Strain and Ferroelectric-Field Effects Co-mediated Magnetism in (011

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Strain and Ferroelectric-field Effects Co-mediated Magnetism in (011)CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Multiferroic Heterostructures Ping Wang, Chao Jin, Dongxing Zheng, Dong Li, Junlu Gong, Peng Li, and Haili Bai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07584 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 22, 2016

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ACS Applied Materials & Interfaces

Strain and Ferroelectric-field Effects Co-mediated Magnetism in (011)-CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Multiferroic Heterostructures

Ping Wang,1 Chao Jin,1* Dongxing Zheng,1 Dong Li,1 Junlu Gong,1 Peng Li,2 and Haili Bai1

1

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing

Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, Tianjin 300354, PRC

2

Division of Physical Science and Engineering, King Abdullah University of Science

and Technology (KAUST), Thuwal 23955, Kingdom of Saudi Arabia

*

Author to whom all correspondence should be addressed. E-mail: [email protected]

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ABSTRACT

Electric-field mediated magnetism was investigated in CoFe2O4 (CFO, deposited by reactive cosputtering under different oxygen flow rates) films fabricated on (011)-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrates. Ascribed to the volatile strain effect of PMN-PT, the magnetization of the CFO films decreases along [01-1] direction whereas it increases along [100] direction under the electric field, which attributes to the octahedron distortion in the spinel ferrite. Moreover, a non-volatile mediation was obtained in the CFO film with low oxygen flow rate (4 sccm), deriving from the ferroelectric-field effect, in which the magnetization is different after removing the positive and negative fields. The cooperation of the two effects produces four different magnetization states in the CFO film with low oxygen flow rate (4 sccm), compared to the only two different states in the CFO film with high oxygen flow rate (10 sccm). It is suggested that the ferroelectric-field effect is related to the oxygen vacancies in CFO films.

Keywords: multiferroic heterostructures, CoFe2O4 films, strain, ferroelectric-field effect, oxygen vacancy

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INTRODUCTION

Electric-field mediated magnetism has kindled a tremendous amount of research interests in recent years due to its importance for exploiting dense, fast, energy-efficient, nonvolatile and multistate electronic memory devices based on magnetoelectric (ME) coupling.1‒4 Due to the weak room temperature intrinsic ME coupling in single-phase multiferroics,5 people shift their interests to multiferroic heterostructures which can achieve the strong ME coupling in straightforward layer-by-layer structures.6,7 Hence, electric-field mediated magnetism has been widely studied in multiferroic heterostructures by combining ferro(i)magnetic with ferroelectric or piezoelectric materials.8‒12 There are different ways to achieve the electric-field mediation of magnetism in multiferroic heterostructures. One is the strain-mediated ME effect, which can effectively mediate the magnetic anisotropy and further modulate the magnetization and magnetic coercive field.10,11 The magnetization

change

ratio

could

reach

as

much

as

90%

in

the

Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 under the modulation of strain.10 A 580 Oe of magnetic

coercive

field

change

was

also

observed

in

CoFe2O4/

Pb(Mg1/3Nb2/3)0.62Ti0.38O3 due to the strain effect.11 Another one is the ferroelectric-field effect, inducing charges accumulation/dissipation at the interface and producing spin dependent mediation, measured by ferromagnetic resonance,12‒14 −3− ACS Paragon Plus Environment


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which can also lead to the change of magnetic coercive field and effective magnetic field

in

La0.8Sr0.2MnO3/Pb(Zr0.2Ti0.8)O3

and

Co92Zr8/Pb(Mg1/3Nb2/3)0.7Ti0.3O3

heterostructures.14,15 Recently, the Pb(Mg1/3Nb2/3)1–xTixO3 substrate has drawn wide attention for strain

mediation

due

to

the

large

piezoelectric

coefficient.

For

the

(011)-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT), the piezoelectric coefficients can even reach ~ −3100 pC/N (d31) along the [100] direction and ~ 1400 pC/N (d32) along the [01-1] direction, respectively.16 The effective strain mediated magnetism by combining ferro(i)magnetic thin film and Pb(Mg1/3Nb2/3)1–xTixO3 had been achieved.10,11,17–21 On the other hand, ferroelectric-field effect mediated magnetism had also been realized in ferro(i)magnetic/Pb(Mg1/3Nb2/3)1–xTixO3 heterostructures based on the excellent ferroelectric characteristic of Pb(Mg1/3Nb2/3)1–xTixO3.14,22 The ferroelectric-field effect was reported to be able to produce spin-dependent screening, electronic bonding change, magnetic phase transition and so on, which modulated the magnetism

at surface

ferroelectric-field

effectively.22‒24

effects

mediated

If

the

combination

magnetism

can

be

of

strain

and

achieved

in

ferromagnetic/ferroelectric heterostructures, the regulation effect of electric field on the magnetism is expected to be improved.

