- Email: [email protected]
BTO composite thin films
Accepted Manuscript Title: Voltage-driven hysteretic changes in magnetization in multiferroic Co/BTO composite thin films Author: Li Shu Jia-Mian Hu Yao Gao Liang Wu Jing Ma Y.H. Lin C.W. Nan PII: DOI: Reference:
S0169-4332(14)01243-4 http://dx.doi.org/doi:10.1016/j.apsusc.2014.05.202 APSUSC 28021
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-3-2014 25-5-2014 28-5-2014
Please cite this article as: L. Shu, J.-M. Hu, Y. Gao, L. Wu, J. Ma, Y.H. Lin, C.W. Nan, Voltage-driven hysteretic changes in magnetization in multiferroic Co/BTO composite thin films, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.202 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Voltage‐driven hysteretic changes in magnetization in multiferroic Co/BTO composite thin films
ip t
Li Shu, Jia-Mian Hu, Yao Gao, Liang Wu, Jing Ma*, Y. H. Lin, C. W. Nan
School of Materials Science and Engineering, and State Key Lab of New Ceramics and
Email: [email protected]
us
*
cr
Fine Processing, Tsinghua University, Beijing 100084, China
an
Abstract
Multiferroic Co/BaTiO3 layered composite thin film were grown on Nb-doped SrTiO3
M
single crystal by pulse laser deposition, in which the polycrystalline Co film was annealed
d
under magnetic field to induce in-plane uniaxial magnetic easy axis. Voltage-induced
te
magnetization changes along and perpendicular to the easy axis were measured by
Ac ce p
Magneto-Optical Kerr effect (MOKE) magnetometer without applied magnetic field. These changes in magnetization could be due to magnetoelectric coupling between Co and BaTiO3, and to possibly electro-optical effect of BTO if the MOKE laser could penetrate the top Co film. After excluding the electro-optical interference by analyzing the experimental results within an optical model, a hysteric loop of magnetization versus voltage is identified with a relative change in magnetization of about 8%. Key words Multiferroic composites; Thin film; Magneto-Optical Kerr effect; Electro-optical effect
1 Page 1 of 22
1. Introduction Artificial multiferroic heterostructures of ferroelectric and ferromagnetic layers are of
ip t
increasing interest because the cross-coupling between the magnetic and electric polarizations [1,2] allows novel means of controlling magnetization or polarization. For
cr
example, an electric voltage, rather than magnetic field, can be directly used to control
us
magnetization, known as converse magnetoelectric (ME) coupling, and hence can potentially be utilized to design novel spintronic or magnetic devices with much lower power
an
consumption and higher speed. Examples include voltage-driven magnetic random access
M
memories [3-6] and logic circuits [7]. To ensure non-volatility of these devices, voltage-driven hysteric changes in magnetizations must be obtained in multiferroic
d
heterostructures[8,9]. Although such magnetization-voltage hysteresis loop has been
te
demonstrated in La0.8Sr0.2MnO3/Pb(Zr0.2Ti0.8)O3 layered thin films at low temperature (100
Ac ce p
K) via ferroelectric control of spin-polarized charge densities at the hetero-interface, room-temperature demonstration of this hysteresis loop in multiferroic composite thin films still remains scarce. In this article, we observe voltage-driven hysteretic changes in magnetization in multiferroic Co/BaTiO3 composite thin films by Magneto-Optical Kerr Effect (MOKE) magnetometer [see Fig. 1(a)]. In particular, no external magnetic field is applied throughout the measurement process, thus avoiding the unintended role that the magnetic field may play in voltage-driven magnetization changes as in conventional MOKE measurement [10]. 2. Experiment 2 Page 2 of 22
Heterostructure of the ferroelectric BaTiO3 (BTO) and the ferromagnetic Co was fabricated. The BTO thin film of ~100 nm in thickness was epitaxially grown on (100)
ip t
oriented Nb-doped SrTiO3 (STO) substrate using pulsed laser deposition as before [11], and then transferred to a DC magnetron sputtering system for the deposition of polycrystalline
cr
Co thin film with 20 nm in thickness at 200 °C. During deposition, an external magnetic
us
field of 300 Oe was applied to induce an in-plane magnetic easy axis[12]. The Co film and Nb-doped STO substrate are used as the top and bottom electrodes for ferroelectric
an
(Precision Multiferroic, Radiant Technologies, Inc.) and piezoelectric measurement.
