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Magnetic and electrical transport properties of LaBaCo2O5.5+δ thin films directly integrated on Si (001)
Materials Letters 109 (2013) 143–145
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Magnetic and electrical transport properties of LaBaCo2O5.5+δ thin films directly integrated on Si (001) Ming Liu a,b,n, Chunrui Ma b, Gregory Collins b, Jian Liu b, Yamei Zhang b, Haibin Wang b, Chonglin Chen b a Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, PR China b Department of Physics and Astronomy, University of Texas at San Antonio, TX 78256, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 19 May 2013 Accepted 3 July 2013 Available online 11 July 2013
Perovskite cobaltates LaBaCo2O5.5+δ thin films were directly integrated onto (001) Si substrates by pulsed laser deposition. Microstructural studies from X-ray diffraction reveal that the LaBaCo2O5.5+δ films are polycrystalline. Electrical transport property measurements indicate that the polycrystalline LaBaCo2O5.5+δ films have a semiconductor behavior with a largest magnetoresistance value of 18% at 40 K. Magnetization property measurements show that the polycrystalline films exhibit magnetic behavior similar to its bulk materials of nanoscale ordered LaBaCo2O6 and have larger magnetic moments and magnetic coercive field than the single-crystal LaBaCo2O5.5+δ films. & 2013 Elsevier B.V. All rights reserved.
Keywords: LaBaCo2O5.5+δ Thin films Magnetoresistance Electronic transport
1. Introduction Complex transition-metal oxides exhibit a wide variety of electrical transport, magnetic and optical properties which have fascinated scientists and attracted a considerable amount of engineers′ attention [1–6] Among them, Perovskite cobaltates with chemical formula (Ln,A)CoO3 δ (Ln¼Lanthanide, A¼ Alkaline earth metal) have shown various exciting phenomena varying from their excellent mixed ionic/ electronic conducting properties to their fascinating strong correlation and the spin-states interactions at low temperature [7–10]. Especially, the order or disorder of the A-site cations can significantly induce the different oxidation states of cobalt (Co2+/Co3+/Co4+) and electronic spin states, which can generate various material characteristics and physical properties. The double perovskite cobaltates LaBaCo2O5.5+δ (LBCO) have a very small difference of the A-site cations′ radii between La3+ and Ba2+ resulting in the trend to form two different A-site cation atomic arrangements: the ordered or disordered phase. Various exciting electrical and magnetic properties have been achieved in previous studies due to these interactions [11–19]. Recently, highly epitaxial single crystalline thin films of LBCO were fabricated on SrTiO3, [14,19] LaAlO3, [15,16] and MgO, [17,18] and these films were found to possess an extraordinary sensitivity to reducing/oxidizing environments and an exceedingly fast redox reaction at high temperature. Furthermore, at low temperatures,
n Corresponding author at: Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, PR China. Tel.: +86 29 82668679; fax: +86 29 82668794. E-mail address: [email protected] (M. Liu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.07.013
these highly epitaxial LBCO thin films grown exhibit a large MR, which is much greater than would be expected from comparisons with the various phases (varying from disordering, nano-ordering, and ordering phases) of its bulk material [10–12]. In this letter, we report the achievement of the directed integration of LBCO thin films with (001) Si substrates and find that these polycrystalline films exhibit a different magnetic behavior comparing to single-crystal LBCO films, which is similar as the bulk materials of nanoscale ordered LaBaCo2O6. 2. Experimental procedure The LBCO thin films were integrated onto (001) Si substrates by using a KrF excimer pulsed laser deposition system which was homemade system with a special design of high temperature heater ( 950 1C). A laser energy density of 2.0 J/cm2 with a frequency of 5 Hz was adopted during film deposition. A high density, single phase, stoichiometric LaBaCo2O5+δ target was purchased from MTI Crystal Inc. The single-crystal (001) silicon (Si) substrates were selected as the substrate for the film growth. The optimal growth conditions have been determined to be for the deposition at 850 1C with an oxygen pressure of 250 mTorr. Immediately after the deposition, the LBCO films were post-annealed at 850 1C for 15 min in a pure oxygen pressure of 200 Torr, then slowly cooled down to room temperature with a rate of 5 1C/min. 3. Results and discussion X-ray diffraction (XRD) was performed to systematically examine the microstructure, and crystallinity of the LBCO films. Fig. 1 is
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M. Liu et al. / Materials Letters 109 (2013) 143–145
the XRD θ–2θ scanning pattern of the as-grown LBCO films on (001) Si showing all the peaks from the LBCO phases and (001) Si substrate. It is revealed that the films are polycrystalline, which
Fig. 1. A typical XRD pattern of the LBCO films on (001) Si substrates, which shows that the LBCO films are polycrystalline.