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Generally, transition metal oxides are characterized by ground states with entangled spin, charge, lattice, and orbital degrees of freedom due to strong electronic correlations,25 which exhibit large sensitivity to strain, pressure, and chemical or electron doping.26 Spinel ferrite films are extensively used as ferromagnetic layers in multiferroic heterostructures.27–31 Spinel ferrite CoFe2O4 (CFO) owns large magnetic anisotropy and magnetostriction.28,29 Previous studies had convinced that the magnetic anisotropy of CFO can be mediated effectively by strain.11,30,31 In addition, the magnetism and oxygen vacancies are intimately correlated due to the superexchange interaction in the spinel ferrite.32‒34 In our previous work, we had found that the oxygen vacancies in the CFO films can migrate driving by electric fields.35 Hence, the oxygen vacancies played an important role in electric field mediated magnetism of CFO films.36,37 In this work, we fabricated the CFO/PMN-PT heterostructures to study the co-effect of strain and ferroelectric-field mediated magnetism. The strain effect induces the octahedron distortion in the spinel ferrite. The ferroelectric-field effect is related to the oxygen vacancies in CFO films. The cooperation of the two effects produces four different regulation states.

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EXPERIMENTAL DETAILS

Sample Preparation. The CFO films were grown under two different oxygen flow rates on single crystal PMN-PT (011) substrates with thickness of 0.2 mm by DC magnetron reactive cosputtering. The Fe (99.99%) and Co (99.9%) targets with a diameter of 60 mm were sputtered using two DC sputtering sources, keeping sputtering power at ~70 and 35 W, respectively. The substrate temperature was kept at ~500 oC during the deposition. The chamber pressure was maintained at 2 Pa in a gas mixture of Ar (99.999%) and O2 (99.999%) in the process. The Ar, O2 flow rates were kept at 100, 4 sccm for CFO-1 film, and 100, 10 sccm for CFO-2 film, respectively. Then the CFO-1 film was cooled down to room temperature in the vacuum, while the CFO-2 film was cooled down to room temperature still in a gas mixture of Ar, O2 with flow rates of 100, 10 sccm. Structural and Ferroelectric Characterization Measurements. The thickness of the CFO films was ~30 nm measured by a Dektak 6M surface profiler. The structure of the CFO films was characterized by X-ray diffraction (XRD, Cu Kα source, from 10o to 100o in step of 0.02o). The surface morphology was observed with Bruker Multimode 8 atom force microscopy (AFM). The polarization hysteresis loop of the PMN-PT substrate was measured by a TF Analyzer 1000 at 1 Hz. The rotation of the ferroelectric domains and ramp curve (amplitude) of the PMN-PT substrates were −6− ACS Paragon Plus Environment

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observed with piezoelectric force microscopy (PFM, AFM equipped with Nanoscope V controller). Magnetic Measurement under Electric Fields. The magnetic properties were measured along the in-plane [01-1] and [100] directions by the Quantum Design magnetic property measurement system (SQUID-VSM). The voltage across the CFO/PMN-PT structures was applied in situ on the sample along the [011] direction perpendicular to the sample surface by a Keithley 2400 source meter instrument during the magnetic measurement, with the Ta and Ag layers serving as the positive and negative electrodes, respectively. All the measurements were performed at room temperature.