M
MOKE magnetometer (NanoMOKE2, Durham Magneto Optics Ltd., UK) was utilized to detect local magnetism variations of the Co thin film [see Fig. 2(a)]. The induced uniaxial
d
magnetic easy axis is evidenced by the square-shaped magnetic hysteresis loop in which the
te
applied magnetic field H is collinear with the easy axis [i.e., Ω=0°, see Fig. 2(b)], as well as
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the linear-like hard-axis loop in which the H is perpendicular to the easy axis (Ω=90°). Furthermore, a previously developed AC voltage (V) modulated MOKE technique is employed to detect voltage-induced Kerr signal changes under no applied magnetic field, which can ensure a pure electric-field modulated magnetism [10]. 3. Results and Discussion
The X-ray diffraction pattern of the heterostructure is shown in Fig. 1(a), clearly demonstrating a (001) orientation for the BTO thin film. The (002) peak of BTO indicates the presence of out-of-plane ferroelectric domain. Figure 1(b) shows the surface morphology of the BTO film by atomic force microscope, demonstrating a root-mean-square surface 3 Page 3 of 22
roughness of about 1 nm. A well-defined ferroelectric hysteresis loop is obtained at 1000 Hz [see Fig. 1(c)], and local ferroelectric domain switching behaviors in an area of 6×6 μm2 are
ip t
clearly imaged [see Fig. 1(d)]. Figure 3 shows the AC-voltage (sinusoidal wave, 13 Hz)-induced relative changes in the
cr
total Kerr signal, i.e., ΔI/I. The curve measured along the easy axis (Ω=0°) shows an evident
us
hysteresis (see the two remnant states at V=0), while the curve along the hard-axis (Ω=90°) is linear-like with little hysteresis. Surprisingly, the hard-axis curve even shows a slightly
an
bigger Kerr signal changes than the easy axis curve (Fig. 3), though the remnant
M
magnetization (i.e., H=0 for such AC-voltage testing) should almost be zero along the hard axis [see Fig. 2(b)]. Therefore, the observed remarkable Kerr signal changes along the hard
d
easy should not be largely based on voltage-induced changes in magnetization (i.e., converse
te
ME coupling). Another possible cause is the voltage-induced changes in the surface
Ac ce p
reflectivity of the BTO film, also known as the electric-optical effect [13,14], because the present Co film thickness (20 nm) may not completely prevent the penetration of the laser beam. Thus we need to first extract this electric-optical contribution by analyzing experimentally measured Kerr signal changes within an optical model. Then the voltage-induced magnetization changes along the easy axis can be obtained by excluding such electric-optical contribution. Based on a previously developed model [13], the voltage-induced Kerr signal changes from electric-optical effect (ΔIEO/I) and the converse ME effect (ΔIME/I) can be expressed as 2 ΔI EO ΔRss sin ψ + 2rps Δrss sinψ cosψ = I Rss sin 2 ψ
(1) 4 Page 4 of 22
2 ΔI ME ΔR ps cos ψ + 2rss Δrps sinψ cosψ = I Rss sin 2 ψ
(2)
where Rss and rss ( Rss = rss2 ) are the reflectance and reflection coefficient for the
ip t
longitudinally polarized Kerr vector Ess; and Rps and rps for the orthogonally polarized Kerr
between the laser analyzer and the Eps, as shown in Fig. 4(a) [15].
