Magnetic Field, Temperature, and Time Controlled Manipulation of

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Magnetic Field, Temperature, and Time Controlled Manipulation of Switching Mechanism in NiO Film: Evidence of Large Magnetoconductance S. Das, S. Majumdar, and S. Giri* Department of Solid State Physics, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: February 1, 2010; ReVised Manuscript ReceiVed: March 10, 2010

NiO film with ∼83 nm average thickness is fabricated by sol-gel dip coating technique. The sharp switching between low and high conducting states having a large ratio of currents (∼107) at low temperature is observed in the NiO film. We clearly demonstrate that switching effect can be tuned by controlling external parameters such as magnetic field, thermal excitation, and time duration of the applied voltage. Magnetic field induced switching is involved with the extraordinary large magnetoconductance (∼98% at 20 K). Large magnetoconductance is significant for the spintronic applications. The resistive switching memory controlled by the external parameters associated with the signature of large magnetoconductance is fascinating for the multifunctional applications. Introduction Recently, metal/oxide/metal (MOM) nanostructures have been recognized as promising candidates for the next generation of two-terminal electronic devices. A new memory concept has been introduced as resistive random access memory (RRAM), which has several advantages compared to other nonvolatile memory devices for fast writing times, high densities, and low operating voltages. The most fundamental issue in RRAM technology is different hysteretic behaviors observed in the current-voltage (I-V) characteristics for different RRAM systems.1,2 The uncertainty is exacerbated by the difficulties in characterizing the physical parameters responsible for the electrical switching, because the active regions of the devices are extremely small and buried under the metal contact. Several mechanisms such as filamentary effect,3-6 charge-trap model,1,7,8 insulator-metal transition,9,10 diffusion of oxygen vacancies,11,12 and schottky effect13,14 have been proposed to interpret the switching behavior, although most of these models leave unsolved problems. In fact, understanding the switching mechanism is highly desirable in controlling the switching effect for technological applications. Switching behavior characterized by different modes of hysteresis in the I-V characteristics has been reported in various materials, including metal oxides,3-6,10,12 phosphide,15 and organics.16-18 Semiconducting oxide, NiO, is a well-known resistance-switching material where investigations on NiO having MOM nanostructure have been revisited several times for settling different issues on the mechanism of the resistive switching memories (RSM).3-6 Since the resistive switching effect in nanostructured film has tremendous technological applications, the control of the switching mechanism using external parameters is another important issue that needs to be explored extensively. In this article we are motivated on this issue of controlling switching mechanism by external parameters such as magnetic field, thermal excitation, and time duration of the applied voltage. Bulk NiO is antiferromagnetic (AFM) having a Ne´el temperature around 525 K.19 Nanostructured NiO * To whom correspondence should be addressed. E-mail: sspsg2@ iacs.res.in.

displays varieties of magnetic properties from disordered magnetic to antiferromagnetic behavior.20 Recent results exhibit that conductivity of the NiO film is strongly sensitive to the magnetic field.21 We observe here an extraordinarily large magnetoconductance in NiO film. In this paper, we demonstrate that large magnetoconductance can be exploited for controlling the switching mechanism in NiO film. In addition to the magnetic field controlled manipulation of switching effect, we further demonstrate that thermal excitation and time duration of applied voltage can also be used as important parameters in manipulating switching effect. We suggest that coexistence of large magnetoconductance and switching effect associated with a large ratio (∼107) between high and low conducting states is significant for the multifunctional applications. Experimental Section NiO film was deposited on a cleaned Si (100) single crystal by the sol-gel dip coating technique.22 Si (100) substrate was dipped in a solution composed of Ni2+ source. Nickel nitrate having 0.04 M concentration was added and homogenized with dehydrated alcohol. Polyethylene glycol (molecular weight, 20 000) was then added to the solution in the ratio of 70 mg of polyethylene glycol with 1 mL of 0.04 M active solution. Homogeneous solution was filtered and finally it was used for the chemical deposition. After each layer deposition the prepared substrate was dried at 100 °C for 20 min followed by a 20 min annealing at 500 °C in air. The layer deposition and subsequent heat treatments were done successively 25 times. The powder X-ray diffraction pattern of the film measured in a Seifert XRD 3000P diffractometer with Cu KR radiation was recorded to check the single polycrystalline phase of NiO film. The surface topology of NiO film was probed by the atomic force microscopy with use of a microscope, Veeco-diCP II. Two and three-dimensional images were analyzed by WSXM 4.0.23 Gold electrodes were deposited on the top of the NiO film with an ion coater (Eiko Engineering, Japan). Electrical connections in between Au contact and Cu wires were fabricated by using air drying silver paint. The silver electrodes along with the sample were cured at 150 °C for 4 h. All the electrical measurements were carried out without any device encapsulation