RESULTS AND DISCUSSION

The XRD θ–2θ pattern of the CFO-1 film on the PMN-PT (011) substrate is given in Fig. 1(a). The bulk CFO has a cubic unit cell with lattice parameter of 8.39 Å.38‒40 The lattice parameter of PMN-PT is 4.02 Å with rhombohedral phase.4,31,41,42 Only the (022), (044) diffraction peaks of the CFO film and the (011), (022) diffraction peaks of the PMN-PT substrate (supporting information, Fig. S1) are observed, indicating that the film is highly oriented along the [011] direction without any other impurity phases. Based on Bragg equation, the out-of-plane ([011] direction) −7− ACS Paragon Plus Environment


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interplanar space of CFO-1 film can be calculated and it is 5.89 Å. Moreover, CFO-2 film is [011] oriented with weak diffraction reflections of other planes of CFO and a few oxide impurities (supporting information, Fig. S1). The AFM images convince that the CFO-1 and CFO-2 films are uniform with the average roughness of 1.37 and 1.03 nm, respectively (supporting information, Fig. S2). Figure 1(b) shows the room temperature polarization hysteresis loop of the PMN-PT substrate. The PMN-PT substrate has perfect ferroelectric performance with the remnant polarization (PR) of ~24 µC/cm2 and coercive field (EC) of ~5 kV/cm, respectively. The larger PR and the less EC benefit polarization reversal and further achieving mediation of magnetism by small electric field comparing to the other ferroelectric materials, such as the Pb(Zr0.2Ti0.8)O3 and BiFeO3 (the coercive fields are about hundred kV/cm).15,43,44 Moreover, the PR, EC of the CFO-1/PMN-PT and CFO-2/PMN-PT heterostructures are ~37 µC/cm2, ~4.7 kV/cm and ~30 µC/cm2, ~3.6 kV/cm (supporting information, Fig. S3). The ferroelectricity is still perfectly present in CFO-1/PMN-PT and CFO-2/PMN-PT heterostructures. The PFM phase image of the PMN-PT substrate is displayed in Fig. 1(c). The relative bright and dark contrasts represent the domain with polarization states pointing upward and downward. After applying a positive probe bias switching (+10 V) in 3×3 µm2 area of the PMN-PT substrate, the polarization state point downward in the 3×3 µm2 area changes into the polarization state point upward, which also illustrates the polarization reversal of the PMN-PT −8− ACS Paragon Plus Environment

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substrate, as shown in the inset of Fig. 1(c). The piezoelectricity of the PMN-PT substrate was also certified by measuring the PFM ramp curve (supporting information, Fig. S4). Figure 1(d) shows the schematic of the CFO/PMN-PT

(b)

30 15 0 -15

4.0 sccm

2

(a)

P (µC/cm )

PMN-PT (022)

CFO (044)

CFO (022)

PMN-PT (011)

heterostructures and measurement configuration. Intensity (arb. units)

-30 20

40 60 80 2θ (degree)

100 -10

-5

0

5

(c)

(d) Ta CFO PMN-PT Ag

z

y

x [1 0

1 µm

10

E (kV/cm)

[011]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0]

[01 -1]

1 µm

FIG. 1. (a) XRD θ–2θ scan pattern of the CFO-1/PMN-PT heterostructure. (b) Room temperature polarization hysteresis loop of the PMN-PT substrate. (c) PFM phase image of the PMN-PT substrate in 6×6 µm2 area, the inset (labeled by blue square) shows the PFM phase image after applying a positive probe bias switching (+10 V) in 3×3 µm2 area (labeled by red square) of the PMN-PT substrate, the relative bright and dark contrasts respectively refer to the domain with polarization states point upward and downward. (d) Schematic of the CFO/PMN-PT heterostructures and measurement configuration. The Ta and Ag layers serve as the positive and negative electrodes, respectively. −9− ACS Paragon Plus Environment


3

M (emu/cm )

400

MR

64 48

200

(a)

32

4.0 sccm

-0.2 0.0 0.2

0

0 kV/cm +10 kV/cm +0 kV/cm -10 kV/cm -0 kV/cm

-200

6

HC

0 -6

-400

-0.6 -0.4 -0.2

-50

-25

0

25

50

H (kOe) 400

3

M (emu/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MR

48 36

200

(b)

24 -0.2

0

0.0

-200

10.0 sccm

0.2

0 kV/cm +10 kV/cm +0 kV/cm -10 kV/cm -0 kV/cm

10

HC

0 -10

-400

-0.6

-50

-25

0

-0.4

25

-0.2

50

H (kOe)

FIG. 2. Magnetization curves of (a) the CFO-1/PMN-PT and (b) the CFO-2/PMN-PT heterostructures measured at 300 K in five states. The value of electric field applied was 10 kV/cm. The electric and magnetic fields were applied along out-of-plane [011] and in-plane [01-1] directions, respectively. The insets (upper left and bottom right) show the enlarged images of magnetization curves to clearly distinguish the changes of MR and HC. TAB. 1. The values of HC and MR in five states of the CFO/PMN-PT heterostructures. Electric field applied (kV/cm)

