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Electric-field Control of Magnetism in Co40Fe40B20/(1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 Multiferroic Heterostructures with Different Ferroelectric Phases Yan Liu, Yonggang Zhao, Peisen Li, Sen Zhang, Dalai Li, Hao Wu, Aitian Chen, Yang Xu, X.F. Han, Shiyan Li, Di Lin, and Haosu Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10233 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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ACS Applied Materials & Interfaces

Electric-field Control of Magnetism in Co40Fe40B20/(1x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 Multiferroic Heterostructures with Different Ferroelectric Phases Yan Liu, †,‡ Yonggang Zhao,*, †,‡ Peisen Li,§ Sen Zhang,|| Dalai Li,⊥ Hao Wu,⊥ Aitian Chen, †,‡ Yang Xu,# X. F. Han,⊥ Shiyan li,# Di Lin,▽ and Haosu Luo.▽ †

Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics,

Tsinghua University, Beijing 100084, China ‡

Collaborative Innovation Center of Quantum Matter, Beijing 100084, China

§

College of Mechatronics and Automation, National University of Defense Technology,

Changsha 410073, China ||

College of Science, National University of Defense Technology, Changsha 410073, China

⊥

Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences,

Beijing 100190, China #

State Key Laboratory of Surface Physics and Department of Physics, Fudan University,

Shanghai 200433, China ▽

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China

ACS Paragon Plus Environment

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KEYWORDS:

multiferroic,

nonvolatile,

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electric-field-controlled-magnetization,

phase,

temperature

ABSTRACT: Electric-field control of magnetism in multiferroic heterostructures composed of Co40Fe40B20

(CoFeB)

and

(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3

(PMN-xPT)

with

different

ferroelectric phases via changing composition and temperature is explored. It is demonstrated that the nonvolatile loop-like bipolar-electric-field-controlled magnetization, previously found in the CoFeB/PMN-xPT heterostructures with PMN-xPT in the rhombohedral (R) phase around morphotropic phase boundary (MPB), also occurs for PMN-xPTs with both R phase far away from MPB and monoclinic (M) phase, suggesting that the phenomenon is the common feature of CoFeB/PMN-xPT multiferroic heterostructures for PMN-xPT with different phases. The magnitude of the effect changes with increasing temperature and volatile bipolar-electric-fieldcontrolled magnetization with a butterfly-like behavior occurs when ferroelectric phase changes to the tetragonal phase (T). Moreover, for the R-phase sample with x = 0.18, an abrupt and giant increase of magnetization is observed at a characteristic temperature in the temperature dependence of magnetization. These results are discussed in terms of coupling between magnetism and ferroelectric domains including macro and microdomains for different ferroelectric phases. This work is helpful for understanding the phenomena of electric-field control of magnetism in FM/FE multiferroic heterostructures and is also important for applications.


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INTRODUCTION Electric-field control of magnetism is crucial for exploiting dense, fast, and nonvolatile random access memory with reduced energy consumption. A promising way to control magnetism via electric fields is using the ferromagnetic (FM)/ferroelectric (FE) multiferroic heterostructures.1-4 Several approaches have been demonstrated to realize electric-field control of magnetism via magnetoelectric (ME) coupling, including manipulations of exchange bias effect,1-6 charge carrier density7 or exchange coupling8 at the interface of FE/FM and strain.9-17 Among them, the strain-mediated FM/FE multiferroic heterostructures have been widely studied due to the various choices of FM and FE materials and remarkable ME coupling.1-3,9-17 It should be mentioned that there also have been some reports on combination of exchange-biased systems and ferroelectric materials to achieve electric-field-controlled magnetization reversal,18-21 and electrical manipulations of reversible magnetization reversal has been realized at zero magnetic field.21 To obtain large ME effect in the strain-mediated FM/FE multiferroic heterostructures, FE materials with large piezo strains are favored. In this regard, the relaxor ferroelectric (1x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystals with excellent piezoelectric property, especially those near the morphotropic phase boundary (MPB),22-24 have been widely used as the ferroelectric substrate in FM/FE multiferroic heterostructures. However, most of the previous reports on electric-field control of magnetism in FM/PMN-xPT through strain mediated interaction are volatile.9-11,25 While nonvolatile electric-field control of magnetism is required for applications of information storage. There have been a few reports on the nonvolatile electricfield control of magnetism in the strain-mediated FM/FE multiferroic heterostructures.12,14,26,27 It was demonstrated that the electric-field-induced remanent strain or irreversible ferroelectric


