Resistive Switching and the Local Electric Field in Bi0.85-x

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Article 0.85-

Resistive Switching and the Local Electric Field in the Bi Pr REFe Mn O/CuFeO (RE=Sr, Dy) Bilayered Thin Films x

0.15

x

0.97

0.03

3

2

4

Zhongwei Yue, Guoqiang Tan, Huijun Ren, Ao Xia, Dan Shao, Meiyou Guo, Wei Yang, and Zhengjun Chai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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ACS Applied Materials & Interfaces

Resistive

Switching

and

the

Local

Electric

Field

in

the

Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4 (RE=Sr, Dy) Bilayered Thin Films

Zhongwei Yuea, Guoqiang Tana*, Huijun Renb, Ao Xiaa, Dan Shaoa, Meiyou Guoa, Wei Yanga, Zhengjun Chaia

a

School of Materials Science and Engineering, Shaanxi University of

Science & Technology, Xi’an, Shaanxi 710021, China b

School of Arts and Sciences, Shaanxi University of Science &

Technology, Xi’an, Shaanxi 710021, China *Email address: [email protected] (G. Q. Tan)

ABSTRACT:

Bilayer

Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4

(BPRExFMO/CuFO, RE=Sr, Dy) thin films were prepared onto FTO/glass substrates by the chemical solution deposition method. The structure transition doesn’t appear after ions doping, which is confirmed by XRD and its refined results. The samples remain the trigonal R3c: H structure of BFO phase and tetragonal I41/amd structure of CuFO phase. The asymmetric character of leakage current density curves and resistive switching effects have been explored. And the ions substitution impacts on the resistive switching effects may be due to the existence of the local fields. Under the applied electric field, carries are accumulated and arranged directionally at the interface between the BFO and CuFO layers to form the local electric field. Such local field is affected by ions dopants, and the field compensates or weakens the applied electrical field. The reinforced or weakened resistive behavior is depended on the directions of the local field and injection of electrons. The polarization switching

currents

of

the

BPFMO/CuFO,

BPSFMO/CuFO,

and

BPDFMO/CuFO samples, whose distribution fields are proportional to the local electrical fields, are 0.0070, 0.0049 and 0.0074 A under the positive applied field, respectively. And the remanent polarization is increased to 74.4, 73.5 and

ACS Paragon Plus Environment


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84.3 µC/cm2 of the doped samples respectively. KEYWORDS: BiFeO3/CuFe2O4, bilayered thin films, resistive switching, local electric field, ferroelectric 1. INTRODUCTION BiFeO3 (BFO) is one of lead-free multiferroic materials with a high Curie point (TC~1100K) and a high Néel point (TN~630K)1, which possesses the ferroelectric and ferromagnetic orderings at room temperature simultaneously, and is a potential material for new generation applications, such as sensors, ferroelectric random access memories and microelectromechanical systems2. However, undesired ferroelectric polarization and weak ferromagnetism of the prepared pure BiFeO3 have been revealed3-5. Several methods, such as optimizing the preparation techniques6-8, ion substitutions5,

9-15

and the

formation of composite16-19, have been applied to solve those deficiencies and to promote the practical application. But as the previous reports20-21, the discouraging ferromagnetism was obtained along with excellent ferroelectricity of the BFO films via ion substitution. Some studies22-24 about the combinations of the ion substitutions and formation of heterostructure to improve the ferroelectricity and ferromagnetism simultaneously have been reported. CuFe2O4 (CuFO) is one of lead-free semiconductor materials with favorable magnetism, which has been studied in the photocatalysis25-27. Few studies are available on thin films28-29, especially for the multiferroic thin films. Multiferroic properties of bilayer thin films of ions-doped BFO/CuFO thin films are rarely reported. Moreover, BFO thin films which possess giant remnant polarization and room-temperature magnetization usually exhibit p-type conduction as a result of Bi volatilization30-32. It is an ideal candidate for resistance random access memories

(ReRAMs)

with

the

metal-ferroelectric-semiconductor-metal

structure2. The ReRAMs based on resistive switching (RS) have attracted considerable attention because of its advantages in high density and operational speed30, 33. The RS effect which results from polarization reversal