Fig. 2. Temperature dependence of the electrical resistance of the LBCO film in applied magnetic fields of 0 and 7 T. The inset is the applied field dependence of the MR value taken by isothermal magnetoresistance measurement.
also indicate the successfully integration of the LBCO thin films onto (001) Si substrate. The magnetic and electrical transport properties have been performed with a Quantum Design Physical Property Measurement System (PPMS-9). Fig. 2 is the temperature dependence of the electrical resistance for the LBCO films on Si substrates. It can be seen that the resistance of the films increases exponentially with the decrease of the temperature in the entire measurement range of 290–20 K, indicating that the films have a typical semiconductor-like behavior similar to the single-crystal LBCO films studied previously. To further understand the nature of the electrical transport properties, the magnetoresistance (MR) studies were measured at different temperatures under applied magnetic fields using isothermal MR measurements. As seen in the inset of Fig. 2, the MR evolutions were plotted at different temperatures under the applied magnetic field from 0 T to 7 T. The MR values were calculated using MR (%)¼{|R (H)R (0)|/R(0)} 100%, where R (H) and R (0) are the resistance under the application of magnetic field and without magnetic field, respectively. It is evident that the maximum MR value is found at 40 K with a corresponding value of 18% at 7 T, which is slightly smaller than the maximum MR value of LBCO on STO, [13] but still larger than the MR value obtained from various phases of its bulk material [11]. On the other hand, the film is a polycrystalline crystal and so the grain boundary maybe a factor to influence the carrier hopping and increase the carrier scattering of carrier as well as the resistance, although with an applied magnetic field. However, it still successfully opens a way to directly integrate LBCO thin film on Si substrate and improve the physical properties, compared to the bulk material. Magnetization measurements were used to further investigate the physical properties of the LBCO films on Si. Shown in Fig. 3(a) are the temperature dependence of both zero field cool (ZFC) and field cool (FC) magnetic moment measurements in an applied field of 200 Oe. The FC curve exhibits a different magnetic behavior when compared to the single-crystal LBCO films studied previously, but it is similar to the magnetic behavior of the nanoscale-ordered LaBaCo2O6 bulk materials reported by Kundu et al. [17]. It is interesting to note that the paramagnetic-to-ferromagnetic transition temperature (Tc) appear at about 197 K, which is higher than the Tc of all various phases of bulk materials. The magnetic moment is one order of magnitude greater than the single-crystal LBCO films on STO. The negative magnetization observed in the ZFC magnetic moment measurement (the temperature lower than 179 K) may result from the suppression of the spin fluctuations from the nanodomain boundaries, similar to the previous reports of the (La,Sr)CoO3 films [20,21].
Fig. 3. Magnetization measurements showing the magnetic properties of LBCO films on (001)Si. (a) Temperature dependence of the field cooled (FC) and zero-field-cooled (ZFC) magnetizations in an applied magnetic field of 200 Oe; (b) Applied field dependence of the hysteresis on the field scan width measured at the temperature of 137 K, 100 K and 30 K, respectively.