cr
vector Eps that can be reversed after magnetization reversal; and ψ is the azimuth angle
us
When the hard easy is in the incidental plane of laser beam under zero magnetic field,
an
rps is zero. Thus the contributions from the electric-optical effect (ΔIEO/I) and converse ME effect (ΔIME/I) to the relative Kerr signal changes measured along the hard axis, i.e., (ΔI/I)h,
M
are,
(3) (4)
te
ΔI ME =0 I
d
ΔI EO ΔRss = I Rss
Ac ce p
Similarly, the relative Kerr signal changes measured along the easy axis, i.e.,(ΔI/I)e, is also comprised of ΔIEO/I and ΔIME/I, which can be expressed as,
ΔI EO ΔRss (1 + 2rps / rss cotψ ) = I Rss
(5)
2 ΔI ME Δrps ( 2rps cot ψ + 2rss cotψ ) = I Rss
(6)
Given that rps is proportional to the magnetization [16], the relative magnetization change, ΔM/M, can be represented by the change in surface reflectivity, i.e., Δrps/rps, that could be obtained from Eqs. (3-6), i.e.,
5 Page 5 of 22
Δrps / r ps
⎡⎛ ΔI ⎞ ⎛ 2r ⎞ ⎛ ΔI ⎞ ⎤ Rss ⎢⎜ ⎟ − ⎜1 + ps cotψ ⎟ ⎜ ⎟ ⎥ rss ⎠ ⎝ I ⎠h ⎦ ⎣⎝ I ⎠ e ⎝ = 2 2r ps ( rps cot ψ + rss cotψ )
(7)
ip t
The two reflection coefficients rps and rss in Eq. (7) could be obtained by fitting the experimentally measured average Kerr signal changes i.e., ΔI/Iav, as a function of the
cr
analyzer angle ψ [see Fig. 4(b)] before and after reversing magnetization with an applied
us
magnetic field. Before the magnetization reversal [corresponding to the left panel of Fig. 4(a)], the intensity of the laser transmitted through the polarizer, i.e., I+, is given by, 2
= rss2 sin 2 ψ + rps2 cos 2 ψ + 2rss rps sinψ cosψ
an
I + = ( rss sinψ + rps cosψ )
(8)
M
After reversing the magnetization [corresponding to the right panel of Fig. 4(a)], the transmitted laser intensity, i.e., I-,is given by, 2
d
I − = ( rss sinψ − rps cosψ )
(9)
te
= rss2 sin 2 ψ + rps2 cos 2 ψ − 2rss rps sinψ cosψ
Ac ce p
Thus the average transmitted laser intensity, i.e., Iav, can be calculated as, I av =
1 ( I + + I − ) + γ D = rps2 cos 2 ψ + rss2 sin 2 ψ + γ D . 2
(10)
The parameter γD is introduced to represent the depolarization of the incidental laser
because the polarizer is not 100% efficient. Accordingly, the ΔI/Iav can be given as,
4rps rss cosψ sinψ ΔI I + − I − = = 2 I av I av rps cos 2 ψ + rss2 sin 2 ψ + γ D
(11)
By fitting experimental measured ΔIav/I using Eq. (11) [see Fig. 4(b)], rss and rps can be obtained to be 0.85 and 1.25×10-3, respectively, with a γD of 1.1×10-4. The purely voltage-induced relatively change in magnetization (ΔM/M) can now be obtained by estimating corresponding changes in the reflection coefficient Δrps/rps based on 6 Page 6 of 22
Eq. (7). As shown in Fig. 4(c), an evident hysteresis is observed between -5 V and +5 V, with an abrupt change in magnetization of about 8%. This hysteresis should be related to
ip t
irreversible ferroelectric domain switching [17] or charge effect [18,19], but the hidden physics remains further clarification.
cr
4. Conclusions
us
In summary, we have fabricated a multiferroic Co/BTO composite thin film that has an in-plane uniaxial magnetic easy axis using pulse laser deposition and magnetron sputtering.
an
Combined theoretical and experimental analysis have been performed to exclude the
M
contribution of electric-optical effect from the voltage-induced total Kerr signal changes measured using Kerr magnetometer without magnetic field. A purely voltage-driven hysteric
d
magnetization change of up to about 8% has thus been obtained, which promises application
Ac ce p
te
potential as non-volatile magnetoelectric devices.
Acknowledgements
This work was supported by the NSF of China (Grant Nos. 51332001, 11234005 and
51221291), and Beijing Education Committee (Grant No. 20121000301).