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domain effect controls the magnetization, leading to nonvolatile electric-field control of magnetism.12,14,26,27 However, a specific value or range of electric field is required to induce this effect12,14,26,27 and the magnetization shows obvious relaxation with time after removal of electric fields, which are disadvantageous for applications.26 Recently we reported a large and nonvolatile loop-like bipolar-electric-field-controlled-magnetization at room temperature in Co40Fe40B20(CoFeB)/PMN-0.30PT(001),13 and the effect is related to the combined action of 109° FE domain switching and absence of magnetocrystalline anisotropy in CoFeB. Since PMN0.30PT is around the MPB with the highest piezoelectric property and complex phase structures,28-30 it is not clear whether MPB or rhombohedral (R) phase is essential for the large and nonvolatile bipolar-electric-field-controlled-magnetization. It is well-known that PMN-xPT shows rich phases in the temperature-composition (T-x) phase diagram.31,32 Upon increasing x from 0.30, it changes from the R phase to the monoclinic (M) phase coexisting with the R phase or tetragonal (T) phase for x ranging from 0.31 to around 0.34 at 300 K.31 For x larger than 0.35, the T phase is dominant.30,31 While decreasing x from 0.30, the R phase without the MPB emerges for smaller x, and the degree of relaxors property related to the polar nano-regions (PNRs) increases with decreasing x.33 So far, for the (001)-cut PMN-xPTs, only compositions of 0.28 ≤ x ≤ 0.30 around the MPB region have been used in the electric-field control of magnetism in FM/PMN-xPT,9,13,34-36 while electric-field-controlled-magnetization based on PMN-xPT far away from the MPB, and PMN-xPT near MPB but beyond the R phase (e.g. the M phase) are still lacking. In this case, it is worthy to clarify the role of the ferroelecric phase in the electricfield control of magnetism, and more importantly whether MPB is essential for the large and nonvolatile bipolar-electric-field-controlled-magnetization reported in the CoFeB/PMN0.30PT(001).13 Since ferroelectric phase of PMN-xPT also changes with temperature,31 we can


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use temperature as another way to explore the effect of ferroelectric-phase change on electricfield control of magnetism in FM/PMN-xPT. Moreover, for the poled PMN-xPT with small x, there is a sharp macro-micro domain transition at a certain temperature (Td) for the heating process.37,38 The sharp change of the macrodomain state to the microdomain state provides a good platform for the research of ferroelectric domain size effect with the unique size change (macro to micro) in the FM-FE domain coupling system, which has not been addressed before. These studies will enrich the knowledge of electric-field control of magnetism in FM/PMN-xPT and should be also significant for applications. In this paper, we study electric-field control of magnetism in CoFeB/PMN-xPT heterostructures with x = 0.18, 0.28, 0.33, and 0.36, corresponding to R phase far away from the MPB, R phase, M phase and T phase around the MPB, respectively.31,39 The large and nonvolatile bipolar-electric-field-controlled-magnetization occurs for samples with x = 0.18, 0.28, 0.33, demonstrating that MPB is not essential for this effect. While volatile bipolar-electricfield-controlled-magnetization with a butterfly-like behavior was observed for sample with x = 0.36, suggesting that different mechanism is involved in the T-phase sample. The magnitude of electric-field-controlled-magnetization changes with increasing temperature and dramatic change occurs around the phase transition temperature. Moreover, the temperature dependence of magnetization for x = 0.18 sample shows an abrupt and giant increase at the macro-micro domain transition for the heating process. We discussed these results by considering the interaction between magnetism and ferroelectric domains and/or domain switching for different ferroelectric phases of PMN-xPT. EXPERIMENTAL SECTION