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in ferroelectric materials shows the low and high conductive states. So the positive or negative polarization charge at the ferroelectric-semiconductor interface is induced by two opposite states33. At the interface, the carries are accumulated or depleted, resulting in different local conductivities34. As a semiconductor material, CuFO can be combined with BFO to form ferroelectric-semiconductor heterojunction, resulting in less efficient screening effects of the bound charges in semiconductors, because of lower concentration of carriers35. Consequently, the energy band bending is induced at the interface. The effect could change the potential profile, which helps to obtain a high or low RS state35. Therefore it would be of considerable interest to establish ions-doped BFO/CuFO thin films. In

this

paper,

we

prepared

Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4

bilayer

thin

(BPRExFMO/CuFO,

films

of

RE=Sr,

Dy)

based on the favorable results of our previous work20-21. The influence of Pr, Sr, Dy and Mn doping the crystal, the leakage current density and multiferroic properies of the thin films were investigated. We also studied the RS effects on the ions-doped BFO/CuFO ferroelectric semiconductor heterojunctions, which has

not

been

studied

hitherto

in

those

thin

films

with

metal-ferroelectric-semiconductor-metal structure. And the resistive hysteresis behavior, the large remanent polarization and the improved ferromagnetic properties were observed. 2. EXPERIMENTAL SECTION 2.1. Preparation Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4 (BPRExFMO/CuFO, RE=Sr, Dy) layered thin films were deposited onto FTO/glass substrates by chemical solution deposition method. The CuFe2O4 (CuFO) precursor solution with 0.2 mol/L was prepared by dissolving Cu(NO3)23H2O and Fe(NO3)39H2O (Cu: Fe=1: 2.2) in the mixture of 2-methoxyethanol and acetic anhydride (3: 1 in volume). The substrate was irradiated with ultraviolet light to decrease the contact angle before deposition. The CFO films were deposited by spin coating



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at 4000 rpm for 15 s, preheated at 180 ℃ for 10 min and annealed at 620 ℃ in a furnace for 20 min to crystallize. These steps were repeated several times to get CuFO films with desired thickness. Similarly, Bi(NO3)35H2O, Pr(NO3)36H2O,

Sr(NO3)2,

Dy(NO3)36H2O,

C4H6MnO44H2O

and

Fe(NO3)39H2O were used as the original materials to obtain the Bi0.85Pr0.15Fe0.97Mn0.03O3 (BPFMO), Bi0.81Pr0.15Dy0.04Fe0.97Mn0.03O3 (BPDFMO), Bi0.83Pr0.15Sr0.02Fe0.97Mn0.03O3 (BPSFMO) precursor solutions (0.3 mol/L). They were synthesized on the top of CuFO thin films by using the same process, but the annealing parameters were 550 ℃ for 10 min. Finally, the BPFMO/CuFO, BPDFMO/CuFO and BPSFMO/CuFO thin films were obtained. 2.2. Characterization The crystalline structure of thin films was analyzed by using D/max-2200 X-ray diffractometer (grazing incidence mode), and the data were refined by a Maud program. The dielectric measurements were carried using an Agilent E4980A concise LCR meter. An Agilent B2900 was adopted to obtain the leakage current densities of the thin films, and the ferroelectric measurements were performed using a Ferroelectric Analyzer (TF2000). The magnetic hysteresis loops were acquired by using the superconducting quantum interference magnetic measuring system (SQUID MPMS-XL-7). 3. RESULTS and DISCUSSION Fig. 1 (a) plots the XRD patterns of the BFO/CuFO, BPFMO/CuFO, BPDFMO/CuFO and BPSFMO/CuFO bilayer thin films. Both BFO phase and CuFO phase are found to be polycrystalline in nature, and no impurities were detected. For comparison, the partially magnified patterns around 2θ=32°, 36°, 39°, 57° of all films are also presented in Fig. 1 (b) to observe the variation of thin films’ diffraction peaks clearly. Some changes of diffraction peaks occur along with the introduction of ions into BFO/CuFO thin films. The doublets (104)/(110), (006)/(202) and (018)/(300) peaks are clearly separated in BFO/CuFO. Then, the doubly-split peaks merge to form the single (110), (202) and (300) peaks in the co-doped thin films. This phenomenon indicates that


