Td ¼ 300 C to 222 6 35 nm for Td ¼ 450 C. BFO deposited at 500 C exhibits bimodal size distributions: the larger grains with average size of 281 6 40 nm and smaller grain of 121 6 22 nm, which are related to the coexistence of perovskite BFO phase with Bi2O3 and Fe2O3

phases in the films. The particles observed in SEM image are grains connected with high-angle grain boundaries, but dcsd measured from XRD analysis is the size of substructures within a grain. Therefore, the grain size obtained in SEM image is much larger than dcsd. The grain growth caused by high-temperature deposition also increases surface roughness. Figure 3 presents XRD patterns of BFO films grown on Pt/Ti/SiO2/Si(100) substrates at Td ¼ 400 C with different thickness. Clearly, they are identified to be well-crystallized BFO with random orientation for all studied samples, and no secondary phase is found. When BFO thickness is increased, dcsd of BFO (001) and BFO (110) grains are gradually enlarged from 8.4 nm and 9.4 nm for t ¼ 50 nm to 39.2 nm and 42.1 nm for t ¼ 400 nm, respectively, revealing an improved crystallinity (or reduced defect density) of BFO films. Figure 4 shows SEM images of BFO films grown on Pt/ Ti/SiO2/Si(100) substrates at Td ¼ 400 C with different thickness. It is observed that all BFO films display the dense morphology. With the increase of BFO thickness, the average grain size increases from 92 6 19 nm for t ¼ 50 nm to 248 6 26 nm for t ¼ 400 nm, and the surface morphology is roughened. Similarly, these sizes are much larger than dcsd measured from FWHM in XRD analysis. Figure 5 depicts magnetic hysteresis loops of 5 nm Co layer on 200 nm BFO films at different Td ¼ 350–450 C and

FIG. 2. SEM images of 200-nm BFO films grown on Pt/Ti/SiO2/Si(100) substrates at various deposition temperatures.

FIG. 4. SEM images of BFO films grown on Pt/Ti/SiO2/Si(100) substrates at Td ¼ 400 C with different thicknesses.


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FIG. 5. (Color online) Magnetic hysteresis loops of Co/BFO films with (a) t ¼ 200 nm and Td ¼ 350 C, (b) t ¼ 200 nm and Td ¼ 400 C, (c) t ¼ 200 nm and Td ¼ 450 C, and (d) t ¼ 400 nm and Td ¼ 400 C.

400 nm BFO films at Td ¼ 400 C. Strong FM behavior with asymmetric hysteresis loop, which exhibits a shift in the center position toward negative field, is found for these samples through field cooling from 370 C to RT at the external field of 2 kOe. Large HEB values of 308–400 Oe are obtained, and are significantly larger than the epitaxial FM/BFO bilayers (15–150 Oe).11–13 The dependence of Td on the exchange bias field (HEB) and coercivity (Hc) is summarized in Fig. 6(a). HEB increases from 277 Oe to 400 Oe with increasing Td from 300 to 450 C where the quality of the perovskite structure is optimized as mentioned previously. When secondary phases appears at Td ¼ 500 C, HEB decreases drastically to about 110 Oe. The direct relation between crystallinity and HEB suggests that the behavior of exchange bias is determined by the magnetoctystalline anisotropy of AFM order of BFO. Roughened surface may also enhance HEB, especially for BFO with (001) texture.13 Moreover, the increased surface roughness could be responsible for the enhancement of Hc from 1201 to 2650 Oe. The localized shape anisotropy of FM layer provides excess magnetic pinning sites in addition to the AFM–FM coupling, increasing the impedance of domain wall motion.17 Figure 6(b) shows HEB and Hc of Co/BFO films as function of BFO thickness. With the increase of BFO thickness, HEB increases from 0 Oe from t ¼ 50 nm to 377 Oe for t ¼ 200 nm, and then slightly decreases to 360 Oe for t ¼ 400 nm. The former might be ascribed to the enhanced crystallinity of BFO films and the roughened surface. The latter might be resulted from increased grain size.18,19 On the other hand, similar to the dependence of Td on the Hc, the increase of Hc with increasing BFO thickness from 50 Oe for t ¼ 50 nm to 3632 Oe for t ¼ 400 nm results from the roughened surface with BFO thickness. Different exchange bias behavior is obtained in Fe/BFO films. As shown in Fig. 6(b), HEB increases from 0 Oe from t ¼ 50 nm to 15 Oe for t ¼ 400 nm with the increase of BFO thickness, and it is one order lower than the Co/BFO system. The macroscopic model for the general exchange bias19 pre-

FIG. 6. (Color online) (a) HEB and Hc of Co/BFO films as function of deposition temperature and (b) HEB and Hc of FM/BFO films as function of BFO thickness (FM ¼ Co and Fe).

dicted that HEB depends on whether the domain wall is formed on the AFM or FM side of the interface, i.e., HEB ! (KAFMAAFM)1/2/MFMtFM or HEB ! (KFMAFM)1/2/MFMtFM, where KAFM, KFM, AAFM, and AFM are the magnetic anisotropy constant and the exchange stiffness of AFM layer and FM layer, respectively, and MFM and tFM the saturation magnetization and thickness of FM layer, respectively. Accordingly, much larger HEB in Co/BFO films than Fe/BFO films might be attributed to much higher magnetocrystalline anisotropy and exchange stiffness and lower saturation magnetization for Co than Fe. This paper is supported by National Science Council, Taiwan under Grant Nos. NSC-98-2112 -M-029-001-MY3 and NSC-100-2112 -M-029-002-MY3. 1

J. R. Teague et al., Solid-State Commun. 8, 1073 (1970). J. Wang et al., Science 299, 1719 (2003). 3 G. Catalan and J. F. Scott, Adv. Mater. 21, 2463 (2009). 4 H. Bea et al., J. Phys.: Condens. Matter 20, 434221 (2008). 5 M. Bibes and A. Barthelemy, Nature Mater. 7, 425 (2008). 6 Y. H. Chu et al., Nature Mater. 7, 478 (2008). 7 R. Thomas et al., J. Phys.: Condens. Matter 22, 423201 (2010). 8 H. Bea et al., Appl. Phys. Lett. 87, 072508 (2005). 9 S. Dong et al., Phys. Rev. Lett. 103, 127201 (2009). 10 K. L. Livesey, Phys. Rev. B 82, 064408 (2010). 11 H. Bea et al., Phys. Rev. Lett. 100, 017204 (2008). 12 L. W. Martin et al., Nano Lett. 8, 2050 (2008). 13 J. Dho and M. G. Blamire, J. Appl. Phys. 106, 073914 (2009). 14 J. Dho et al., Adv. Mater. 18, 1445 (2006). 15 H. Naganuma et al., J. Appl. Phys. 109, 07D736 (2011). 16 Y. H. Lee et al., Appl. Phys. Lett. 87, 172901 (2005). 17 M. T. Johnson et al., Rep. Prog. Phys. 59, 1409 (1996). 18 A. P. Malozemoff, J. Appl. Phys. 63, 3874 (1988). 19 J. Nogues et al., Phys. Rep. 422, 65 (2005). 2
