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Article

Charge Transfer, Orbital Reconstruction and Induced Magnetic Coupling in Manganite/BiFeO Heterostructures 3

Yongmei Liang, Xingkun Ning, Zhan Jie Wang, Bin He, Yu Bai, Xinguo Zhao, Wei Liu, and Zhidong Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03972 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Charge Transfer, Orbital Reconstruction and Induced Magnetic Coupling in Manganite/BiFeO3 Heterostructures

Yongmei Liang,†,‡ Xingkun Ning,†,§ Zhanjie Wang,*,†,‡ Bin He,† Yu Bai,† Xinguo Zhao,†,‡ Wei Liu,†,‡ Zhidong Zhang†,‡ †

Shenyang National Laboratory for Materials Science, Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), 72 Wenhua Road, Shenyang 110016, China

‡

School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang 110016, China

§

Hebei Key Lab of Optic-electronic Information and Materials, The College of Physics Science and Technology, Hebei University, Baoding 071002, China

1

ACS Paragon Plus Environment


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ABSTRACT La0.7Sr0.3MnO3/BiFeO3 (LSMO/BFO) and LaMnO3/BiFeO3 (LMO/BFO) heterostructures were prepared on (001)-oriented SrTiO3 (STO) substrates by pulsed-laser deposition, and the magnetic exchange coupling and the charge transfer procedure at the heterointerfaces were investigated. The LSMO/BFO and LMO/BFO heterostructures exhibited an exchange bias with a magnetic field HEB = 46 Oe and HEB = 30 Oe, respectively. The charge transfer procedure is determined from Fe ions to Mn ions at the LSMO/BFO interface, conversely from Mn ions to Fe ions at the LMO/BFO interface. The charge transfer from Fe ions to Mn ions can lead to stronger interfacial magnetic coupling. According to the charge transfer mode, we describe possible microscopic scenarios of orbital reconstruction at the LSMO/BFO and LMO/BFO interfaces. Our work sheds light on the origin of the exchange bias effect in the manganite/BFO heterostructures.

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INTRODUCTION The exchange bias effect has attracted considerable attention due to its various applications in high-density magnetic recording, magnetic sensors, spin valves for magnetic recording read heads and non-volatile memory devices.1-3 In recent years, the exchange bias has also been observed in La0.7Sr0.3MnO3/BiFeO3 (LSMO/BFO) heterostructures.4-9 Moreover, the LSMO/BFO heterostructure exhibits stronger magnetoelectric coupling than BFO films due to that ferromagnetic (FM) order can couple to ferroelectric (FE) order indirectly through antiferromagnetic (AFM) order.10-13 Combining

the

magnetoelectric

coupling

with

ferromagnetic

coupling

in

ferromagnetic/multiferroic heterostructures provides an efficient electric-field control of magnetic properties, which will be of great significance in the field of spintronics.14-23 Extensive researches have led to the notion that the exchange bias effect is induced by uncompensated interfacial spin-pinning effects at the FM/AFM interface.2,24-25 However, the mechanism of the uncompensated interfacial spin-pinning cannot explain the exchange bias of the LSMO/BFO heterostructure, because BFO is well known as a compensated G-type AFM material.26-27 Some theoretical models, such as the Dzyaloshinskii-Moriya interaction have been proposed to describe the exchange bias of the LSMO/BFO heterostructure due to the FM/G-AFM interfaces.26 Yu et al. reported that the electronic orbital reconstruction at the interface was the origin of the exchange bias in the LSMO/BFO heterostructure.5 Calderón et al. calculated the induced magnetic moment in the BFO based on charge transfer arising from the LSMO/BFO interface, and 3