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the substitution induces structural distortion due to the different ionic radii between Pr3+, Dy3+, Sr2+ and Bi3+ ions, Mn2+ and Fe3+ ions7. At the same time, all the peaks of BFO phase are slightly shifted toward a higher angle with doping different ions, which can be attributed to the decreased volume of cells according to the results in Table 1. In addition, the (012) peak presents various change. The same phenomenon was observed in other researches4, 10, 36-37, it indicated that the peak intensity and the crystal orientation can be influenced by the different ions doping. Thus the decreased (012) peak intensity of BPDFMO/CFO and BPSFMO/CFO by comparison with BPFMO/CuFO film can be attributed to the suppression of Dy and Sr ions doping.

Fig. 1 XRD patterns (a), partially magnified patterns (b) and Rietveld analyses of X-ray diffraction data (c) of BPRExFMO/CuFO (RE=Sr, Dy) composite films. In order to confirm the structure evolution of doped thin films, the Rietveld refinement of the XRD patterns of BPRExFMO/CuFO (RE=Sr, Dy) composite



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films has been performed using Maud software, and the results are plotted in Fig. 1 (c). From the refined results, it can be determined that all the CuFO phases are tetragonal I41/amd structure while the BFO phases are trigonal R3c: H structure. The detailed Rietveld refinement parameters of the films are listed in Table 1. That means the structure of the doped thin films doesn’t change, but only the structure distortion appears. This distortion can influence the ferroelectric properties and ferromagnetic properties38. Table 1 Lattice parameters of BPRExFMO/CuFO (RE=Sr, Dy) composite films. Lattice parameters Samples

Structures

Space Groups

Volume

Rw (%)

a (Å)

c (Å)

s (Å3)

Trigonal(BFO)

R3c:H(89%)

5.5713

13.8453

372.17

Tetragonal(CuFO)

I41/amd(11%)

5.8100

8.7100

294.02

Trigonal(BFO)

R3c: H(79%)

5.5777

13.7518

370.51

Tetragonal(CuFO)

I41(21%)

5.8197

8.7888

297.67

Trigonal(BFO)

R3c: H(85%)

5.5561

13.7317

367.11

Tetragonal(CuFO)

I41/amd(15%)

5.8246

8.7640

297.33

Trigonal(BFO)

R3c: H(81%)

5.5723

13.7553

369.89

Tetragonal(CuFO)

I41/amd(19%)

5.8167

8.6844

293.83

BFO/CuFO

9.95

BPFMO/CuFO

8.03

9.02

BPDFMO/CuFO

BPSFMO/CuFO

9.66

Fig. 2 (a) shows the frequency dependent dielectric constants of Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4 (BPRExFMO/CuFO, RE=Sr, Dy) thin films. The dielectric dispersion is exhibited by all the samples with the frequency below 100 kHz and the constants of the doped thin films are obviously lower than that of BFO/CuFO at this frequency region. It is mainly because of space charge polarization39-40. In case of BFO, various defects ( VO•• , VBi′′′ etc) are induced by the volatilization of Bi. At lower frequency, the bigger dielectric constant is attributed to the migration of charge carries in the direction of the electric field up to grain boundaries, where the charge carries are accumulated, making contribution to polarization. But at higher frequency, the dielectric constants of all the thin films tend to be definite values. The


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values have a small difference because the space charges have no enough time to respond to the electric field change, which causes the decrease of probability to reach grain boundaries, leaving a little contribution to the constants. From the frequency spectra of dielectric loss (Fig. 2(b)), the dielectric loss peaks exist in all the thin films. A dielectric loss peak appears in the BFO/CuFO film around 10 kHz while that are present in the doped thin films at the frequency of 1~2 kHz. When the frequency gets close to 1 MHz, the loss values tend to be increased. Those features are different from those of the single layer BFO thin films that have been reported20, 28. Compared with the frequency spectra of the dielectric constants, the frequency of the loss peaks is consistent with that of the drastically decreased dielectric constant. This phenomenon observed in the report about Bi0.8La0.2FeO3/CoFe2O4 bilayered thin films41 is a kind of dielectric relaxation and can be classified as Wagner-Maxwell dielectric relaxation42. The observed dielectric relaxation can be attributed to different resistivity and permittivity of the layers involved42.

