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Investigation of magneto-electric effects in (PMN-PT) @ NiFe2O4 core shell nanostructures and nanocomposites for non volatile memory applications
Journal Pre-proofs Investigation of magneto-electric effects in (PMN-PT) @ NiFe2O4 core shell nanostructures and nanocomposites for non volatile memory applications Anju Ahlawat, Azam Ali Khan, Pratik Desmukh, Mandar Shirolkar, Jieni Li, Haiqian Wang, S. Satapathy, A.K. Karnal PII: DOI: Reference:
S0167-577X(19)31714-8 https://doi.org/10.1016/j.matlet.2019.127082 MLBLUE 127082
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
23 October 2019 22 November 2019 26 November 2019
Please cite this article as: A. Ahlawat, A. Ali Khan, P. Desmukh, M. Shirolkar, J. Li, H. Wang, S. Satapathy, A.K. Karnal, Investigation of magneto-electric effects in (PMN-PT) @ NiFe2O4 core shell nanostructures and nanocomposites for non volatile memory applications, Materials Letters (2019), doi: https://doi.org/10.1016/ j.matlet.2019.127082
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Investigation of magneto-electric effects in (PMN-PT) @ NiFe2O4 core shell nanostructures and nanocomposites for non volatile memory applications Anju Ahlawat1, Azam Ali Khan1,2, Pratik Desmukh1,2, Mandar Shirolkar3,4, Jieni Li4, Haiqian Wang4, S. Satapathy1,2, and A. K. Karnal1,2 1Laser
and Functional Materials Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India 2Homi
Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai400094, India 3Symbiosis
Center for Nanoscience and Nanotechnology (SCNN), Symbiosis International (Deemed
University) (SIU), Lavale, Pune 412115, Maharashtra, India 4Hefei
National Laboratory for Physical Sciences at the Microscale, University of Science and
Technology of China, Hefei, Anhui 230026, China
Abstract We studied structural, ferroelectric, magnetic and magneto-electric (ME) properties of (0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3) (PMN-PT)/NiFe2O4 nanocomposite and core shell nanostructures (with ferrimagnetic NiFe2O4 core and ferroelectric PMN-PT shell). Morphology of core shell nanostructures and nanocomposites have been analyzed using transmission electron microscopy and scanning electron microscopy. Both the PMNPT)/NiFe2O4 samples reveal magneto electric effect at room temperature. The ME coupling coefficient (α) was found to be two order of magnitude higher for PMN-PT)/NiFe2O4 nanocomposites in comparison to the core shell nanostructures. The PMN-PT/NiFe2O4 nano composite showed repeatable and reversible switching of α (α > 0 and α < 0) in response to the applied positive and negative electric- pulses E = 20 kV/cm. The response of α was recorded for eighteen cycles of electric pulses (single pulse duration=100 seconds). The binary information can be stored in two states of α (α > 0 and α < 0) in non volatile manner. The PMN-PT/NiFe2O4 nanocomposite exhibited retention of α for ∼104 seconds and hence it can be used for non volatile memory applications. The principle of using α for memory storage may offer numerous benefits such as low power consumption, avoid destructive reading process, and easy writing/reading process. Keywords: NiFe2O4; PMN-PT; Core shell; Nanocomposites; Magnetoelectric response
1
1. Introduction Fast growing information technologies demand memory devices with non-volatility, high reading/writing speed, low cost, high density, large endurance and low power consumption [1,2]. Multiferroic materials hold great potential towards spintronics and non-volatile memory devices [3-5]. In this view engineered artificial multiferroic heterostructures with strong magnetoelectric (ME) coupling are appealing and of great technological significance due
to
the
interface-related
and
proximity
effects
[6-8].