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Sample Preparation. Samples with Au/Ta (5 nm)/Co40Fe40B20 (20 nm)/PMN-xPT (0.5 mm)/Au structure were investigated. PMN-PT single crystals were grown using the modified Bridgeman technique and the details have been described in Ref. 40. PMN-PT substrates are (001)-cut with an error range within 0.5° and one-side-polished with a size of 5 mm × 5 mm × 0.5 mm. The substrates were unpoled before deposition of magnetic films. CoFeB soft magnetic films were deposited in a multisource ultrahigh vacuum magnetron sputtering system with a base vacuum of 1 × 10–6 Pa without a magnetic field. Au layers with a thickness of 300 nm were sputtered on both the top and bottom of the structures as electrodes. Magnetic Measurement Under Electric Fields. Electric fields were applied along the [001] direction perpendicular to the sample surface with a Keithley 6517A voltage source, and the magnetic measurements were carried out along the in-plane [110] and [1-10] directions, respectively, in a magnetic property measurement system (MPMS) with in situ electric fields. Strain-Electric Field Measurement. The strain gauge was pasted on the sample surface with glue along the in-plane diagonal direction. The resistance of strain gauge was measured with a constant current of 1 mA using a Keithley 2400 as a current source and voltmeter. Electric fields were also applied along the out-of-plane direction as the magnetic measurement. Ferroelectric Phase Characterization and Dielectric Permittivity Measurements. Ferroelectric phases for PMN-xPT substrates were characterized by X-ray diffraction (XRD) on a D/max-2500 X-ray diffractometer with a Cu Kα1 radiation. Dielectric permittivity measurements were executed by a WK 6500B precision impedance analyzer at various frequencies. RESULTS AND DISCUSSION


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Au Ta CoFeB

(a)

R [001]

(200)

PMN-PT

E

[010]

Intensity (arb.units)

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(b) x = 0.36 x = 0.33 x = 0.28 x = 0.18

[100]

A

44.6

44.8

45.0 45.2 2θ (deg)

45.4

Figure 1. (a) Schematic of the sample structure and experimental configuration. (b) Selected regions of XRD diffraction patterns (without Kα2 component) for PMN-xPT substrates. Figure 1a is a schematic of the sample configuration, and the detailed experimental methods are in Experimental Section. We used the (001)-cut PMN-xPT single crystals with measured compositions of x = 0.18, 0.28, 0.33, and 0.36 as the piezoelectric substrates, and amorphous CoFeB film. Since segregation during growth of PMN-xPT single crystals inevitably results in a deviation of the real composition from the nominal one,40,41 we used electron probe microanalyzer (EPMA) to check the composition uniformity and obtained the real compositions of PMN-xPT single crystals (Table S1, Supporting Information). The X-ray diffraction profiles around the pseudo-cubic (200) diffraction peak for PMN-xPTs with x = 0.18, 0.28, 0.33, and 0.36 measured at 300 K are shown in Figure 1b. Both x = 0.18 and x = 0.28 single crystals display singlet (200) pseudocubic diffraction peak, indicating that both samples predominately locate in the R phase.30,31 It is noticed that the (200) peak broadens remarkably when passing from x = 0.18 to x = 0.28, which has been taken as an indication of the appearance of a monoclinic phase.30,42 For x = 0.33 single crystal, a shoulder appears on the higher angle side, consistent with the characteristics of the M phase, and it may also contain some T phase.30,31 For


7


x = 0.36 single crystal, the singlet (200) pseudocubic reflection splits into two distinct peaks, indicating that the T phase becomes dominant.30 Therefore, PMN-xPT single crystals used in this work with x = 0.18, 0.28, 0.33, and 0.36, include both various ferroelectric phases around the MPB (x = 0.28, 0.33, 0.36), and R phase far away from the MPB (x = 0.18).31,39

(a)

1100

(b)

x = 0.18

x = 0.28

3 M (emu/cm )

3 M (emu/cm )

1000

900

1000

900

800 800

-8

-4

0 4 E (kV/cm)

8

-8

(c) 1050

780

x = 0.33

3 M (emu/cm )

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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900

-4

0 4 E (kV/cm)

8

0 E (kV/cm)

8

(d) x = 0.36

760

750 740 600

-8

-4

0 E (kV/cm)

4

8

-8

-4

4

Figure 2. Electric-field control of the in-plane magnetization of CoFeB/PMN-xPT heterostructures along the [1-10] direction at 300 K for (a) x = 0.18, (b) x = 0.28, (c) x = 0.33 and (d) x = 0.36, respectively. Figure 2a is the magnetization-electric field (M-E) curve for x = 0.18 sample, which displays the large and nonvolatile bipolar-electric-field-controlled-magnetization with a loop-like behavior, indicating that the nonvolatile M-E behavior also exists in x = 0.18 sample, which is