Fig. 2 Dependence of dielectric constants (a) and dielectric loss (b) with frequency of Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4 (BPRExFMO/CuFO, RE=Sr, Dy) composite films. The leakage current densities of Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4



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(BPRExFMO/CuFO, RE=Sr, Dy) composite films are illustrated in Fig. 3. At -200 kV/cm bias electric field, the leakage current densities are 3.08×10-2 A/cm2, 3.45×10-4 A/cm2, 1.21×10-4 A/cm2 and 5.64×10-4 A/cm2, respectively. By comparing with the BFO/CuFO thin films, it’s obvious that the current densities of BPFMO/CuFO, BPDFMO/CuFO and BPSFMO/CuFO thin films are two orders of magnitude lower than those of undoped thin films under negative field. It is clear that all the curves reveal asymmetric feature under positive and negative fields, which obviously resembles those in the reports about bilayered thin films2, 35, 43. In addition, the minimum values of the leakage current show up at the negative bias electric field instead of zero electric field. Therefore, the above mentioned phenomena of current densities may be attributed to the introduction of CuFO and the formation of the interface between CuFO and BFO layers, and it can be inferred that the dipoles reversal and migration of carriers are different in the different electric field directions.

Fig. 3 The leakage current densities of BPRExFMO/CuFO (RE=Sr, Dy) composite films. To explain the above features of currents, the sweep dc voltage is applied to measure the leakage current, and the curves are shown in Fig. 4. As can be seen, the leakage currents density curves of all the samples don’t overlap under the electric fields with different directions. It illustrates that there are


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distinct RS behaviors in all the bilayered thin films in this system, which facilitates the thin films to perform different resistive states under different bias electric fields. And the similar phenomenon also occurs in some reports2, 28, 34. Under the negative applied field, the status of high and low RS (HRS and LRS) of the synthesized thin films can be expressed as RS ratio (RHRS/RLRS) which depends on the leakage currents, as shown in Fig. 5. The maximum ratio of the undoped BFO/CuFO thin film is 158.87, and it decreases to 2.07, 2.35 and 2.38 respectively which are 76.75, 67.60 and 66.75 times lower than that BFO/CuFO film. It may be related to the local electrical field which relates to the variation of carries (including the concentrations and the species) induced by ions substitution, affecting the injection of electrons which brings about great change of ratio values.

Fig. 4 The leakage current densities of BFO/CuFO (a), BPFMO/CuFO (b), BPDFMO/CuFO (c) and BPSFMO/CuFO (d) composite films with cyclic voltage.



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Fig. 5 RS ratio as a function of negative field of (a) BFO/CuFO and (b) BPRExFMO/CuFO (RE=Sr, Dy) thin films. The polarization of paraelectric phase CuFO is unable to switch under the applied field28. And the interface polarization coupling between the nonswitchable polarization of CuFO phase and the switchable polarization of BFO phase exists in the bilayered thin films. Thus the exchange behavior of BFO ferroelectric polarization charges will be influenced by the feature of CuFO. Moreover, BFO has plenty of oxygen vacancies for its p-type conduction43. When the voltage is changed from negative to positive, the dipoles are turned into the directional arrangement under the negative electric field, as seen in Fig. 4 and Fig. 6 (a). The electrons are injected into BFO and combine with oxygen vacancies to facilitate the injection of electrons. At the same time, due to the formation of the interface, the positive carries are accumulated at the side close to BFO. It is in favor of the migration of electrons, which weakens the resistive behavior under the negative electric field. At the moment of loading positive electric field, dipoles and carries complexes in the interface still remain this state (see Fig. 6 (b)). So the injection of electrons are blocked, increasing the potential barrier43. But with the injection of electrons, dipoles in BFO and the local electric field are switched to the opposite direction, and the potential barrier decreases to certain extents (seen in Fig. 6 (c)). Hence the leakage currents are reduced as a consequence of highly resistive behavior. Meanwhile, the minimum values of currents in all the specimens don’t