Several
type
of
ferroelectric/ferromagnetic composites in different stacking geometries such as particulate, columnar structures, multilayers and core−shell nanostructures [9-11] have been reported. The ME effect in such heterostructures are originated due to strain interactions at the interface and therefore stacking/assembling two phases (piezoelectric and magnetostrictive) in different geometry could strongly affect the strain transfer across interface and consequently ME coupling. Efforts are being made to develop strong ME coupled multiferroic composites for utilization in non volatile memory (NVM) applications. Recently, a new concept of information storage (using states of ME coupling coefficient α, rather than resistance, magnetization etc.) has been invented for fast reading/writing process[2,3]. The applied positive and negative electric pulses induce reversible switching of α between two states named as “0” and “1 (e.g α > 0 and α < 0). The concept of utilizing these two state of α for information storage in NVM devices results low power consumption, easy reading/writing process as compared to the available nonvolatile RAMs, e.g. Ferroelectric (Fe) RAM [12] resistive switching memories (RRAM) [13] and phase change memories (PRAM) [14]. Only few multiferroic composites have been explored for the NVM applications by using the principle of ME coupling coefficient α switching for memory storage [1,2,10]. Hence, it is essential to search for suitable ME materials to be used for NVM applications by using the principle of α switching. In this letter, we studied ME properties in (0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3)(PMNPT)/NiFe2O4
core-shell
nanostructures
and
compared
with
PMN-PT/NiFe2O4
nanocomposites (0-3 type particulate). We demonstrated the use of PMN-PT/NiFe2O4 nanocomposites for non volatile memory applications by using the principle of electric field controlled reversible switching of ME coupling coefficient α. Hitherto, PMN-PT/NiFe2O4 nanocomposite have not been used for NVM applications. Hence the present work reveals important finding for NVM storage devices. 2
2. Experimental Section PMN-PT/NFO composite nanostructures were prepared in two forms; 0-3 type nanocomposite (NC) and core shell (CS) nanostructures. Initially, the nanopowders of NFO and PMN-PT were synthesized using sol-gel method [11-12]. The 0-3 type particulate 0.5PMN-PT0.5/NFO nanocomposites was prepared by mixing parent powders of nano NFO and PMN-PT in 1:1 ratio. After 5 hours of mixing, the powder was pelletized and sintered at 800ºC for 4 hours. For preparing core shell, as prepared NFO nanoparticles (.01 mg) in the 10 ml solution of PMN-PT were added to separately prepared PMN-PT solution and sonicated with continuous heating until gel formation. The obtained gels were dried and calcined at 800ºC for 2 hours. The details of characterization techniques are summarized in supplementary file. 3. Results and Discussion X-ray diffraction (XRD) patterns of the PMN-PT/NFO core shell nanostructures, nanocomposite and individual phases are shown in Fig. 1(a). All the diffraction peaks correspond to the reference parent compounds of NFO and PMN-PT and no extra peak was detected. NFO and PMN-PT are crystallized in cubic (space group Fd3m) and tetragonal symmetries (space group P4mm). Figure. 1 (b) shows formation of core shell of PMNPT@NFO composite, where NFO (~100 nm) core is surrounded by PMN-PT shell. Figure 1(c) and (d) shows enlarged view of core shell interface marked as 1, 2 and 3. Figure 1(e) reveals formation of 0-3 type particulate nanocomposite of PMN-PT/NFO where small size nanoparticles correspond to NFO (~100 nm) and bigger size PMN-PT particles of order of ~1-2 micron. Figure 2 (a) and (b) represent frequency dependent dielectric constant (ε)׳, tan δ and conductivity (ac) at room temperature for CS and NC, respectively. The ε׳, ac and tan δ values for CS is larger than that of the NC. This might be due to pores or defects formation at grain boundaries in CS nanostructures as compared to that of NC [13]. Figure 2(c) shows room temperature polarization vs electric field (P-E) hysteresis loops for CS and NC PMNPT/NFO, respectively. The NC sample exhibits well saturated loop with maximum polarization of 20.5 µCcm−2. However, CS reveals lossy ferroelectric behavior which is usually due to leakage current. Figure 2(d) reveals magnetic isotherms for CS and NC PMNPT/NFO at room temperature. Ferromagnetic behavior with saturation magnetic moment of 9 emu/g and 9 emu/g is evident for CS and NC. The coercivity and remnant magnetization are 5 and 50 Oe for CS and NC respectively. The higher coercivity for CS can be explained as
3