Physical quantity MR (emu/cm ) HC (Oe) MR (emu/cm3)

0 60.20 455 40.31

+10 33.19 265 34.39

‒10 36.38 273 34.24

+0 51.67 379 40.22

‒0 53.84 395 40.57

HC (Oe)

487

428

418

473

492

3

CFO-1 CFO-2

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Figure 2 displays the magnetization curves (M–H curves) of the CFO/PMN-PT heterostructures at 300 K in 5 states, labeled as state “0”, “+”, “+0”, “‒”, “‒0”. The state “0”, “+”, “+0”, “‒”, “‒0” respectively represents the one applying no electric field, applying positive electric field, removing positive electric field, applying negative electric field, and removing negative electric field. The value of the electric field applied was 10 kV/cm. The electric and magnetic fields were along out-of-plane [011] direction and in-plane [01-1] direction, respectively. The insets (upper left and bottom right) in the Figs. 2(a) and 2(b) show the enlarged images of the M–H curves to clearly distinguish the changes of remnant magnetization (MR) and magnetic coercive field (HC). The MR and HC of the CFO-1 and CFO-2 films both decrease under both positive and negative electric fields (Tab. 1). Furthermore, the reductions of MR and HC of the CFO-1 film under positive electric field are larger than those under negative electric field. After removing the electric fields, the decreased MR and HC recover whereas both values are still less than those without applying electric fields. But the reductions of MR and HC of the CFO-2 film are almost same under positive and negative electric fields (Tab. 1). After removing the electric fields, the decreased MR and HC approximately return back to those values without applying electric fields.



0.90

3 1

0.85

-10

H//[01-1]

0

90

180

(c)

1.00

270

0.95 0.90

-10

H//[01-1]

150

300

450

t (s)

600

1.01

3 4

1.00

2

-10

H//[100]

0

10

0

0

1

0

360

10 sccm

4 sccm 1.02

10

150

300

(d)

450

600

10 sccm

1.02 1.01

0 1.00 -10

M(E)/M(0)

0

(b)

10

M(E)/M(0)

4

2

0.95

10

E (kV/cm)

M(E)/M(0)

4 sccm

E (kV/cm)

(a)

1.00

M(E)/M(0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H//[100]

0

150

300

450

600

t (s)

FIG. 3. Time dependent normalized M(E)/M(0) curves of the CFO/PMN-PT heterostructures with the magnetic field of 1 kOe at 300 K. The electric field (10 kV/cm) was applied along out-of-plane [011] direction. (a), (b) M(E)/M(0)–t curves of the CFO-1/PMN-PT heterostructure with the magnetic fields along in-plane [01-1] and [100] directions, respectively. (c), (d) M(E)/M(0)–t curves of the CFO-2/PMN-PT heterostructure with the magnetic fields along in-plane [01-1] and [100] directions, respectively.

Figure 3 shows the time dependent normalized M(E)/M(0) of CFO/PMN-PT heterostructures with the magnetic field of 1 kOe at 300 K, where M(E) is the magnetization under electric field, M(0) the magnetization without applying electric field, and t the time. Figures 3(a) and 3(b) show the M(E)/M(0)–t curves of the CFO-1/PMN-PT heterostructure with the magnetic fields along in-plane [01-1] and


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[100] directions, respectively. There are several characteristics can be observed. Firstly, the magnetization decreases in [01-1] direction whereas it increases in [100] direction. Secondly, the magnetization under positive electric fields is lower (higher) than that under negative electric fields in [01-1] direction (in [100] direction). Thirdly, the magnetization after removing positive electric fields is also different from that after removing negative electric fields. Hence, four different magnetization states were obtained under different electric fields. Moreover, Figures 3(c) and 3(d) present the M(E)/M(0)–t curves of the CFO-2/PMN-PT heterostructure with the magnetic fields along in-plane [01-1] and [100] directions, respectively. In this situation, the magnetization decreases along [01-1] direction but increase [100] direction with applying electric fields as well. Nevertheless, we can hardly observe any difference between positive and negative electric fields. As a comparison to four magnetization states in sample CFO-1, only two different magnetization states were detected in sample CFO-2.



