Magnetoresistance Behavior of Conducting Filaments in Resistive

Mar 7, 2017 - The resistive switching (RS) effect in various materials has attracted much attention due to its interesting physics and potential for a...
56 downloads 9 Views 4MB Size
Download PDF
Research Article www.acsami.org

Magnetoresistance Behavior of Conducting Filaments in ResistiveSwitching NiO with Different Resistance States Diyang Zhao,†,§ Shuang Qiao,†,§ Yuxiang Luo,†,∥ Aitian Chen,†,§ Pengfei Zhang,†,§ Ping Zheng,‡ Zhong Sun,†,§ Minghua Guo,†,§ Fu-kuo Chiang,‡ Jian Wu,†,§ Jianlin Luo,‡,§ Jianqi Li,‡,§ Satoshi Kokado,⊥ Yayu Wang,†,§ and Yonggang Zhao*,†,§ †

Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China ‡ Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China § Collaborative Innovation Center of Quantum Matter, Beijing 100084, China ∥ Shandong Institute of Aerospace Electronics Technology, Yantai 264670, China ⊥ Department of Electronics and Materials Science, Graduate School of Integrated Science and Technology, Shizuoka University, Hamamatsu 432-8561, Japan S Supporting Information *

ABSTRACT: The resistive switching (RS) effect in various materials has attracted much attention due to its interesting physics and potential for applications. NiO is an important system and its RS effect has been generally explained by the formation/rupture of Ni-related conducting filaments. These filaments are unique since they are formed by an electroforming process, so it is interesting to explore their magnetoresistance (MR) behavior, which can also shed light on unsolved issues such as the nature of the filaments and their evolution in the RS process, and this behavior is also important for multifunctional devices. Here, we focus on MR behavior in NiO RS films with different resistance states. Rich and interesting MR behaviors have been observed, including the normal and anomalous anisotropic magnetoresistance and tunneling magnetoresistance, which provide new insights into the nature of the filaments and their evolution in the RS process. Firstprinciples calculation reveals the essential role of oxygen migration into the filaments during the RESET process and can account for the experimental results. Our work provides a new avenue for exploration of the conducting filaments in resistive switching materials and is significant for understanding the mechanism of RS effect and multifunctional devices. KEYWORDS: resistive switching effect, anisotropic magnetoresistance, tunneling magnetoresistance, conducting filaments, first-principles calculation

■

magnetoresistance,21 and superconductivity,19 have been shown because of the various types of filaments. The conducting filament model can be classified mainly into three types: electrochemical metallization mechanism (ECM), valence change mechanism (VCM), and thermochemical mechanism (TCM).3 Conducting filaments related to ECM and VCM have been characterized directly by in situ transmission electron microscopy (TEM), with examples of Ag filaments in oxides12,22 for ECM and Magnéli-phase filaments in TiO213 for VCM. In contrast, the nature of conducting filaments in NiO, a model material for TCM, is still in question with various descriptions, such as Ni atomic chains,23 oxygen vacancy chains,24−26 Ni-rich suboxide phases,27,28 and Ni metal phase,21

INTRODUCTION In recent years, the resistive switching (RS) effect has drawn enormous attention from both academic and industrial communities due to its interesting physics as well as promising applications.1−4 It has been demonstrated that a wide variety of materials,5 such as metal oxides,6−8 chalcogenides,9 and organic materials,10 show RS behavior. So far the mechanisms of RS, which are related to some important scientific issues and also significant for applications of RS, have not been solved.11 Diverse mechanisms have been proposed to explain the observed RS effect, including formation/rupture of conducting filaments,12−14 alteration of Schottky barrier,15 and trapping/ detrapping of charge carriers.16 Among them, the conducting filament model is the most extensive and popular one, which involves unique physics, including nonequilibrium steady states17 and strongly interacting electrons.17,18 Rich and interesting phenomena, such as conductance quantization,19,20 © 2017 American Chemical Society

Received: December 22, 2016 Accepted: March 7, 2017 Published: March 7, 2017 10835

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of Au/NiO/Pt stack and experimental setup of magnetoresistance measurements of in-plane and out-of-plane modes. (b) I− V curve of resistive switching with a multistep RESET process (black curve) and the corresponding resistance values (red curve). (c) HRTEM image of one segment of filaments. Lattice fringes in the yellow-line region are around 3.6 Å, identical to double spacing of Ni (002).

proposed on the basis of experimental results21,24,27,28 or theoretical calculations.23,25 In fact, previous experimental results with ex situ TEM only provided information on the existence of filaments in NiO films21,27,29 and could not show that they are really related to resistive switching. Electrical transport properties, including magnetoresistance (MR) behavior, are directly related to the very conducting filament(s) involved in the RS process of NiO, since both resistive switching and magnetoresistance are related to electrical transport. In our previous work,21 we have given convincing evidence of the involvement of metallic Ni filaments in the RS process of NiO through MR results, demonstrating MR as an effective experimental method to uncover the nature of the conducting filaments in NiO. However, regarding the conducting filaments involved in the RS process of NiO, some critical questions30 remain to be solved, such as the composition and size of the filaments, microscopic evolution process of the filaments in the RS process, and the location of filament rupture. On the other hand, the conducting filaments in NiO formed in the RS process are unique and quite different from artificial Ni nanowires,31−34 so it is interesting and meaningful to further explore their MR behavior. In this paper, MR behavior of the conducting filaments in NiO with different resistance states was systematically studied for the first time. For the low-resistance state (LRS), we observed the normal anisotropic magnetoresistance (AMR) behavior, similar to that of Ni nanowires with a large or small diameter. For samples in the high-resistance state (HRS) with different resistances, both normal and anomalous AMR behavior (with the opposite sign of AMR ratio with respect

to the normal AMR) were observed. In some cases, we even observed coexistence of normal and anomalous AMR behavior, with a change from anomalous to normal AMR behavior with decreasing temperature as well as the tunneling magnetoresistance (TMR). This rich and interesting MR behavior has never been observed in bulk Ni or artificial Ni nanowires, or even in any other materials before. To understand the interesting MR behavior of the conducting filaments in NiO with different resistance states, first-principles calculations were carried out to obtain the partial density of states (PDOS) for Ni filaments by considering migration of oxygen into the hot region (the high-temperature region of Ni filaments due to Joule heating), as well as migration of Ni ions in the reverse direction ignored in the previous work, during the RESET process. Based on the experimental results and theoretical calculation, some important information has been obtained about size of the conducting filament, evolution of the filament during the RESET process, and location of filament rupture. This work provides a new way to uncover the nature of conducting filaments in NiO and other TCM systems (including nonmagnetic systems) through MR behavior. Moreover, the coexistence of RS effect and rich MR behavior demonstrates resistance modulation by both electric fields and magnetic fields in one simple device, which is significant for developing nanoscale multifunctional devices.