follows: NFO nanoparticles are isolated from each other due to PMN-PT shell in CS while for NC the cluster formation of NFO nanoparticles occurs and hence coercivity reduced. The ME coupling was measured by measuring dc magnetic field dependence of magnetoelectric coupling coefficient (α) for CS and NC samples as shown schematically in the Fig. 3(a). The α vs dc magnetic field curves (measured in the presence of 1 Oe ac magnetic field) illustrate non linear behaviour for both the samples (Fig. 3(b) and 3(c)). For NC, the value of α is zero at Hdc=0 and reaches a peak (24.5 mV/cm-Oe at Hdc= 0.5 KOe) and then decreases (at Hd c=5 kOe) because magnetization of NFO is saturated. However, the maximum value of α is reduced for CS (2.1 mV/cm-Oe at Hdc= 1 KOe and 0.44 mV/cm-Oe at Hdc=o). Indeed, the value of piezo coefficients depends on the direction of magnetization and polarization respectively, therefore induced voltage due to ME coupling depends on the magnetic and electric dipole interactions [13]. Further the response of α was measured with respect to the applied electric pulse E (as shown in Fig. 3(d)). PMN-PT/NFO nanocomposite exhibit reversible and repeatable switching of α (e.g α > 0 and α < 0) in response to positive and negative electric pulse E = 20 kVcm−1. Due to ME coupling, change in direction of magnetization (M) would bring a change in polarization (P), and vice versa. Hence the sign of α depends on the direction of P, i.e the sign of α = dP/dH α > 0 for positive P when the direction of M remains unchanged, and α < 0 for negative P. Subsequently, the digital information can be written by applying electric field to control the direction of P, similar to that in FeRAMs. The two values of α can be recorded in terms of bits “0” and “1” (e.g α > 0 as “1” and α < 0 as “0”) for memory applications. The information can be read out by measuring α using a single coil (generating a small magnetic field (Δ H)) surrounding the array of such memory elements and detecting the induced the induced voltage (Δ V). Hence the present geometry (Fig. 3(a)) behaves as single memory element and Fig. 3(d) shows non volatile nature of binary information storage because sign of α retains its value until application of next electric pulse E. The retention of α as a function of time was measured and figure 3(e) demonstrate α retention time for 104 seconds. Therefore, the PMN-PT/NFO nanocomposite show stable switching characteristics for potential NVM applications. However, such kind of ME coefficient switching could not be found for PMNPT/NFO core shell nanostructures. This may be due to poor strain transfer at interface and proximity effects. Different stacking geometries of PMN-PT and NFO may leads to the different strain transfer mechanism between two phases, which might have caused the 4
reduced magnetoelectric coupling in core shell nanostructures. Further efforts need to be attempted for fabrication and testing of PMN-PT/NFO core shell nanostructures for NVM applications. 4. Conclusions: In summary, PMN-PT@(NiFe2O4 core shell nanostructures and
0-3 type PMN-
PT/NiFe2O4 nanocomposite were fabricated. The nanocomposites show improved ferroelectric properties and ME voltage coefficient α for nanocomposites as compared to the core shell nanostructures. The electric-field-induced reversible switching (two level) of the α was obtained for PMN-PT/NiFe2O4 nanocomposite. The principle of using the ME coefficient α for digital information storage in non volatile memories offers efficient and convenient writing/reading operations. Acknowledgement: The authors would like to acknowledge DST for financial support.
Figures: Figure 1. (a) X ray diffraction pattern of NFO, PMN-PT, PMN-PT@NFO core shell nanostructures. P and N correspond to the PMN-PT and NFO reflections; (b) Transmission electron microscopy images of PMN-PT@NFO core shell nanostructures, (c) & (d) represent enlarged view of marked areas 1, 2 and 3 respectively; (e) Scanning electron microscopy images of PMN-PT/NFO nanocomposite.
5
Figure 2 . Frequency dependent dielectric constant (ε)׳, tan δ and conductivity (ac) of PMNPT@NFO cores shell (CS) (a); and for PMN-PT/NFO nanocomposite (NC) (b); Polarization Vs electric field hysteresis loop for PMN-PT/NFO CS and NC (c); room temperature magnetization vs magnetic field curves for PMN-PT/NFO CS and NC samples (d).
6
Figure 3. Schematic configuration of the ME voltage coefficient α measurement (a); ME voltage coefficient vs applied dc-bias magnetic field curves measured in the presence of 1 Oe ac magnetic field for PMN-PT/NFO core shell nanostructure (b) and PMN-PT/NFO nanocomposite (c); the repeatable switching of ME voltage coefficient by applying electric pulse E as a function of time measured for PMN-PT/ NFO samples (d); retention of α as a function of time (e).
