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Multilevel Resistance Switching Memory in La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) Heterostructure by Combined Straintronics-Spintronics Weiping Zhou, Yuanqiang Xiong, Zhengming Zhang, Dunhui Wang, Weishi Tan, Qingqi Cao, Zhenghong Qian, and Youwei Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11392 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 12, 2016

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ACS Applied Materials & Interfaces

Multilevel Resistance Switching Memory in La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) Heterostructure by Combined StraintronicsSpintronics Weiping Zhou,†,‡,∥Yuanqiang Xiong, ‡,∥ Zhengming Zhang,§ Dunhui Wang,*, ‡,∥ Weishi Tan, † Qingqi Cao, ‡,∥ Zhenghong Qian,⊥ and Youwei Du‡,∥

†Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China ‡National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China ∥Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China §School of Physical and Mathematical Science, Nanjing Tech University, Nanjing 211816, People’s Republic of China

⊥Center for Integrated Spintronic Devices (CISD), Hangzhou Dianzi University, Hangzhou, 310018, People’s Republic of China

ACS Paragon Plus Environment

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KEYWORDS: multilevel memory, straintronics, spintronics, non-volatile, resistance switching, heterostructure

ABSTRACT: We demonstrate a memory device with multi-fields switchable multilevel states at room

temperature

based

on

the

integration

of

straintronics

and

spintronics

in

La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) (011) heterostructure. By precisely controlling the electric field applied on PMN-PT substrate, multiple non-volatile resistance states can be generated in La2/3Ba1/3MnO3 film which can be ascribed to the strain-modulated metalinsulator transition and phase separation of manganite. Furthermore, due to the strong coupling between spin and charge degrees of freedom, the resistance of La2/3Ba1/3MnO3 film can be readily modulated by magnetic field over a broad temperature range. Therefore, by combining electroresistance and magnetoresistance effects, multilevel resistance states with excellent retention and endurance properties can be achieved at room temperature with the coactions of electric and magnetic fields. The incorporation of ferroelastic strain, magnetic and resistive properties in memory cell suggests a promising approach for multistate, high density and lowpower consumption electronic memory devices.

1. INTRODUCTION With the rapid progress in information technology, the pursuit of high density and multistate data storage memory has attracted ever-increasing attention. To satisfy the demand of increasing storage capacity, multilevel storage of at least three states is an effective method to store more data into each single memory cell, thus significantly enhancing the data density without reducing


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the feature dimensions.1 On the other hand, storage device with memory cells responses to multiple physical stimuli would benefit for multifunctional applications. Among these storage techniques, resistive switching is a fascinating route to realize multilevel states through controlling the formation and rupture of conductive filaments, which has been achieved in various oxide-based systems.2-4 However, due to the random nature of the conductive filaments formation mechanism, resistive switching memories suffer from issue of reliability.3,5 Besides, most of these resistive switching devices are only operated by single electrical stimulus, limiting their multifunction and storage capacity.6,7 Another promising strategy to construct four states memory is multiferroic tunneling junctions which are composed of ferromagnetic electrodes separated by ferroelectric tunnel barrier.8,9 However, these multilevel resistance states switched by electric and magnetic fields mostly operate at low temperature, restricting the practical device applications.10-12 Nevertheless, the concept of combination magnetic and electric properties inspires us to explore alternative avenues for achieving multilevel and multifunctional data storage in multiferroic materials. In magnetoelectric composites consisting of ferromagnetic and piezoelectric elements, strain which can be electrically controlled is extensively explored for realizing converse magnetoelectric coupling.13-17 Furthermore, by fabricating various kinds of functional films on piezoelectric substrates, the role of strain has been extended as a knob to manipulate various physical parameters through electric field.18-22 This research field is termed as “straintronics” and proposed for potential applications in low power consumption logic and memory devices.23-26 One of the most important issue in straintronics is to manipulate the resistance state of the functional film on piezoelectric substrate via electric-field-induced strain, which has been demonstrated in several strong correlated oxide systems.19,25,27-30 However, most of these


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researches are focused on the resistance switching without magnetic field response. At the same time, we have noticed that the resistance of some strong correlated materials can be modulated by magnetic field as well. Thus, it is worth looking forward to the integration of these effects in one system for constructing storage device with multi-fields switchable multilevel states. La1xBaxMnO3

belongs to the family of strong correlated manganites with intrinsic coupling between

lattice, charge, orbital, and spin degrees of freedom, exhibiting an abundant of physical properties such as high Curie temperature, metal-insulator transition and colossal magnetoresistance effect.31-33 Moreover, it is reported that the magnetic and transport properties of La1-xBaxMnO3 films are very sensitive to epitaxial strain via deposited on different substrates.34 Among this series compounds, La2/3Ba1/3MnO3 (LBMO) exhibits the highest Curie temperature and large magnetoresistance effect at room temperature.35 Based on these properties, it is fascinating to choose LBMO as the functional film for investigating magnetic and electric fields co-modulated resistance to achieve multistate memory device at room temperature. Another key issue in straintronics is how to retain the strain state after removing the electric field.