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appear at zero electric field and the same phenomenon can also be observed by the other authors33, 44. The electric fields of minimum leakage current under different direction electric fields are -18.49/17.36 kV/cm, -34.41/32.07 kV/cm, -43.50/43.20 kV/cm and -42.49/41.64kV/cm. The values with little change are considered to be the same at the opposite electric field. So it is deduced that the interface is built up between BFO and CuFO layers and the local electric field exists in the depletion region. The carries, such as charges, charged defects and defects complexes, are accumulated at the interface45 to form the local electric field in the depletion region46, as shown in Fig. 6 (a). When the applied field is changed from negative value to positive value, the local electric field is weakened in the applied field, revealing a negative offset practical field due to the direction of the local electric field being opposite to the applied field. While the field varies from positive value to negative value, the lowest point is shifted toward the negative electric field due to the existence of the positive local field, as in Fig. 6 (c). Thus the local electric field does exist in the interface and it’s direction changes with the applied electric field.

Fig. 6 Schematic energy band diagrams and sketches of the bilayered thin films under different bias electric field conditions: (a) negative electric field; (b) positive electric field; (c) positive electric field; (d) negative electric field.



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Fig. 7 P-E hysteresis loops (a) and polarization currents loops (b) of BPRExFMO/CuFO (RE=Sr, Dy) composite films. The P-E hysteresis loops of Bi0.85-xPr0.15RExFe0.97Mn0.03O3/CuFe2O4 (BPRExFMO/CuFO, RE=Sr, Dy) composite films at room temperature and 1 kHz are depicted in Fig. 7 (a), and the inset is the loop of BFO/CuFO film. The P-E hysteresis loop of undoped thin film manifests as a circular curve with low breakdown field, and it is difficult to polarize due to the large leakage current in this thin films. Thus the remanent polarization value of 2.5 µC/cm2 is not the intrinsic characteristic, it is the contribution of the leakage compensation. On the contrary, the breakdown electric field and ferroelectric properties of the doped thin films increase obviously. Under the maximum applied electric field of 910 kV/cm, the remanent polarization Pr are 74.4 µC/cm2, 73.5 µC/cm2 and 84.3 µC/cm2 respectively, it increases 29.4 times at least. And the coercive fields Ec are 450 kV/cm, 614 kV/cm, 610 kV/cm respectively. As shown in Fig. 7 (b), the distinct switching peaks of samples around the Ec are observed. The polarization switching currents of the BPFMO/CuFO, BPDFMO/CuFO and BPSFMO/CuFO thin films are 0.0070, 0.0049 and 0.0074 A under positive applied field, respectively. The results of the samples for this test have demonstrated that the ions doping is an effective method to improve the ferroelectric properties in the bilayered thin films. The ferroelectric properties of all the BFO/CuFO based thin films are commonly considered originated from the distortion of BFO phase23, 47. However, the switching peaks in doped thin films show different breadths. And the broad distribution of switching fields


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compared to Ec results in a strongly rounded hysteresis loops which are the consequence of the formation of the local fields between BFO and additional layer with lower dielectric constant48. It can be found out that the local field’s values are consistent with the width of switching peaks. These local fields affect the domain switching to some extent, and the local electric fields are proportional to field distribution of current peaks. It means that the distribution of switching fields appear to be broad since part of the switching induced by the local field occurs already before the sign of the switching field occurring48.

Fig. 8 M-H hysteresis loops of (a) BPRExFMO/CuFO (RE=Sr, Dy) composite films and (b) CuFO thin film. Fig. 8 (a) presents the measured magnetic hysteresis loops at room temperature of the prepared samples, with the applied magnetic field of 8 kOe. The observation exhibits reduced ferromagnetic properties in the doped thin films, the remanent magnetization (Mr) of all species are 17.8 emu/cm3, 15.2 emu/cm3, 13.2 emu/cm3 and 15.8 emu/cm3, which is 19.7 times at least bigger than that of BFO (0.67 emu/cm3)20. The coercive field (Hc) are 610 Oe, 634 Oe, 754 Oe and 725 Oe. Based on our previous works20-21, 28, both the BFO and ions-doped BFO thin films possess weak ferromagnetism. It means that the ferromagnetic properties of the bilayered thin films improved by comparison with BFO. The magnetic hysteresis loop of single-layered CuFO film is plotted in Fig. 8 (b), and the Mr and Hc are 72 emu/cm3 and 800 Oe, respectively. According to literature’s report49-50, it can be illustrated that the increased magnetism mainly derive from CuFO layer rather than BFO layer. As