■

EXPERIMENTAL SECTION

Device Fabrication. We fabricated polycrystalline NiO thin films with 300 nm thickness on Pt/TiO2/SiO2/Si substrates using pulsed laser deposition (PLD) with a KrF excimer laser beam (248 nm). During deposition, the substrate temperature was fixed at 500 °C, 10836

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces

Figure 2. AMR behavior of LRS and HRS at 300 K. (a) Normal AMR behavior with positive AMR ratio of LRS. (b) No change in OP resistivity of LRS for some other samples. (c) Normal AMR behavior with positive AMR ratio of HRS. (d) Anomalous AMR behavior with negative AMR ratio of HRS for some other samples. (e) Temperature dependence of electrical transport of HRS with normal and anomalous AMR behavior, for which the resistance is 171.92 and 146.87 Ω, respectively. (f) RRR values of 26 sets of data measured on different top electrodes of several NiO samples between 10 and 300 K with noteworthy AMR behaviors. oxygen pressure was 0.45 Torr, and the laser fluence and frequency were set to 1.2 J·cm−2 and 3 Hz, respectively. Then an array of Au top electrodes, with diameter 100 μm and thickness 100 nm, was deposited on the NiO thin film by magnetron sputtering to get the Au/NiO/Pt capacitor structure. Experimental Characterization. The current−voltage (I−V) sweep and direct current (dc) conduction characteristic in this work were performed by a Keithley 2400 source meter. Magnetoresistance of the samples was measured from 10 to 300 K with 500 μA current and maximum sweeping magnetic field of 10 kOe, by use of a system of electrical measurements in combination with a superconducting quantum interference device (SQUID) magnetometer. The magnetic fields were applied parallel [in-plane (IP) with the electrical current perpendicular to the magnetization] or perpendicular [out-of-plane (OP) with the electrical current parallel to the magnetization] to the surface of NiO thin film. The cross-sectional NiO thin film in the LRS was thinned by abrasive papers and plasma ion milling for

microstructural observations. TEM observations were performed with a FEI Tecnai F20 at 200 kV. Density Functional Theory Calculations. The features of this system were imitated by performing first-principles calculation with Vienna ab initio simulation package (VASP) based on density functional theory (DFT). The electron−ion interaction and exchange−correlation function were described by the projector augmented wave method and the generalized gradient approximation of Perdew−Burke−Ernzerhof (GGA-PBE), respectively. To ensure the accuracy of the computation, a 450 eV plane-wave basis cutoff, dense Monkhorst−Pack k meshes as 1 × 1 × 9, and 10−5 eV total energy precision of self-consistent iterations were chosen. For the simulation of Ni filament evolution, we used a supercell (lattice constant set as 10.572 × 10.572 × 3.524), adding 5 Å vacuum space in both x and y directions and relaxed the atomic coordination until the residual forces became smaller than 0.01 eV/Å. In the perfect Ni wire and Ni defect cases, ferromagnetic order was set, while in the O doping case, with O 10837

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces as an interstitial or substitutional impurity, antiferromagnetic order was carried out.

could not observe the AMR behavior with a negative MROP, similar to that of bulk Ni.35 As shown in Figure 1b, NiO devices can be triggered from the LRS to the HRS with different resistances through a multistep RESET process. It is interesting to explore MR behavior of NiO samples in HRS with different resistances. We found that some samples show normal AMR behavior at room temperature, as shown in Figure 2c, similar to that observed for samples in the LRS (Figure 2a). Surprisingly, other samples show anomalous AMR behavior at room temperature with a negative AMR ratio, as shown in Figure 2d (an expanded view of Figure 2c,d at low magnetic fields is shown in Figure S1); namely, MRIP is negative and MROP is positive, totally contrary to the normal AMR effect exhibited by Ni bulk35 or Ni nanowires.31−34 It should be mentioned that negative AMR ratio has been usually found in half-metallic ferromagnets,39−41 while it has never been observed in strong or weak ferromagnets, such as Ni, Co, and Fe, except Fe4N.36,42 So the observation of anomalous AMR for the Ni conducting filaments in NiO with HRS is rather unusual. Moreover, one sample usually shows either normal AMR behavior31−35 or anomalous AMR behavior.39−42 However, we observed both normal and anomalous AMR behavior for different HRS through separate RESET processes between the same top electrode (TE) and the bottom electrode (BE). It means that one NiO device can exhibit normal or anomalous AMR behavior just by RESET, which has never been reported before. Furthermore, we could enhance the emerging chance of the expected AMR behavior through modulation of the compliance current (ICC) in the SET process. In detail, the higher ICC of the SET process favors anomalous AMR of the HRS after the subsequent RESET process, while the lower ICC of the SET process favors normal AMR of the HRS after the subsequent RESET process. To get more insights into the different HRS and their correlation with AMR behavior, we measured the temperature dependence of resistance for different HRS. Figure 2e shows the temperature dependence of normalized resistances for the HRS with normal (Figure 2c) and anomalous (Figure 2d) AMR behavior, suggesting their difference in the residual resistivity ratio (RRR), defined as R (300 K)/R (10 K).43 We have a total of 26 sets of AMR data measured on different TEs of several NiO samples between 10 and 300 K, from which we plot resistance versus RRR with corresponding AMR behavior, as shown in Figure 2f. It is obvious that the sign of AMR ratio has no correlation with resistance of the HRS; however, it displays a strong correlation with the RRR value of the HRS, that is, RRR values of those HRS with anomalous AMR behavior are generally larger than those with normal AMR behavior. The different AMR behavior of the HRS is likely to result from the distinctions in composition or structure of Ni filaments originating from the RESET process. Thus, investigation of the origins resulting in different signs of AMR ratio can provide important information on the microscopic evolution mechanism of Ni filaments during the RESET process. So far, the nature of the RESET process in the NiO film has not yet been completely elucidated.30 Most researchers believe that the RESET process is caused by oxygen ion migrationinduced reoxidation of the conducting filaments23,24,26,44 without considering the migration of Ni ions. However, for the oxidation of Ni, it has been well established both experimentally and theoretically that, below 800 °C, the outward diffusion of Ni ions (produced due to ionization at

