Control of room-temperature defect-mediated ferromagnetism in VO2 films

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Acta Materialia 59 (2011) 6362–6368 www.elsevier.com/locate/actamat

Control of room-temperature defect-mediated ferromagnetism in VO2 films Tsung-Han Yang ⇑, Sudhakar Nori, Siddhartha Mal, Jagdish Narayan NSF Center for Advanced Materials and Smart Structures, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA Received 9 February 2011; accepted 30 June 2011 Available online 2 August 2011

Abstract We report interesting ferromagnetic properties and their control in a vanadium-based oxide system driven by stoichiometric defects. Vanadium oxide (VO2) thin films were grown on c-plane sapphire substrates by a pulsed laser deposition technique under different ambient conditions. The ferromagnetism of the epitaxial VO2 films can be switched on and off by altering the cooling ambient parameters. In addition, the saturated magnetic moments and coercivity of the VO2 films were found to be a function of the oxygen partial pressure during the growth process. The room-temperature ferromagnetic properties of VO2 films were correlated with the nature of the microstructure and the growth parameters. The origin of the induced magnetic properties are qualitatively understood to stem from intrinsic structural and stoichiometric defects. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ferromagnetic; Vanadium dioxide; Semiconductor to Metal Transition (SMT)

1. Introduction Vanadium dioxide (VO2) has been widely investigated because of several interesting features associated with its near-room-temperature semiconductor to metal transition (SMT) exhibiting abrupt changes in its optical and electrical properties [1,2]. The unique SMT characteristics accompany the concomitant phase transformation from a monoclinic phase (semiconducting phase) to a tetragonal phase (metallic phase) at a critical temperature of 68 °C for bulk single crystal VO2 [3,4]. Earlier studies on VO2 films were focused essentially on electrical and optical properties that probe the SMT characteristics, which, in turn, are strongly dependent on the nature of the VO2 microstructure, strain and the point defects [5,6]. Although the rich display of electronic properties portrays VO2 as a potentially viable candidate for smart windows [7], ultrafast sensors [8] and other practical applications [9,10], the ⇑ Corresponding author.

E-mail address: [email protected] (T.-H. Yang).

magnetic properties are equally important for integrated multifunctional devices. However, there are few studies on magnetic properties in this otherwise important class of electronic materials. On the other hand, other classes of oxide-based dilute magnetic semiconductors, such as ZnO, MgO, In2O3 and SnO2, have magnetic moments because of unpaired 3d electron spinning, which can interact with the host atoms or the magnetic impurities, impurity–defect complexes or intrinsic defect complexes [11]. Due to this interaction, pure metal oxides with non-stoichiometric defects have been reported to show room-temperature ferromagnetic (RTFM) properties [12]. Recently, we reported stoichiometry-related defects in VO2 owing to the narrow stability range of different phases, such as V2O3, V3O5, V4O7, V5O9, VO2 and V2O5, in which the oxygen to vanadium atom ratio varies from 1.5 to 2.5 [13]. The saturated magnetization and coercivity were estimated to be 18 emu cm3 and 40 Oe at room temperature, respectively [14]. It is envisaged that the observed ferromagnetism has its origin in the valence charge defects with unpaired electrons (V3+) in VO2 thin film. Further, the

1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.06.047

T.-H. Yang et al. / Acta Materialia 59 (2011) 6362–6368 Table 1 Different growth and cooling ambient pressures for VO2 films grown on c-sapphire. Growth pressure

Cooling ambient

A B C

pO2 = 103 torr pO2 = 102 torr pO2 = 102 torr

Vacuum pO2 = 102 torr Vacuum

ferromagnetism of VO2 can be improved by a Cr2O3 buffer layer while the SMT characteristics are preserved [15]. The enhanced saturation magnetic moment of 30 emu cm3 and the coercivity of 86 Oe was explained by the antiferromagnetic/ferromagnetic (FM) coupling between the Cr2O3 layer and the VO2 film [15]. The motivation behind the present work was to determine if there is any direct evidence between the native stoichiometric defects and magnetic properties leading to the ferromagnetic coupling. It turns out that the post-growth, cooling-cycle ambient parameters have a direct bearing on the physical properties of these materials. In this paper, we report interesting magnetic studies on as-grown and

(a)Sample A

Al2O3(006)

VO2(002)/(020)

Counts (a.u.)