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reported23, many factors, like annealing temperature and mismatch strain, have effect on the ferromagnetic properties. Since same temperature has been set up to annealing treatment, so the annealing temperature can be excluded out of the influence of ferromagnetic properties. Therefore, the slightly decreased magnetic properties of the doped bilayer films might be attributed to the stress that results from mismatch strain50, originating from structure distortion that caused by ions doping. 4. CONCLUSION In summary, the bilayer BPRExFMO/CuFO (RE=Sr, Dy) thin films were fabricated onto FTO/glass substrates by the chemical solution deposition method. The ions doped samples have no structure transformation, they retain the trigonal R3c: H structure of BFO phase and tetragonal I41/amd structure of CuFO phase in all thin films. However, the leakage current density curves possess the asymmetric character under positive and negative bias electric field owning to RS effect. The RS ratio value of BFO/CuFO thin film is apparently different from that of doped thin films, the RS effect may be affected by the variation of the local fields induced by ions dopants. Under the applied electric field, the accumulation and directional arrangement of carries at the bilayer interface facilitate the formation of the local electric field which is affected by ions dopants, and the local electrical fields with different directions compensate or weaken the applied field. The opposite direction between the local field and the injection of electrons can lower the RS behavior, while the same direction can strengthen the RS behavior. The distribution fields of the polarization switching currents in the doped species are proportional to the local electric fields. The significantly improved ferroelectric properties induced by the ions doping are observed. The positive switching currents and remanent polarization of BPFMO/CuFO, BPSFMO/CuFO, and BPDFMO/CuFO films are 0.0070 A, 0.0049 A, 0.0074 A and 74.4 µC/cm2, 73.5µC/cm2, 84.3 µC/cm2, respectively. ACKNOWLEDGMENTS


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This work is supported by the Project of the National Natural Science Foundation of China (Grant No. 51372145); the Academic Leaders Funding Scheme of Shaanxi University of Science & Technology (2013XSD06); the Graduate Innovation Fund of Shaanxi University of Science &Technology (SUST-A04). REFERENCES (1) Tu, C. S.; Chen, C. S.; Chen, P. Y.; Wei, H. H.; Schmidt, V. H.; Lin, C. Y.; Anthoniappen, J.; Lee, J. M. Enhanced Photovoltaic Effects in A-site Samarium Doped BiFeO3 Ceramics: The Roles of Domain Structure and Electronic State. J. Eur. Ceram. Soc. 2016, 36, 1149-1157. (2) Wu, J. G.; Wang, J.; Xiao, D. Q.; Zhu, J. G. BiFeO3/Zn1-xMnxO Bilayered Thin Films. Appl. Surf. Sci. 2011, 258, 1390-1394. (3) El Bahraoui, T.; Slimani Tlemçani, T.; Taibi, M.; Zaarour, H.; El Bey, A.; Belayachi, A.; Tiburcio Silver, A.; Schmerber, G.; El Naggar, A. M.; Albassam, A. A.; Lakshminarayana, G.; Dinia, A.; Abd-Lefdil, M. Characterization of Multiferroic Bi1−xEuxFeO3 Powders Prepared by Sol-gel Method. Mater. Lett. 2016, 182, 151-154. (4) Hu, Z. Q.; Li, M. Y.; Liu, J.; Pei, L.; Wang, J.; Yu, B. F.; Zhao, X. Z. Structural Transition and Multiferroic Properties of Eu-Doped BiFeO3 Thin Films. J. Am. Ceram. Soc. 2010, 93, 2743-2747. (5) Singh, K.; Singh, S. K.; Kaur, D. Tunable Multiferroic Properties of Mn Substituted BiFeO3 Thin Films. Ceram, Int. 2016, 42, 13432-13441. (6) Park, T. J.; Papaefthymiou, G. C.; Viescas, A. J.; Moodenbaugh, A. R.; Wong, S. S. Size-dependent Magnetic Properties

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Resistive Switching and the Local Electric Field in Bi0.85-x

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