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considered that the charge transfer was the origin of the electric-field controlled exchange bias in the LSMO/BFO heterostructure.3 Mi et al. calculated the interfacial electronic structure of the LSMO/BFO heterostructure by using the first-principles method and concluded that charge accumulation and depletion between Fe and Mn occur at the interfacial regions.28-30 However, these important theoretical researches result need to be verified by experiments. Therefore, to better understand the mechanism of exchange bias in the LSMO/BFO heterostructure, it is important experimentally to investigate the charge transfer at the LSMO/BFO interface. On

the

other

hand,

in

recent

years,

charge

transfer

at

ferromagnetic/Pauli-paramagnetic (FM/PM) interfaces such as LaMnO3/LaNiO3,31 La0.7Ca0.3MnO3/LaNiO3,32 and LSMO/LaNiO3,33 has been studied, and is believed to induce the exchange bias in these FM/PM heterostructures. The charge transfer may also take place at the LSMO/BFO interface, where LSMO is a conductor and BFO is a semiconductor, and the two layers form metal/semiconductor (M/S) contact. For LSMO and BFO, the Fermi level of BFO is higher than that of LSMO because of the different work functions of LSMO (4.9 eV)34 and BFO (4.7 eV).35 Thus electrons in BFO may be transferred into LSMO due to the higher Fermi level of BFO than that of LSMO. Similarly, charge transfer may also exist at the LaMnO3/BiFeO3 (LMO/BFO) interface, because

the

insulator

(LMO)

and

semiconductor

(BFO)

layers

form

insulator/semiconductor (I/S) contact. In this heterostructure, the Fermi level of LMO is also different from that of BFO. However, the charge transfer procedure at theses two 4


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interfaces may be different due to the different interfacial nature. Therefore, exploring the charge transfer at the LSMO/BFO and LMO/BFO interfaces and comparing their difference are helpful to understand the mechanism of exchange bias in the manganite/BFO heterostructures. In this study, the LSMO/BFO and LMO/BFO heterostructures are epitaxially grown on SrTiO3 (STO) substrates by pulsed laser deposition and their exchange bias and charge transfer at the LSMO/BFO and LMO/BFO interfaces are investigated. The results show that exchange bias occurs at both LSMO/BFO and LMO/BFO interfaces, but the strength is different. The charge transfer procedure is determined from Fe ions to Mn ions at the LSMO/BFO interface, conversely from Mn ions to Fe ions at the LMO/BFO interface. It is obvious that the charge transfer between the Mn ions and Fe ions and its direction will determine the strength of interfacial ferromagnetic coupling at the manganite/BFO interfaces.

EXPERIMENTAL SECTION La0.7Sr0.3MnO3/BiFeO3

(LSMO/BFO)

and

LaMnO3/BiFeO3

(LMO/BFO)

heterostructures were prepared by pulsed laser deposition (PLD) on (001)STO single-crystal substrates, using a KrF excimer laser (λ = 248 nm). The laser flux was approximately 1.2 J/cm2 and the frequency was 2-5 Hz. The bottom layer LSMO or LMO with a thickness of 6 nm was deposited firstly on the STO substrate at 700 °C under an pressure of 30 Pa of pure O2, and then the BFO layer with a thickness of 40 nm was 5



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grown on the LSMO or LMO layer at 700 °C under an pressure of 10 Pa of pure O2. The magnitude of the exchange bias HEB is inversely proportional to the thickness of the ferromagnetic layer.4 So when we prepare the samples, we choose to deposite thinner LSMO or LMO layer (6 nm) to obtain obvious exchange bias effect. To remove oxygen vacancies, the samples were annealed in pure O2 under 5×104 Pa. The crystal structure and orientation of the heterostructures were characterized by X-ray diffraction (XRD) (Rigaku, D/max-2000, Cu Kα radiation). The interfaces and epitaxial relationship in the heterostructures were studied by transmission electron microscopy (TEM, F20, Tecnai, Netherlands). X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250; Al Kα source, 1486.6 eV, Energy step: 0.1 eV, Resolution: 400 meV) was chosen to detect the electronic structure and valence states around the interface region in the heterostructures. Magnetization measurements were carried out from 5 K to 370 K by using the superconducting quantum interference device magnetometer (SQUID).

RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the LSMO/BFO and LMO/BFO heterostructures on the (001)STO substrate. For comparison, the XRD patterns of the single LSMO (24 nm) and LMO (24 nm) films on the (001)STO substrate are also illustrated in Figure 1. The LSMO and LMO films with the (00l) orientation have epitaxially grown on the STO substrate. For the LSMO/BFO and LMO/BFO heterostructures, the diffraction peak of LSMO or LMO could not be detected in the 6


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XRD patterns due to that their thickness (6 nm) is too thin. However, the (00l) peaks of BFO corresponding to the diffraction peaks of substrate can be detected in the XRD patterns of both heterostructures, implying that the LSMO/BFO and LMO/BFO heterostructures were epitaxially grown on the substrate. This can also be further confirmed by TEM results. Figure 2a and 2b show typical HRTEM images of the cross section of LSMO/BFO and LMO/BFO heterostructures. The interfaces of the LSMO/BFO and LMO/BFO are clear and sharp. The LSMO and LMO layers have grown epitaxially on the STO substrate, and the BFO layer has grown epitaxially on the LSMO or LMO layer. Figure 2c and 2d show Fast Fourier Transform (FFT) patterns transformed from the HRTEM images of the LSMO/BFO and LMO/BFO heterostructures. The orientation relationships between the LSMO or LMO layer and the STO substrate is (001)LSMO//(001)STO or (001)LMO//(001)STO, and between LSMO or LMO layer and BFO layer is (001)LSMO//(001)BFO or (001)LMO//(001)BFO. The d-spacing of out-of-plane for the BFO layer in the LSMO/BFO and LMO/BFO heterostructures calculated from the FFT patterns is 4.05 and 4.06 Å, respectively, which are larger than that of the bulk BFO (3.96 Å),36 indicating that the BFO layer in both LSMO/BFO and LMO/BFO heterostructures is under an in-plane compressive strain state. In addition, the d-spacing of out-of-plane of the LSMO (dLSMO(001) ~ 3.84 Å) and LMO (dLMO(001) ~ 3.85 Å) layers in the LSMO/BFO and LMO/BFO heterostructures are both shrank compared with the bulk values (3.87 Å33 for the LSMO and 3.91 Å37 for the LMO). Moreover, the d-spacing of in-plane of the LSMO (dLSMO(100) ~ 3.89 Å) and LMO (dLMO(100) ~ 3.94 Å) layers in 7



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the LSMO/BFO and LMO/BFO heterostructures are both elongated compared with the bulk values. The shrinkage of the d-spacing of out-of-plane and the elongation of the d-spacing of in-plane of the LSMO and LMO layers in the LSMO/BFO and LMO/BFO heterostructures indicate that both LSMO and LMO layers undergo an in-plane tensile strain state. Combined with the XRD results, it is clear that the high-quality LSMO/BFO and LMO/BFO heterostructures have been fabricated for studying their natures.

Figure 1

8


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Figure 2 The temperature dependences of the magnetization after field cooling was measured in an in-plane magnetic field of 100 Oe for the samples, and the results are shown in Figure 3. The magnetization decreases with the increase of temperature and a FM-PM transition can be observed. The Curie temperature (TC) determined from the peaks in the dM/dT-T curves is 290 K and 320 K for the LSMO/BFO heterostructure and the pure LSMO film, and 150 K and 125 K for the LMO/BFO heterostructure and the pure LMO film, respectively. Namely, the TC for the LSMO in LSMO/BFO heterostructure decreased, but for the LMO in LMO/BFO heterostructure increased. It is clear that the ferromagnetic interaction at the LSMO/BFO and LMO/BFO interfaces is different, which may be due to the difference in the electron transfer integral of Mn3+–O–Mn4+ .38 9



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Figure 3

Figure 4 Figure 4 shows the magnetic hysteresis loops of the LSMO/BFO and LMO/BFO 10