10.1021/jp100947r  2010 American Chemical Society Published on Web 03/22/2010

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Figure 1. (a) X-ray powder diffraction pattern at room temperature. Two- (b) and three-dimensional (c) images of NiO film, using atomic force microscopy.

with use of a source meter (Keithley, 2400) coupled with a computer by the GPIB network. Closed cycle cryocooler (Janis Research Co. Inc., USA) attached with a temperature controller (Lakesore, model 331) was used for the temperature variation and an electromagnet (Polytronic Corp., India) was used for the application of magnetic field. Results and Discussions To confirm structural properties the powder X-ray diffraction pattern of the film was recorded in a diffractometer using Cu KR radiation, shown in Figure 1a. Peaks corresponding to (111), (200), and (220) planes of polycrystalline NiO film are assigned

Das et al. in the X-ray diffraction pattern without any trace amount of impurity phases. The average crystalline size of the NiO particles in the film is ∼25 nm, which was estimated from the broadening of the diffraction peaks by using Scherrer’s formula.24 The surface topology of NiO film was probed by atomic force microscopy (AFM). Two- and three-dimensional images are shown in Figure 1, panels b and c, respectively. Figure 1b exhibits the compactness and morphology of the grains of NiO particles. As seen in the figure the film is composed of nearly spherical shaped particles having a wide distribution of particle size. Figure 1c reveals the surface roughness of the film. Average roughness is found to be ca. 10 nm. The average thickness of the film was determined by scratching the film from the substrate and was found to be ∼83 nm (see Figure 1S in the Supporting Information). The I-V curve measured in between (20 V at 20 K is shown in Figure 2a. Current in the I-V curve increases abruptly above a certain bias voltage (VTh) and a sharp change in conductance is found with a considerably high ratio, ∼107 between high and low conducting states. After switching the current does not follow the same path while applied voltage was reversed below VTh. When measurement was performed in negative polarity of the applied voltage, the feature in the I-V curve repeats with exactly the same value of VTh and the current in the high conducting state shown in Figure 2a. We carefully note that the curves at a fixed temperature were highly reproducible. We further note that VTh strongly depends on the sweep rate of the applied voltage. The value of VTh is shifted toward lower voltage when the sweep rate was slower as shown in Figure 2b. In addition, a second switching is noticed in the I-V curve at higher switching voltage, which is highlighted in Figure 2b in the case of slower sweeping rate. The evidence of a second

Figure 2. (a) I-V curve measured at 20 K exhibits sharp switching between low and high conducting states. (b) Switching effect at 20 K for different sweep rates of voltage. (c) Switching effect at different temperatures. The inset of panel c exhibits the threshold voltage (VTh) as a function of temperature.

Manipulation of Switching Mechanism in NiO Film

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Figure 3. (a) Switching effect at 20 K measured in different external magnetic fields. (b) Threshold voltage (VTh) as a function of magnetic field (H). (c) Conductance (G) scaled by G0 (G at H ) 0) with H. The inset of panel c shows the magnetoconductance, MC ) (G - G0)/G0 with H.

switching is also observed in the time dependence of I-V characteristics (see Figure 4b, as well as Figure 3S in the Supporting Information). We further note that the second switching effect smears out with increasing temperature. The sweep rate dependent results are in accordance with the switching behavior in epitaxial magnetite film grown in MgO (100) substrate where local Joule heating has been proposed to interpret the hysteresis in resistive switching.25 Joule heating is typically described as Q ) i2Rt, i being the current passing through the circuit and Q being the heat produced during time t. Thus, a slower sweep causes larger t than the faster sweep rate where larger t leads to more Joule heating. An example of a switching mechanism as a function of temperature is displayed in Figure 2c where VTh decreases systematically with increasing temperature, shown in the inset of Figure 2c. A sharp switching effect is not clearly visible or smeared out above ∼250 K. Nonlinear I-V curves are observed above 250 K (Figure 2S in the Supporting Information). An example of the nonlinear I-V curve at 300 K is also displayed in Figure 2c. In accordance with the previous reports in NiO film, the switching from low to high conducting states at VTh at a particular set temperature (Tset) can be interpreted by opening the voltage driven filamentary4-6 or percolative conducting3 paths in nanoscale. When sweeping voltage starts from 0 up to the voltage, V < VTh, the system remains approximately at Tset due to the comparatively small amount of Joule heating. For V > VTh, the system is driven into the high conducting state. With the sudden increase in conductance or current, Joule heating of the conducting channel is much larger, elevating the local effective temperature of the channel to Thigh from Tset. Nonsteady-state warming of the sample continues further in the high conducting state. As the voltage is swept back below VTh, the conducting