■

RESULTS AND DISCUSSION A schematic configuration of the Au/NiO/Pt devices studied in this work is depicted in Figure 1a. We observed reversible bistable resistive switching through a voltage sweep as indicated in Figure 1b, after a forming process on a pristine sample to get the LRS. All samples show unipolar RS behavior, and the RESET process often exhibits multiple steps. We also carried out high-resolution transmission electron microscopy (HRTEM) investigations on samples with clear LRS to identify the presence of filaments, as typically shown in the image of Figure 1c. Lattice spaces in the filament exhibit visible changes and often show a regular interspacing of around 3.6 Å, that is, double spacing of Ni (002), suggesting the existence of Ni metal phase in our NiO films; this structural feature has been well illustrated in our previous work.21 As schematically illustrated in Figure 1a, MR measurements were performed in two modes: in-plane (IP) mode, with electrical current perpendicular to magnetization, and out-of-plane (OP) mode, with electrical current parallel to magnetization, with a constant dc current of 500 μA. For samples in the LRS, as shown in Figure 2a, normal AMR behavior was observed at 300 K, which exhibits positive IP MR (MRIP), defined as [R⊥ − R⊥(Hsat)]/ R⊥(Hsat), and a negative OP MR (MROP), defined as [R∥ − R∥(Hsat)]/R∥(Hsat), where R⊥ and R⊥(Hsat) are the resistances for IP and R∥ and R∥(Hsat) are the resistances for OP (Figure 1a) at zero magnetic field and saturation magnetic field, respectively.31 The behavior shown in Figure 2a is similar to that of the bulk Ni35 with a positive AMR ratio. The AMR ratio is generally defined by use of resistivity as36 ρ − ρ⊥ Δρ = ρ ρ⊥

(1)

In contrast to the normal AMR shown in Figure 2a, no MROP was observed for some other samples as shown in Figure 2b (an expanded view of Figure 2a,b at low magnetic fields is shown in Figure S1). This can be attributed to the strong diameter dependence of AMR behavior, which has been studied in artificial Ni nanowires.31 There have been some reports on the AMR behaviors of the artificial Ni nanowires indicating that, for thin Ni nanowires with diameters less than 60 nm, the OP resistivity does not change with magnetic field showing zero MROP,31,32 which has been explained by considering that the only stable magnetic configuration is single domain over the entire length of nanowires.31 In the case of Ni nanowires with diameters larger than 100 nm, the AMR effect shows behavior31,33,34 similar to that of bulk Ni35 due to the multidomain magnetic configuration.31 In fact, for the sizes of Ni filaments in NiO films, there have been a few direct TEM observations,21,27,37 from which one can deduce that it is in the range 5−50 nm; however, these observations only reflect the situation in tiny areas and these filaments may not be those that are really related to the RS effect. There has also been an indirect estimation by extracting the conducting filament size by a physical conduction model,38 which indicated that the filament diameter ranges from 1 to 20 nm. Nevertheless, our AMR results of the LRS suggest that the sizes of Ni filaments that are really related to the RS effect in NiO films vary in a larger range and may be even larger than 100 nm; otherwise, we 10838

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces the interface) through the NiO formed around Ni is the dominant transport process45,46 with a slight inward diffusion of oxygen ions,47 while over 800 °C, oxygen ion transport also has to be taken into consideration.47 Therefore, the migration of Ni ions during the RESET process should be considered, which has been neglected in previous work.23,24,44 We propose the following scenario for the RESET process. The current through Ni filaments leads to Joule heating, which makes Ni ions migrate out of the hot region (Ni filaments) and oxygen ions migrate in the reverse direction.30 These Ni ions result from breaking of Ni bonds45,48 in the Ni filaments, and the O ions result from the thermal decomposition effect of surrounding oxygen-rich clusters such as Ni2O3 produced in the SET process.24,44 The Ni ions combine with O ions into Ni−O bonds near the surface of the Ni filaments, resulting in thinner Ni filaments with higher resistance, namely, HRS. As a consequence, Ni vacancies and O defects are introduced into the thinner Ni filaments, which could influence the AMR behavior. On the basis of the preceding scenario for the RESET process, we can explain the strong correlation between sign of the AMR ratio and RRR value of the HRS as follows. We have shown that the size of Ni filament in NiO films varies over a large range in this work, and it was reported in the literature that higher temperature due to Joule heating is needed to reset the Ni conducting filaments with relatively large diameters.49,50 For the thinner Ni filaments in the LRS, the RESET process only introduces Ni vacancies with few O defects into the Ni filaments, due to the lower temperatures required for the RESET process.45,46,49,50 As a result, the AMR behavior of the corresponding HRS is normal as the LRS. In contrast, for the thicker Ni filaments in the LRS, the RESET process introduces both Ni vacancies and O defects into the Ni filaments due to the higher temperatures required for the RESET process. These O defects change the band structure and Fermi level position of the filaments, resulting in changes of the dominant s−d scattering in Ni filaments,36,51 which leads to sign reversal of the AMR ratio;36,52 that is, anomalous AMR behavior. This picture is consistent with the fact that the RRR value of metal nanowire is directly proportional to its diameter:53,54 thinner Ni filaments show smaller RRR values, while larger ones show larger RRR values. This picture is supported by our firstprinciples calculation and theoretical analysis, as will be shown later. In addition, we found the relative magnitudes of MROP and MRIP of the AMR behavior are diverse for different samples due to many factors, including diverse angles between the current and the easy axes of different Ni filaments; these are discussed in detail in Supporting Information section S4. We also studied the temperature dependence of AMR for our samples. Generally, the signs of AMR ratio of the LRS and HRS do not change with decreasing temperature (Figures S2 and S3), while the high-field (above 3000 Oe) OP MR behavior changes with decreasing temperature; that is, the OP resistance decreases at 300 K but increases at 10 K with increasing magnetic field (Supporting Information section S5). However, the sign of the AMR ratio for some HRS amazingly changes from negative (anomalous AMR) to positive (normal AMR) with decreasing temperature; that is, MRIP changes from negative to positive with decreasing temperature, as shown in Figure 3, and MROP shows a reverse change (Figure S6). This unusual AMR behavior for magnetic materials has only been observed in the half-metallic ferromagnet Fe3O440,55 and was explained by considering temperature dependence of the

Figure 3. Sign change of AMR ratio with decreasing temperature for some HRS. (a−d) Evolution of MRIP from negative to positive with decreasing temperature. (e) One more MR measurement when the temperature is increased back to 300 K. The resistance value and the MRIP change back to the original state in panel a, indicating the reliability of the sign change with decreasing temperature.

density of states (DOS) ratio at the Fermi level of the up spin to the down spin, where the temperature dependence of the DOS ratio was due to temperature-dependent exchange splitting energy.36,40 In addition, Figure 3 shows an abnormal temperature dependence of the coercive field (HC, magnetic field corresponding to the peak of AMR curve) of the Ni filament. HC increases with decreasing temperature from 300 to 100 K (Figure 3a−c), while it decreases at 10 K (Figure 3d), which violates the conventional rule that HC increases with decreasing temperature.56 There have been several reports on the abnormal temperature dependence of HC, which was generally attributed to nonmonotonic evolution of magnetocrystalline anisotropy with decreasing temperature.57−59 To summarize, we have observed rich AMR behavior of Ni conducting filaments in one NiO RS sample, including normal AMR behavior, anomalous AMR behavior, and sign change of the AMR ratio with decreasing temperature, by means of the 10839