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heterostructures at 5 K after field cooling (FC) from 370 K under an in-plane applied magnetic field of +2 kOe as compared with the pure LSMO and LMO films. Magnetic fields above ±10 kOe were applied to attain saturated magnetization (Ms). The LSMO/BFO and LMO/BFO heterostructures show a strong enhancement of the coercive field with respect to the pure LSMO and LMO films. In addition, the LSMO/BFO and LMO/BFO heterostructures show the exchange bias effect with the characteristic that the hysteresis loops shifted along the negative direction of the X axis. The cooling field was 2 kOe, and its direction was the same as the positive direction of the X axis. Thus, the shift of the hysteresis loop is opposite to the direction of the cooling field, as expected from conventional exchange bias behavior. The absolute values of the exchange bias field (HEB) and coercivity field (HC) are calculated according to HEB = |H1+ H2|/2 and HC = |H1- H2|/2, respectively, where H1 and H2 represent the negative and positive fields at M = 0. For the LSMO/BFO heterostructure, a large HEB of about 46 Oe is observed, which is much larger than that of the LMO/BFO heterostructure (30 Oe). The enhancement of coercivity and the observed exchange bias in the heterostructures indicate that the interfacial

magnetic

coupling

exists

in

the

LSMO/BFO

and

LMO/BFO

heterostructures.39-40 The larger HEB in the LSMO/BFO heterostructure is due to the stronger interfacial magnetic coupling and will be discussed below in detail. Namely, the strength of interfacial magnetic coupling is different in the LSMO/BFO and LMO/BFO heterostructures.

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Figure 5 We also measured the hysteresis loops at each temperature after field cooling from 370 K. Figure 5 shows the temperature dependences of HEB and HC for the LSMO/BFO and LMO/BFO heterostructures. For both heterostructures, the HEB decreases with increasing temperature and diminishes to zero over the blocking temperature (TB). The relation between HEB and temperature can be described by the following formula: HEB = H0 exp(-T/T0)

(1)

where H0 is the extrapolation of HEB at 0 K and T0 is a constant. The TB of about 65 K is observed in the LSMO/BFO heterostructure, and it is much higher than that in the LMO/BFO heterostructure (20 K). The temperature dependent behaviors of HC for the LSMO/BFO and LMO/BFO heterostructures are also shown in Figures 5a and 5b. The 12


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HC of the LSMO/BFO and LMO/BFO heterostructures decreases linearly with increasing temperature. In addition, it should be noted that there is a crossover at TB in HC fitting line, which agrees with the recent findings in perovskite manganite, such as the FM/AFM heterostructure of LSMO/NiO,39 the FM/PM heterostructure of LCMO/LNO,32 and the FM/AFM heterostructure of LSMO/BFO.5 The higher TB in the LSMO/BFO heterostructure indicates that the interfacial magnetic coupling in which is stronger than that in the LMO/BFO heterostructure. Namely, the strength of interfacial magnetic coupling is different in the LSMO/BFO and LMO/BFO heterostructures.

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Figure 6 To understand the mechanism of exchange bias in the manganite/BFO heterostructures, we investigated the charge transfer at the LSMO/BFO and LMO/BFO interface regions. The chemical valence states of the elements across the LSMO/BFO and LMO/BFO interfaces were studied by XPS. Figure 6 shows the detailed core-level spectra of Fe 2p for analyzing the Fe valence states. The Fe 2p spectra are fitted very well with the peaks at the energies of 709.5 eV, 710.5 eV and 713.1 eV, which correspond to the peak position of Fe2+, Fe3+ and Fe4+ respectively.41-43 All the core-level spectra were fitted with a Gaussian–Lorentzian mix with a Shirley background44-45 to ensure the quality of the peak fitting. The ratio of Fe2+, Fe3+ and Fe4+ was estimated by the integrated peak area based on the equation: CA/CB = IASB/IBSA, where CA and CB denote the concentrations of ions of corresponding valence states, SA and SB represent the sensitivity factors of the Fe ion in the samples, and IA and IB are the integrated peak areas after background correction.46 Then the Fe valence states in the manganite/BFO