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FIG. 4. (a) Unit cell and spontaneous polarizations of (011) oriented PMN-PT substrate. (b)‒(d) Schematics of polarization by the positive electric field induced strain in PMN-PT. (e)‒(g) Schematics of polarization by the negative electric field induced strain in PMN-PT. (h), (i) Schematics of strain effect on Co2+ ions at octahedral sites, (h) Octahedron in the CFO film without strain, (i) Octahedron distortion in the CFO film with strain. (j), (k) Schematics of ferroelectric-field effect, and the oxygen vacancy distributions at the CFO/PMN-PT interface under positive (j) and negative (k) electric fields.

To illustrate the results of the M‒H curves and M(E)/M(0)–t curves, the piezoelectricity and ferroelectricity of PMN-PT substrate should be taken into account, which plays an important role in electric field mediated magnetism.10,14,17‒22 For the PMN-PT with rhombohedral (R) phase, the spontaneous polarizations are along the directions, which lie along the diagonals of the (011) and (01-1) planes in (011)-cut case,10,45 as shown in Fig. 4(a). When a positive electric field is applied to pole the PMN-PT substrate, the polarizations are rotated towards the [0-1-1] direction with only two of them, as presented in the Fig. 4(b). With the electric field increasing, the polarizations become more obvious and finally along [0-1-1] direction, as presented in Figs. 4(c) and 4(d), which leads to a tensile strain along the [01-1] direction and compressive strain along the [100] direction.10 The negative electric − 14 − ACS Paragon Plus Environment

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field pole the PMN-PT similarly as the positive electric field, just towards the [011] direction, which also produces a tensile strain along the [01-1] direction and a compressive strain along the [100] direction, as shown in Figs. 4(e)‒4(g). The CFO has an inverse spinel structure, with the Co2+ cations occupying octahedral sites and the Fe3+ cations occupying both the tetrahedral (A) and octahedral (B) sites.46 The Co2+ ions at B sites are responsible for the magnetic anisotropy of the CFO material.47 The anisotropic strain-field induced by the PMN-PT substrate promotes the distortion of the octahedron, increasing the bonds along the [01-1] direction and decreasing the bonds along the [100] direction, as shown in the Figs. 4(h) and 4(i). The strain is related to the spin and orbital degrees of freedom.48 The occupation of 3d orbitals and the overlapping of electron clouds between adjacent atoms choose preferential orientation due to the distortion by the anisotropic strain.49 So the magnetic anisotropy of CFO is changed under strain effect. The stress anisotropy energy is described by E = K cos 2 θ , the anisotropy constant is described by

K = −3λσ / 2 , σ = Y ε , where cos θ

is the directional cosine of the

magnetization vector along the film normal, λ the magnetostriction coefficient of CFO, σ the stress, Y the Young’s modulus, ε the strain.28,31 CFO has negative magnetostriction along both [01-1] and [100] direction.29 When the CFO film suffers tensile strain, σ > 0 , K > 0 , E > 0 , the energy increases along [01-1] direction. The magnetic moments switch away from the [01-1] direction more easily. The sample − 15 − ACS Paragon Plus Environment


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becomes harder to be magnetized along [01-1] direction. As a result, MR, HC and magnetization decrease under electric fields due to the tensile strain along the [01-1] direction (see Figs. 3(a) and 3(c)), consistent with the results of Wang et al and Yang et al.11,31 On the contrary, CFO film is under compressive strain along [100] direction, the sample becomes easier to be magnetized, which leads to the increase of the magnetization along the [100] direction (see Figs. 3(b) and 3(d)). In principle, the strains under positive and negative electric fields should be the same,50 as shown in Fig. 4. The magnetism mediations of the CFO-2 film are almost the same when we applied positive and negative electric fields. However, those of the CFO-1 film show a distinct difference, as clearly shown in Fig. 3. Further, the magnetization of CFO-1 film after removing positive electric fields is also different from that after removing negative electric fields. Therefore, this observation probably indicates the existence of another non-volatile mediation besides the volatile strain in the CFO-1 film, but this non-volatile is weak in the CFO-2 film. Besides piezoelectricity, the ferroelectricity is crucial in the PMN-PT substrates, which can also realize mediation of magnetism by ferroelectric-field effect.12,17,22 Notably, the non-volatile mediation exclusively appears in the CFO film under the O2 flow rate of 4 sccm (see Figs. 3(a) and 3(b)). Here, ascribed to vacuum-annealing at high temperatures,35,51 a large number of the oxygen vacancies presents in the CFO-1 film under 4 sccm of O2 flow rate. Whereas, few oxygen vacancies exist in the CFO-2 − 16 − ACS Paragon Plus Environment