channel still remains in the high conducting state because the channel is at the elevated effective temperature, Thigh. Thus, hysteresis in the I-V curve is observed in the NiO film. We further performed a new scheme of measurement of switching effect in the presence of external magnetic field at 20 K. Each I-V curve exhibiting switching effect was recorded at a fixed field where magnetic field was varied until 10 kOe. The I-V curves at selective fields at 20 K are shown in Figure 3a. With the application of 0.5 kOe field a considerable decrease of VTh is observed, which decreases further with increasing magnetic field (H). A plot of VTh vs H is shown in Figure 3b. For H g 6.0 kOe the switching effect is not visible or smeared out at 20 K. Two examples of I-V curves for H g 6.0 kOe are shown in Figure 3a. Conductance (G ) dI/dV) scaled by G0 (G at H ) 0) measured at 20 V is plotted with H at 20 K, which is shown in Figure 3c. We note that conductance decreases sharply with the application of very small H, then it decreases further very slowly for 1.0 e H e 4.0 kOe, and finally shows a saturating trend for H > 4.0 kOe. Magnetoconductance (MC) defined as (G - G0)/G0 is also plotted in the inset of Figure 3c. The MC displays almost similar features of G/G0 with H. Strikingly, an extraordinary large MC ≈ 98% is observed at a small field, H ≈ 1.0 kOe. We also have noted a large MC effect even at room temperature where ∼77% of MC is observed at 50 kOe and 20 V (Figure 4S in the Supporting Information). In the case of a low dimensional magnetic system such as the NiO film, the magnetic anisotropy is typically much higher than the bulk counterpart where large magnetic anisotropy might be involved with the large magnetoconductance in the NiO film. A large MC effect in the NiO film is fascinating for the spintronic applications. As a result of the application of magnetic field, resistance (R) increases (or conductance decreases)

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Das et al. voltage displayed in Figure 2b. To check the stability of current in the ON and OFF states with time at 20 K we applied 16 V, waited for 100 s, and then decreased to +4 V. At +4 V the current was recorded with time, which provides the time variation of the ON state as displayed in Figure 4d by the opened circles. After this measurement -16 V was applied, with waiting for 100 s, and again the voltage was returned to +4 V. Current was then immediately recorded with time at +4 V, providing the time variation of the OFF state, which is also shown in Figure 4d by the filled circles. Excellent time stability of the high and low conducting states over a wide range of time is observed at different temperatures. Conclusions

Figure 4. (a) Pulse sequence of applied voltage where a sequence of one cycle composed of write-read-erase-read pulse sequence is highlighted within the arrows. (b) Measured current pulses are displayed and current pulses around applied read voltage pulses are also highlighted, exhibiting memory effect. (c) The ratio between currents at ON and OFF states (ION/IOFF) is plotted with duration of pulses (tSW). (d) Time (t) dependence of high and low conducting states. Experimental protocols are described in the text.