7
References [ ] J. X. Shen, J. Z. Cong, D. S. Shang, Y. S. Chai, S. P. Shen. K. Zhai, Y. Sun, Sci. Rep. 6
(2016) 4473-34478. [2] P. Lu, D. Shang, J. Shen,Y. Chai Y. C.Yang, K. Zhai, J. Cong, S. Shen, Y. Sun, Appl. Phys. Lett. 109 (2016) 252902-252907. [3] M. M. Shirolkar, C. Hao, S. Yin, M. Li and H. Wang,. Appl. Phys. Lett. 102 (2013) 243501-243505. [4] A. Ahlawat, S. Satapathy, P. Deshmukh, M. M. Shirolkar,A. K. Sinha, and A. K. Karnal, Appl. Phys. Lett. 111 (2017) 262902-262907. [5] J. X. Shen, J. Z. Cong, Y. S. Chai, D. S Shang, S. P. Shen, K Zhai, Y.Tian, and Y. Sun, Phys. Rev. Appl. 6 (2016) 021001-021005. [6] M. M. Shirolkar, J. Li, X. Dong, M. Li, Phys. Chem. Chem. Phys. 19 (2017) 26085-26097. [7] R. C. Peng, J. J. Wang, J. M. Hu, L. Q. Chen, and C .W. Nan, 106 (2015) 142901. [8] J. J. Wang, J. M. Hu, J. Ma, J. X. Zhang, L. Q. Chen, and C. W. Nan, Sci. Rep. 4 (2014) 7507. [9] H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. SalamancaRiba, S. R. Shinde, S. B. Ogale, F. Bai, D. Viehland, Y. Jia, D. G. Schlom, M. Wuttig, A. Roytburd, and R. Ramesh, Science 303(2004) 661. [10] A. Ahlawat, S. Satapathy, M.M. Shirolkar, J. Li, A.A. Khan, P. Deshmukh, H. Wang, R.J. Choudhary, A.K. Karnal, A.C.S. Appl, Nano Mater. 1, (2018) 3196-3203. [11] J. S. Andrew, J. D. Starr and M. A. K. Budi, Scripta Materialia 74 (2014) 38–43. [12] R. Waser, M. Aono, Nat. Mater.6 (2007) 833-840. [13] G. Sreenivasulu, J. Zhang, R. Zhang, M. Popov, V. Petrov, G. Srinivasn, Materails 18 (2018) 18- 33.
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Author’s Contribution:
Anju Ahlawat formulated the background of the manuscript, performed experiments, analyzed the data and prepared the manuscript. Azam Ali Khan and Pratik Desmukh helped in the synthesis of the samples, supported in data collection of XRD, Dielectric, ferroelectric and magnetic measurements. Mandar Shirolkar carried out TEM measurements. Haiqian Wang supervised the Magnetoelectric measurements. Jieni Li and Mandar Shirolkar performed Magnetoelectric measurements and helped in data analysis. S. Satapathy and A. K. Karnal put suggestions and corrected the manuscript.
9
Hightlights
Synthesis of PMN-PT/NiFe2O4 core shell nanostructures and nanocomposites. Study of structural, ferroelectric, magnetic and magneto-electric response. Utilization of PMN-PT/ NiFe2O4nanocomposites for non volatile memory applications. Possible memory devices with low power consumption and easy write/readout process.
