Emergent Magnetic Phenomenon with Unconventional Structure in Epitaxial Manganate Thin Films

Abstract A variety of emergent phenomena are enabled by interface engineering in the complex oxides heterostructures. While extensive attention is attracted to LaMnO3 (LMO) thin films for observing the control of functionalities at its interface with substrate, the nature of the magnetic phases in the thin film is, however, controversial. Here, it is reported that the ferromagnetism in two and five unit cells thick LMO films epitaxially deposited on (001)‐SrTiO3 substrates, a ferromagnetic/ferromagnetic coupling in eight and ten unit cells ones, and a striking ferromagnetic/antiferromagnetic pinning effect with apparent positive exchange bias in 15 and 20 unit cells ones are observed. This novel phenomenon in both 15 and 20 unit cells films indicates a coexistence of three magnetic orderings in a single LMO film. The high‐resolution scanning transmission electron microscopy suggests a P21/n to Pbnm symmetry transition from interface to surface, with the spatial stratification of MnO6 octahedral morphology, corresponding to different magnetic orderings. These results can shed some new lights on manipulating the functionality of oxides by interface engineering.


Introduction
Artificial oxide heterostructures with chemically abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. [1][2][3][4][5][6][7][8] Various interesting properties and diverse phase diagrams have been demonstrated in LaMnO3 (LMO) thin films, multilayers, and superlattices, making the interfaces between LMO and substrates become an ideal candidate for discovering new phenomena for controlling functionalities. [9][10][11][12] Stoichiometric LMO bulk is known to be a layer-type (A-type) antiferromagnet, in which Mn 3+ is a Jahn-Teller ion with a occupancy and the in-plane interaction between adjacent Mn ions is ferromagnetic (FM) while the out-of-plane one is antiferromagnetic (AFM). [13] In contrast to bulk LMO, there has been lots of controversy about magnetic phase in thin films. While some studies describe the appearance of FM behavior in stoichiometric thin films, [9,14,15] other reports have shown that antiferromagnetic order staying in the films thinner than 5 unit cells (u.c.), and ferromagnetic phase in films thicker than 5 u.c.. [9,13] Meanwhile, almost all experiments have unveiled an insulating feature [9,12,13,16,17] for LMO films in both ferromagnetic and antiferromagnetic phases, although many theoretical reports have predicted that LMO thin films possess a Pbnm structure and with a ferromagnetic metallic phase. [18,19] Numerous efforts have been devoted to eliminate this paradox by reducing sample symmetry [15] or confirming electronic phase separation, [14] among which H. J. Xiang et. al [15] firstly introduced unconventional P21/n structure in LMO to explain this ferromagnetic insulating feature.
However, this novel P21/n structure in LMO films has not been experimentally observed so far.
In this paper, we firstly report an experimental observation of ferromagnetic insulating state in ultra-thin LMO films with thickness of 2 and 5 u.c.. We demonstrate a ferromagnetic/ferromagnetic (FM/FM) coupling in 8 and 10 u.c. films, indicating the appearance of a harder FM state in the region above 5 u.c.. In addition to the FM/FM coupling, a striking ferromagnetic/antiferromagnetic (FM/AFM) pinning effect with remarkable positive exchange bias is shown in 15 and 20 u.c. thin films, indicating an antiferromagnetic order appearing in the layer above 10 u.c. away from the interface.
To reveal the evolution of magnetic property in different regions, we show the characterizations by high-resolution scanning transmission electron microscopy (STEM), which display a transition from P21/n to Pbnm structure, corresponding to the evolution of FM to AFM. Our study demonstrates that different magnetic orderings, diverse octahedral morphologies, and various exchange couplings can be achieved in a single oxide by controlling their dimensionality.