Here,

(011)-oriented

0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3

(PMN-PT) is

selected

as

piezoelectric substrate for its electric field controllable reversible and non-volatile strain.36 In this paper, multilevel resistance states with electric and magnetic fields excitation have been demonstrated at room temperature due to the combination of electroresistance and magnetoresistance effects, which paves the way for novel high density hybrid straintronicsspintronics memory devices.

2. EXPERIMENTAL SECTION


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The LBMO film was fabricated on (011)-oriented PMN-PT single crystal substrate with dimensions of 3 mm [01-1]×5 mm [100]×0.5 mm [011] by pulsed laser deposition method. During growth, the substrate temperature was kept at 750 ℃, the background oxygen pressure was 25 Pa and the laser repetition rate was 3 Hz. After deposition, the film was cooled down slowly in oxygen pressure of 4×104 Pa to reduce possible oxygen deficiency. X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements were performed to characterize the structure and surface topography of the film. Scanning electron microscope (SEM) study was conducted to determine the film thickness. The transport property was measured by standard four probes method in a closed cycled helium temperature control system. The in-situ electric field was applied along the thickness direction of the PMN-PT substrate by a source meter (Keithley, model 2410).

3. RESULTS AND DISCCUSION


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Figure 1. (a) XRD pattern of LBMO/PMN-PT (011) heterostructure measured at room temperature. (b) The schematic diagram of LBMO film for the electric field control of resistance measurements. (c) The AFM image of LBMO film with an area of 3×3 µm2. (d) The crosssection SEM image of LBMO/PMN-PT (011) heterostructure. The XRD θ-2θ scan of LBMO/PMN-PT (011) heterostructure is shown in Figure 1a. It is clear that only the diffraction peaks of substrate and film can be observed, suggesting the single phase and highly (011)-oriented nature of the film. The out-of-plane lattice constant of LBMO film calculated from these (011) diffraction peaks is determined to be ~2.743 Å which is smaller than the bulk value (~2.761 Å), indicating that the film is subjected to a compressive strain (-0.65%) along the out-of-plane direction. This result is consistent with the larger lattice parameter of the PMN-PT substrate (~4.02 Å) compared with that of LBMO bulk (~3.91 Å).37 Figure 1b presents the schematic diagram of the device for resistance measurements with in-situ electric field applied along the thickness direction of PMN-PT substrate and magnetic field applied in the film (011) plane. The PMN-PT substrate was first poled under electric field of 8 kV/cm for ten minutes at 300 K before conducting the electric field control of resistance measurements. The AFM image shown in Figure 1c displays a smooth surface with the root mean square roughness about 3.35 nm. Figure 1d shows the cross-section SEM image of LBMO/PMN-PT (011) heterostructure, which reveals the uniform thickness (about 115 nm) of the film and a clear interface.


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Figure 2. (a) The relative modulation of resistance as a function of symmetric bipolar electric field at room temperature. (b) The relative modulation of resistance as a function of asymmetric bipolar electric fields with different positive amplitudes at room temperature. Figure 2a demonstrates the modulation of resistance as a function of sweeping electric fields for LBMO film at 300 K. By applying a triangular electric field with amplitude of 8 kV/cm on PMN-PT substrate, the resistance of LBMO film displays a butterfly-like loop with respect to electric field, which is similar to the shape of strain vs. electric field loop observed in PMN-PT substrate,38 indicating that the modulation of resistance in LBMO film can be mainly ascribed to the electric-field-induced strain. The maximum modulation of resistance is observed at the coercive field 1.7 kV/cm with a relative enhancement of 21.4%, which is defined as ∆R / R = [ R ( E ) − R ( E = −8 　 kV/ cm)] / R ( E = −8 　 kV/ cm) . This value is larger than that reported

in other systems at room temperature,19,28-30 suggesting that LBMO film is extremely sensitive to strain. Remnant resistance cannot be obtained in this symmetric bipolar electric field case due to the 180˚ ferroelectric domain switching, which results in zero remnant strain. However, as shown in Figure 2b, by cycling an asymmetric bipolar electric field with positive amplitude of 1.5 kV/cm (less than the coercive field) on the PMN-PT substrate, a hysteresis-like loop of the