Sample

6363

(b)Sample B

Al2O3(0012)

VO2(004)/(040) Al2O3(0012)

Al2O3(006)

VO2(004)

VO2(002)

(c)Sample C

Al2O3(0012)

Al2O3(006) VO2(004)

VO2(002) 30

35

40

80

85

90

95

100

2θ (degrees) Fig. 1. XRD pattern of the as-deposited vanadium oxide films (samples A, B and C): (a) sample A (growth ambient: pO2 = 103 torr; cooling ambient: vacuum); (b) sample B (growth ambient: pO2 = 102 torr; cooling ambient: pO2 = 102 torr); and (c) sample C (growth ambient: pO2 = 102 torr; cooling ambient: vacuum).

Fig. 2. (a) Cross-section TEM image of a VO2 thin film (sample C); inset: a high-resolution TEM image of the film–substrate interface. (b) SAD pattern, taken from the interface region, showing an epitaxial relationship: VO2 (0 0 2) k Al2O3 (0 0 0 6) and VO2 [0 1 0] k Al2O3 ½2 1 1 0. (c) The atomic arrangement between the basal planes of the VO2 (0 0 2) and Al2O3 (0 0 0 6). It shows that the misfittings along Al2O3 ½2 1 1 0 and Al2O3 ½1 0 1 0 are 4.2 and 17.6%, respectively. (d) A pole figure (2h = 27.795° W = 44.93°), showing three sets of (0 1 1) planes. It indicates that there are three in-plan orientations of VO2 (0 0 2) grown on c-sapphire due to the different symmetries.

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Fig. 3. (a) Bright-field TEM image of VO2 films (sample C). (b) High-resolution image of the VO2 films. It presents two types of VO2 grain (A and B area). The FFTs of these two areas are shown in (c and d), respectively. This confirms A area (c) as (0 0 2), with a high defect content. (d) Area B comprises (0 2 0) textured VO2 grains.

undoped epitaxial thin films of VO2 on c-plane sapphire substrates by altering the cooling cycles and ambient parameters. In addition, the RTFM is also found to vary with the oxygen partial pressure during the growth and post-growth stages. The observed magnetic properties are discussed and correlated with structural characteristics and defects in the absence of impurity content as determined by high-resolution transmission microscopy analysis and X-ray photoelectron spectroscopy (XPS). 2. Experimental Vanadium dioxide films were deposited by a pulsed laser deposition system using a KrF Excimer laser of 248 nm (s = 25 ns). The energy density and repetition rate were kept at 3–4 J cm2 and 10 Hz, respectively. The VO2 target was prepared by sintering compacted VO2 powder at 1000 °C in an argon atmosphere. Prior to deposition, the sapphire (0 0 0 1) substrates were ultrasonically cleaned sequentially in methanol, acetone and methanol solutions. The depositions were done at a substrate temperature of 600 °C. Table 1 summarizes the various growth and postgrowth stages and the associated parameters related to the film deposition, such as the oxygen partial pressure

(pO2). Samples A and C were grown under oxygen partial pressures of 103 and 102 torr, respectively, and cooled under vacuum to room temperature. However, sample B was grown and cooled under an oxygen partial pressure of 102 torr. The structural characterization of the films was done using X-ray diffraction (XRD) and transmission electron microscopy (TEM) (JEOL 2000FX and JEOL 2010F). The valence states of vanadium ion in VO2 films were examined by XPS in a MAC 2 using Mg/Al target. The field dependence of magnetization was measured using a physical property measurement system in conjunction with a vibrating sample magnetometer attachment. Hysteresis measurements were performed by scanning the magnetic fields from 10 to +10 kOe at different temperatures ranging from 10 to 360 K. 3. Results and discussion Fig. 1a and c shows the XRD patterns (h–2h scan) of VO2 films grown on c-sapphire under oxygen pressures of 103 (sample A) and 102 (sample C) torr, which were cooled in the vacuum. For a comparison, Fig. 1b shows the XRD pattern from sample B, which was deposited and cooled under an oxygen pressure of 102 torr. It is