M. Liu et al. / Materials Letters 109 (2013) 143–145
Hysteresis loop measurements have also been used to investigate the magnetic phenomena, as seen in the Fig. 3(b). It can be seen that the coercive field (Hc) increases with the decreasing of the temperature and showed a value of 5.7 kOe at 30 K which is larger than the value of nanoscale ordered LBCO phase of bulk materials ( 4.2 kOe), corresponding a shift towards to the hard magnetic material property. These results may be from the fact that the nanoscale ordered phase is dominant in the LBCO films on Si. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
In summary, we have directly integrated the LBCO thin films onto (100) Si substrates by pulsed laser deposition. Microstructural investigations reveal that the LBCO films are polycrystalline. Electrical transport property measurements show that the polycrystalline LBCO films on Si have a semiconductor behavior with a largest MR value of 18% at 40 K. Magnetization property measurements reveal that the films exhibit magnetic behavior similar to the nanoscale ordered LaBaCo2O6 bulk materials and obtain larger magnetic moments and magnetic coercive field than the single-crystal LaBaCo2O5.5+δ films.
[10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgment This research was partially supported by the National Science Foundation under NSF-NIRT-0709293, and the Natural Science Foundation of China under 18110225. Also, Dr. Ming Liu and Ms. Chunrui Ma would like to acknowledge the support from the “China Scholarship Council” for their PhD researches at UTSA.
145
[18] [19] [20] [21]
Jacobson AJ. Chem Mater 2010;22(660–674):111. Taskin AA, Lavrov AN, Yoichi A. Appl Phys Lett 2005;86:091910/1–3. De Souza RA, Kilner JA. Solid State Ionics 1998;106:175–87. Luo GP GP, Wang YS, Chen SY, Liou Y, Heilman AK, Min NB, Chen CL, Chu CW. Appl Phys Lett 2000;76:1908–10. Li CY, Wang YQ, Cai RS, Chen YZ, Sun JR. Mater Lett 2013;95:70–3. Jalili H, Han JH, Kuru Y, Cai ZH, Yildiz B. J Phys Chem Lett 2011;2:801–7. Fauth F, Suard E, Caignaert V, Domenge’s B, Mirebeau I, Keller L. Eur Phys J B 2001;21:163–74. Zhang L, Liu GH, Li YB, Xu JR, Zhu XB, Sun YP. Mater Lett 2008;62: 1322–4. Yuan Z, Liu J, Chen CL, Wang CH, Luo XG, Chen XH, Kim GT, Huang DX, Wang SS, Jacobson AJ, Donner W. Appl Phys Lett 2007;90:212111/1–3. Fauth F, Suard E, Caignaert V. Phys Rev B 2002;65:060401/1–4. Rautama EL, Caignaert V, Boullay P, Kundu AK, Pralong V, Karppinen M, Ritter C, Raveau B. Chem Mater 2009;21:102–9. Rautama EL, Boullay P, Kundu AK, Caignaert V, Pralong V, Karppinen M, Raveau B. Chem Mater 2008;20:2742–50. Kundu AK, Rautama EL, Boullay P, Caignaert V, Pralong V, Raveau B. Phys Rev B 2007;76:184432/1–4. Liu M, Liu J, Collins G, Ma CR, Chen CL, He J, Jiang JC, Meletis EI, Jacobson AJ, Zhang QY. Appl Phys Lett 2010;96:132106/1–3. Liu J, Collins G, Liu M, Chen CL, Jiang JC, Meletis EI, Zhang QY, Dong C. Appl Phys Lett 2010;97:094101/1–3. Liu J, Liu M, Collins G, Chen CL, Jiang XJ, Gong WQ, Jacobson AJ, He J, Jiang JC, Meletis EI. Chem Mater 2010;22:799–802. Ma CR, Liu M, Collins G, Liu J, Zhang YM, Chen CL, He J, Jiang JC, Meletis EI. Appl Phys Lett 2012;101:021602/1–4. Liu M, Ma CR, Liu J, Collins G, Chen CL, He J, Jiang JC, Meletis EI, Sun L, Jacobson AJ, Whangbo MH. ACS Appl Mater Interfaces 2012;4:5524–8. Ma CR, Liu M, Collins G, Wang HB, Bao SY, Xu X, Enriquez E, Chen CL, Lin Y, Whangbo MH. ACS Appl Mater Interfaces 2013;5:451–5. Luo GP, Wang YS, Chen SY, Heilman AK, Chen CL, Chu CW, Liou Y, Ming NB. Appl Phys Lett 2000;76:1908–10. Kwon C, Gim Y, Fan Y, Hundley MF, Roper JM, Arendt PN, Jia QX. Appl Phys Lett 1998;73:695/1–3.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Magnetic and electrical transport properties of LaBaCo2O5.5+δ thin films directly integrated on Si (001) Ming Liu a,b,n, Chunrui Ma b, Gregory Collins b, Jian Liu b, Yamei Zhang b, Haibin Wang b, Chonglin Chen b a Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, PR China b Department of Physics and Astronomy, University of Texas at San Antonio, TX 78256, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 19 May 2013 Accepted 3 July 2013 Available online 11 July 2013