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces

Figure 4. Superposed IP AMR behavior of HRS and statistics. (a−c) Superposed IP AMR behavior at (a) 300, (b) 100, and (c) 10 K, showing features of both normal AMR behavior with peaks and anomalous AMR behavior with troughs. (d) Measured distribution of MRIP(H = 0) at 300 K. Each pillar is centered at MRIP(H = 0) ± 0.1‰.

actually a joint effect of multifilaments. Even so, the superposed AMR curves can be observed only when two or more filaments show comparable resistance; otherwise, the observed MR behaviors usually reflect magnetoresistance of the single filament with the lowest resistance, because of the dominant role in electrical transport of this filament among all the filaments in parallel. In addition to the diverse AMR behaviors of the HRS mentioned above, we also observed TMR in some samples for the first time. Figure 5a,b shows the IP and OP MR behaviors at 300 K. It can be seen that the OP MR curve, in addition to the sharp peaks related to AMR, also shows two obvious plateaus, which is the typical characteristics of TMR.61 Therefore, we divide the MR curve into two parts with a red curve and a green curve as shown in Figure 5c,d, of which the red curves are apparently related to the normal AMR with the corresponding IP and OP behaviors, respectively. The green curve for OP (Figure 5d) shows the typical R−H loops, similar to that observed in a magnetic tunnel junction (MTJ) with an external magnetic field (Hext) applied along the easy-axis direction, while the green curve for IP (Figure 5c) shows R−H peaks, similar to that observed in the MTJ with H ext perpendicular to the easy-axis direction.61 It has been demonstrated in our previous work that the magnetic easy axis of Ni filaments is along their axes, namely, the OP direction.21 The schematics of the magnetic and electrical configurations of Ni filaments are illustrated in the insets of Figure 5c,d, and the detailed results measured at 200, 100, and 10 K are shown in Figure S8a. Furthermore, Figure 5e shows the temperature dependence of the MR ratio of the OP green

RESET process. It is noteworthy that this particular coexistence of three kinds of AMR behavior has never been reported in any other materials or systems before. Moreover, we also observed a strange IP MR behavior for the HRS, showing features of both normal AMR behavior with peaks and anomalous AMR behavior with troughs (Figure 4a− c). This complex MR behavior can be regarded as a superposition of a normal AMR curve and an anomalous AMR curve (section S7, Figure S7a). The separation method for these two curves is discussed in section S7, through which the RRR values and temperature dependence of MRIP(H = 0) {[R⊥(H = 0) − R⊥(Hsat)]/R⊥(Hsat)} of the two separated curves match our experimental results for normal and anomalous AMR behavior, respectively (Figure S7b), indicating the separation method is reasonable and effective. In fact, we fabricated dozens of NiO samples and performed MR measurements nearly 100 times, using the procedure described in the Experimental Section. The measured distribution of MRIP(H = 0) at 300 K is shown in Figure 4d. The distribution is very broad with a peak near zero, indicating a quarter of the data points (after counting all the points) almost do not exhibit measurable MR. Among the remaining three-quarters of the data points, some exhibit positive MRIP (normal AMR) and others exhibit negative MRIP (anomalous AMR). The superposed AMR curves and the statistics can be explained by the multifilamentary model,21,60 implying the coexistence of two types of filaments that show the normal and anomalous AMR behavior, respectively. The two types of filaments are different consequences of the RESET process and jointly contribute to electrical transport, thus the MR curves we obtained are 10840

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces

Figure 5. TMR behavior of HRS. (a, b) IP and OP MR behaviors at 300 K. (c, d) Separation of IP and OP MR curves corresponding to panels a and b into AMR behavior (red curves) and TMR behavior (green curves). (e) Temperature dependence of MR ratio of OP green curves. (f) Remarkable TMR behavior with a large ratio of 1.8% at 300 K.

connection of Ni conducting filaments takes place at the anodic site of the filaments25,64 or in the bulk.65 Our observation of TMR in this work demonstrates that an insulating gap (most probably NiO) can be produced in a Ni filament during the RESET process, forming a Ni/insulator/Ni nano-MTJ structure. As the two ferromagnetic (FM) layers (Ni segments) are essential to MTJ structure, we can deduce that the rupture region should be ultrathin and located in the Ni filament (bulk) rather than at either end of the Ni filament. The observation of TMR in the NiO RS sample is a new breakthrough to some extent in the resistive switching study of NiO films, since it not only demonstrates the location of the rupture region for Ni filament but also shows significance for applications.

plateaus shown in Figure 5d and Figure S8a, indicating that the MR ratio exhibits an obvious decrease with increasing temperature, and the behavior is consistent with that reported for TMR.62 Hence, the green curves are related to the TMR behavior of the ruptured Ni filaments. Likewise, the temperature dependence of the MRIP(H = 0) [MROP(H = 0)] and the IP [OP] coercive field (HC, magnetic field corresponding to the peak of MR curve) of the red curves (Figure S8b) are also consistent with those of AMR reported in the literature.63 More interestingly, we even observed a remarkable TMR with a large ratio of 1.8% at 300 K as shown in Figure 5f. The observation of TMR can provide important information on the location of the rupture region of Ni filaments, which is not clear because there have been too few direct observations or indirect analysis with convincing evidence to confirm the location of the rupture region. Actually, it is still controversial whether the rupture and 10841

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces

Figure 6. Partial density of states (PDOS) of Ni filaments. (a) PDOS of perfect Ni. (b) Different structures of the Ni supercell with an oxygen defect from the c-axis view without vacuum space: (1) doping an O atom as an interstitial impurity or (2) replacing a Ni atom by an O atom as a substitutional impurity. The red sphere represents an O atom while the gray ones show Ni atoms. (c, d) PDOS of structures with O defect in panel b; the vertical dashed line indicates the Fermi energy.

