APPLIED PHYSICS LETTERS 98, 022105 共2011兲
Epitaxial VO2 / Cr2O3 / sapphire heterostructure for multifunctional applications Tsung-Han Yang,1,a兲 S. Mal,1 C. Jin,2 R. J. Narayan,2 and J. Narayan1 1
Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907, USA 2 Joint Department of Biomedical Engineering, University of North Carolina, Chapel Hill, North Carolina 27599, USA
共Received 7 September 2010; accepted 22 December 2010; published online 11 January 2011兲 In this letter, we report integration of magnetic and ultrafast-transition properties of VO2 films with antiferromagnetic 共AFM兲 Cr2O3 template layer in the epitaxial VO2 / Cr2O3 / Al2O3 heterostructure The Cr2O3 is an AFM material, which can pin the spin momentum of ferromagnetic VO2, in addition to providing epitaxial template. Thus, the magnetic properties of VO2 films grown with Cr2O3 buffer layer can be improved for multifunctional magnetic tunnel junctions and sensor applications. Electrical resistivity measurements as a function of temperature showed a sharp transition width 共1.94 ° C兲, with a small hysteresis width 共5.7 ° C兲, and large resistance change 共⬃3.8⫻ 104兲 across the semiconductor to metal transition 共SMT兲. We discuss the correlations of the magnetic properties and SMT characteristics with epitaxial growth and formation of twins. © 2011 American Institute of Physics. 关doi:10.1063/1.3541649兴 Multifunctionality of a material will play a critical role in the design of next-generation solid state devices.1,2 VO2 has attracted the most interest because it shows an ultrafast semiconductor-to-metal transition 共SMT兲 at ⬃68 ° C.3,4 The SMT accompanies a phase transformation from monoclinic 共P21 / c兲 to tetragonal 共P42 / mnm兲 with a large change in electrical resistance and optical reflectance.5,6 Thus, VO2 has been proved the potential for integration of electrical and optical switching properties on the same chip.6,7 In this letter, we report integration of ferromagnetic 共FM兲 VO2-films with antiferromagnetic 共AFM兲 chromium oxide 共Cr2O3兲 on c-sapphire. The Cr2O3 is AFM material,8,9 which can interact with the spin momentum of FM VO2. Due to AFM/FM interaction, the saturation magnetization and coercivity of VO2 in the FM hysteresis loop10,11 can be improved for multifunctional device applications. Therefore, this epitaxial VO2 / Cr2O3 / Al2O3 heterostructure has a great potential for applications in magnetic field as well as in electrical and optical switching devices. In addition, we also studied the role of twin boundaries on the SMT characteristics of VO2 films as the epitaxy on Cr2O3 results in the formation of twins in a controlled way.12 It should be noted that the sharpest SMT in VO2 was observed in bulk single crystals where the resistance drop approaches five orders of magnitude with a negligible hysteresis 共⌬H ⬍ 2 ° C兲.5 SMT characteristics of epitaxial VO2 films deteriorate due to the presence of strains and residual defects. For example, the epitaxial VO2 films show a sharp transition with a larger thermal width 共⌬H ⬍ 2 – 5 ° C兲 and a smaller amplitude change in resistance 共⌬A ⬃3–4 orders兲.13 In the polycrystalline VO2 films, the resistance drop exhibits up to approximately two orders of magnitude with a broad transition.14 With the proposed heterostructure we plan to improve ferromagnetism of VO2, while preserving the SMT characteristics of VO2. a兲
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The epitaxial VO2 and Cr2O3 films were grown on c-sapphire by pulsed laser deposition 共KrF excimer laser of 248 nm, energy density of 3 – 4 J / cm2, pulse width of 25 ns, and repetition rate of 10 Hz兲. The substrate temperature 共TS兲 used for growth of Cr2O3 films on c-sapphire was 650 ° C. The TS was reduced to 600 ° C for the VO2 grown on Cr2O3 layers. The oxygen pressure used for depositions was 10 and 5 mTorr during VO2 and Cr2O3 growth, respectively. A Rigaku diffractometer, using a Cu K␣ radiation 共 = 1.54 Å兲, was used for performing -2 scans. A Philips X-Pert Pro MRD x-ray diffractometer was used for detailed -scan and pole figure to determine in-plane and out of plane orientations of VO2 films. The root mean square roughness was obtained by a caliber atomic force microscope 共Veeco Inc.兲 using silicon nanoprobe cantilevers as scanning probes. The acoustic tapping mode was used. The SMT characteristics were investigated by electrical resistance performed by HP4155B semiconductor parameter analyzer in the temperature of 25– 90 ° C were performed by a HP4155B semiconductor parameter analyzer. Magnetic measurements at room temperature were carried out using an alternating gradient magnetometer 共Princeton Measurements Inc.兲. The transmission spectra were measured using a U-3010 UV-visible dualbeam spectrometer 共Hitachi兲 at 25 and 90 ° C in the wavelength range of 200–900 nm. Figure 1 shows x-ray diffraction patterns 共-2 scan兲 for the VO2 films grown on c-sapphire buffered by Cr2O3. It shows the 共006兲 and 共0012兲 peaks from the substrate at 2 value of 41.70° and 90.78°, respectively. The peaks at 39.27° and 84.27° correspond to 共006兲 and 共0012兲 of rhombohedral Cr2O3.15 The peak at 39.89° can be attributed to reflection from either the 共020兲 or 共002兲 plane of VO2 because the same d-spacing, resulting in the same Bragg angles. The peak at 85.77° is the second order reflection from these planes. It is not possible to study the epitaxial relationship of VO2 films on c-sapphire buffered by Cr2O3 layers from the -2 scan only.3 This problem required further analysis of structure of the films using phi-scan and pole figure to iden-
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FIG. 1. XRD patterns of VO2 films grown on Cr2O3 buffer layer with c-sapphire substrates.