7 Page 7 of 22
References: [1] C. A. F. Vaz, J Hoffman, C. H. Ahn, R. Ramesh, Magnetoelectric coupling effects in
ip t
multiferroic complex oxide composite structures, Adv. Mater. 22 (2010) 2900-2918. [2] J. Ma, J. M. Hu, Z. Li, C. W. Nan, Recent progress in multiferroic magnetoelectric
cr
composites: from bulk to thin films, Adv. Mater. 23 (2011) 1062-1087.
us
[3] M. Bibes, A. Barthélémy, Multiferroics: towards a magnetoelectric memory. Nat. Mater. 7(2008) 425-426.
an
[4] J. M. Hu, Z. Li, L. Q. Chen, C. W. Nan, High-density magnetoresistive random access
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memory operating at ultralow voltage at room temperature, Nat. Commun. 2 (2011) 553. [5] M. Liu, S. D. Li, O. Obi, J. Lou, S. Rand, N. X. Sun, Electric field modulation of
d
magnetoresistance in multiferroic heterostructures for ultralow power electronics, Appl.
te
Phys. Lett. 98 (2011) 222509; M. Liu, O. Obi, J. Lou, S. D. Li, X. Xing, G. M. Yang, N.
Ac ce p
X. Sun, Tunable magnetoresistance devices based on multiferroic heterostructures, J. Appl. Phys. 109 (2011) 07D913.
[6] A. Brandlmaier, S. Geprägs, G. Woltersdorf, R. Gross, S. T. B. Goennenwein, Nonvolatile, reversible electric-field controlled switching of remnant magnetization in multifunctional ferromagnetic-ferroelectric hybrids, J. Appl. Phys. 110 (2011) 043913.
[7] J. M. Hu, Z. Li, Y. H. Lin, C. W. Nan, A magnetoelectric logic gate, Phys. Status Solidi (RRL) 4 (2010) 106-108. [8] C. A. F. Vaz, Electric field control of magnetism in multiferroic heterostructures, J. Phys.: Condensed Matt. 24 (2012) 333201. 8 Page 8 of 22
[9] A. Chiolerio, M. Quaglio, A. Lamberti, F. Celegato, D. Balma, P. Allia, Magnetoelastic coupling in multilayered ferroelectric/ferromagnetic thin films: A quantitative evaluation,
ip t
Appl. Sur. Sci., 258, (2012) 8072-8077. [10] Z. Li, J. M. Hu, L. Shu, Y. Zhang, Y. Gao, Y. Shen, Y. H. Lin, and C. W. Nan, A simple for
direct
observation
of
the
converse
magnetoelectric
effect
in
cr
method
us
magnetic/ferroelectric composite thin films, J. Appl. Phys. 110 (2011) 096106.
[11] L. Shu, Z. Li, J. Ma, Y Gao, L Gu, Y Shen, Y H Lin, C W Nan, Thickness-dependent
an
voltage-modulated magnetism in multiferroic heterostructures, Appl. Phys. Lett. 100
M
(2012) 022405.
[12] M. Liu, J. Lou, S. D. Li, N. X. Sun, E-Field Control of Exchange Bias and Deterministic
te
Mater. 21 (2011) 2593-2598.
d
Magnetization Switching in AFM/FM/FE Multiferroic Heterostructures, Adv. Funct.
Ac ce p
[13] L. Shu, Y. Gao, J. M. Hu, Z. Li, Y. Shen, Y. H. Lin, C. W. Nan, Evaluating the electro-optical effect in alternating current-voltage-modulated Kerr response for multiferroic heterostructures, J. Appl. Phys. 114 (2013) 204102.
[14] J. Schubert, M. Siegert, M. Fardmanesh, W. Zander, M. Prompers, C. Buchal, J. Lisoni, C. H. Lei, Superconducting and electro-optical thin films prepared by pulsed laser deposition technique, Appl. Sur. Sci., 168, (2000) 208-214. [15] P. R. Cantwell, U. J. Gibson, D. A. Allwood, H. A. Macleod, Optical coatings for improved contrast in longitudinal magneto-optic Kerr effect measurements, J. Appl. Phys. 100 (2006) 093910. 9 Page 9 of 22
[16] Z. J. Yang, M. R. Scheinfein, Combined three axis surface magnetooptical Kerr effects in the study of surface and ultrathin film magnetism, J. Appl. Phys. 6810 (1993) 74.
via magnetoelectric effect, J. Appli. Phys. 110 (2011) 043919.