■

In the estimation for Ni, m↓*/m↑* = 1 could be chosen,66 and then eq 4 is simplified as

THEORETICAL CALCULATIONS Regarding the origin of AMR effect, it is related to scattering of conduction electrons into the split d bands, and there is a relationship between the sign of the AMR ratio and the s−d scattering process.36 According to theoretical work by Kokado et al.,36 we can evaluate the AMR ratios by use of eq 2 with γ = 0.01: ⎛ ⎞ Δρ 1 1 ∝ γ(D↑d − D↓d )⎜⎜ − ⎟⎟ ρ ρ↑ ⎠ ⎝ ρ↓

(2)

ρσ = ρsσ + ρsσ → dσ

(3)

σ = ↑ or ↓

⎛ D↑s ⎞2 ⎟⎟ = ⎜⎜ ρs ↑ ⎝ D↓s ⎠ ρs ↓

Therefore, we can judge the sign of the AMR ratio from the sp and d orbital DOS of Ni filaments with structural or component variation, which is related to the evolution of Ni filaments during the RESET process. To get the DOS of Ni filaments, we carried out firstprinciples calculation within density functional theory (DFT). First, it should be pointed out that our theoretical calculation is completely different from the previous reports,23,25 which focused on the effect of oxygen vacancy on the density of states (DOS) of NiO supercell. However, because of the convincing evidence of TEM observation and AMR results for metallic Ni filaments, which are supposed to be the dominant electrical transport paths, we constructed a Ni supercell with vacuum layers (details are described in the Experimental Section) to simulate the Ni filaments, and we systematically calculated the varying DOS, induced by the outward migration of Ni ions and/or the inward migration of oxygen ions, to account for the observed diverse AMR behaviors.

where Dσd is the DOS of each d state with the σ spin at the Fermi energy EF, ρsσ is the resistivity of the conduction state (consisting of 4s and 4p states for Ni, named as s) with the σ spin, and ρsσ→dσ is the resistivity due to s−d scattering. In addition, we can obtain the relationship between ρs↑→d↑ and ρs↓→d↓ through D↑d and D↓d,36 while the relationship between ρs↑ and ρs↓ can be estimated by ⎛ m * ⎞ 4 ⎛ D s ⎞2 ↓ ⎟⎟ ⎜⎜ ↑ ⎟⎟ = ⎜⎜ ρs ↑ ⎝ m↑* ⎠ ⎝ D↓s ⎠

(5)

ρs ↓

(4) 10842

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces

ρs↓/ρs↑ and ρs↑→d↑/ρs↓→d↓ with decreasing temperature are too small to reverse the relationship between ρ↑ and ρ↑ (either ρ↑ < ρ↓ or ρ↑ > ρ↓), and that is why the sign of the AMR ratio remains unchanged with decreasing temperature (Figures S2 and S3). However, similar to the case of Fe3O4,36 the relationship between ρ↑ and ρ↓ could be occasionally changed in rare cases with decreasing temperature, leading to a sign change of AMR ratio with decreasing temperature as mentioned above (Figure 3). From the aforementioned experimental and calculation results in the present work, the conducting filament model of NiO and other similar TCM materials is improved and complemented, especially the microscopic evolution process of Ni filaments during the RESET process, as schematically shown in Figure 7. Multifilaments with diameters in a range of tens to

We first calculated the PDOS of perfect Ni (without defects) and the result is shown in Figure 6a, which is similar to that of previous work.67 From Figure 6a and eq 5, we can deduce ρs↑ < ρs↓ due to (D↑s/D↓s)2 = 1.61 and ρs↑→d↑ < ρs↓→d↓ due to D↑d < D↓d. The AMR ratio is thus positive because of D↑d < D↓d and ρ↑ < ρ↓, which originates from ρs↑ < ρs↓ and ρs↑→d↑ < ρs↓→d↓. The sign of the AMR ratio is consistent with that of the LRS (Figure 2a) related to the Ni filaments, as well as the experimental results of bulk Ni35 and artificial Ni nanowires.31−34 For the RESET process, we proposed that outward migration of Ni ions and/or the inward migration of oxygen ions are involved, on the basis of experimental results. In order to get insight into the RESET process, we introduced some defects on the Ni supercell to calculate the influence of outward migration of Ni ions and/or inward migration of oxygen ions on the PDOS of Ni filaments. First, the relationships D↑d < D↓d and D↑s > D↓s are unchanged when only Ni vacancies are introduced into the Ni supercell (Figure S9a), which means the outward migration of Ni ions away from the Ni filaments. Thus, the AMR ratio remains positive on account of the invariant D↑d < D↓d and ρ↑ < ρ↓, and this could be reasonable because of the weak interaction between atoms in metals.68 However, when an oxygen atom is doped into the Ni supercell as an interstitial impurity, as schematically illustrated in the left lattice structure of Figure 6b, to investigate the effect of inward migration of oxygen ion on Ni filaments, the DOS of d band changes remarkably, as shown in Figure 6c, due to the strong interaction between the active O atom and neighboring Ni atoms, which can dramatically influence the magnetic moment for Ni atoms. From Figure 6c, it can be seen that ρs↑ < ρs↓ still holds because of (D↑s/D↓s)2 = 1.35, while ρs↑→d↑ > ρs↓→d↓ is deduced due to D↑d > D↓d, which is totally different from that of perfect Ni filaments (Figure 6a). Then imitating the study method on Fe4N in the work of Kokado et al.,36 the AMR ratio turns to be negative (anomalous AMR) owing to D↑d < D↓d and ρ↑ < ρ↓with the assumption of ρs↓→d↓/ρs↑ < 0.30, which is reasonable (Supporting Information section S10). In addition, the explanation of negative MRIP and positive MROP for the anomalous AMR is presented in Supporting Information section S11. The conclusion is still true if we change the doping site of the O atom as an interstitial impurity as shown in Figure S9b. Furthermore, when an O atom replaces a Ni atom as a substitutional impurity, schematically illustrated in the right structure of Figure 6b, it means coexistence of outward migration of Ni ions and inward migration of oxygen ions. From Figure 6d, we can deduce ρs↑ < ρs↓ due to (D↑s/D↓s)2 = 1.13 and ρs↑→d↑ > ρs↓→d↓ due to D↑d > D↓d; the AMR ratio can also be negative (anomalous AMR) with ρ↑ < ρ↓ if we assume ρs↓→d↓/ρs↑ < 0.11. Similar results can be obtained when changing the replacement site of the O atom a little bit farther away from the edge, as shown in Figure S9c. On the basis of this systematic analysis, we can reach the conclusion that doping O atoms into Ni filaments as an interstitial impurity or as a substitutional impurity could make the AMR ratios negative, suggesting that inward migration of oxygen ions during the RESET process is the key factor for appearance of anomalous AMR when Ni filaments are in HRS, which is in good agreement with our picture for the RESET process described earlier. To account for the temperature dependence of the AMR ratio, as we know, the values of (D↑s/D↓s)2 and D↑d/D↓d change as temperature decreases because of the temperature dependence of exchange splitting.69 Generally speaking, the changes of

Figure 7. Schematic diagram of microscopic evolution process of Ni filaments during the RESET process. (a) LRS with multifilaments of various diameters after a forming process. (b) Different consequences of the RESET process for the thicker and thinner Ni filaments. (c) Ruptures of Ni filaments and generation of insulating gaps in Ni filaments due to further reaction. 10843