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far away from the MPB region.31,39 The ratio of (M+0-M-0)/M-0 is about 30%, where M+0 is the magnetization after removing +8 kV/cm, and M-0 is the magnetization after removing -8 kV/cm. This result illustrates that PMN-xPT substrates far away from the MPB region can also show the large nonvolatile M-E modulation, implying that MPB is not essential for this effect. Figure 2b shows the M-E curve for x = 0.28 sample, showing the nonvolatile bipolar-electric-fieldcontrolled-magnetization with a loop-like behavior, consistent with our previous report,13 with a modulation ratio around 20%. The M-E curve for x = 0.33 sample is shown in Figure 2c, which also displays the nonvolatile bipolar-electric-field-controlled-magnetization with a modulation ratio around 40%. However, its loop-like behavior is a little bit different from those of x = 0.18 and 0.28 samples shown in Figure 2a and 2b, and can be regarded as a mixture of the loop-like and butterfly-like behaviors since x = 0.33 substrate contain both the M and T phase.31,43 The butterfly-like behavior will be described in detail later for x = 0.36 sample with the T phase. The magnetoelectric coefficients around the polarization switching for x = 0.18, 0.28 and 0.33 samples all reach the order of 10-6 s/m (Figure S1, Supporting Information), which is comparable to the previous report.13 Figure 2d shows the M-E behavior for x = 0.36 sample, exhibiting a butterfly-like behavior, which is volatile with small variations of magnetization under electric fields. This result suggests that the nonvolatile bipolar-electric-field-controlled-magnetization is absent for CoFeB/PMN-xPT with the T phase. So it can be concluded that CoFeB/PMN-xPT with x = 0.18, 0.28 and 0.33 show the nonvolatile bipolar-electric-field-controlled-magnetization with a loop-like behavior. While x = 0.36 (T phase) sample shows the volatile bipolar-electricfield-controlled-magnetization with a butterfly-like behavior. For the origin of the nonvolatile bipolar-electric-field-controlled-magnetization with a looplike behavior, we have demonstrated for CoFeB/PMN-0.30PT(001) that it originates from the


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combined action of 109° FE domain switching and absence of magnetocrystalline anisotropy in CoFeB.13 The net polarization projection along the in-plane [1-10] direction changes around the coercive electric field due to the 109° FE domain switching, and remains until the reverse process occurs, which produces two in-plane strain states in PMN-0.30PT. This, combined with the isotropic CoFeB, leads to the loop-like M-E behavior. This mechanism should also apply to the samples with x = 0.18 and 0.28 of this work since they also have the R phase with 109° FE domain switching, similar to that of PMN-0.30PT. It should be mentioned that for the 71° and 180° FE domain switching in the (001)-cut PMN-xPT with the R phase, the in-plane polarization projection is unchanged, leading to the volatile M-E curve with a butterfly-like behavior in CoFeB/PMN-xPT(001). The net 109° domain switching acts as an important role in the nonvolatile electric field control of magnetization, and is believed to originate from some defects in PMN-PT substrates, which lower the energy barrier of the 109° domain switching.44 For x = 0.33 sample, it mainly contains the M phase with a small amount of the T phase,31,43,45 and displays MA phase after poled.45 Since MA phase deviates slightly from the R phase in PMN0.33PT43 (Figure S2, Supporting Information), the nonvolatile M-E curve with a loop-like behavior for x = 0.33 sample should also be explained by the mechanism for the R-phase case since MA phase has FE domain switching close to 109°, quite similar to the R phase (Figure S2, Supporting Information). For x = 0.36 sample, it is in the T phase and the polarization reversal process is different (S3 of Supporting Information), which leads to the volatile bipolar-electricfield-controlled-magnetization with a butterfly-like behavior. In addition, it has been reported that electric field results in the phase transition from the R phase to MA, MC phase or even T phase for PMN-0.30PT,29 and the R-MA phase transition happens around 2 kV/cm.29 The composition of CoFeB/PMN-0.28PT used in our work is very close to PMN-0.30PT, so the R-