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Fig. 4. Comparison between the field-dependent magnetization of VO2 films cooled under a vacuum (ferromagnetic, samples A and C) and 10 mtorr of oxygen partial pressure (diamagnetic, sample B) at 300 K. The plot clearly emphasizes that the ferromagnetism disappears upon cooling under an oxygen partial pressure. The top inset shows the hysteresis loops of the ferromagnetic film (vacuum cooled, sample A) at 10, 100 and 300 K, which show significant hysteresis (ferromagnetic signature) at all temperatures.

not possible to distinguish between (0 2 0) and (0 0 2) from the XRD patterns (h–2h scans), as both reflections have the same d-spacing in the monoclinic structure. However, by combing h–2h scans with cross-section TEM results, it is possible to distinguish the (0 2 0) and (0 0 2) peaks separately. These peaks, at 2h = 39.87 and 85.89°, are attributed to (0 0 2) and (0 0 4) of monoclinic VO2, respectively, in both 102 torr samples (B and C). Since growth orientations are fixed during initial growth conditions, subsequent cooling in a vacuum or pO2 = 102 torr should not alter the epitaxial orientation relationships. On the other hand, the 103 torr sample (A) cooled in a vacuum shows a broad peak near 2h = 39.87° with a larger full width at half maximum. This feature has been explained (using both XRD and cross-section TEM techniques) due to the presence of both (0 2 0) and (0 0 2) grains in sample A (Fig. 1a). In order to gain a better insight into the details of the inplane growth orientation and microstructure of VO2 films grown under pO2 = 102 torr (sample C), a systematic analysis of TEM images and selected-area diffraction (SAD) patterns were carried out. Fig. 2a shows a cross-section TEM image of the VO2 film (sample C) deposited on c-sapphire under an oxygen pressure of 102 torr and cooled in a vacuum. The thickness of the VO2 film was estimated to be 200 nm. The inset of Fig. 2a is a high-resolution image taken near the VO2/Al2O3 interface. From the SAD patterns (shown in Fig. 2b), the zone direction of the VO2 films and the Al2O3 substrate are [0 1 0] and ½2  1 1 0, respec-

tively. The growth orientation of VO2 films on c-sapphire is further confirmed to be (0 0 2). In addition, the (0 0 2) ˚. d-spacing of the VO2 film was determined to be 2.27 A Fig. 2d is an XRD pole figure of the VO2 film, using (0 1 1) reflections of monoclinic VO2 (2h = 27.795°). It shows six peaks (three sets of diffracted peaks), indicating three in-plane orientations of VO2 (0 0 2) grown on c-sapphire. Also, each of the VO2 grains is rotated 60° with respect to the others. The in-plane arrangement of VO2 unit cells on the c-sapphire substrate can be realized, as shown in Fig. 2c. In addition, the epitaxial relationship can be written as: (0 2 0)f k (0 0 0 6)s, [1 0 0]f k ½1 0  1 0s and [0 1 0]f k ½2 1 1 0s . The large misfit varying from 17.6% in the ½1 0 1 0s direction to 4.2% in the ½2  1 1 0s direction is accommodated by the paradigm of domain matching epitaxy, where integral multiples of planes match across the VO2 film/sapphire interface [16]. Fig. 3a is bright-field TEM image of VO2 films grown on c-sapphire under an oxygen partial pressure of 103 torr (sample A). The figure shows that the VO2 film is 200 nm thick. The high-resolution image shown in Fig. 3b clearly resolves two types of VO2 grains. These two VO2 grain types (marked by A and B) are distributed uniformly with in the films. The fast Fourier transformation (FFT) diffraction patterns are shown in Fig. 3c and d for A and B area, respectively. The diffraction patterns show that the A and B areas are along the (0 0 2) and (0 2 0) orientations of VO2, respectively. This indicates that the lower oxygen pressure promotes the growth of (0 2 0)-

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6

M (emu/cm3)

3

0

-3

-6 -10000

-5000

0

5000

10000

H (Oe) Fig. 5. Isothermal field-dependent magnetization plotted for the VO2 films grown under 103 torr oxygen partial pressure (sample A) at three different temperatures, 10, 100 and 300 K. The saturation magnetization decreases while the coercivity increases as the temperature is decreased from room temperature to 10 K. The top left inset shows a comparison between low-field M–H loops for the two samples of VO2 films grown under 102 (sample C) and 103 torr (sample A) oxygen partial pressures at several temperatures. Plotted in the bottom right inset is the temperature dependence of magnetization of the VO2 films grown under 102 torr (sample C), indicative of a high Curie transition temperature.