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heterostructures can be calculated from the fitted peaks of the Fe 2p3/2 peak. As shown in Figure 6a, the peak of the Fe3+ is absolutely dominant for the BFO layer but becomes less dominant when approaching the LSMO/BFO interface. And the appearance of 20% Fe4+ demonstrates a tendency toward Fe4+ for the interfacial Fe3+ in the LSMO/BFO heterostructure. Additionally, as illustrated in Figure 6b, a large amount of Fe2+ appears at the interface region of the LMO/BFO heterostructure, which means the the decrease of the Fe3+ ion number and the increase of Fe2+ ion number when approaching the LMO/BFO interface. On the basis of the test results, it is clear that the charge transfer with the type of Fe3+ - 1e → Fe4+ and Fe3+ + 1e → Fe2+ occurred at the interface region for the LSMO/BFO and LMO/BFO heterostructures, respectively.

15



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Figure 7 To deeply explore the charge transfer procedure at the manganite/BFO interface, it is essential to analyze the Mn valence states. The Mn 3s core-level spectra of the LSMO/BFO and LMO/BFO heterostructures have been used to analyze the Mn valence states.46 Figure 7 shows the Mn 3s core level spectra with the characteristic of different splitting energy for the LSMO/BFO and LMO/BFO heterostructures. The exchange splitting is derived from the formation of two states after photoionization due to 3d-3s exchange interaction: a high-spin state with the spins of the 3s and the 3d electrons parallel at the lower binding energy labeled as 3S(1) and a low-spin state with antiparallel spin alignment at the higher binding energy labeled as 3S(2).47-48 The energy separation (∆E) between the splitting peaks can be expressed by the following formula: ∆E = (2S+1) J3s-3d

(2)

where S represents the total spin moment and J3s-3d represents the effective exchange integral between Mn 3s and Mn 3d states.49 The valence states of Mn have a linear

16


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anti-dependence on the Mn 3s exchange splitting based on the linear equation: υMn = 9.67 – 1.27∆E3s/eV (υMn is the valence states of Mn, ∆E3s is the 3s energy splitting).46 Therefore, the changed trend of Mn valence can be derived from the 3s energy splitting ∆E. As shown in the Figure 7a, the splitting magnitude of Mn 3s for the interface of LSMO/BFO (∆E ~ 5.98 eV) is larger than that of LSMO (∆E ~ 5.5 eV), which means the decrease of the Mn4+ ion number and the increase of Mn3+ ion number in the LSMO/BFO heterostructure. For the LMO/BFO heterostructure, the data demonstrates a clear decrease of the Mn 3s exchange splitting from ∆E ~ 6.3 eV for the LMO layer to ∆E ~ 6.0 eV at the LMO/BFO interface, which means the increase of Mn4+ ion number and decrease of Mn3+ ion number in the LMO/BFO heterostructure. The electron transfer integral of Mn3+–O–Mn4+ in the LSMO/BFO heterostructure decreased as a result of decrease of Mn4+ ion number.38 On the contrary, the electron transfer integral of Mn3+–O–Mn4+ in the LMO/BFO heterostructure increased as a result of increase of Mn4+ ion number. Thus, the Curie temperature of the LSMO/BFO heterostructure decreased compared with the pure LSMO film, while the Curie temperature of the LMO/BFO heterostructure increased compared with the pure LMO film. Recalling the increase of Fe valence at the interface as proved by the detailed core-level spectra of Fe 2p, it is reasonable to conclude that the charge transfer from Fe ions to Mn ions occurs at the LSMO/BFO interface. On the contrary, charge transfer from Mn ions to Fe ions takes place at the LMO/BFO interface. In summary, the charge transfer procedure at the manganite/BFO interfaces can be described as Fe3+ + Mn4+ → Fe4+ + Mn3+ and Fe3+ + 17



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Mn3+ → Fe2+ + Mn4+ for the LSMO/BFO and LMO/BFO heterostructures, respectively.