10

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MA phase transition should also occur, but this will not affect our results since MA phase should produce the loop-like M-E behavior similar to the R phase as mentioned above. Besides, the M-E curve of CoFeB/PMN-0.28PT (Figure 2b) does not show abrupt changes with increasing electric field, suggesting that R phase and the induced M phase are comparable in changing strain and the resultant magnetization change within our studied electric-field range. It should be noted that in our experiment, the maximum electric field used is about 8 kV/cm, which is not large enough to cause the FE phase transition of MAMC and MCT.46 The magnetization changes in CoFeB/PMN-xPT structures can be attributed to the changes of magnetic anisotropy under electric fields as displayed in our previous work.13 The strain-electric field curves (Figure S4, Supporting Information) show that electric-field control of magnetism in CoFeB/PMN-xPT is related to the strain-transfer mechanism.9,44 The strain transferred to the CoFeB films induces the variation of the magnetic anisotropy under electric fields,13 which is reflected by the squareness (Mr/Ms) of magnetic hysteresis (M-H) loops in Figure S5. It can be seen that the curves of Mr/Ms versus electric field display the correlated behavior with M-E curves, indicating that the change of magnetic anisotropy induces the magnetization change along the measured direction. It should be pointed out that the ferroelectric field effect (FEF)7,47 should be minor in the electric field control of magnetization in our samples because the effective length of FEF effect is only 1-2 unit cells near the interface for metals,1 which is much smaller than the thickness of CoFeB film (20 nm) in our sample, so it can not explain the large modulation in our results. Furthermore, we studied the M-E behavior of the sample with CoFeB (20 nm)/Pt (35 nm)/PMN-0.28PT structures, and the result is shown in Figure S6, which reveals that the thick Pt layer does not affect the modulation, suggesting that FEF effect is not important for our samples.


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(a)

1100 3

M (emu/cm )

3

M (emu/cm )

1050

900 x = 0.18

750

200 K 300 K 320 K

600

(b)

1000 900

x = 0.28 200 K 300 K 340 K

800 700

-8

-4

0 E (kV/cm)

4

8

-8

x = 0.33

750

200 K 300 K 340 K

600 -4

0 E (kV/cm)

4

8

Temperature (K)

3

900

-8

-4

0 E (kV/cm)

4

8

(d)

(c) 1050 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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500

C

400

Butterfly 300

Loop 200

R 100 10

15

20

T

Mix

M 25

30

35

40

45

50

x (%)

Figure 3. Electric-field control of the in-plane magnetization of CoFeB/PMN-xPT heterostructures along the [1-10] direction at different temperatures: (a) M-E curves for x = 0.18 sample at 200, 300 and 320 K, respectively. (b) & (c) M-E curves for x = 0.28 and x = 0.33 samples at 200, 300 and 340 K, respectively. (d) The phase diagram of electric-field control of magnetism with the compositions and temperatures involved in our work and the ferroelectric phase diagram of PMN-xPT was adapted from Refs. 31 and 32. C, R, M and T refer to the cubic, rhombohedral, monoclinic and tetragonal phase, respectively. Figures 3a-3c are the M-E curves for x = 0.18, 0.28 and 0.33 samples at different temperatures measured in the heating process, and the temperatures were chosen to be lower than the corresponding Curie temperatures, which were calculated to be 353, 403 and 428 K, respectively.48 It should be mentioned that we are more interested in the nonvolatile bipolar-


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electric-field-controlled-magnetization, so we only chose x = 0.18, 0.28 and 0.33 samples for the temperature-dependence study. Figure 3d shows the schematic T-x phase diagram adapted from the literature31,32 with marks for the selected compositions and temperatures in this work. Figure 3a illustrates the M-E curves for x = 0.18 sample measured at different temperatures. From 200 K to 320 K, the M-E curves remain loop-like, and the difference of remanent magnetizations under zero electric field decreases. However, it can be noticed that the M-E curves at 300 K and 320 K show a slight upward on the negative electric-field side, indicating that the electrostrictive effect, which contributes to the butterfly-like S-E curve, begins to appear, suggesting that the contribution of the butterfly-like M-E behavior increases with increasing temperature. The M-E curve measured at 340 K, which is near TC, is shown in Figure S7a (Supporting Information). For x = 0.28 and 0.33 samples, the corresponding values of TC are higher than the limit of our measurement system, so we carried out the study from 200 K to 340 K below TC. Figure 3b illustrates the M-E curves for x = 0.28 sample measured at different temperatures. The contribution of the butterfly-like M-E behavior increases with increasing temperature as shown on the negative electric field side, while the contribution of the loop-like M-E behavior decreases. For x = 0.33 sample as shown in Figure 3c, it shows the nonvolatile loop-like M-E behavior at 200 K, changes to a mixture of the nonvolatile loop-like M-E behavior and volatile butterfly-like M-E behavior at 300 K, and then becomes volatile butterfly-like M-E behavior at 340 K due to the transition from the FE M phase to the FE T phase31,32,49 as shown in Figure 3d. Based on the above experimental results, we can draw the ‘phase diagram’ (Figure 3d) of electric-field-controlled-magnetization marked by ‘loop’ for the R phase and M phase dominated regions filled by the green background, and ‘butterfly’ for the T phase region with the yellow background, to account for the temperature dependence of M-E behavior for samples with


13


different x. For PMN-0.18PT, it is in the R phase at low temperatures,32 and evolves into the cubic phase at around 340 K, as discussed in S7 of Supporting Information. The decrease of contribution of the loop-like M-E behavior with increasing temperature suggests that portion of 109° domain switching decreases with increasing temperature.