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film fabricated with 10 sccm of O2 flow rate and cooled down to room temperature in the same atmosphere. Therefore, the non-volatile mediation, absent in the CFO film under the O2 flow rate of 10 sccm (see Figs. 3(c) and 3(d)), shows strong relevance to the oxygen vacancies. The positive electric field accumulates electrons, and negative electric field accumulates holes on the upper surface of PMN-PT in the CFO-1/PMN-PT heterostructure, respectively. The electrons/holes will attract/repel the oxygen vacancies at the CFO-1/PMN-PT interface, leading to different oxygen vacancy distributions. The oxygen vacancies are accumulated near or away from CFO/PMN-PT interface, as shown in Figs. 4(j) and 4(k). In the spinel ferrite, the magnetism and oxygen vacancies are correlated due to the A-O-B, A-O-A and B-O-B superexchange interactions.32‒34 The redistributions of oxygen vacancy under the positive and negative electric fields can affect the superexchange interactions of A-O-B, A-O-A and B-O-B in the CFO films. The influences of oxygen vacancies for superexchange interactions are different when they are accumulated in the local region and dispersed in a large region. Hence, it is possible that the numbers of A-O-B, A-O-A and B-O-B superexchange interactions are changed in the local region with high density of oxygen vacancies. In addition, the oxygen vacancy plays an important part in the bond angle and the valence of the transition metal in spinel ferrite.51 The local bond angles of A-O-B, A-O-A and B-O-B and local valences (Fe2+\3+, Co2+\3+) impact the spin structures of CFO films. − 17 − ACS Paragon Plus Environment


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The destabilization of spins was able to mediate magnetic anisotropy.22,33 Moreover, as shown in Figs. 3(a) and 3(b), it can be seen that the magnetization decrease/increase is a relaxation process, which might relate to the response progress of the oxygen vacancies redistribution under the electric fields.

CONCLUSION

The co-mediation of magnetism by the volatile anisotropic strain and non-volatile ferroelectric-field effect has been achieved under electric fields in the CFO/PMN-PT heterostructures. The tensile strain along [01-1] direction reduces the magnetization while the compressive strain along [100] direction enhances the magnetization under the electric fields. Comparing to the only two different regulation magnetization states in the CFO film deposited under higher oxygen flow rate, four different regulation states are presence in the CFO film fabricated under lower oxygen flow rate due to the combined ferroelectric-field and strain effects, which indicates that the ferroelectric-field effect is related to the oxygen vacancies. The four different regulation magnetization states are expected to have great potential applications in multistate electronic memory devices.


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ASSOCIATED CONTENT

*Supporting Information XRD θ–2θ scans of the PMN-PT substrate and the CFO-2/PMN-PT heterostructure, AFM images of the CFO-1/PMN-PT and the CFO-2/PMN-PT heterostructures, Room temperature polarization hysteresis loops of the CFO-1/PMN-PT and CFO-2/PMN-PT heterostructures, PFM ramp curve (amplitude) of the pure PMN-PT substrate.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contributions P.W. and C.J. designed the outline of the manuscript and wrote the main manuscript text. D.Z., D.L., J.G., P.L. and H.B. contributed detailed discussions and revisions. All authors reviewed the manuscript.

Notes The authors declare no competing ﬁnancial interest.



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ACKNOWLEDGEMENTS

C.J. would like to acknowledge the support of National Natural Science Foundation of China (11304221 and 11434006) and Natural Science Foundation of Tianjin City (13JCZDJC32800).


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Multiferroic

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Table of Contents Graphic (TOC)

0.95

0 0.90

3 1

0.85

-10

H//[01-1]

0

90

180

270

1.02 (b)

0

3 4

1.00

2

150

-10 300

450

under electric fields

(d)

H//[100]

0

Co Co

strain effect

10

1

1.01

O

[01-1] [01-1] [01-1]

360

4 sccm

O Co

[100]

4

2

(c)

10

E (kV/cm)

M(E)/M(0)

4 sccm

E (kV/cm)

(a)

1.00

M(E)/M(0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


+E

-E

----------

++++++++++

O vacancy

CFO PMN-PT

---------ferroelectric-field effect

++++++++++

600

t (s)

− 29 −


Strain and Ferroelectric-Field Effects Co-mediated Magnetism in (011

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