ip t
[17] Jing. W, Jing. M, L. Zheng, Switchable voltage control of the magnetic coercive field
cr
[18] H. J. A. Molegraaf, J. Hoffman, C. A. F. Vaz, S. Gariglio, D. van der Mare, C. H. Ahn,
us
J. M. Triscone, Magnetoelectric effects in complex oxides with competing ground states, Adv. Mater. 21 (2009) 3470-3474.
an
[19] M. Zhernenkov, M. R. Fitzsimmons, C. Jerzy, J. Majewski, I. Tudosa, E. E. Fullerton,
M
Electric-field modification of magnetism in a thin CoPd film, Phys. Rev. B 82 (2010)
Ac ce p
te
d
024420.
10 Page 10 of 22
Figure captions Figure 1(a) X-ray diffraction pattern of the multiferroic Co (20 nm)/ BTO (100 nm)/Nb-STO thin-film heterostructure. (b) Atomic force microscopic topography image (top view) of the
ip t
BTO film. (c) Ferroelectric hysteresis loop of the BTO film. (d) The out-of-plane piezoelectric force microscopy phase image (an area of 6×6 μm2) with the BTO film first
cr
being poled at +10 V (dark contrast), then at -10 V (bright contrast in the center area of 2×2
us
μm2).
Figure 2 (a) Schematics of the multiferroic Co/BTO/Nb-STO thin-film heterostructure and
an
the voltage-modified MOKE magnetometer. (b) Magnetic Kerr hysteresis loops measured
M
along the EA and HA.
te
zero magnetic field.
d
Figure 3 The AC-voltage-induced relative changes in the total Kerr signal measured under
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Figure 4(a) Vector diagrams for the field amplitudes of the reflected laser beam from the Co film surface before and after magnetization reversal (b) Experimentally measured average Kerr signal changes, i.e., ΔIav/I, and corresponding fitting results by Eq. (11). (c) Purely voltage-induced hysteric changes in magnetization.
11 Page 11 of 22
Highlights Multiferroic Co/BTO thin film with in-plane uniaxial magnetic easy axis was grown by
ip t
PLD and magnetron sputtering. Contribution of electric-optical effect was excluded from voltage-induced magnetization
cr
changes by theoretical and experimental analysis.
us
A purely voltage-driven hysteric magnetization change of up to about 8% has thus been
Ac ce p
te
d
M
an
obtained.
12 Page 12 of 22
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Page 13 of 22
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Page 14 of 22
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Page 15 of 22
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Page 16 of 22
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Page 17 of 22
Ac
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ed
ce pt
Ac
M an
cr
us
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Ac
Page 20 of 22
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Page 22 of 22
S0169-4332(14)01243-4 http://dx.doi.org/doi:10.1016/j.apsusc.2014.05.202 APSUSC 28021
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-3-2014 25-5-2014 28-5-2014
Please cite this article as: L. Shu, J.-M. Hu, Y. Gao, L. Wu, J. Ma, Y.H. Lin, C.W. Nan, Voltage-driven hysteretic changes in magnetization in multiferroic Co/BTO composite thin films, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.202 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Voltage‐driven hysteretic changes in magnetization in multiferroic Co/BTO composite thin films
ip t
Li Shu, Jia-Mian Hu, Yao Gao, Liang Wu, Jing Ma*, Y. H. Lin, C. W. Nan
School of Materials Science and Engineering, and State Key Lab of New Ceramics and
Email: [email protected]
us
*
cr
Fine Processing, Tsinghua University, Beijing 100084, China
an
Abstract
Multiferroic Co/BaTiO3 layered composite thin film were grown on Nb-doped SrTiO3
M
single crystal by pulse laser deposition, in which the polycrystalline Co film was annealed
d
under magnetic field to induce in-plane uniaxial magnetic easy axis. Voltage-induced
te
magnetization changes along and perpendicular to the easy axis were measured by
Ac ce p
Magneto-Optical Kerr effect (MOKE) magnetometer without applied magnetic field. These changes in magnetization could be due to magnetoelectric coupling between Co and BaTiO3, and to possibly electro-optical effect of BTO if the MOKE laser could penetrate the top Co film. After excluding the electro-optical interference by analyzing the experimental results within an optical model, a hysteric loop of magnetization versus voltage is identified with a relative change in magnetization of about 8%. Key words Multiferroic composites; Thin film; Magneto-Optical Kerr effect; Electro-optical effect