Perovskite cobaltates LaBaCo2O5.5+δ thin films were directly integrated onto (001) Si substrates by pulsed laser deposition. Microstructural studies from X-ray diffraction reveal that the LaBaCo2O5.5+δ films are polycrystalline. Electrical transport property measurements indicate that the polycrystalline LaBaCo2O5.5+δ films have a semiconductor behavior with a largest magnetoresistance value of 18% at 40 K. Magnetization property measurements show that the polycrystalline films exhibit magnetic behavior similar to its bulk materials of nanoscale ordered LaBaCo2O6 and have larger magnetic moments and magnetic coercive field than the single-crystal LaBaCo2O5.5+δ films. & 2013 Elsevier B.V. All rights reserved.
Keywords: LaBaCo2O5.5+δ Thin films Magnetoresistance Electronic transport
1. Introduction Complex transition-metal oxides exhibit a wide variety of electrical transport, magnetic and optical properties which have fascinated scientists and attracted a considerable amount of engineers′ attention [1–6] Among them, Perovskite cobaltates with chemical formula (Ln,A)CoO3 δ (Ln¼Lanthanide, A¼ Alkaline earth metal) have shown various exciting phenomena varying from their excellent mixed ionic/ electronic conducting properties to their fascinating strong correlation and the spin-states interactions at low temperature [7–10]. Especially, the order or disorder of the A-site cations can significantly induce the different oxidation states of cobalt (Co2+/Co3+/Co4+) and electronic spin states, which can generate various material characteristics and physical properties. The double perovskite cobaltates LaBaCo2O5.5+δ (LBCO) have a very small difference of the A-site cations′ radii between La3+ and Ba2+ resulting in the trend to form two different A-site cation atomic arrangements: the ordered or disordered phase. Various exciting electrical and magnetic properties have been achieved in previous studies due to these interactions [11–19]. Recently, highly epitaxial single crystalline thin films of LBCO were fabricated on SrTiO3, [14,19] LaAlO3, [15,16] and MgO, [17,18] and these films were found to possess an extraordinary sensitivity to reducing/oxidizing environments and an exceedingly fast redox reaction at high temperature. Furthermore, at low temperatures,
n Corresponding author at: Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, PR China. Tel.: +86 29 82668679; fax: +86 29 82668794. E-mail address: [email protected] (M. Liu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.07.013
these highly epitaxial LBCO thin films grown exhibit a large MR, which is much greater than would be expected from comparisons with the various phases (varying from disordering, nano-ordering, and ordering phases) of its bulk material [10–12]. In this letter, we report the achievement of the directed integration of LBCO thin films with (001) Si substrates and find that these polycrystalline films exhibit a different magnetic behavior comparing to single-crystal LBCO films, which is similar as the bulk materials of nanoscale ordered LaBaCo2O6. 2. Experimental procedure The LBCO thin films were integrated onto (001) Si substrates by using a KrF excimer pulsed laser deposition system which was homemade system with a special design of high temperature heater ( 950 1C). A laser energy density of 2.0 J/cm2 with a frequency of 5 Hz was adopted during film deposition. A high density, single phase, stoichiometric LaBaCo2O5+δ target was purchased from MTI Crystal Inc. The single-crystal (001) silicon (Si) substrates were selected as the substrate for the film growth. The optimal growth conditions have been determined to be for the deposition at 850 1C with an oxygen pressure of 250 mTorr. Immediately after the deposition, the LBCO films were post-annealed at 850 1C for 15 min in a pure oxygen pressure of 200 Torr, then slowly cooled down to room temperature with a rate of 5 1C/min. 3. Results and discussion X-ray diffraction (XRD) was performed to systematically examine the microstructure, and crystallinity of the LBCO films. Fig. 1 is
144
M. Liu et al. / Materials Letters 109 (2013) 143–145
the XRD θ–2θ scanning pattern of the as-grown LBCO films on (001) Si showing all the peaks from the LBCO phases and (001) Si substrate. It is revealed that the films are polycrystalline, which
Fig. 1. A typical XRD pattern of the LBCO films on (001) Si substrates, which shows that the LBCO films are polycrystalline.