tify the growth orientation of VO2 and Cr2O3 films. To confirm the orientations of VO2 on c-sapphire buffered by Cr2O3, -scan measurements 共2 = 26.864°兲 were performed for VO2 共110兲 planes, which are inclined at 43.1° from 共020兲 and 68.4° from 共002兲 planes. The choice of VO2 共110兲 plane for the -scan was based on its significant separation in values from other diffracting planes, which avoided any interference from other peaks. The absence of peaks in the -scan measurements 关Fig. 2共a兲兴 of 共002兲 indicates that the XRD 共-2 scan, Fig. 1兲 peak at 39.89° must correspond to VO2 共020兲 not VO2 共002兲. Figure 2共b兲 shows results from the -scans of 共104兲 lattice planes of Cr2O3 and Al2O3 and 共011兲 planes of VO2 thin films. The threefold symmetry observed in -scan of Cr2O3 共104兲 planes illustrates that the Cr2O3 is rhombohedral and grows epitaxially on c-sapphire. It should be noted that the Cr2O3 共104兲 reflection is parallel to the sapphire 共104兲 reflection, therefore, the epitaxial relationships 关shown in Fig. 3共a兲兴 between Cr2O3 and Al2O3 ¯¯10兴 储 Cr O 关21 ¯¯10兴 and are determined to be Al2O3关21 2 3 ¯ 0兴 储 Cr O 关101 ¯ 0兴. In addition, the pole figure Al2O3关101 2 3 关Fig. 3共a兲兴 of Cr2O3 共104兲 plane shows only three peaks 共 = 38°兲, confirming the single-crystalline nature of Cr2O3 films on c-sapphire. The -scan reflections 关Fig. 2共b兲兴 from the 共011兲 plane of VO2 thin films shows three sets of peaks of VO2 共011兲 and each set is rotated by 60° with respect to each other. These three in-plane orientations of VO2 grains on Cr2O3 buffer layer are confirmed by the pole
FIG. 3. 共Color online兲 共a兲 Schematic showing the orientations of rhombohedral Cr2O3 unit cell on rhombohedral Al2O3. The pole figure of Cr2O3 共104兲 planes was presented. 共b兲 Schematic showing the orientations of monoclinic VO2 unit cell on Cr2O3 unit cell. The pole figure of VO2 共011兲 planes was performed.