1 Page 1 of 22
1. Introduction Artificial multiferroic heterostructures of ferroelectric and ferromagnetic layers are of
ip t
increasing interest because the cross-coupling between the magnetic and electric polarizations [1,2] allows novel means of controlling magnetization or polarization. For
cr
example, an electric voltage, rather than magnetic field, can be directly used to control
us
magnetization, known as converse magnetoelectric (ME) coupling, and hence can potentially be utilized to design novel spintronic or magnetic devices with much lower power
an
consumption and higher speed. Examples include voltage-driven magnetic random access
M
memories [3-6] and logic circuits [7]. To ensure non-volatility of these devices, voltage-driven hysteric changes in magnetizations must be obtained in multiferroic
d
heterostructures[8,9]. Although such magnetization-voltage hysteresis loop has been
te
demonstrated in La0.8Sr0.2MnO3/Pb(Zr0.2Ti0.8)O3 layered thin films at low temperature (100
Ac ce p
K) via ferroelectric control of spin-polarized charge densities at the hetero-interface, room-temperature demonstration of this hysteresis loop in multiferroic composite thin films still remains scarce. In this article, we observe voltage-driven hysteretic changes in magnetization in multiferroic Co/BaTiO3 composite thin films by Magneto-Optical Kerr Effect (MOKE) magnetometer [see Fig. 1(a)]. In particular, no external magnetic field is applied throughout the measurement process, thus avoiding the unintended role that the magnetic field may play in voltage-driven magnetization changes as in conventional MOKE measurement [10]. 2. Experiment 2 Page 2 of 22
Heterostructure of the ferroelectric BaTiO3 (BTO) and the ferromagnetic Co was fabricated. The BTO thin film of ~100 nm in thickness was epitaxially grown on (100)
ip t
oriented Nb-doped SrTiO3 (STO) substrate using pulsed laser deposition as before [11], and then transferred to a DC magnetron sputtering system for the deposition of polycrystalline
cr
Co thin film with 20 nm in thickness at 200 °C. During deposition, an external magnetic
us
field of 300 Oe was applied to induce an in-plane magnetic easy axis[12]. The Co film and Nb-doped STO substrate are used as the top and bottom electrodes for ferroelectric
an
(Precision Multiferroic, Radiant Technologies, Inc.) and piezoelectric measurement.
M
MOKE magnetometer (NanoMOKE2, Durham Magneto Optics Ltd., UK) was utilized to detect local magnetism variations of the Co thin film [see Fig. 2(a)]. The induced uniaxial
d
magnetic easy axis is evidenced by the square-shaped magnetic hysteresis loop in which the
te
applied magnetic field H is collinear with the easy axis [i.e., Ω=0°, see Fig. 2(b)], as well as
Ac ce p
the linear-like hard-axis loop in which the H is perpendicular to the easy axis (Ω=90°). Furthermore, a previously developed AC voltage (V) modulated MOKE technique is employed to detect voltage-induced Kerr signal changes under no applied magnetic field, which can ensure a pure electric-field modulated magnetism [10]. 3. Results and Discussion
The X-ray diffraction pattern of the heterostructure is shown in Fig. 1(a), clearly demonstrating a (001) orientation for the BTO thin film. The (002) peak of BTO indicates the presence of out-of-plane ferroelectric domain. Figure 1(b) shows the surface morphology of the BTO film by atomic force microscope, demonstrating a root-mean-square surface 3 Page 3 of 22
roughness of about 1 nm. A well-defined ferroelectric hysteresis loop is obtained at 1000 Hz [see Fig. 1(c)], and local ferroelectric domain switching behaviors in an area of 6×6 μm2 are
ip t
clearly imaged [see Fig. 1(d)]. Figure 3 shows the AC-voltage (sinusoidal wave, 13 Hz)-induced relative changes in the
cr
total Kerr signal, i.e., ΔI/I. The curve measured along the easy axis (Ω=0°) shows an evident
us
hysteresis (see the two remnant states at V=0), while the curve along the hard-axis (Ω=90°) is linear-like with little hysteresis. Surprisingly, the hard-axis curve even shows a slightly
an
bigger Kerr signal changes than the easy axis curve (Fig. 3), though the remnant
M