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resistance vs. electric field is obtained. It is obvious that the resistance does not return back to its initial state when the electric field is back to zero unless a negative electric field is applied, indicating the non-volatile switching in LBMO/PMN-PT (011) structure. In this scenario, two distinct resistance states A and D, which are denoted as the low resistance state and the high resistance state at zero electric field, respectively, are observed in the device. Moreover, by adjusting the positive amplitude (1.3 kV/cm and 1.4 kV/cm) of the asymmetric bipolar electric fields, other hysteresis loops of ∆ R / R vs. electric field with intermediate remnant resistance states B and C can be obtained. Base on this result, we demonstrate that the resistance of LBMO film can be reversibly switched between these four non-volatile states through the application of appropriate electric field pulses to the device, as shown in Figure 3. When an electric field pulse of 1.5 kV/cm is applied to the device, the resistance state of LBMO film is switched from A to D. It is worth noting that the D state can keep stable with the switch off the electric field until a negative electric field pulse of -4 kV/cm is applied. By modulating the amplitude of positive electric field pulses to 1.3 kV/cm or 1.4 kV/cm without varying the negative electric field pulses, the resistance states B and C of LBMO film can be generated. Therefore, the resistance states stored in this structure are non-volatile and can be read out in non-destructive manner, which would have potential application in designing low power consuming memory devices.19,26,28,30


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Figure 3. The non-volatile switching of LBMO film resistance between A, B, C and D four states under the electric field pulses at 300 K.

Figure 4. (a) The resistance of LBMO film as a function of temperature under two remnant strain states and the temperature dependence of relative resistance modulation between those two states. (b) The strain of PMN-PT (011) substrate as a function of asymmetric bipolar electric field with different maximum positive amplitude. The inset illustrates the ferroelastic domain states in remnant strain states A and D where ferroelectric polarization points to the out-of-plane direction and ferroelectric polarization stays in-plane, respectively.


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It is known that LBMO exhibits metal-insulator transition near the Curie temperature due to the coexistence and competition between ferromagnetic metallic phase and paramagnetic insulating phase, and the phase separation is very sensitive to external perturbations such as magnetic field and epitaxial strain.33,34 In order to investigate the electric-field-controlled strain impact on the metal-insulator transition behavior, the resistance of LBMO film as a function of temperature in two remnant strain states of PMN-PT substrate, which correspond to two remnant resistance states A and D in Figure 3, has been measured. As shown in Figure 4a, obviously, these two curves are separated in the measured temperature range and both of them display a metal-insulator transition behavior. Moreover, the metal-insulator transition temperature of LBMO film can be notably increased about 6 K when the strain state of PMN-PT substrate is transformed from state D to A by an electric field. While the relative resistance modulation between these non-volatile states calculated by ∆R / RA = ( RD − RA ) / RA ×100% is increased with decreasing temperature, exhibiting large tunability in a wide temperature range. The straininduced modulation of transport property in LBMO/PMN-PT (011) heterostructure can be understood in the scenario of Jahn-Teller effect and phase separation. By switching the strain state of PMN-PT substrate from A to D by an electric field, an in-plane tensile strain is generated due to the 71˚/109˚ ferroelastic domain switching,28,36 and the strain is transferred to the LBMO film across the interface. It is reported that the in-plane tensile strain in LBMO film would enhance the Jahn-Teller electron-lattice coupling strength and localize the conducting eg electrons, which favors the paramagnetic insulating phase and suppresses the ferromagnetic metallic phase.39,40 Therefore, the resistance in state A is decreased compared with that in state D for the reduction of electron-lattice coupling strength and promotion of the conduction of eg electrons.


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It is reported that the rhombohedral phase of PMN-PT single crystal has eight spontaneous ferroelectric polarization directions along the orientations.29,36 For (011)-cut single crystal, four of them point to the out-of-plane direction and the others are in-plane.36 To understand the observed non-volatile resistance modulation in this structure, the in-plane strain of PMN-PT substrate as a function of electric field has been measured. As shown in Figure 4b, when an asymmetric bipolar electric field is swept from zero to 1.5 kV/cm on the negatively poled PMN-PT substrate, the polarization undergoes 71˚/109˚ domain switching with the vectors changing from two downward directions to four possible in-plane directions, leading to a significant in-plane strain which can be remained if the electric field is back to zero. Afterwards, when the electric field is decreased from zero to -4 kV/cm, the in-plane polarization rotates back to the downward directions, and the strain recovers to its initial state once the electric field is removed.29,36 As a result, a hysteresis-like loop of strain vs. electric field with remnant strain states D and A can be observed. Moreover, by adjusting the maximum positive electric field amplitude to 1.3 kV/cm, a minor strain vs. electric field loop with an intermediated remnant strain can be obtained, indicating that the proportion of the 71˚/109˚ switched ferroelastic domain, i.e., the remnant strain value, can be controlled by applying appropriate electric field to the substrate.36 These strain curves are similar with that resistance curves present in Figure 2b, indicating that the observed non-volatile resistance states can be attributed to the evolution of the ferroelectric/ferroelastic domain structure induced non-volatile strain in PMN-PT substrate through modification of Jahn-Teller electron-lattice coupling.