Fig. 2. Temperature dependence of the electrical resistance of the LBCO film in applied magnetic fields of 0 and 7 T. The inset is the applied field dependence of the MR value taken by isothermal magnetoresistance measurement.
also indicate the successfully integration of the LBCO thin films onto (001) Si substrate. The magnetic and electrical transport properties have been performed with a Quantum Design Physical Property Measurement System (PPMS-9). Fig. 2 is the temperature dependence of the electrical resistance for the LBCO films on Si substrates. It can be seen that the resistance of the films increases exponentially with the decrease of the temperature in the entire measurement range of 290–20 K, indicating that the films have a typical semiconductor-like behavior similar to the single-crystal LBCO films studied previously. To further understand the nature of the electrical transport properties, the magnetoresistance (MR) studies were measured at different temperatures under applied magnetic fields using isothermal MR measurements. As seen in the inset of Fig. 2, the MR evolutions were plotted at different temperatures under the applied magnetic field from 0 T to 7 T. The MR values were calculated using MR (%)¼{|R (H)R (0)|/R(0)} 100%, where R (H) and R (0) are the resistance under the application of magnetic field and without magnetic field, respectively. It is evident that the maximum MR value is found at 40 K with a corresponding value of 18% at 7 T, which is slightly smaller than the maximum MR value of LBCO on STO, [13] but still larger than the MR value obtained from various phases of its bulk material [11]. On the other hand, the film is a polycrystalline crystal and so the grain boundary maybe a factor to influence the carrier hopping and increase the carrier scattering of carrier as well as the resistance, although with an applied magnetic field. However, it still successfully opens a way to directly integrate LBCO thin film on Si substrate and improve the physical properties, compared to the bulk material. Magnetization measurements were used to further investigate the physical properties of the LBCO films on Si. Shown in Fig. 3(a) are the temperature dependence of both zero field cool (ZFC) and field cool (FC) magnetic moment measurements in an applied field of 200 Oe. The FC curve exhibits a different magnetic behavior when compared to the single-crystal LBCO films studied previously, but it is similar to the magnetic behavior of the nanoscale-ordered LaBaCo2O6 bulk materials reported by Kundu et al. [17]. It is interesting to note that the paramagnetic-to-ferromagnetic transition temperature (Tc) appear at about 197 K, which is higher than the Tc of all various phases of bulk materials. The magnetic moment is one order of magnitude greater than the single-crystal LBCO films on STO. The negative magnetization observed in the ZFC magnetic moment measurement (the temperature lower than 179 K) may result from the suppression of the spin fluctuations from the nanodomain boundaries, similar to the previous reports of the (La,Sr)CoO3 films [20,21].
Fig. 3. Magnetization measurements showing the magnetic properties of LBCO films on (001)Si. (a) Temperature dependence of the field cooled (FC) and zero-field-cooled (ZFC) magnetizations in an applied magnetic field of 200 Oe; (b) Applied field dependence of the hysteresis on the field scan width measured at the temperature of 137 K, 100 K and 30 K, respectively.
M. Liu et al. / Materials Letters 109 (2013) 143–145
Hysteresis loop measurements have also been used to investigate the magnetic phenomena, as seen in the Fig. 3(b). It can be seen that the coercive field (Hc) increases with the decreasing of the temperature and showed a value of 5.7 kOe at 30 K which is larger than the value of nanoscale ordered LBCO phase of bulk materials ( 4.2 kOe), corresponding a shift towards to the hard magnetic material property. These results may be from the fact that the nanoscale ordered phase is dominant in the LBCO films on Si. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
In summary, we have directly integrated the LBCO thin films onto (100) Si substrates by pulsed laser deposition. Microstructural investigations reveal that the LBCO films are polycrystalline. Electrical transport property measurements show that the polycrystalline LBCO films on Si have a semiconductor behavior with a largest MR value of 18% at 40 K. Magnetization property measurements reveal that the films exhibit magnetic behavior similar to the nanoscale ordered LaBaCo2O6 bulk materials and obtain larger magnetic moments and magnetic coercive field than the single-crystal LaBaCo2O5.5+δ films.