figure, shown in Fig. 3共b兲 of VO2 共011兲 plane. It should be noted that VO2 共011兲 reflections are rotated by 30° from Al2O3 共104兲 reflections, as shown by the -scan reflections of Al2O3 and VO2 关shown in Fig. 2共b兲兴. From these results, the epitaxial relationship between Cr2O3 and ¯ 0兴 储 VO 关100兴 and VO2 is established to be Cr2O3关101 2 ¯ 00兴 储 VO 关001兴 as shown in Fig. 3共b兲. The Cr O 关11 2
3
2
epitaxial relationships between Al2O3, Cr2O3, and VO2 are presented as Al2O3共0006兲 储 Cr2O3共0006兲 储 VO2共020兲 and ¯ 00兴 储 Cr O 关11 ¯ 00兴 储 VO 关001兴. Al O 关11 2
3
2
3
2
Figure 4共a兲 shows the electrical resistance as a function of temperature for VO2 films grown on Cr2O3 buffered c-sapphire substrate. The derivative and Gaussian fit curves 关Fig. 4共a兲 inset兴 were used to obtain the SMT characteristics.16 The transition temperatures for VO2 monoclinic ⇔VO2 tetragonal phase were determined to be 73.4 and 67.7 ° C during heating and cooling cycles, respectively. Therefore, the thermal hysteresis 共⌬H兲 associated with this reversible SMT was determined to be 5.7 ° C. The respective value for sharpness 共⌬T兲 of SMT was 1.94 ° C. The amplitude 共⌬A兲 of SMT, defined as the ratio of the electrical resistance at 35 and 90 ° C, was 3.8⫻ 104 for VO2 films grown on Cr2O3 buffered on c-sapphire. The SMT characteristics such as sharpness 共⌬T兲 and amplitude 共⌬A兲 are similar to those of VO2 films directly grown on c-sapphire.12
FIG. 2. 共Color online兲 共a兲 -scan measurements of 共110兲 planes corresponded to 共002兲 and 共020兲 growth orientations, respectively. 共b兲 The -scans of VO2 / Cr2O3 / Al2O3 samples for 共104兲 planes of Al2O3 共top, solid line兲 and C2O3 共top, square兲 and 共011兲 planes of VO2 共bottom兲, respectively.
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FIG. 4. 共Color online兲 共a兲 Electrical resistance of the VO2 thin films. Inset: the derivatives of logR共T兲 for the heating 共square兲 and cooling 共triangle兲 cycles are shown. Symbols represent data points and the lines represent Gaussian fitting. 共b兲 Hysteresis loop of VO2 films grown Cr2O3 buffer layer on c-sapphire. Inset: rms roughness of VO2 / Cr2O3 / C-sapphire heterostructure. The average rms is estimated by atomic force microscope to be ⬃15 nm. 共c兲 Optical transmittance for VO2 / Cr2O3 / C-sapphire heterostructure at 25 and 90 ° C in the wavelength range of 200–900 nm.
Magnetic measurements were performed on VO2 films as-grown Cr2O3 thin films buffered on c-sapphire. Figure 4共b兲 shows the room temperature magnetization data plotted as a function of the applied magnetic field. The saturated magnetic moment of VO2 films on Cr2O3 template layer buffered on c-sapphire was determined to be ⬃30 emu/ cm3 and the coercivity was ⬃86 Oe. The surface morphology 关shown in the Fig. 4共b兲 inset兴 was studied by atomic force microscope. The average root mean square 共rms兲 roughness was ⬃15 nm and VO2 grain size was ⬃100 nm. Figure 4共c兲 shows the transmittance spectrum measured at T = 25 and 90 ° C in the wavelength range of 200–900 nm. It is clearly shown that the transmittance at 800 nm decreased by a factor of 5 when temperature increased to 90 ° C. Since the granular structure in these VO2 films is separated by coherent twin boundaries, we envisage that tensile strain plays a more prominent role in enhancing the energy barrier for the monoclinic to tetragonal transformation. For isolated grains or grains isolated by large angle boundaries, the probability to trigger the structural phase transition becomes less because the number of defects within the grains is reduced.13,17,18 The tensile strain along monoclinic VO2 c-direction causes an increased distance between vanadium ion pair 共V+4-V+4兲.19 Based on epitaxial relationships, there is 5.9% tensile misfit along VO2 c-direction 关Fig. 3共b兲兴. Therefore, the increase of ⬃5.4 ° C in transition temperature can be attributed to the nanosize 共⬃100 nm兲 VO2 grain and the tensile strain along VO2 c-direction. In our previous