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces hundreds of nanometers are formed in the NiO film during the forming process, leading to the LRS (Figure 7a). Figure 7b shows the microscopic evolution process of Ni filaments in the RESET process, including the evolution of sizes and compositions, induced by the outward migration of Ni ions and/or the inward migration of oxygen ions. The RESET process only introduces Ni vacancies with few O defects into the thinner Ni filaments (the right one) due to the lower temperatures required for the RESET process, while it introduces both Ni vacancies and O defects into the thicker Ni filaments (the middle one) due to the higher temperatures required for the RESET process. Moreover, the O defects in the thicker Ni filaments (Figure 7b) can remarkably influence the DOS of Ni filaments in the HRS at low temperatures, leading to anomalous AMR behavior. Further reaction can lead to the rupture of Ni filaments and generate insulating gaps in the Ni filaments (Figure 7c), forming a nano-MTJ that exhibits TMR behavior.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Professor Jianwang Cai and Professor Yizheng for valuable discussions. This work was supported by National Science Foundation of China (Grants 11134007 51572150) and the 973 project of the Ministry of Science Technology of China (Grant 2015CB921402).

■

CONCLUSIONS In summary, rich and interesting AMR behavior was observed for the unique Ni conducting filaments formed in NiO RS samples, including normal AMR with a positive AMR ratio (two different OP MR behaviors in the LRS), anomalous AMR with a negative AMR ratio, superposition of normal and anomalous AMR, and even sign change of the AMR ratio with decreasing temperature. We also observed TMR behavior in some samples with the HRS, which is a new breakthrough, indicating a nano-MTJ can be realized in the Ni filament just by the RESET process. Furthermore, through the experimental results and first-principles calculation, we provide an in-depth understanding of the Ni conducting filament in NiO RS samples, such as the size of Ni filaments related to the RS effect, microscopic evolution process of the filaments in the RS process, and the location of the filaments’ rupture region. The combination of multistep RS effect and rich MR behavior in NiO RS films provides a new avenue for multifunctional devices. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16458. Additional text, nine figures, and one table showing expanded view of AMR behavior at low magnetic fields, AMR behavior of LRS and HRS, magnitudes of MROP and MRIP of anomalous AMR behavior, high-field OP MR behavior, OP MR behavior at different temperatures, separation method for superposed AMR behavior, detailed results of TMR behavior, calculation results for PDOS of Ni supercell, estimation of ρs↓→d↓/ρs↑, and explanation of negative MRIP and positive MROP for anomalous AMR (PDF)

■

REFERENCES

(1) Waser, R.; Aono, M. Nanoionics-Based Resistive Switching Memories. Nat. Mater. 2007, 6, 833−840. (2) Sawa, A. Resistive Switching in Transition Metal Oxides. Mater. Today 2008, 11, 28−36. (3) Waser, R.; Dittmann, R.; Staikov, G.; Szot, K. Redox-Based Resistive Switching Memories - Nanoionic Mechanisms, Prospects, and Challenges. Adv. Mater. 2009, 21, 2632−2663. (4) Kim, K. M.; Jeong, D. S.; Hwang, C. S. Nanofilamentary Resistive Switching in Binary Oxide System; A Review on the Present Status and Outlook. Nanotechnology 2011, 22, No. 254002. (5) Jeong, D. S.; Thomas, R.; Katiyar, R. S.; Scott, J. F.; Kohlstedt, H.; Petraru, A.; Hwang, C. S. Emerging Memories: Resistive Switching Mechanisms and Current Status. Rep. Prog. Phys. 2012, 75, No. 076502. (6) Argall, F. Switching Phenomena in Titanium Oxide Thin Films. Solid-State Electron. 1968, 11, 535−541. (7) Gibbons, J. F.; Beadle, W. E. Switching Properties of Thin NiO Films. Solid-State Electron. 1964, 7, 785−790. (8) Hiatt, W. R.; Hickmott, T. W. Bistable Switching in Niobium Oxide Diodes. Appl. Phys. Lett. 1965, 6, 106−108. (9) Das, B. C.; Pal, A. J. Memory Applications and Electrical Bistability of Semiconducting Nanoparticles: Do the Phenomena Depend on Bandgap? Small 2008, 4, 542−547. (10) Lebedev, E.; Forero, S.; Brutting, W.; Schwoerer, M. Switching Effect in Poly (p-Phenylenevinylene). Synth. Met. 2000, 111-112, 345− 347. (11) Yang, J. J.; Strukov, D. B.; Stewart, D. R. Memristive Devices for Computing. Nat. Nanotechnol. 2013, 8, 13−24. (12) Yang, Y. C.; Pan, F.; Liu, Q.; Liu, M.; Zeng, F. Fully RoomTemperature-Fabricated Nonvolatile Resistive Memory for Ultrafast and High-Density Memory Application. Nano Lett. 2009, 9, 1636− 1643. (13) Kwon, D.; Kim, K. M.; Jang, J. H.; Jeon, J. M.; Lee, M. H.; Kim, G. H.; Li, X.; Park, G.; Lee, B.; Han, S.; Kim, M.; Hwang, C. S. Atomic Structure of Conducting Nanofilaments in TiO2 Resistive Switching Memory. Nat. Nanotechnol. 2010, 5, 148−153. (14) Sun, Z.; Zhao, Y.; He, M.; Gu, L.; Ma, C.; Jin, K.; Zhao, D.; Luo, N.; Zhang, Q.; Wang, N.; Duan, W.; Nan, C. Deterministic Role of Concentration Surplus of Cation Vacancy over Anion Vacancy in Bipolar Memristive NiO. ACS Appl. Mater. Interfaces 2016, 8, 11583− 11591. (15) Jia, C. H.; Dong, Q. C.; Zhang, W. F. Effect of Incorporating Copper on Resistive Switching Properties of ZnO Films. J. Alloys Compd. 2012, 520, 250−254. (16) Jiang, X. L.; Zhao, Y. G.; Zhang, X.; Zhu, M. H.; Zhang, H. Y.; Shang, D. S.; Sun, J. R. Trap State Controlled Bipolar Resistive Switching Effect and Electronic Transport in LaAlO3/Nb:SrTiO3 Heterostructures. Appl. Phys. Lett. 2013, 102, No. 233501. (17) Li, J.; Aron, C.; Kotliar, G.; Han, J. E. Electric-Field-Driven Resistive Switching in the Dissipative Hubbard Model. Phys. Rev. Lett. 2015, 114, No. 226403. (18) Peng, H. Y.; Li, Y. F.; Lin, W. N.; Wang, Y. Z.; Gao, X. Y.; Wu, T. Deterministic Conversion Between Memory and Threshold Resistive Switching via Tuning the Strong Electron Correlation. Sci. Rep. 2012, 2, No. 442.