Figure 8

Figure 9 According to the above results, we now explore the physical origin of the charge transfer procedure at the LSMO/BFO and LMO/BFO interfaces. In conventional semiconductor, the occurrence of the charge transfer is due to the the difference of Fermi level between two layers of the heterostructure. However, in strongly correlated electron oxides, the the difference of Fermi level is not enough to explain the origin of the charge transfer because the charge degree of freedom is coupled to the orbital and spin degrees of freedom. So we choose the valence-band offset (VBO) theory to explain the the 18


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physical origin of the charge transfer. The charge transfer procedure is dependent on the negative or positive values of the VBO.38 The VBO at the interface region for the LSMO/BFO and LMO/BFO heterostructures can be obtained from the following equation proposed by Kraut et al.:50 ∆EVBO(A/B) = (ECL(A-I) – ECL(B-I)) – [(ECL(A) – EV(A)) – (ECL(B) – EV(B))]

(3)

where ∆EVBO(A/B) represents the VBO of the BFO layer relative to the manganites (LSMO, LMO) (where A represents BFO and B represents manganites), ECL(A–I) − ECL(B–I) is the energy separation between the Fe 2p and Mn 2p core levels (CLs) in the manganite/BFO heterostructure, and ECL(A) − EV(A) and ECL(B) − EV(B) refer to the energy separations of the core levels relative to valence-band maximum (VBM) for BFO and the manganite. The overall error bar for the VBO values is ±100 meV with the help of the accuracy of ± 20 meV for Fe 2p and Mn 2p spectra, which are intense and narrow. The valence band edge of the pure LSMO, LMO and BFO films is shown in Figure 8. The VBM of the pure LSMO, LMO and BFO films was determined by the intersection of the linear fitting line of the valence band spectra and the the extended base line, which can help us get an uncertainty of less than 0.1 eV.51 The VBM of the pure LSMO, LMO and BFO films is 0.03 ± 0.1, 1.45 ± 0.1, and 1.15 ± 0.1 eV, respectively, as illustrated in Figure 8. According to Equation (3), the VBO values are calculated to be +0.92 ± 0.1 eV and -0.35 ± 0.1 eV for the LSMO/BFO and LMO/BFO heterostructures, respectively. As illustrated in Figure 9, the positive VBO value of the LSMO/BFO heterostructure manifests that the valence band of BFO shifted to a higher binding energy relative to LSMO, and the charge 19



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transfer direction was from Fe ions to Mn ions. On the contrary, the negative VBO value of the LMO/BFO heterostructure indicates that the valence band of BFO shifted to a lower binding energy relative to LMO, and the charge transfer direction was from Mn ions to Fe ions. The occurrence of the charge transfer at the manganite/BFO interfaces is due to the valence band edge shifting, which will flatten barrier height of energy at the interfaces. And the direction of the valence band edge shifting determines the procedure of the charge transfer at the manganite/BFO interfaces. As a result, the charge transfer direction was from Fe ions to Mn ions at the LSMO/BFO interface, but from Mn ions to Fe ions at the LMO/BFO interface.

Figure 10 We now turn to explore the correlation between charge transfer and magnetic