considerably, which leads to the further enhancement of Joule heating (∝i2Rt) in the high conducting state, keeping other parameters almost fixed. Thus, field induced result exhibits an analogous feature observed due to thermal excitation as seen in Figure 2c. We carefully note that hysteretic behavior of the I-V curve exactly repeats while different parameters such as set temperature, time of applied voltage, and applied magnetic field are kept fixed. The reproducible hysteresis in the I-V curve is significant, which typically gives rise to the stable memory effect in the two-terminal devices, and memory effect can be tuned by external parameters. To investigate the memory effect a particular sequence of voltage pulses, viz., write-read-erase-read (W-R-E-R) having different pulse duration (tSW), was applied at 20 K and the corresponding measured current pulses are recorded. Here, +16 V was applied for the write pulse, which was read by +4 V pulse. Then -16 V was applied as an erase pulse, which was followed by another read pulse at +4 V as seen in Figure 4a. Although both the read pulses were applied at +4 V having pulse duration tSW ) 0.5 s; a large difference between measured currents after write and erase pulses is observed which are highlighted in Figure 4b by the ON and OFF states, respectively. The excellent memory effect was further checked with different tSW values where pulse duration was decreased to 0.1 s (see Figure 4S of the Supporting Information for tSW ) 0.2 s). The minimum value of tSW ) 0.1 s used in this measurement protocol is the limitation of our experimental setup. The plot of ION/IOFF as a function of tSW is displayed in Figure 4c, where ION and IOFF are the values of current in the ON and OFF states, respectively. The values of ION/IOFF decrease sharply with increasing tSW and remain unchanged for tSW g 2 s. Here, the new scheme of measurements provides valuable information on the resistive switching mechanism in NiO film where faster switching provides significantly larger values of the ION/IOFF ratio. In addition to the memory effect, we interestingly observe the signature of the second switching effect, which is highlighted in Figure 4b and more prominently in Figure 3S (see the Supporting Information) and has also been observed for the continuous sweep of the applied

In conclusion, we observe a sharp switching effect involved with a large ratio (∼107) between low and high conducting states at 20 K. The threshold voltage in the switching effect, above which a sharp transition from low conducting to high conducting states takes place, shifts toward low voltage with increasing temperature and switching is smeared out above 250 K. We demonstrate that the threshold voltage can be tuned by thermal excitation, duration of voltage pulses, and external magnetic field. The magnetic field induced switching effect involves extraordinarily large magnetoconductance (∼98%), which is fascinating for spintronics applications. Formation and rapture of conducting paths in nanoscale or nanofilaments controlled by Joule heating are suggested to interpret the switching mechanism in NiO film. Acknowledgment. S.G. wishes to thank DST (Project No. SR/S2/CMP-46/2003), India for the financial support. S.D. thanks CSIR, India for a fellowship. S.G. also wishes to thank the Unit on Nano Science & Technology (UNANST-DST) at IACS, Kolkata for providing the AFM facility. Supporting Information Available: The AFM image showing the average height of the NiO film (Figure 1S), the I-V curves at different temperatures in between 250 and 300 K (Figure 2S), voltage and current pulses having pulse duration ∼0.2 s with the time, showing the memory effect (Figure 3S), and percentage of magnetoconductance with magnetic field at 300 K measured at 20 V (Figure 4S). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ignatieva, A.; Wu, N. J.; Chen, X.; Nian, Y. B.; Papagianni, C.; Liu, S. Q.; Strozier, J. Phase Transitions 2008, 81, 791. (2) Waser, R.; Aono, M. Nat. Mater. 2007, 6, 833. (3) Chang, S. H.; Lee, J. S.; Chae, S. C.; Lee, S. B.; Liu, C.; Kahng, B.; Kim, D.-W.; Noh, T. W. Phys. ReV. Lett. 2009, 102, 026801. (4) Park, J. G.; Nam, W. S.; Seo, S. H.; Kim, Y. G.; Oh, Y. H.; Lee, G. S.; Paik, U. G. Nano Lett. 2009, 9, 1713. (5) Lee, S. B.; Chae, S. C.; Chang, S. H.; Noh, T. W. Appl. Phys. Lett. 2009, 94, 173504. (6) Shima, H.; Takano, F.; Akinaga, H.; Tamai, Y.; Inoue, I. H.; Takagi, H. Appl. Phys. Lett. 2007, 91, 012901. (7) Simmons, J. G.; Verderber, R. R. Proc. R. Soc. London, Ser. A. 1967, 301, 77. (8) Bozano, L. D.; Kean, B. W.; Beinhoff, M.; Carter, K. R.; Rice, P. M.; Scott, J. C. AdV. Funct. Mater. 2005, 15, 1933. (9) Rozenberg, M. J.; Inoue, I. H.; Sanchez, M. J. Phys. ReV. Lett. 2004, 92, 178302. (10) Asamitsu, A.; Tomioka, Y.; Kuwahara, H.; Tokura, Y. Nature 1997, 388, 50. (11) Nian, Y. B.; Strozier, J.; Wu, N. J.; Chen, X.; Ignatiev, A. Phys. ReV. Lett. 2007, 98, 146403. (12) Yang, J. J.; Pickett, M. D.; Li, X.; Ohlberg, D. A. A.; Stewart, D. R.; Williams, R. S. Nat. Nanotechnol. 2008, 3, 429.

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