(a)

160 Td

900

x = 0.18

∆ M/ML (%)

3

M (emu/cm )

1050

750 600

5 Oe 10 Oe 20 Oe

450 150 1200

200

250

300 T (K)

350

(b)

140 120 100

x = 0.18

80 5

400

10

15

20

H (Oe)

25.0k

(c)

(d)

Td= 329 K

800 20.0k 300 K

0

15.0k

-400 -800

x = 0.18

0.1 kHz 1 kHz 10 kHz

30k 20k 10k

340 K

0

10.0k

-1200 -40

Tm= 353 K T (K) 200 300 400

ε

x = 0.18

400

ε

3

M (emu/cm )

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-20

0 H (Oe)

20

40

320

340

360

T (K)

Figure 4. (a) Temperature dependence of the in-plane magnetization for the poled CoFeB/PMN0.18PT with an applied magnetic field of 5 Oe (squares), 10 Oe (triangles) and 20 Oe (circles), respectively, along the [1-10] direction. (b) The relative change of magnetization with ∆M/ML for H = 5, 10, 20 Oe. (c) M-H loops along the [1-10] direction for the poled CoFeB/PMN-0.18PT at 300 K and 340 K. (d) Dielectric permittivity versus temperature for the poled PMN-0.18PT single crystal with an enlarged view. The inset is the dielectric permittivity over the measured temperature range from 180 K to 400 K.


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It is well-known that PMN-xPT single crystals show a remarkable relaxor behavior and the degree of relaxors increases with decreasing x.33 For relaxor ferroelectrics, PNRs play an important role, which are determined by composition, temperature etc.33,50 External electric fields higher than the critical field can induce a ferroelectric transition with a long-range ferroelectric order, which retrieves to the relaxor ferroelectrics at a temperature Td in the heating process.33 This transition also corresponds to the change from the macrodomain state to the microdomain state or depolarization of relaxors, revealed by the dielectric anomaly with a sharp peak in the temperature dependence of dielectric permittivity.37,38 To investigate the effect of this macrodomain to microdomain transition in PMN-xPT on the magnetization, poled samples were used to study the magnetization versus temperature (M-T) behavior. PMN-0.18PT single crystal was chosen because it shows a remarkable relaxor behavior and its Td is close to room temperature.28 Figure 4a shows the M-T curves for CoFeB/PMN-0.18PT heterostructures measured in different magnetic fields, after poled by an electric field of -8 kV/cm at 180 K and then removed before heating. Interestingly, the magnetization shows a giant jump at 330 K in the M-T curves. The relative change of magnetization ΔM/ML can reach 150%, 130% and 75% for H = 5, 10, 20 Oe (ΔM = M340 K M300 K, and ML = M300 K), respectively, as shown in Figure 5b. To get information on the change of magnetic anisotropy around this transition, we measured the M-H loops at 300 K and 340 K for the poled CoFeB/PMN-0.18PT heterostructure and the results are shown in Figure 4c, which displays a drastic change of magnetic anisotropy from 300 K to 340 K, i.e. change from hard axis to easy axis. Figure 4d shows the anomaly in the temperature dependence of dielectric permittivity (ε-T) for PMN-0.18PT single crystal after the same poling process as that in Figure 4a. The inset is the whole curve of ε-T throughout the studied temperature range measured with a


