Exchange Bias Effect and Orbital Reconstruction in (001)-Oriented

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Exchange bias effect and orbital reconstruction in (001)-oriented LaMnO3/LaNiO3 superlattices Guowei Zhou, Zhi Yan, Yuhao Bai, Julu Zang, Zhi-Yong Quan, Shifei Qi, and Xiaohong Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14503 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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ACS Applied Materials & Interfaces

Exchange bias effect and orbital reconstruction in (001)-oriented LaMnO3/LaNiO3 superlattices

Guowei Zhou,†,‡ Zhi Yan,† Yuhao Bai,‡ Julu Zang,† Zhiyong Quan,†,‡ Shifei Qi,*,†,‡ and Xiaohong Xu*,†,‡ †

School of Chemistry and Materials Science of Shanxi Normal University & Key Laboratory of

Magnetic Molecules and Magnetic Information Materials of Ministry of Education, Linfen 041004, China ‡

Research Institute of Materials Science of Shanxi Normal University & Collaborative Innovation

Center for Shanxi Advanced Permanent Magnetic Materials and Techonology, Linfen 041004, China

ABSTRACT: Paramagnetic LaNiO3-based heterostructures have been attracting the attention of researches, especially since the interesting exchange bias effect has been observed in (111)-oriented LaMnO3/LaNiO3 superlattices. However, this effect is not expected to occur in the (001) direction superlattices. In this paper, we report the observation

of

an

unexpected

exchange

bias

effect

in

(001)-oriented

(LaMnO3)3/(LaNiO3)t superlattices. The orbits of interfacial Mn/Ni ions preferentially occupy the strain stabilized x2-y2 in ultrathin LaNiO3 (LNO) layers (t ≤ 4 unit cells). Conversely, as the LNO layer becomes thicker (t ≥ 6 unit cells), the exchange bias effect is absent and the orbits are reconstructed to form the 3z2-r2 preferential occupancy. The absence of the exchange bias in thicker LNO-based superlattices is

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attributed to the interfacial charge transfer suppressed by orbital reconstruction as a consequence of the increasing LNO thickness. In the thinner LNO-based superlattices, the larger charge transfer results in stronger localized magnetic moments for the cause of the exchange bias effect. These results provide a useful interpretation of the relationship between macroscopic magnetic properties and the microscopic electronic structure in oxide-based heterostructures. KEYWORDS: orbital reconstruction, charge transfer, exchange bias, LNO thickness, superlattices

1. INTRODUCTION The recent advances in thin film growth techniques allow the fabrication of transition metal oxide heterostructures with atomically sharp interfaces, which have attracted the attention of research due to novel phenomena such as high temperature superconductivity

and

interfacial

magnetism.1,2

Among

these

phenomena,

understanding and controlling the interactions between charge, spin, orbital and lattice degrees of freedom in transition metal oxides constitute one of the main topic of modern condensed matter physics.3 For example, in the LaAlO3/SrTiO3 interface, the presence of high mobility conductivity and ferromagnetism was attributed to the interface electronic reconstruction.4 In the (Y, Ca)Ba2Cu3O7/La0.67Ca0.33MnO3 system, the variation of orbital ordering and ferromagnetism in the interfacial CuO2 layers were caused by the interfacial charge transfer. The presence of ferromagnetism in interfacial Ti ions was induced by charge transfer from interfacial Mn ions in

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LaMnO3/SrTiO3 superlattices.5,6 As far as different oxide materials are concerned, titanates, cuprates, and manganites have been extensively investigated, while nickelate heterostructures have received less attention.7-9 Perovskite nickelates are a peculiar family of compounds which display temperature-driven transitions from a metal paramagnetic state to an insulator with antiferromagnetic properties, expect for the LaNiO3 compound. LaNiO3 is always a metal and paramagnet lacking any other temperature dependent ordering phenomena in its bulk form.10 In