10
S0167-577X(19)31714-8 https://doi.org/10.1016/j.matlet.2019.127082 MLBLUE 127082
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
23 October 2019 22 November 2019 26 November 2019
Please cite this article as: A. Ahlawat, A. Ali Khan, P. Desmukh, M. Shirolkar, J. Li, H. Wang, S. Satapathy, A.K. Karnal, Investigation of magneto-electric effects in (PMN-PT) @ NiFe2O4 core shell nanostructures and nanocomposites for non volatile memory applications, Materials Letters (2019), doi: https://doi.org/10.1016/ j.matlet.2019.127082
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Investigation of magneto-electric effects in (PMN-PT) @ NiFe2O4 core shell nanostructures and nanocomposites for non volatile memory applications Anju Ahlawat1, Azam Ali Khan1,2, Pratik Desmukh1,2, Mandar Shirolkar3,4, Jieni Li4, Haiqian Wang4, S. Satapathy1,2, and A. K. Karnal1,2 1Laser
and Functional Materials Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India 2Homi
Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai400094, India 3Symbiosis
Center for Nanoscience and Nanotechnology (SCNN), Symbiosis International (Deemed
University) (SIU), Lavale, Pune 412115, Maharashtra, India 4Hefei
National Laboratory for Physical Sciences at the Microscale, University of Science and
Technology of China, Hefei, Anhui 230026, China
Abstract We studied structural, ferroelectric, magnetic and magneto-electric (ME) properties of (0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3) (PMN-PT)/NiFe2O4 nanocomposite and core shell nanostructures (with ferrimagnetic NiFe2O4 core and ferroelectric PMN-PT shell). Morphology of core shell nanostructures and nanocomposites have been analyzed using transmission electron microscopy and scanning electron microscopy. Both the PMNPT)/NiFe2O4 samples reveal magneto electric effect at room temperature. The ME coupling coefficient (α) was found to be two order of magnitude higher for PMN-PT)/NiFe2O4 nanocomposites in comparison to the core shell nanostructures. The PMN-PT/NiFe2O4 nano composite showed repeatable and reversible switching of α (α > 0 and α < 0) in response to the applied positive and negative electric- pulses E = 20 kV/cm. The response of α was recorded for eighteen cycles of electric pulses (single pulse duration=100 seconds). The binary information can be stored in two states of α (α > 0 and α < 0) in non volatile manner. The PMN-PT/NiFe2O4 nanocomposite exhibited retention of α for ∼104 seconds and hence it can be used for non volatile memory applications. The principle of using α for memory storage may offer numerous benefits such as low power consumption, avoid destructive reading process, and easy writing/reading process. Keywords: NiFe2O4; PMN-PT; Core shell; Nanocomposites; Magnetoelectric response
1
1. Introduction Fast growing information technologies demand memory devices with non-volatility, high reading/writing speed, low cost, high density, large endurance and low power consumption [1,2]. Multiferroic materials hold great potential towards spintronics and non-volatile memory devices [3-5]. In this view engineered artificial multiferroic heterostructures with strong magnetoelectric (ME) coupling are appealing and of great technological significance due
to
the
interface-related
and
proximity
effects
[6-8].
Several
type
of
ferroelectric/ferromagnetic composites in different stacking geometries such as particulate, columnar structures, multilayers and core−shell nanostructures [9-11] have been reported. The ME effect in such heterostructures are originated due to strain interactions at the interface and therefore stacking/assembling two phases (piezoelectric and magnetostrictive) in different geometry could strongly affect the strain transfer across interface and consequently ME coupling. Efforts are being made to develop strong ME coupled multiferroic composites for utilization in non volatile memory (NVM) applications. Recently, a new concept of information storage (using states of ME coupling coefficient α, rather than resistance, magnetization etc.) has been invented for fast reading/writing process[2,3]. The applied positive and negative electric pulses induce reversible switching of α between two states named as “0” and “1 (e.g α > 0 and α < 0). The concept of utilizing these two state of α for information storage in NVM devices results low power consumption, easy reading/writing process as compared to the available nonvolatile RAMs, e.g. Ferroelectric (Fe) RAM [12] resistive switching memories (RRAM) [13] and phase change memories (PRAM) [14]. Only few multiferroic composites have been explored for the NVM applications by using the principle of ME coupling coefficient α switching for memory storage [1,2,10]. Hence, it is essential to search for suitable ME materials to be used for NVM applications by using the principle of α switching. In this letter, we studied ME properties in (0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3)(PMNPT)/NiFe2O4