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heating rate of 1℃/min at a frequency f = 0.1, 1, 10 kHz, respectively. From Figure 4d, it is obvious that a sharp dielectric anomaly occurs at around 330 K (Td), which is consistent with the giant jump in the M-T curves mentioned above. Since this anomaly is related to the change from the macrodomain state to the microdomain state,37,38 the giant jump of magnetization should originate from it. It should be mentioned that the dielectric permittivity becomes more frequency dispersive above Td, reflecting the relaxor behavior in the microdomain state due to the PNRs.33 To understand the giant jump of magnetization (Figure 4a), a schematic of the mechanism is shown in Figure 5. For T < Td (Figure 5a), PMN-0.18PT is in the macrodomain state due to poling and the sizes of the FE domains are relatively large as shown by the green ellipses with the in-plane projections of polarizations (black arrows). For T > Td (Figure 5b), the macrodomains change into the microdomains, which are nano-meter sized domains or PNRs. The coupling between FE and FM domains are shown in Figure 5c and 5d. For T < Td (Figure 5c), the directions of magnetic moments are decided by both the easy axis of EA-FE (black double-head arrows) and easy axis of EA-GH (purple double-head arrows), of which the EA-FE comes from the strain due to the polarization of the FE macrodomains via the magnetoelastic energy, and the EA-GH comes from the growth process of CoFeB. For T > Td (Figure 5d), the EA-FE disappears due to the transition from the macrodomains to microdomains, and the directions of magnetic moments are only decided by the EA-GH. Under external magnetic field along the [1-10] direction as shown in Figure 5e and 5f, the directions of magnetic moments rotate to the direction of magnetic field with some angles, which depend on magnetic field. This schematic of the mechanism can account for the giant jump of magnetization shown in Figure 4a. When temperature goes above Td, the EA-FE related to the FE macrodomains disappears, so the magnetic moments align to the direction of the magnetic field, which is along the easy axis of


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EA-GH, so magnetizations under magnetic fields of 5, 10, and 20 Oe jump to the values which approximately equal to the saturation magnetization. This is consistent with the M-H loop (340 K) in Figure 4c, which shows that magnetizations under magnetic fields of 5, 10, and 20 Oe approximately equal to the saturation magnetization. It is noticed that magnetization also decreases with increasing temperature before Td (Figure 4a). This may be related to the gradual change of FE macrodomains with increasing temperature since the dielectric permittivity shows gradual increase before Td as shown in the inset of Figure 4d. All these results suggest that the FE-FM coupling is strongly related to the domain state, and the transtion from FE macrodomains to microdomains quenches the FE-FM coupling, resulting in a sharp magnetization jump around Td .


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Figure 5. Schematic for both FE domains and the corresponding FM domains morphology of poled CoFeB/PMN-0.18PT heterostructure for temperatures below and above Td with applied external magnetic field along the [1-10] direction. The green ellipses in (a), (b) and shallow green ellipses of (c)-(f) refer to the FE macrodomains (T < Td) and FE microdomains (T > Td), and the inside black arrows refer to the polarization directions. The grey layers in (c)-(f) refer to


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the FM film, and the coupling between FE domains and FM domains is expressed by the superposition of the top FM layer and the bottom FE layer. The three figures on the left show the case for T < Td, while the right ones for T > Td. (c) and (d) are illustrations for domain coupling of FE and FM layers. The black double-head arrow EA-FE indicates the easy axis induced by the FE macro-domains at T < Td, and the purple double-head arrow EA-GH stands for the easy axis of the FM layer due to film growth. EA-GH exists in FM layer for temperatures both below and above Td. (e) and (f) illustrate the direction of magnetic moments in the FM layer after applying external magnetic fields along the [1-10] direction. The aqua arrow represents the direction of the external magnetic field, and the dark blue arrow denotes the magnetic moment for domains at T < Td, and single domain at T > Td. The directions of magnetic moments are decided by the direction of EA-FE, EA-GH, and the external magnetic field for T < Td. While for T > Td, it is only affected by the external magnetic field and EA-GH because of the disappearance of the FE macrodomains. CONCLUSIONS We demonstrate a large and nonvolatile bipolar-electric-field-controlled-magnetization in CoFeB/PMN-xPT(001) with x = 0.18, 0.28, 0.33, suggesting that MPB is not essential for this effect. Volatile bipolar-electric-field-controlled-magnetization with a butterfly-like behavior was observed for sample with x = 0.36, suggesting that different mechanism is involved in the sample with the T phase. The magnitude of electric-field-controlled-magnetization changes with increasing temperature and dramatic change occurs around the phase transition temperature. The temperature dependence of magnetization for x = 0.18 sample shows an abrupt and giant increase at the macro-micro domain transition for the heating process. This is the first time to demonstrate the effect of ferroelectric-relaxor transition with a dramatic macro-micro domain