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coupling as well as the resulting exchange bias effect in the LSMO/BFO and LMO/BFO heterostructures. The orbital reconstruction which is closely related to charge transfer should be discussed.32 Based on the experimental results of charge transfer mode of the 3d ions as mentioned above, we describe possible microscopic scenarios for the correlation between charge transfer and orbital reconstruction. Both Mn3+ ions and Mn4+ ions exist in LSMO and LMO films. And the Mn3+ ions and Mn4+ ions possess electronic configurations of t2g3eg1 and t2g3 states, respectively. Namely, there is no electron in 3d-eg orbitals (dx2-y2 orbital and d3z2-r2 orbital) of Mn4+ ions. And the coupling strength between the 3d-t2g of Mn4+ and Fe3+ ions is weak,32-33 so the hybridization of 3d orbitals of Mn4+ and Fe3+ ions across the interface is negligible. As a result, only Mn3+ ions are considering in the schematic energy diagrams of the orbital reconstruction at the LSMO/BFO and LMO/BFO interfaces, as illustrated in Figure 10. The energy level of the 3d-t2g orbitals of Mn ions and Fe ions across the interface is almost the same as the inner layer of the LSMO (LMO) and BFO due to the weak coupling strength between the 3d-t2g of Mn ions and Fe ions.32-33 When considering interfacial orbital reconstruction, the orbital occupancy of LSMO (LMO) and BFO before reconstruction is another key factor. The Fe3+ and Mn3+ ions possess electronic configurations of low spin t2g3eg2 and low spin t2g3eg1 states, respectively.5,33 As illustrated in Figure 10a, the energy level of BFO is higher than that of LSMO due to the lager VBM value of BFO than that of LSMO. As mentioned above, the TEM results have been revealed that the BFO layer is under the in-plane compressive strain state, and LSMO layer is under the in-plane tensile strain 21



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state in the LSMO/BFO heterostructure. Therefore, dx2-y2 orbital rather than d3z2-r2 orbital is energetically preferred to occupy by electron for LSMO due to the in-plane tensile strain, while dx2-y2 orbital is higher in the energy than d3z2-r2 orbital for BFO due to the in-plane compressive strain.33,52 At the interface, the covalent bond of Fe-O-Mn forms as the charge transfer from Fe ions to Mn ions happens. The strong hybridization between Fe dx2-y2 orbital (higher energy) and Mn d3z2−r2 orbital (higher energy) forms the bonding orbital (lower energy) and the corresponding antibonding orbital (higher energy) at the interface. The hybridization of the Mn dx2-y2 orbital (lower energy) and Fe d3z2-r2 orbital (lower energy) and the hybridization of 3d-t2g orbitals of Mn and Fe across the interface are negligible due to the weak coupling strength between them.33 After hybridization, the electron from Fe dx2-y2 orbital would take the bonding orbital (the lowest energy level) to favor the energy stable. In this scenario, the electrons are transferred from Fe dx2-y2 orbital to the molecular bonding orbital shared by Fe and Mn at the interface, as shown in Figure 10a. Namely, the charge transfer from Fe ions to Mn ions occurs in the LSMO/BFO heterostructure. As the eg electrons are closely related to the double-exchange interaction mode, the orbital reconstruction at the interface and the filling of electrons would affect the interfacial magnetic coupling. The FM order at the interface is determined by the double exchange interaction formed at the interface through the covalent bond of Fe-O-Mn which causes exchange bias coupling at the LSMO/BFO interface. Figure 10b shows schematic energy diagrams of the orbital reconstruction at the 22


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LMO/BFO interface. The energy level of LMO is higher than that of BFO due to the lager VBM value of LMO than that of BFO. As mentioned above, the TEM results have been revealed that the BFO layer is under the in-plane compressive strain state, and LMO layer is under the in-plane tensile strain state in the LMO/BFO heterostructure. Therefore, dx2-y2 orbital rather than d3z2-r2 orbital is energetically preferred to occupy by electron for LMO due to the in-plane tensile strain, while dx2-y2 orbital is higher in the energy than d3z2-r2 orbital for BFO due to the in-plane compressive strain. At the interface, the covalent bond of Mn-O-Fe forms when the charge transfer from Mn ions to Fe ions occurs. Then the strong hybridization between Mn d3z2-r2 orbital (higher energy) and Fe dx2-y2 (higher energy) orbital would form the bonding orbital and antibonding orbital. The hybridization of the Mn dx2-y2 orbital (lower energy) and Fe d3z2-r2 orbital (lower energy) and the hybridization of 3d-t2g orbitals of Mn and Fe across the interface are negligible. In this scenario, the electrons are transferred from Mn dx2-y2 orbital to an intermediate state of Mn d3z2-r2 orbital, and then finally to the molecular bonding orbital shared by Fe and Mn at the interface.33 Namely, the charge transfer from Mn ions to Fe ions occurs in the LMO/BFO heterostructure. The eg electron of the Mn hops to Fe, favoring the energy stable. The FM order at the interface is determined by the double exchange interaction formed at the interface through the covalent bond of Mn-O-Fe. The strong exchange bias effect and higher TB in the LSMO/BFO heterostructure suggest that the magnetic coupling at the interface of LSMO/BFO is stronger than that at the interface of LMO/BFO. Therefore, charge transfer from Fe ions to Mn ions is in favor of stronger 23