core-shell
nanostructures
and
compared
with
PMN-PT/NiFe2O4
nanocomposites (0-3 type particulate). We demonstrated the use of PMN-PT/NiFe2O4 nanocomposites for non volatile memory applications by using the principle of electric field controlled reversible switching of ME coupling coefficient α. Hitherto, PMN-PT/NiFe2O4 nanocomposite have not been used for NVM applications. Hence the present work reveals important finding for NVM storage devices. 2
2. Experimental Section PMN-PT/NFO composite nanostructures were prepared in two forms; 0-3 type nanocomposite (NC) and core shell (CS) nanostructures. Initially, the nanopowders of NFO and PMN-PT were synthesized using sol-gel method [11-12]. The 0-3 type particulate 0.5PMN-PT0.5/NFO nanocomposites was prepared by mixing parent powders of nano NFO and PMN-PT in 1:1 ratio. After 5 hours of mixing, the powder was pelletized and sintered at 800ºC for 4 hours. For preparing core shell, as prepared NFO nanoparticles (.01 mg) in the 10 ml solution of PMN-PT were added to separately prepared PMN-PT solution and sonicated with continuous heating until gel formation. The obtained gels were dried and calcined at 800ºC for 2 hours. The details of characterization techniques are summarized in supplementary file. 3. Results and Discussion X-ray diffraction (XRD) patterns of the PMN-PT/NFO core shell nanostructures, nanocomposite and individual phases are shown in Fig. 1(a). All the diffraction peaks correspond to the reference parent compounds of NFO and PMN-PT and no extra peak was detected. NFO and PMN-PT are crystallized in cubic (space group Fd3m) and tetragonal symmetries (space group P4mm). Figure. 1 (b) shows formation of core shell of PMNPT@NFO composite, where NFO (~100 nm) core is surrounded by PMN-PT shell. Figure 1(c) and (d) shows enlarged view of core shell interface marked as 1, 2 and 3. Figure 1(e) reveals formation of 0-3 type particulate nanocomposite of PMN-PT/NFO where small size nanoparticles correspond to NFO (~100 nm) and bigger size PMN-PT particles of order of ~1-2 micron. Figure 2 (a) and (b) represent frequency dependent dielectric constant (ε)׳, tan δ and conductivity (ac) at room temperature for CS and NC, respectively. The ε׳, ac and tan δ values for CS is larger than that of the NC. This might be due to pores or defects formation at grain boundaries in CS nanostructures as compared to that of NC [13]. Figure 2(c) shows room temperature polarization vs electric field (P-E) hysteresis loops for CS and NC PMNPT/NFO, respectively. The NC sample exhibits well saturated loop with maximum polarization of 20.5 µCcm−2. However, CS reveals lossy ferroelectric behavior which is usually due to leakage current. Figure 2(d) reveals magnetic isotherms for CS and NC PMNPT/NFO at room temperature. Ferromagnetic behavior with saturation magnetic moment of 9 emu/g and 9 emu/g is evident for CS and NC. The coercivity and remnant magnetization are 5 and 50 Oe for CS and NC respectively. The higher coercivity for CS can be explained as
3
follows: NFO nanoparticles are isolated from each other due to PMN-PT shell in CS while for NC the cluster formation of NFO nanoparticles occurs and hence coercivity reduced. The ME coupling was measured by measuring dc magnetic field dependence of magnetoelectric coupling coefficient (α) for CS and NC samples as shown schematically in the Fig. 3(a). The α vs dc magnetic field curves (measured in the presence of 1 Oe ac magnetic field) illustrate non linear behaviour for both the samples (Fig. 3(b) and 3(c)). For NC, the value of α is zero at Hdc=0 and reaches a peak (24.5 mV/cm-Oe at Hdc= 0.5 KOe) and then decreases (at Hd c=5 kOe) because magnetization of NFO is saturated. However, the maximum value of α is reduced for CS (2.1 mV/cm-Oe at Hdc= 1 KOe and 0.44 mV/cm-Oe at Hdc=o). Indeed, the value of piezo coefficients depends on the direction of magnetization and polarization respectively, therefore induced voltage due to ME coupling depends on the magnetic and electric dipole interactions [13]. Further the response of α was measured with respect to the applied electric pulse E (as shown in Fig. 3(d)). PMN-PT/NFO nanocomposite exhibit reversible and repeatable switching of α (e.g α > 0 and α < 0) in response to positive and negative electric pulse E = 20 kVcm−1. Due to ME coupling, change in direction of magnetization (M) would bring a change in polarization (P), and vice versa. Hence the sign of α depends on the direction of P, i.e the sign of α = dP/dH α > 0 for positive P when the direction of M remains unchanged, and α < 0 for negative P. Subsequently, the digital information can be written by applying electric field to control the direction of P, similar to that in FeRAMs. The two values of α can be recorded in terms of bits “0” and “1” (e.g α > 0 as “1” and α < 0 as “0”) for memory applications. The information can be read out by measuring α using a single coil (generating a small magnetic field (Δ H)) surrounding the array of such memory elements and detecting the induced the induced voltage (Δ V). Hence the present geometry (Fig. 3(a)) behaves as single memory element and Fig. 3(d) shows non volatile nature of binary information storage because sign of α retains its value until application of next electric pulse E. The retention of α as a function of time was measured and figure 3(e) demonstrate α retention time for 104 seconds. Therefore, the PMN-PT/NFO nanocomposite show stable switching characteristics for potential NVM applications. However, such kind of ME coefficient switching could not be found for PMNPT/NFO core shell nanostructures. This may be due to poor strain transfer at interface and proximity effects. Different stacking geometries of PMN-PT and NFO may leads to the different strain transfer mechanism between two phases, which might have caused the 4
reduced magnetoelectric coupling in core shell nanostructures. Further efforts need to be attempted for fabrication and testing of PMN-PT/NFO core shell nanostructures for NVM applications. 4. Conclusions: In summary, PMN-PT@(NiFe2O4 core shell nanostructures and
0-3 type PMN-
PT/NiFe2O4 nanocomposite were fabricated. The nanocomposites show improved ferroelectric properties and ME voltage coefficient α for nanocomposites as compared to the core shell nanostructures. The electric-field-induced reversible switching (two level) of the α was obtained for PMN-PT/NiFe2O4 nanocomposite. The principle of using the ME coefficient α for digital information storage in non volatile memories offers efficient and convenient writing/reading operations. Acknowledgement: The authors would like to acknowledge DST for financial support.
Figures: Figure 1. (a) X ray diffraction pattern of NFO, PMN-PT, PMN-PT@NFO core shell nanostructures. P and N correspond to the PMN-PT and NFO reflections; (b) Transmission electron microscopy images of PMN-PT@NFO core shell nanostructures, (c) & (d) represent enlarged view of marked areas 1, 2 and 3 respectively; (e) Scanning electron microscopy images of PMN-PT/NFO nanocomposite.
5
Figure 2 . Frequency dependent dielectric constant (ε)׳, tan δ and conductivity (ac) of PMNPT@NFO cores shell (CS) (a); and for PMN-PT/NFO nanocomposite (NC) (b); Polarization Vs electric field hysteresis loop for PMN-PT/NFO CS and NC (c); room temperature magnetization vs magnetic field curves for PMN-PT/NFO CS and NC samples (d).
6
Figure 3. Schematic configuration of the ME voltage coefficient α measurement (a); ME voltage coefficient vs applied dc-bias magnetic field curves measured in the presence of 1 Oe ac magnetic field for PMN-PT/NFO core shell nanostructure (b) and PMN-PT/NFO nanocomposite (c); the repeatable switching of ME voltage coefficient by applying electric pulse E as a function of time measured for PMN-PT/ NFO samples (d); retention of α as a function of time (e).
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References [ ] J. X. Shen, J. Z. Cong, D. S. Shang, Y. S. Chai, S. P. Shen. K. Zhai, Y. Sun, Sci. Rep. 6
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Author’s Contribution:
Anju Ahlawat formulated the background of the manuscript, performed experiments, analyzed the data and prepared the manuscript. Azam Ali Khan and Pratik Desmukh helped in the synthesis of the samples, supported in data collection of XRD, Dielectric, ferroelectric and magnetic measurements. Mandar Shirolkar carried out TEM measurements. Haiqian Wang supervised the Magnetoelectric measurements. Jieni Li and Mandar Shirolkar performed Magnetoelectric measurements and helped in data analysis. S. Satapathy and A. K. Karnal put suggestions and corrected the manuscript.
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Hightlights
Synthesis of PMN-PT/NiFe2O4 core shell nanostructures and nanocomposites. Study of structural, ferroelectric, magnetic and magneto-electric response. Utilization of PMN-PT/ NiFe2O4nanocomposites for non volatile memory applications. Possible memory devices with low power consumption and easy write/readout process.
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