Results and Discussion
The crystalline quality of LMO films with different thicknesses is ensured by highresolution x-ray diffraction (XRD) patterns in Figure 1a (Figure 2a). This ferromagnetism in 2 u.c. films is firstly obtained, whereas it was reported as an antiferromagnetic layer by other groups. [9,13,14] This ferromagnetic feature has also been found in 5 u.c. samples displayed in Figure   2b. With increasing film thickness up to 8 or 10 u.c., interestingly, the MH loops become  [10,20,21] following the FML2 in the films. To be mentioned later, we denote the layer thicker than 11 u.c. as the antiferromagnetic layer (AFML). The existence of this antiferromagnetic order is also verified by the abrupt drops in the MH loops around zero magnetic field, indicated by red circles (Figure 2g). These drops correspond to the spin flip of the AFML when subjected to a certain external magnetic field. [21][22][23] This antiferromagnetic feature is analogous to the antiferromagnetic property of LMO bulk. [24,25] Figure S1b) with increasing thickness where the ratio of Mn 2+ /Mn 3+ becomes smaller. This distribution is also in good agreement with the enhancement of the O 1s peak at 529.8 eV with increasing thickness (Figure S1c) which represents higher Mn oxidation states than +2. [26] The appearance of Mn 2+ may attribute to the electronic reconstruction at the interface between STO and LMO. It is known that the net charge imbalance between two adjacent sublayers will inevitably lead to the polar catastrophe. Therefore, the charge transfer between two transition metal ions happens to neutralize the charged interfaces. [17,27] For an ultrathin LMO film with a thickness below 2 u.c., we believe that surface effect may also contribute to add free electrons, e.g. oxygen vacancies or strain-induced nonstoichiometry, into LMO film, leading the partial Mn 3+ ions change to Mn 2+ ions. [28,29,30,31] To reveal the mechanism of our magnetic results, the unit-cell-resolved structural evolution is characterized with high-resolution scanning transmission electron microscopy (STEM). Figure Figure 3b, and the corresponding oxygen octahedra are schematically illustrated in Figure 3c using Visualization for Electronic and Structural Analysis (VESTA) program, [31] where the octahedra manifest vast diversity from each layer. We carried out statistics over 50 atomic columns and 22 atomic rows from the STEM image shown in Figure 3e using DigitalMicrograph. The statistical example for a single row in FML1, FML2, and AFML is shown in Figure S2. For the substrate, generally, the cubic STO presents virginal TiO6 octahedra and belongs to typical Pm3 m space group. In both FML1 and FML2, two kinds of octahedra, denoted as OC1 and OC2, appear alternatively in G type manner [15] with three-dimensional alternation of OC1 and OC2. Figure 3d  c axis (Lz) of the octahedra denoted as OC2 for one row and OC1 for the nearest rows.
Lz (Lx) is always longer (shorter) in OC1 than the one in OC2, shown as the zigzag curve in Figure 3d for both FML1 and FML2. In Figure 3d, we find that both OC1 and OC2 are c-axially elongated and Lz is longer than Lx for FML1 due to the compressing effect from the substrate. This characteristic is verified by the statistics of STEM characterization for 5 u.c. LMO films displayed in Figure S3. However, for FML2, Lz is longer (shorter) than Lx in OC1 (OC2). The alternation of OC1 and OC2 in FML1 matches perfectly with the octahedral spatial arrangement of LMO in the P21/n structure predicted by H. J. Xiang et. al.. [15] Further evidence on the octahedra tilt is illustrated suggest that the thickness-driven octahedral tilt within LMO layer is uniformly distributed parallel to the STO substrate. In addition, the insulating behaviors measured for our films shown in Figure S5 is in good agreement with the insulating phase for ferromagnetic P21/n structure. The gradual structural transition in LMO films can be attributed to the symmetry-mismatch between STO (cubic) and LMO (orthorhombic). [9,13] The relaxation of shear strain will inevitably change the crystallographic symmetry of LMO films with increasing thickness. This substrate/film lattice and symmetry mismatch is the main source of the octahedral rotation modification and Jahn-Teller distortion. [12,14,[32][33][34] The evolution of MnO6 octahedra changes bond length and angle, thus affecting the balance between the intra-atomic exchange interaction energy and crystal field energy through the structural distortion.
As a result, the electronic ground state is modified, leading to the observed emerging magnetic phenomenon in our LMO films. [34][35][36][37] In AFML, Lx (Lz) remains roughly the same for OC1 and OC2, with Lz being longer than Lx. The similarity of OC1 and OC2 in AFML with c-axially stretching and ab plane compressing remains, which is described as normal Q3 distortion mode. [25,38] The octahedra in AFML manifest the heaviest rotation ( Figure S4) and the film demonstrates the Pbnm structure. Above relative variations of Lx and Lz in FML1, FML2, and AFML refer to the ferromagnetic, harder ferromagnetic, and antiferromagnetic phases in the films, respectively. The octahedra are schematically plotted in Figure 4 for P21/n ( Figure 4a) and Pbnm (Figure 4b), respectively, as well as the occupation of Mn 3d orbitals (Figures   4c-d). In the alternation of OC1 and OC2, the single eg electron of OC1 occupies the orbital with lower energy, and the eg electron of OC2, however, occupies the orbital as shown in Figures 4a and 4c. Three-dimensionally alternating / orbital order gives rise to the ferromagnetism in FML1 and FML2 according to Goodenough-Kanamori rules. Detail explanations can be found in Reference [15]. Since Lz is always larger than Lx in the same octahedron in FML1, and their relative length alternates three-dimensionally in FML2 (Figure 3d) In AFML, as shown in Figures 4b and 4d, the hybrid orbital Mn ion interacts with the empty orbital of adjacent Mn through O2p orbital with a negative exchange integral along c axis, resulting in an antiferromagnetic super-exchange interaction. [25,39,40] As a result, this layer shows an A-type antiferromagnetic feature like the bulk. We note that the emergent magnetic phenomenon in this work has not been reported by other groups. We believe that the correct stoichiometry and negligible oxygen vacancy present in our LMO film may be one of the key factors that control the magnetic ground state.

Conclusion
We studied the evolution of the insulating magnetic LMO thin films and found that

Experimental Section
Sample synthesis: The LMO films of various thickness were deposited on (001)oriented STO substrates at 680 ℃ using a pulsed laser deposition (PLD) system. During the deposition, the target-substrate distance was set to of 7.2 cm and the oxygen partial pressure was adjusted to 0.1 Pa. A XeCl laser with central wavelength of 308 nm was employed to provide a laser energy density of 1.2 J/cm 2 at a frequency repetition of 3 Hz. After the growth, the LMO films were annealed in situ for 7.5 min in a vacuum of ~2 × 10 -3 Pa to eliminate excess oxygen, and then were cooled down to room temperature at a rate of 15 ℃/min. Electron energy loss spectroscopy: The unit-cell-resolved EELS spectra were collected using STEM at the same time with structural characterization of these films.
Statistical analysis: We imported the STEM images in Figure 3e and Figure S3a into DigitalMicrograph to obtain the coordinates of each atoms. Using their coordinates,  Figure S3 shows