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magnetic coupling in the manganite/BFO heterostructures.

CONCLUSIONS In this study, the LSMO/BFO and LMO/BFO heterostructures have been grown on the STO substrate. The HEB and TB values in the LSMO/BFO heterostructure are larger/higher than those in the LMO/BFO heterostructure, indicating that the magnetic coupling at the LSMO/BFO interface is stronger than that at the LMO/BFO interface. The XPS study results demonstrate that the charge transfer occurs at the LSMO/BFO and LMO/BFO interfaces, and the charge transfer procedure can be described as electron hopping from Fe ions to Mn ions in the LSMO/BFO heterostructure, while from Mn ions to Fe ions in the LMO/BFO heterostructure. Moreover, the charge transfer from Fe ions to Mn ions can lead to stronger interfacial magnetic coupling. The charge transfer, which is associated with orbital reconstruction, is also the origin of the EB effect in the manganite/BFO heterostructures.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] Notes The authors declare no competing finacial interest.

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ACKNOWLEDGMENTS This research was supported by the Hundred Talents Program of the Chinese Academy of Sciences and the National Natural Science Foundation of China (No. 51172238).

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FIGURE CAPTIONS: Figure 1 XRD patterns of the LSMO/BFO and LMO/BFO heterostructures, and the LSMO and LMO pure films. Figure 2 High-resolution TEM (HRTEM) images of (a) the LSMO/BFO and (b) the LMO/BFO heterostructures. Fast Fourier Transform (FFT) patterns of (c) the LSMO/BFO and (d) the LMO/BFO heterostructures transformed from the HRTEM images, respectively. Figure 3 Temperature dependence of the magnetization measured in an in-plane magnetic field of 100 Oe for the LSMO/BFO and LMO/BFO heterostructures, and the LSMO and LMO pure films. Figure 4 Magnetic hysteresis loops measured at 5 K after FC of (a) the LSMO/BFO 32


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heterostructure and the pure LSMO film, (b) the LMO/BFO heterostructure and the pure LMO film. Figure 5 Exchange-bias field (HEB) and coercivity (HC) of (a) the LSMO/BFO heterostructure and (b) the LMO/BFO heterostructure as a function of temperature. The exponential fitting of HEB and linear fitting of HC as a function of temperature is noted by red lines and blue lines, respectively. Figure 6 Fe 2p-spectra recorded from the BFO layer to the interface of the manganite for (a) the LSMO/BFO heterostructure, (b) the LMO/BFO heterostructure. Figure 7 (a) Mn 3s core-level XPS spectra of the LSMO/BFO interface and the LSMO layer. (b) Mn 3s core-level XPS spectra of the LMO/BFO interface and the LMO layer. Figure 8 Valence-spectra for the pure LSMO, LMO and BFO films with linear fits used to determine the VBM. Figure 9 Schematic diagrams of the VBO for the LSMO/BFO and LMO/BFO heterostructures. Figure 10 Schematic energy diagrams of orbital reconstruction at (a) the LSMO/BFO interface and (b) the LMO/BFO interface.

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Charge Transfer, Orbital Reconstruction, and ... - ACS Publications

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