LaNiO3-based

superlattices,

theoretical

works

have

suggested

that

antiferromagnetism and high temperature superconductivity may be stabilized, which inspired a wealth of experimental and theoretical researches.11,12 Taking Gibert et al.’s work as an example, the exchange bias (EB) effect can be observed in LaMnO3/LaNiO3 superlattices (SLs) grown along the cubic perovskite (111) orientation, while it was not observed in SLs grown along the (001) direction.13 Since the LaMnO3 (LMO) layer is an A-type antiferromagnetic (AFM) insulator and the LNO layer is a paramagnetic (PM) metal,14-17 the EB observed in this superlattice is different from what occurs in traditional ferromagnetic/antiferromagnetic systems. The reasons why this unexpected EB was observed in the (111) direction and not in the (001) orientation have been studied by detecting the interfacial charge and spin via element specific X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD).18-25 Piamonteze et al. compared two different orientations and found a larger charge transfer for (111) superlattices compared to (001),21 which further confirms the origin of the EB effect from (111) superlattices rather than (001).

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However, Hoffman et al. found a considerable charge transfer between Ni and Mn in (001) stacking, although the exchange bias had never been mentioned in the literature.19 Therefore, there is still a lack of adequate information to confirm whether EB does exist or not in (001)-oriented LMO/LNO superlattices and of deeper investigation in order to achieve a deeper understanding of this system. In this work, LMO/LNO superlattices were grown along the (001) orientation and their magnetic properties are measured. The unexpected exchange bias effect is observed indeed in the SLs, in contrast with the experimental observations by Gibert et al..13 This unexpected exchange bias effect is explained by measuring the electronic transport properties and by analyzing the results from X-ray absorption spectroscopy and linear dichroism. Furthermore, for different thickness of the LNO layer, we demonstrate that the variation of orbital occupancy in the interfacial Mn/Ni ions indirectly influences the value of local magnetic moments, which generates the intrinsic correlation between the microscopic electronic structure and the macroscopic magnetic properties.

2. EXPERIMENTAL SECTION Fully epitaxial [LaMnO3(3)/LaNiO3(t)]10 superlattices with 2 ≤ t ≤ 15 unit cells (u.c.) were grown on (001) SrTiO3 (STO) substrates by pulsed laser deposition (see Figure S1). All superlattices studied here have a terminating top layer of LaMnO3. The stoichiometric bulk LMO material is in an A-type antiferromagnetic insulating ground state, while often exhibiting ferromagnetic behavior due to the presence of strain in

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the thin film, and its Curie temperature is around 120 K (see Figure S2). Prior to the growth, the substrates were etched in a commercial buffered oxide etchant solution for 15 minutes in order to form a TiO2 terminated surface (see Figure S3). Details on the sample preparation can be found in our previous work.20 The superlattices crystal structure and epitaxy were characterized by conventional x-ray diffraction (XRD) technique and high-angle annular dark field scanning transmission electron microscope (HAADF-STEM). The Mn L-edge, the Ni L-edge and the O K-edge x-ray absorption spectroscopy (XAS) and x-ray linear dichroism (XLD) measurements were performed ex situ at Beamline BL12B-a of the National Synchrotron Radiation Laboratory (NSRL) in total electron yield mode (TEY) at room temperature. The spectra were normalized in order that the L3 pre-edge and L2 post-edge have coincident intensities for the two polarizations. After that, the pre-edge spectral region was set to zero and the peak at the L3 edge was set to one. The XLD signals were determined by the difference between the XAS in-plane (E//a) and out-of-plane (E//c) components. Magnetic properties were measured in a vibrating sample magnetometer in physical properties measurement system (PPMS-VSM) with in-plane applied magnetic field, and hysteresis curves were obtained after subtracting the diamagnetic background. Transport measurements were performed in the Van der Pauw four-point probe configuration in the temperature range from 5 to 300 K with a Quantum Design PPMS. We also select the two probe method of Electrical Transport Option (ETO) component of PPMS in order to measure higher resistance for the insulating (3-2) superlattice.

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The first-principles calculations were performed within the framework of density functional theory (DFT) and using the projector-augmented wave (PAW) method as implemented in the VASP code. We adopted the generalized gradient approximation (GGA) for treating the exchange-correlation interaction. The atomic geometries were fully optimized until the Hellmann-Feynman forces on each ion were

Exchange Bias Effect and Orbital Reconstruction in (001)-Oriented

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