■

■

Wu the and and

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Aitian Chen: 0000-0003-3535-9470 Yonggang Zhao: 0000-0002-7803-7378 10844

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces (19) Zhu, X.; Su, W.; Liu, Y.; Hu, B.; Pan, L.; Lu, W.; Zhang, J.; Li, R. Observation of Conductance Quantization in Oxide-Based Resistive Switching Memory. Adv. Mater. 2012, 24, 3941−3946. (20) Sasakura, H.; Nishi, Y.; Kimoto, T. Temperature Dependence of Conductance in NiO-based Resistive Switching Memory Showing Two Modes in the Forming Process. Appl. Phys. Lett. 2015, 107, No. 233510. (21) Luo, Y.; Zhao, D.; Zhao, Y.; Chiang, F.; Chen, P.; Guo, M.; Luo, N.; Jiang, X.; Miao, P.; Sun, Y.; Chen, A.; Lin, Z.; Li, J.; Duan, W.; Cai, J.; Wang, Y. Evolution of Ni Nanofilaments and Electromagnetic Coupling in the Resistive Switching of NiO. Nanoscale 2015, 7, 642− 649. (22) Yang, Y.; Gao, P.; Gaba, S.; Chang, T.; Pan, X.; Lu, W. Observation of Conducting Filament Growth in Nanoscale Resistive Memories. Nat. Commun. 2012, 3, No. 732. (23) Lee, H. D.; Magyari-Kope, B.; Nishi, Y. Model of Metallic Filament Formation and Rupture in NiO for Unipolar Switching. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, No. 193202. (24) Chen, Y. S.; Kang, J. F.; Chen, B.; Gao, B.; Liu, L. F.; Liu, X. Y.; Wang, Y. Y.; Wu, L.; Yu, H. Y.; Wang, J. Y.; Chen, Q.; Wang, E. G. Microscopic Mechanism for Unipolar Resistive Switching Behaviour of Nickel Oxides. J. Phys. D: Appl. Phys. 2012, 45, No. 065303. (25) Sarhan, A.; Nakanishi, H.; Dino, W. A.; Kishi, H.; Kasai, H. Oxygen Vacancy Effects on Electronic Structure of Pt/NiO/Pt Capacitor-Like System. Surf. Sci. 2012, 606, 239−246. (26) He, L.; Liao, Z.; Wu, H.; Tian, X.; Xu, D.; Cross, G. L. W.; Duesberg, G. S.; Shvets, I. V.; Yu, D. Memory and Threshold Resistance Switching in Ni/NiO Core-Shell Nanowires. Nano Lett. 2011, 11, 4601−4606. (27) Park, G.; Li, X.; Kim, D.; Jung, R.; Lee, M.; Seo, S. Observation of Electric-Field Induced Ni Filament Channels in Polycrystalline NiOx Film. Appl. Phys. Lett. 2007, 91, No. 222103. (28) Ielmini, D.; Nardi, F.; Cagli, C. Physical Models of SizeDependent Nanofilament Formation and Rupture in NiO Resistive Switching Memories. Nanotechnology 2011, 22, No. 254022. (29) Lee, M.; Han, S.; Jeon, S. H.; Park, B. H.; Kang, B. S.; Ahn, S.; Kim, K. H.; Lee, C. B.; Kim, C. J.; Yoo, I.; Seo, D. H.; Li, X.; Park, J.; Lee, J.; Park, Y. Electrical Manipulation of Nanofilaments in Transition-Metal Oxides for Resistance-Based Memory. Nano Lett. 2009, 9, 1476−1481. (30) Ielmini, D.; Bruchhaus, R.; Waser, R. Thermochemical Resistive Switching: Materials, Mechanisms, and Scaling Projections. Phase Transitions 2011, 84, 570−602. (31) Yoo, B. Y.; Rheem, Y. W.; Beyermann, W. P.; Myung, N. V. Magnetically Assembled 30 nm Diameter Nickel Nanowire with Ferromagnetic Electrodes. Nanotechnology 2006, 17, 2512−2517. (32) Ohgai, T.; Hoffer, X.; Fabian, A.; Gravier, L.; Ansermet, J. P. Electrochemical Synthesis and Magnetoresistance Properties of Ni, Co and Co/Cu Nanowires in a Nanoporous Anodic Oxide Layer on Metallic Aluminium. J. Mater. Chem. 2003, 13, 2530−2534. (33) Abdi, A. N.; Bucher, J. P. Magnetoresistance and Switching of Electrochemically Etched Ni Wires. Appl. Phys. Lett. 2003, 82, 430− 432. (34) Chan, K. T.; Kan, J. J.; Doran, C.; Ouyang, L.; Smith, D. J.; Fullerton, E. E. Oriented Growth of Single-Crystal Ni Nanowires onto Amorphous SiO2. Nano Lett. 2010, 10, 5070−5075. (35) Mcguire, T. R.; Aboaf, J. A.; Klokholm, E. Negative Anisotropic Magnetoresistance in 3D Metals and Alloys Containing Iridium. IEEE Trans. Magn. 1984, 20, 972−974. (36) Kokado, S.; Tsunoda, M.; Harigaya, K.; Sakuma, A. Anisotropic Magnetoresistance Effects in Fe, Co, Ni, Fe4N, and Half-Metallic Ferromagnet: A Systematic Analysis. J. Phys. Soc. Jpn. 2012, 81, No. 024705. (37) Fujii, T.; Arita, M.; Hamada, K.; Takahashi, Y.; Sakaguchi, N. Insitu Transmission Electron Microscopy of Conductive Filaments in NiO Resistance Random Access Memory and its Analysis. J. Appl. Phys. 2013, 113, No. 083701.