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change on magnetism for multiferroic heterostructures. These results were explained by considering the interaction between magnetism and ferroelectric domains and/or domain switching for different ferroelectric phases of PMN-xPT. Our work enriches the knowledge of electric-field control of magnetism in FM/FE multiferroic heterostructures and is also significant for applications. ASSOCIATED CONTENT Supporting Information Composition characterization of the PMN-xPT single crystals by EPMA, the magnetoelectric coefficients for CoFeB/PMN-xPT heterostructures, illustration of R phase, MA phase, T phase and the corresponding polarization switching process, strain measurements for PMN-xPT substrates at 300 K, the magnetic hysteresis loops and Mr/Ms under electric fields for CoFeB/PMN-xPT heterostructures, M-E behavior of CoFeB/Pt/PMN-0.28PT heterostructure, ME behavior of CoFeB/PMN-0.18PT heterostructure at 340 K, M-E behaviors of CoFeB/PMNxPT heterostructures along the perpendicular [110] direction at different temperatures, temperature dependence of the in-plane magnetization for CoFeB/PMN-0.18PT and CoFeB/Si heterostructures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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This work was supported by the 973 project of the Ministry of Science and Technology of China (Grant No. 2015CB921402), National Science Foundation of China (Grant Nos. 11134007, 11304385) and Special Fund of Tsinghua for Basic Research (Grant No. 201110810625), Open Research Fund Program of the State Key Laboratory of Low Dimensional Quantum Physics (Grant No. KF201412). REFERENCES (1) Vaz, C. A. F. Electric Field Control of Magnetism in Multiferroic Heterostructures. J. Phys.: Condens. Matter 2012, 24, 333201. (2) Ma, J.; Hu, J. M.; Li, Z.; Nan, C. W. Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films. Adv. Mater. 2011, 23, 1062-1087. (3) Lu, C. L.; Hu, W. J.; Tian, Y. F.; Wu, T. Multiferroic Oxide Thin Films and Heterostructures. Appl. Phys. Rev. 2015, 2, 021304. (4) Heron, J. T.; Schlom, D. G.; Ramesh, R. Electric Field Control of Magnetisim Using BiFeO3-based Heterostructures. Appl. Phys. Rev. 2014, 1, 021303. (5) Chu, Y. H.; Martin, L. W.; Holcomb, M. B.; Gajek, M.; Han, S. J.; He, Q.; Balke, N.; Yang, C. -H.; Lee, D.; Hu, W.; Zhan, Q.; Yang, P. L.; Fraile-Rodrguez, A.; Scholl, A.; Wang, S. X.; Ramesh, R. Electric-field Control of Local Ferromagnetism Using a Magnetoelectric Multiferroic. Nature Mater. 2008, 7, 478–482. (6) Heron, J. T.; Bosse, J. L.; He, Q.; Gao, Y.; Trassin, M.; Ye, L.; Clarkson, J. D.; Wang, C.; Liu, J.; Salahuddin, S.; Ralph, D. C.; Schlom, D. G.; Iniguez, J.; Huey, B. D.; Ramesh, R. Deterministic Switching of Ferromagnetism at Room Temperature Using an Electric Field. Nature 2014, 516, 370-373.


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(47) Tsai, W. C.; Liao, S. C.; Huang, K. F.; Wang, D. S.; Lai, C. H. Nonvolatile Electric-field Modulation

of

Magnetic

Anisotropy

in

Perpendicularly

Magnetized

L10-

FePt/(001)[Pb(Mg1/3Nb2/3)]0.7-(PbTiO3)0.3 Heterostructures. Appl. Phys. Lett. 2013, 103, 252405. (48) Feng, Z. Y.; Luo, H. S.; Guo, Y. P.; He, T. H.; Xu, H. Q. Dependence of High Electricfield-induced Strain on the Composition and Orientation of Pb(Mg1/3Nb2/3)O3-PbTiO3 Crystals. Solid State Commun. 2003, 126, 347-351. (49) Zhang, S. J.; Li, F. High Performance Ferroelectric Relaxor-PbTiO3 Single Crystals: Status and Perspective. J. Appl. Phys. 2012, 111, 031301. (50) Shvartsman, V. V.; Kholkin, A. L. Domain Structure of 0.8Pb(Mg1/3Nb2/3)O3-0.2PbTiO3 Studied by Piezoresponse Force Microscopy. Phys. Rev. B 2004, 69, 014102.


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Table of Contents 1050 500

C

400

Butterfly 300

Loop 200

R 100 10

15

20

T

Mix

30

35

900

Td

x = 0.18

750 600

5 Oe 10 Oe 20 Oe

450

M 25

M (emu/cm 3)

Temperature (K)

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


40

45

50

150

200

250

x (%)

300

350

400

T (K)


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Electric-Field Control of Magnetism in Co40 ... - ACS Publications

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