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Figure 5. (a) The magnetic field dependence of resistance under two remnant strain states A and D at temperature of 300 K. (b) The temperature dependence of magnetoresistance value under magnetic field of 1 T in these two remnant strain states A and D. Besides response to electric field, the LBMO film resistance can be readily modulated by magnetic field due to the strong interplay between spin and charge degrees of freedom. It is reported that the magnetoresistance effect observed in manganite films is closely related with the phase separation phenomenon, i.e., the coexistence of ferromagnetic metallic phase and paramagnetic insulating phase.41 The magnetic field would alter the subtle balance between these two electronic phases, resulting in a maximum magnetoresistance value near Curie temperature due to the strong competition. Figure 5a shows the magnetic field dependence of resistance for LBMO film under two remnant strain states of PMN-PT substrate at temperature of 300 K. It is clear that the resistance of LBMO film at remnant strain states A and D are distinctly apart from each other and both of them are suppressed with increasing magnetic field. The value of magnetoresistance MR = [ R ( H ) − R (0)] / R (0) × 100% under magnetic field of 1 T at room temperature is as large as -13%. Furthermore, the temperature dependence of magnetoresistance under magnetic field of 1 T in these two remnant strain states has been measured. As shown in


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Figure 5b, LBMO film exhibits large magnetoresistance over a broad temperature range including room temperature, which would be favorable for spintronics device. It is interesting to notice that the magnetoresistance value in strain state D is larger compared with that in state A and the enhancement is most obvious near the metal-insulator transition temperature, demonstrating that strain has an impact on the magnetoresistance effect of LBMO film. The strain modulated magnetoresistance effect can be ascribed to the strain induced modification of phase separation and phase competition in LBMO film. Since the LBMO film suffers a tensile strain in state D, the volume fraction of paramagnetic insulating phase is increased at the expense of ferromagnetic metallic phase, which would promote the MR because of the enhanced phase competition.39

Figure 6. (a) The resistance as a function of symmetric bipolar electric field measured under magnetic field of 0 and 1 T. (b) The resistance as a function of asymmetric bipolar electric fields with different positive amplitudes measured at magnetic field of 0 and 1 T. Due to the coexistence of electroresistance and magnetoresistance effects in LBMO/PMN-PT (011) heterostructure, it is reasonable to integrate these effects for achieving multilevel resistance states, which can be switched by electric and magnetic fields. Figure 6a shows the resistance of


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LBMO film as a function of symmetric bipolar electric field (R-E) under magnetic fields of 0 and 1 T at 300 K. With the application of magnetic field of 1 T, the butterfly-like R-E curve dramatically shifts down and distinctly separates with that under zero magnetic field. As shown in Figure 6a, the resistance in LBMO film can be simultaneously manipulated by the magnetic and electric fields. In the asymmetric bipolar electric fields scenario, as shown in Figure 6b, A, B and D three remnant resistance states in the hysteresis-like R-E curves demonstrate for their well distinguished values under zero magnetic field. However, with the application of magnetic field of 1 T, R-E curves move downwards and these resistance states transform into A′, B′ and D′ three states at zero electric field. As a result, six distinct resistance states have been constructed under the combined action of magnetic and electric fields in LBMO/PMN-PT (011) heterostructure at room temperature. Accordingly, the coactions of electric field pulses and magnetic field to reversibly switch these six resistance states of the device have been demonstrated. As shown in Figure 7, the switching characteristic is similar with that exhibit in Figure 3 under zero magnetic field. However, with the application of magnetic field of 1 T, the resistance state of LBMO film shifts from A to A′. Keeping this magnetic field unchanged, a sequence of electric field pulses with amplitude of 1.5 kV/cm (1.3 kV/cm) and -4 kV/cm are applied to the heterostructure, then the resistance state of LBMO film can be switched between D′ (B′) and A′ correspondingly.