magnetization (i.e., H=0 for such AC-voltage testing) should almost be zero along the hard axis [see Fig. 2(b)]. Therefore, the observed remarkable Kerr signal changes along the hard
d
easy should not be largely based on voltage-induced changes in magnetization (i.e., converse
te
ME coupling). Another possible cause is the voltage-induced changes in the surface
Ac ce p
reflectivity of the BTO film, also known as the electric-optical effect [13,14], because the present Co film thickness (20 nm) may not completely prevent the penetration of the laser beam. Thus we need to first extract this electric-optical contribution by analyzing experimentally measured Kerr signal changes within an optical model. Then the voltage-induced magnetization changes along the easy axis can be obtained by excluding such electric-optical contribution. Based on a previously developed model [13], the voltage-induced Kerr signal changes from electric-optical effect (ΔIEO/I) and the converse ME effect (ΔIME/I) can be expressed as 2 ΔI EO ΔRss sin ψ + 2rps Δrss sinψ cosψ = I Rss sin 2 ψ
(1) 4 Page 4 of 22
2 ΔI ME ΔR ps cos ψ + 2rss Δrps sinψ cosψ = I Rss sin 2 ψ
(2)
where Rss and rss ( Rss = rss2 ) are the reflectance and reflection coefficient for the
ip t
longitudinally polarized Kerr vector Ess; and Rps and rps for the orthogonally polarized Kerr
between the laser analyzer and the Eps, as shown in Fig. 4(a) [15].
cr
vector Eps that can be reversed after magnetization reversal; and ψ is the azimuth angle
us
When the hard easy is in the incidental plane of laser beam under zero magnetic field,
an
rps is zero. Thus the contributions from the electric-optical effect (ΔIEO/I) and converse ME effect (ΔIME/I) to the relative Kerr signal changes measured along the hard axis, i.e., (ΔI/I)h,
M
are,
(3) (4)
te
ΔI ME =0 I
d
ΔI EO ΔRss = I Rss
Ac ce p
Similarly, the relative Kerr signal changes measured along the easy axis, i.e.,(ΔI/I)e, is also comprised of ΔIEO/I and ΔIME/I, which can be expressed as,
ΔI EO ΔRss (1 + 2rps / rss cotψ ) = I Rss
(5)
2 ΔI ME Δrps ( 2rps cot ψ + 2rss cotψ ) = I Rss
(6)
Given that rps is proportional to the magnetization [16], the relative magnetization change, ΔM/M, can be represented by the change in surface reflectivity, i.e., Δrps/rps, that could be obtained from Eqs. (3-6), i.e.,
5 Page 5 of 22
Δrps / r ps
⎡⎛ ΔI ⎞ ⎛ 2r ⎞ ⎛ ΔI ⎞ ⎤ Rss ⎢⎜ ⎟ − ⎜1 + ps cotψ ⎟ ⎜ ⎟ ⎥ rss ⎠ ⎝ I ⎠h ⎦ ⎣⎝ I ⎠ e ⎝ = 2 2r ps ( rps cot ψ + rss cotψ )
(7)
ip t
The two reflection coefficients rps and rss in Eq. (7) could be obtained by fitting the experimentally measured average Kerr signal changes i.e., ΔI/Iav, as a function of the
cr
analyzer angle ψ [see Fig. 4(b)] before and after reversing magnetization with an applied
us
magnetic field. Before the magnetization reversal [corresponding to the left panel of Fig. 4(a)], the intensity of the laser transmitted through the polarizer, i.e., I+, is given by, 2
= rss2 sin 2 ψ + rps2 cos 2 ψ + 2rss rps sinψ cosψ
an
I + = ( rss sinψ + rps cosψ )
(8)
M
After reversing the magnetization [corresponding to the right panel of Fig. 4(a)], the transmitted laser intensity, i.e., I-,is given by, 2
d
I − = ( rss sinψ − rps cosψ )
(9)
te
= rss2 sin 2 ψ + rps2 cos 2 ψ − 2rss rps sinψ cosψ
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Thus the average transmitted laser intensity, i.e., Iav, can be calculated as, I av =
1 ( I + + I − ) + γ D = rps2 cos 2 ψ + rss2 sin 2 ψ + γ D . 2
(10)
The parameter γD is introduced to represent the depolarization of the incidental laser
because the polarizer is not 100% efficient. Accordingly, the ΔI/Iav can be given as,
4rps rss cosψ sinψ ΔI I + − I − = = 2 I av I av rps cos 2 ψ + rss2 sin 2 ψ + γ D
(11)
By fitting experimental measured ΔIav/I using Eq. (11) [see Fig. 4(b)], rss and rps can be obtained to be 0.85 and 1.25×10-3, respectively, with a γD of 1.1×10-4. The purely voltage-induced relatively change in magnetization (ΔM/M) can now be obtained by estimating corresponding changes in the reflection coefficient Δrps/rps based on 6 Page 6 of 22
Eq. (7). As shown in Fig. 4(c), an evident hysteresis is observed between -5 V and +5 V, with an abrupt change in magnetization of about 8%. This hysteresis should be related to
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irreversible ferroelectric domain switching [17] or charge effect [18,19], but the hidden physics remains further clarification.