[10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgment This research was partially supported by the National Science Foundation under NSF-NIRT-0709293, and the Natural Science Foundation of China under 18110225. Also, Dr. Ming Liu and Ms. Chunrui Ma would like to acknowledge the support from the “China Scholarship Council” for their PhD researches at UTSA.
145
[18] [19] [20] [21]
Jacobson AJ. Chem Mater 2010;22(660–674):111. Taskin AA, Lavrov AN, Yoichi A. Appl Phys Lett 2005;86:091910/1–3. De Souza RA, Kilner JA. Solid State Ionics 1998;106:175–87. Luo GP GP, Wang YS, Chen SY, Liou Y, Heilman AK, Min NB, Chen CL, Chu CW. Appl Phys Lett 2000;76:1908–10. Li CY, Wang YQ, Cai RS, Chen YZ, Sun JR. Mater Lett 2013;95:70–3. Jalili H, Han JH, Kuru Y, Cai ZH, Yildiz B. J Phys Chem Lett 2011;2:801–7. Fauth F, Suard E, Caignaert V, Domenge’s B, Mirebeau I, Keller L. Eur Phys J B 2001;21:163–74. Zhang L, Liu GH, Li YB, Xu JR, Zhu XB, Sun YP. Mater Lett 2008;62: 1322–4. Yuan Z, Liu J, Chen CL, Wang CH, Luo XG, Chen XH, Kim GT, Huang DX, Wang SS, Jacobson AJ, Donner W. Appl Phys Lett 2007;90:212111/1–3. Fauth F, Suard E, Caignaert V. Phys Rev B 2002;65:060401/1–4. Rautama EL, Caignaert V, Boullay P, Kundu AK, Pralong V, Karppinen M, Ritter C, Raveau B. Chem Mater 2009;21:102–9. Rautama EL, Boullay P, Kundu AK, Caignaert V, Pralong V, Karppinen M, Raveau B. Chem Mater 2008;20:2742–50. Kundu AK, Rautama EL, Boullay P, Caignaert V, Pralong V, Raveau B. Phys Rev B 2007;76:184432/1–4. Liu M, Liu J, Collins G, Ma CR, Chen CL, He J, Jiang JC, Meletis EI, Jacobson AJ, Zhang QY. Appl Phys Lett 2010;96:132106/1–3. Liu J, Collins G, Liu M, Chen CL, Jiang JC, Meletis EI, Zhang QY, Dong C. Appl Phys Lett 2010;97:094101/1–3. Liu J, Liu M, Collins G, Chen CL, Jiang XJ, Gong WQ, Jacobson AJ, He J, Jiang JC, Meletis EI. Chem Mater 2010;22:799–802. Ma CR, Liu M, Collins G, Liu J, Zhang YM, Chen CL, He J, Jiang JC, Meletis EI. Appl Phys Lett 2012;101:021602/1–4. Liu M, Ma CR, Liu J, Collins G, Chen CL, He J, Jiang JC, Meletis EI, Sun L, Jacobson AJ, Whangbo MH. ACS Appl Mater Interfaces 2012;4:5524–8. Ma CR, Liu M, Collins G, Wang HB, Bao SY, Xu X, Enriquez E, Chen CL, Lin Y, Whangbo MH. ACS Appl Mater Interfaces 2013;5:451–5. Luo GP, Wang YS, Chen SY, Heilman AK, Chen CL, Chu CW, Liou Y, Ming NB. Appl Phys Lett 2000;76:1908–10. Kwon C, Gim Y, Fan Y, Hundley MF, Roper JM, Arendt PN, Jia QX. Appl Phys Lett 1998;73:695/1–3.