oriented VO2 grains. In addition, the higher defect concentration can be observed in area A (VO2 (0 0 2) grains). The preferred growth orientation of VO2 films has an important bearing in that it quite often modifies the physical properties. The above observations are important in correlating the growth and structure with the magnetic properties. The field-dependent magnetization data of both the VO2 films cooled under a vacuum (sample C), (ferromagnetic) and under 10 mtorr of oxygen partial pressure (sample B) (diamagnetic) at 300 K are plotted in Fig. 4. It is interesting to note that the RTFM disappears upon cooling under the oxygen partial pressure. We attribute this switching phenomenon to the presence of intrinsic defects, namely vanadium and oxygen vacancies, which play a key role. The top inset shows the hysteresis loops of ferromagnetic film (vacuum cooled) at 10, 100 and 300 K, showing significant hysteresis at all temperatures, emphasizing that the vacuum cooling preserves the FM while the oxygen cooling results in switching to a diamagnetic state. Hysteresis measurements were performed by sweeping the magnetic fields from 10 to +10 kOe at different temperatures in the range from 10 to 300 K. The isothermal field-dependent magnetization data at three different temperatures, 10, 100 and 300 K, is plotted in Fig. 5 for the VO2 films (sample A) grown on c-sapphire and deposited

under 103 torr oxygen partial pressure. The diamagnetic contribution arising due to the sapphire substrate has been deducted. The magnetization (Ms) for the film saturates around 2500 Oe and it varies by a factor of more than two, decreasing from 4.7 emu cm3 at 10 K to 2.0 emu cm3 at room temperature. The sample shows considerable hysteresis and significantly high values of coercivity at all the measured temperatures typical of a ferromagnetic material. The coercive field decreases in a monotonic fashion from a value of 520 Oe at 10 K to a value of about 180 Oe at 300 K. The top left inset shows a comparison of low field magnetization plotted between ±400 Oe, for both the VO2 films grown under 102 and 103 torr oxygen partial pressures at different temperatures. Plotted in the bottom right inset is the variation of the magnetization of VO2 films grown under 102 torr. The extrapolation onto the temperature axis suggests a Curie (transition) temperature well above room temperature, estimated to be around 500 K. The observed RTFM in VO2 films is essentially due to a complex interplay between two vacancy-type defects and the defect-complexes with the neighboring lattice to bring about magnetic coupling. It is important to extract the different valence states and the exact stoichiometry of the VO2 films to gain an insight

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Fig. 6. (a) Full scan of the binding energy in XPS for an oxygen pressure of 102 torr (sample C), showing O (1s), V (2p) and C (1s) peaks occurring at 529.9, 523.3 and 286.6 eV, respectively. (b and c) The V (2p) peak and the fitting curves with different charge states of vanadium ions (V3+, V4+ and V5+) for samples B and C. The detailed compositions of both VO2 films (the ratio of vanadium to oxygen) were determined to be 1.89 and 1.84 for samples B and C, respectively.

into the structure and bonding characteristics. Fig. 6a shows the full scan of binding energy in XPS for VO2 film (sample B) grown on c-sapphire (0 0 0 1) at 600 °C under an oxygen pressure of 102 torr. The oxygen (1s), vanadium (2p) and carbon (1s) peaks were identified respectively at 532, 518 and 285 eV in the full scan of the XPS spectrum; no significant characteristic peaks of any impurities were found. Since valence charge distributions are related to the chemical binding energy shift, the VO2 films under different cooling oxygen pressures were measured from 510 to 535 eV, with a small step of 0.1 eV. Fig. 6b and c presents the cooling conditions for pO2 = 102 torr (sample B) and vacuum (sample C), respectively. The percentage of V3+ decreased significantly when the cooling

condition was changed from a vacuum to an oxygen atmosphere. The percentages of V3+, V4+ and V5+ in sample B (102 torr; cooled in 10 mtorr) are 28, 59 and 13%, respectively. The percentages of V3+, V4+ and V5+ in the case of sample C (102 torr; cooled in a vacuum) are 45, 53, and 2%, respectively. The detailed compositions of the two VO2 films (the ratio of vanadium to oxygen) were determined to be 1.78 and 1.92 for the vacuum and oxygen atmosphere, respectively. This indicates that the stoichiometry of defect concentration is higher in VO2 films cooled in a vacuum (samples A and C) than in an oxygen atmosphere (sample B). This further establishes that a strong correlation exists between the ferromagnetic coupling and the defects present in the films. In other words, the observed