(38) Ielmini, D.; Nardi, F.; Cagli, C.; Lacaita, A. L. Size-Dependent Retention Time in NiO-Based Resistive-Switching Memories. IEEE Electron Device Lett. 2010, 31, 353−355. (39) Endo, T.; Kubota, H.; Miyazaki, T. Magnetoresistance of Co2MnAl1‑xSix Heusler Alloys. Nippon Oyo Jiki Gakkaishi 1999, 23, 1129−1132. (40) Ziese, M. Spontaneous Resistivity Anisotropy and Band Structure of La0.7Ca0.3MnO3 and Fe3O4 Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 1044−1050. (41) Favre-Nicolin, E.; Ranno, L. Anisotropic Magnetoresistance in Manganite Films. J. Magn. Magn. Mater. 2004, 272−276, 1814−1815. (42) Tsunoda, M.; Komasaki, Y.; Kokado, S.; Isogami, S.; Chen, C.; Takahashi, M. Negative Anisotropic Magnetoresistance in Fe4N Film. Appl. Phys. Express 2009, 2, No. 083001. (43) Kim, M. G.; Kim, S. M.; Choi, E. J.; Moon, S. E.; Park, J.; Kim, H. C.; Park, B. H.; Lee, M. J.; Seo, S.; Seo, D. H.; Ahn, S. E.; Yoo, I. K. Study of Transport and Dielectric of Resistive Memory States in NiO Thin Film. Jpn. J. Appl. Phys. 2005, 44, L1301−L1303. (44) Chien, F. S. S.; Wu, Y. T.; Lai, G. L.; Lai, Y. H. Disproportionation and Comproportionation Reactions of Resistive Switching in Polycrystalline NiOx Films. Appl. Phys. Lett. 2011, 98, No. 153513. (45) Atkinson, A. Transport Processes During the Growth of OxideFilms at Elevated-Temperature. Rev. Mod. Phys. 1985, 57, 437−470. (46) Mitra, S. Investigation of Diffusion Mechanisms During NickelOxide Film Growth by an Isotope-Separation Technique. Mater. Sci. Eng., A 1994, A174, 103−110. (47) Chevalier, S.; Desserrey, F.; Larpin, J. Oxygen Transport During the High Temperature Oxidation of Pure Nickel. Oxid. Met. 2005, 64, 219−234. (48) Lawless, K. R. Oxidation of Metals. Rep. Prog. Phys. 1974, 37, 231−306. (49) Nardi, F.; Ielmini, D.; Cagli, C.; Spiga, S.; Fanciulli, M.; Goux, L.; Wouters, D. J. Control of Filament Size and Reduction of Reset Current Below 10μA in NiO Resistance Switching Memories. SolidState Electron. 2011, 58, 42−47. (50) Russo, U.; Ielmini, D.; Cagli, C.; Lacaita, A. L. Self-Accelerated Thermal Dissolution Model for Reset Programming in Unipolar Resistive-Switching Memory (RRAM) Devices. IEEE Trans. Electron Devices 2009, 56, 193−200. (51) Kokado, S.; Tsunoda, M. Anisotropic Magnetoresistance Effect: General Expression of AMR Ratio and Intuitive Explanation for Sign of AMR Ratio. Adv. Mater. Res. 2013, 750−752, 978−982. (52) Potter, R. I. Magnetoresistance Anisotropy in Ferromagnetic NiCu Alloys. Phys. Rev. B 1974, 10, 4626−4636. (53) Mimura, K.; Ishikawa, Y.; Isshiki, M.; Kato, M. Precise PurityEvaluation of High-Purity Copper by Residual Resistivity Ratio. Mater. Trans., JIM 1997, 38, 714−718. (54) Thummes, G.; Kuckhermann, V.; Mende, H. H. Effect of Sample-Size on the Temperature-Dependence of the Resistivity of Copper Below 4.2 K. J. Phys. F: Met. Phys. 1985, 15, L65−L69. (55) Ziese, M.; Blythe, H. J. Magnetoresistance of Magnetite. J. Phys.: Condens. Matter 2000, 12, 13−28. (56) Sharrock, M. P. Measurement and Interpretation of Magnetic Time Effects in Recording Media. IEEE Trans. Magn. 1999, 35, 4414− 4422. (57) Demkin, V. A. Coercive Force and Electrical Resistivity of Polycrytalline Nickel after Low-Temperature Ultrasonic Fatigue. phys. stat. sol. (a) 1991, 128, 375−382. (58) Guzman, J. M.; Alvarez, N.; Salva, H. R.; Mansilla, M. V.; Gomez, J.; Butera, A. Abnormal Temperature Dependence of the Coercive Field in FePt Thin Films. J. Magn. Magn. Mater. 2013, 347, 61−67. (59) Bran, C.; Espejo, A. P.; Palmero, E. M.; Escrig, J.; Vazquez, M. Angular Dependence of Coercivity with Temperature in Co-based Nanowires. J. Magn. Magn. Mater. 2015, 396, 327−332. (60) Son, J. Y.; Shin, Y. H. Direct Observation of Conducting Filaments on Resistive Switching of NiO Thin Films. Appl. Phys. Lett. 2008, 92, No. 222106. 10845

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846

Research Article

ACS Applied Materials & Interfaces (61) Wada, T.; Yamane, T.; Seki, T.; Nozaki, T.; Suzuki, Y.; Kubota, H.; Fukushima, A.; Yuasa, S.; Maehara, H.; Nagamine, Y.; Tsunekawa, K.; Djayaprawira, D. D.; Watanabe, N. Spin-Transfer-Torque-Induced Rf Oscillations in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions Under a Perpendicular Magnetic Field. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, No. 104410. (62) Wang, S. G.; Ward, R. C. C.; Du, G. X.; Han, X. F.; Wang, C.; Kohn, A. Temperature Dependence of Giant Tunnel Magnetoresistance in Epitaxial Fe/MgO/Fe Magnetic Tunnel Junctions. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, No. 180411. (63) van Gorkom, R. P.; Caro, J.; Klapwijk, T. M.; Radelaar, S. Temperature and Angular Dependence of the Anisotropic Magnetoresistance in Epitaxial Fe Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, No. 134432. (64) Kinoshita, K.; Tamura, T.; Aoki, M.; Sugiyama, Y.; Tanaka, H. Bias Polarity Dependent Data Retention of Resistive Random Access Memory Consisting of Binary Transition Metal Oxide. Appl. Phys. Lett. 2006, 89, No. 103509. (65) Goux, L.; Lisoni, J. G.; Jurczak, M.; Wouters, D. J.; Courtade, L.; Muller, C. Coexistence of the Bipolar and Unipolar ResistiveSwitching Modes in NiO Cells Made by Thermal Oxidation of Ni Layers. J. Appl. Phys. 2010, 107, No. 024512. (66) Mathon, J.; Fraitova, D. Spin Dependent Effective Mass of Conduction Electrons in Ferromagnetic Metals. Phys. Status Solidi B 1965, 9, 97−102. (67) Anderson, J. R.; Papaconstantopoulos, D. A.; Boyer, L. L.; Schirber, J. E. Spin-Polarized Band-Structure Calculations for Ni. Phys. Rev. B: Condens. Matter Mater. Phys. 1979, 20, 3172−3185. (68) Coulson, C. A. Note on the Applicability of the Free-Electron Network Model to Metals. Proc. Phys. Soc., London, Sect. A 1954, A67, 608−614. (69) Tripp, J. H. Effect of Temperature on Fermi Surface of Beryllium. Phys. Rev. B 1970, 1, 550−551.

10846

DOI: 10.1021/acsami.6b16458 ACS Appl. Mater. Interfaces 2017, 9, 10835−10846