study,20 the saturated magnetization 共Ms兲 and coercivity 共HC兲 of the epitaxial FM VO2 films on sapphire were observed to be ⬃18 emu/ cm3 and 40 Oe, respectively, at 25 ° C. In this study, the Ms and Hc were increased to ⬃30 emu/ cm3 and 86 Oe, respectively. The increase of Ms and HC is attributed to AFM/FM coupling. This coupling is observed at temperatures T ⬍ TN 共Neel temperature; AFM兲 and T ⬍ TC 共Curie temperature; FM兲. This coupling induces the increases in Ms and HC with an exchange bias 共EB兲, leading to a shift of hysteresis loop along the field axis.21 However, in our case a negligible EB was observed at room temperature because TN of Cr2O3 is around 307 K.22 Thus, VO2 films buffered with Cr2O3 on c-sapphire showed an increase of Ms and HC, leading to multifunctional devices for electrical, magnetic, and optical switching applications. In summary, VO2 films grow epitaxially on c-sap. with Cr2O3 buffer. There are 60° coherent twin boundaries existed in VO2 films. The SMT characteristics of VO2 were determined to be 5.7 ° C, 1.94 ° C, and 3.8⫻ 104 for width, sharp-
ness of thermal hysteresis, and resistance between two phases, respectively. The shift in transition temperature to 73.4 ° C was attributed to the residual tensile strain along monoclinic c-direction and nanosize VO2 grains. Improved saturation magnetic moment of ⬃30 emu/ cm3 and the coercivity of ⬃86 Oe can be explained by the AFM/FM coupling. The transmittance measurements showed a change by a factor of more than 5 as the temperature changed from 25 to 90 ° C. Therefore, this epitaxial VO2 / Cr2O3 / Al2O3 heterostructure can be used for multifunctional applications in magnetic, electrical, and optical devices. This research was sponsored by the National Science Foundation 共Grant No. DMR-0803663兲. S. Torquato, S. Hyun, and A. Donev, Phys. Rev. Lett. 89, 266601 共2002兲. W. Lu and C. M. Lieber, Nature Mater. 6, 841 共2007兲. 3 T. Yang, R. Aggarwal, A. Gupta, H. Zhou, R. J. Narayan, and J. Narayan, J. Appl. Phys. 107, 053514 共2010兲. 4 C. Kübler, H. Ehrke, R. Huber, R. Lopez, A. Halabica, R. F. Haglund, Jr., and A. Leitenstorfer, Phys. Rev. Lett. 99, 116401 共2007兲. 5 T. Yang, C. Jin, H. Zhou, R. J. Narayan, and J. Narayan, Appl. Phys. Lett. 97, 072101 共2010兲. 6 J. Nag and R. F. Haglund, Jr., J. Phys.: Condens. Matter 20, 264016 共2008兲. 7 R. T. Rajendra Kumar, B. Karunagaran, D. Mangalaraj, S. K. Narayandass, P. Manoravi, M. Joseph, and V. Gopal, Sens. Actuators A 107, 62 共2003兲. 8 H. Mändar, T. Uustare, J. Aarik, A. Tarre, and A. Rosental, Thin Solid Films 515, 4570 共2007兲. 9 N. A. Frey, S. Srinath, H. Srikanth, M. Varela, S. Pennycook, G. X. Miao, and A. Gupta, Phys. Rev. B 74, 024420 共2006兲. 10 Y. Takamura, F. Yang, N. Kemik, E. Arenholz, M. D. Biegalski, and H. M. Christen, Phys. Rev. B 80, 180417 共2009兲. 11 H. Ohldag, A. Scholl, F. Nolting, E. Arenholz, S. Maat, A. T. Young, M. Carey, and J. Stöhr, Phys. Rev. Lett. 91, 017203 共2003兲. 12 T. Yang, C. Jin, R. Aggarwal, R. J. Narayan, and J. Narayan, J. Mater. Res. 25, 422 共2010兲. 13 J. Narayan and V. M. Bhosle, J. Appl. Phys. 100, 103524 共2006兲. 14 D. Ruzmetov, G. Gopalakrishnan, C. Ko, V. Narayanamurti, and S. Ramanathan, J. Appl. Phys. 107, 114516 共2010兲. 15 H. McMurdi et al., Powder Diffr. 2, 44 共1987兲. 16 D. Brassard, S. Fourmaux, M. Jean-Jacques, J. C. Kieffer, and M. A. El Khakani, Appl. Phys. Lett. 87, 051910 共2005兲. 17 R. Lopez, L. C. Feldman, and R. F. Haglund, Jr., Phys. Rev. Lett. 93, 177403 共2004兲. 18 R. Lopez, R. F. Haglund, Jr., and L. C. Feldman, Appl. Phys. Lett. 85, 5191 共2004兲. 19 Y. Muraoka, Y. Ueda, and Z. Hiroi, J. Phys. Chem. Solids 63, 965 共2002兲. 20 T. Yang, S. Nori, H. Zhou, and J. Narayan, Appl. Phys. Lett. 95, 102506 共2009兲. 21 J. Nogués and I. K. Schuller, J. Magn. Magn. Mater. 192, 203 共1999兲. 22 J. Sort, V. Langlais, S. Doppiu, B. Dieny, S. Surinach, J. S. Munoz, M. D. Baro, C. Laurent, and J. Nogues, Nanotechnology 15, S211 共2004兲. 1 2