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ferromagnetism in our VO2 films is essentially defect-driven, and these defects indeed play a critical important role in altering the magnetic properties. The induced magnetic properties observed in our films can be qualitatively understood to stem from the original stoichiometric composition and native structural defects. This is clearly seen in Fig. 4. We attribute the origin of this magnetic coupling to the interaction between the differently charged (or valence) states of the vanadium ions that are actively interacting with defects of several types, such as vacancies and defects of both vanadium as well as oxygen. The different charge states of the vanadium, V3+, V5+, V4+, V1+ and V2+, are due to the possible formation of a number of phases, such as V2O3, V2O5, V3O5, V5O9 and VO, as well as the equilibrium phase VO2. It is important to note that the formation of such phases is due to the fact that the ratio of oxygen ions to vanadium ions varies from 1.5 to about 2.5 [13], which manifests in a narrow compositional stability range. Both the concentration of the different valance (or charged) states and the defects can be reliably controlled by the oxygen partial pressure during the growth process. 4. Conclusions In conclusion, the VO2 thin films cooled to room temperature in a vacuum showed RTFM when processed under oxygen partial pressures of 102 and 103 torr. The detailed crystal structure studies reveal that (0 0 2) VO2 films at pO2 = 102 torr can be grown epitaxially on c-sapphire with three different in-plane orientations. The saturated magnetization and coercivity were estimated to be 18 emu cm3 and 40 Oe, respectively, at room temperature. On the other hand, VO2 films grown under an oxygen pressure of 103 torr were polycrystalline, with a mixture of (0 0 2) and (0 2 0) orientations. The value of the saturated magnetic moment and coercivity for the 103 torr

film were estimated to be 2.0 emu cm3 and 180 Oe, respectively, at room temperature. Furthermore, the ferromagnetism of VO2 films grown on c-sapphire can be switched off when grown and cooled in oxygen pressures of 102 torr, establishing clearly that the ferromagnetism of the VO2 system is driven directly by the stoichiometric defects. Acknowledgement This research was sponsored by the National Science Foundation (DMR-0803663, Dr. Lynnette Madsen). References [1] Yang T, Jin C, Aggarwal R, Narayan RJ, Narayan J. J Mater Res 2010;25:422. [2] Qazilbash MM, Brehm M, Chae B, Ho P, Andreev GO, Kim B, et al. Science 2007;318:1750. [3] Nag J, Haglund Jr RF. J Phys: Condens Matter 2008;20:264016. [4] Narayan J, Bhosle VM. J Appl Phys 2006;100:103524. [5] Yang T, Aggarwal R, Gupta A, Zhou H, Narayan RJ, Narayan J. J Appl Phys 2010;107:053514. [6] Yang T, Jin C, Zhou H, Narayan J. Appl Phys Lett 2010;97:072101. [7] Lampert CM. Solar Energy Mater 1984;11:1. [8] Rajendra Kumar RT, Karunagaran B, Mangalaraj D, Narayandass SK, Manoravi P, Joseph M, et al. Sens Actuat A 2003;107:62. [9] Soltani M, Chaker M, Haddad E, Kruzelesky RV. J Vac Sci Technol A 2006;24:612. [10] Zerov V Yu, Malyarov VG. J Opt Technol 2001;68:939. [11] Narayan J, Nori Sudhakar, Pandya DK, Avasthi DK, Smirnov AI. Appl Phys Lett 2008;93:082507. [12] Narayan J, Nori Sudhakar, Ramachandran S, Prater JT. JOM 2009;61(6):76. [13] Lindstro¨m R et al. Electrochim Acta 2006;51:5001. [14] Yang T, Nori S, Zhou H, Narayan J. Appl Phys Lett 2009;95:102506. [15] Yang T, Mal S, Jin C, Narayan RJ, Narayan J. Appl Phys Lett 2011;98:091102. [16] Narayan J, Larson BC. J Appl Phys 2003;93:278.