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Figure 7. The multi-field parallel switching of LBMO film resistance between different states under the coactions of electric field pulses and magnetic field at 300 K.

Figure 8. (a) The endurance characteristic of the six resistance states for LBMO/PMN-PT (011) heterostructure as a function of switching cycles. (b) The retention characteristic of these states for LBMO/PMN-PT (011) heterostructure as a function of time. All the resistance measurements are performed at 300 K.


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From the practical applications point of view, the reliability and stability of resistance states are very important factors for a memory device to maintain its functionality. Here, the endurance and retention properties of the LBMO/PMN-PT (011) structure have been characterized to test its possibility in device application. Cycled electric field pulses sequence of -4 kV/cm, 1.5 kV/cm, -4 kV/cm and 1.3 kV/cm are alternately applied to the PMN-PT substrate, and the resistance of LBMO film is measured between two neighboring pulses. A, B and D states are measured under zero magnetic field while A′, B′ and D′ states are measured under magnetic field of 1 T. The switching endurance characteristic of the device is shown in Figure 8a. It is noted that the values of these six states are quite stable and can be maintained up to 3×103 cycles without noticeable degradation. Figure 8b displays the retention property of the LBMO/PMN-PT device, which is obtained by setting the PMN-PT substrate into different remnant strain states and acquiring the time dependence of resistance of LBMO film. It is clear that no obvious decay of these resistance values can be observed as long as 105 s. Compared with traditional resistive switching in metal-insulator-metal structure with generally mechanism of filamentary conduction,42,43 the resistance switching in LBMO/PMNPT (011) heterostructure exhibits excellent endurance and retention properties, which would be favorable for applications in multilevel memory devices. These results demonstrating in LBMO/PMNPT heterostructure suggest novel potential application in multifunction, high-density multilevel integrated memory devices, which can be miniaturized by arranging memory cell into crossbar array type architectures through nanofabrication technology, similar with that proposed in resistive random access memory (RRAM).44 The magnetic field used in the device as the operation stimuli to increase the stored states in one memory cell would be beneficial to multilevel storage device. Due to the half-select


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problem caused by stray magnetic field,45 the tradeoff between the memory cell size and magnetic field magnitude should be optimized to obtain high density memory device. Therefore, the incorporation of ferroelastic strain, magnetic and resistive properties in our memory cell suggests an alternative promising approach for multistate, high density electronic memory devices.

4. SUMMARY In summary, the multi-fields manipulation of transport property in LBMO/PMN-PT (011) heterostructure has been investigated. It is demonstrated that distinct, reversible and non-volatile resistance states in LBMO film can be obtained by the application of asymmetric bipolar electric field across the PMN-PT substrate. The variation of the metal-insulator transition temperature up to 6 K is realized through switching between two non-volatile strain states by electric field. Large magnetoresistance effect in a wide temperature range with room temperature MR value of -13% is observed in LBMO film. Furthermore, by combining electroresistance and magnetoresistance effects in LBMO/PMN-PT (011) heterostructure, electric and magnetic fields co-controllable multiple resistance states with excellent endurance and retention properties have been achieve at room temperature. These results suggest that our work would have potential applications in multifunctional and high density electronic devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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Author Contributions W.P.Z. carried out the samples preparation and most of the measurements. W.P.Z. and D.H.W. designed the outline of the manuscript and wrote the main manuscript text. All authors contributed to the manuscript with discussions and revisions. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Nos. 2014AA032904 and 2012CB932304). REFERENCES (1) Rozenberg, M. J.; Inoue, I. H.; Sanchez, M. J. Nonvolatile Memory with Multilevel Switching: a Basic Model. Phys. Rev. Lett. 2004, 92, 178302. (2) Sawa, A. Resistive Switching in Transition Metal Oxides. Mater. Today 2008, 11, 28-36. (3) Kim, K. M.; Lee, S. R.; Kim, S.; Chang, M.; Hwang, C. S. Self-Limited Switching in Ta2O5/TaOx Memristors Exhibiting Uniform Multilevel Changes in Resistance. Adv. Funct. Mater. 2015, 25, 1527-1534. (4) Balatti, S.; Larentis, S.; Gilmer, D. C.; Ielmini, D. Multiple Memory States in Resistive Switching Devices through Controlled Size and Orientation of the Conductive Filament. Adv. Mater. 2013, 25, 1474-1478.


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(45) June W. L.; Justin M. S. Magnetic Nanostructures for Advanced Technologies: Fabrication, Metrology and Challenges. J. Phys. D: Appl. Phys. 2011, 44, 303001.


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Table of Contents Graphic


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Figure 2 - American Chemical Society

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