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4. Conclusions
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In summary, we have fabricated a multiferroic Co/BTO composite thin film that has an in-plane uniaxial magnetic easy axis using pulse laser deposition and magnetron sputtering.
an
Combined theoretical and experimental analysis have been performed to exclude the
M
contribution of electric-optical effect from the voltage-induced total Kerr signal changes measured using Kerr magnetometer without magnetic field. A purely voltage-driven hysteric
d
magnetization change of up to about 8% has thus been obtained, which promises application
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te
potential as non-volatile magnetoelectric devices.
Acknowledgements
This work was supported by the NSF of China (Grant Nos. 51332001, 11234005 and
51221291), and Beijing Education Committee (Grant No. 20121000301).
7 Page 7 of 22
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te
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Ac ce p
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[14] J. Schubert, M. Siegert, M. Fardmanesh, W. Zander, M. Prompers, C. Buchal, J. Lisoni, C. H. Lei, Superconducting and electro-optical thin films prepared by pulsed laser deposition technique, Appl. Sur. Sci., 168, (2000) 208-214. [15] P. R. Cantwell, U. J. Gibson, D. A. Allwood, H. A. Macleod, Optical coatings for improved contrast in longitudinal magneto-optic Kerr effect measurements, J. Appl. Phys. 100 (2006) 093910. 9 Page 9 of 22
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M
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te
d
024420.
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Figure captions Figure 1(a) X-ray diffraction pattern of the multiferroic Co (20 nm)/ BTO (100 nm)/Nb-STO thin-film heterostructure. (b) Atomic force microscopic topography image (top view) of the
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BTO film. (c) Ferroelectric hysteresis loop of the BTO film. (d) The out-of-plane piezoelectric force microscopy phase image (an area of 6×6 μm2) with the BTO film first
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being poled at +10 V (dark contrast), then at -10 V (bright contrast in the center area of 2×2
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μm2).
Figure 2 (a) Schematics of the multiferroic Co/BTO/Nb-STO thin-film heterostructure and
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the voltage-modified MOKE magnetometer. (b) Magnetic Kerr hysteresis loops measured
M
along the EA and HA.
te
zero magnetic field.
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Figure 3 The AC-voltage-induced relative changes in the total Kerr signal measured under
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Figure 4(a) Vector diagrams for the field amplitudes of the reflected laser beam from the Co film surface before and after magnetization reversal (b) Experimentally measured average Kerr signal changes, i.e., ΔIav/I, and corresponding fitting results by Eq. (11). (c) Purely voltage-induced hysteric changes in magnetization.
11 Page 11 of 22
Highlights Multiferroic Co/BTO thin film with in-plane uniaxial magnetic easy axis was grown by
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PLD and magnetron sputtering. Contribution of electric-optical effect was excluded from voltage-induced magnetization
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changes by theoretical and experimental analysis.
us
A purely voltage-driven hysteric magnetization change of up to about 8% has thus been
Ac ce p
te
d
M
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obtained.
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