Magnetic Anisotropy Induced by Orbital Occupation States in La0.67Sr0.33MnO3 Film

Interface engineering is an effective and feasible method to regulate the magnetic anisotropy of films by altering interfacial states between different films. Using the technique of pulsed laser deposition, we prepared La 0.67 Sr 0.33 MnO 3 (LSMO) and La 0.67 Sr 0.33 MnO 3 /SrCoO 2.5 (LSMO/SCO) films on the (110) - oriented La 0.3 Sr 0.7 Al 0.65 Ta 0.35 O 3 substrates. By covering the SCO film above the LSMO film, we transformed the easy magnetization axis of LSMO from the [001] axis to the [ 1 1 ̅ 0] axis in the film plane. Based on statistical analyses, we found that the corresponding Mn - Mn ionic distances are different in the two types of LSMO films, causing different distortions of Mn - O octahedron in the LSMO film. In addition, it also induces diverse electronic occupation states in Mn 3+ ions. The e g electron of Mn 3+ occupies 3 z 2 - r 2 and x 2 - y 2 orbitals in the LSMO and LSMO/SCO, respectively. We conclude that the electronic spin reorientation leads to the transformation of the easy magnetization axis in the LSMO films.


Introduction
Transition metal oxide (TMO) films, especially the oxides processed by the interface engineering [1][2][3][4][5][6], have attracted extensive attention in the past decades owing to their diverse properties, such as interface charge transfer [7], two-dimensional electron gas [8], and discrepancy from bulk materials.Among TMOs, manganates exhibit distinctive performances as a result of the novel magnetic and electronic structures induced by the strong correlation between electrons as well as competition among lattice, charge, spin and orbit degrees of freedom, for instance, metal-insulator transition [9], colossal magnetoresistance [10,11], magnetocaloric effects [12], and magnetoelectric coupling multiferroic effect [13].
For an epitaxial film, interfaces [6], strain induced by a mismatch with the substrate [14], and restriction of dimension [15] may break the delicate balance among different interactions and result in exotic properties.A fundamental property of magnetic materials is magnetic anisotropy (MA).MA plays a crucial role in many physical phenomena, including magnetic skyrmions [16], the magnetocaloric effect [17], and Kondo effect [18].There are three types of MA existing in magnetic materials.Magnetocrystalline anisotropy, magnetoelastic coupling anisotropy, and shape anisotropy.Magneiocrystalline anisotropy in independent of grain size and shape while shape anisotropy is the dominant form of anisotropy in small size specimen [19,20].La0.67Sr0.33MnO3(LSMO), as a perovskite-type magnetic material, is a promising candidate for spintronics [21][22][23] material because of its room Curie temperature and 100% spin polarization [9].For epitaxial thin films, different interfacial couplings cause different MA behaviors [1,[24][25][26].MA in LSMO has been proved to be closely related to the occupation state of Mn 3d-orbital electrons [27].In general, the mismatch of lattice constant at interface distorts the MnO6 octahedra of LSMO and controls the electronic occupation state of Mn ions.
Revealing the mechanism of MA in the LSMO film will enrich the corresponding scientific understanding and pave the way to future material designs and device applications of spintronic materials.In this work, we found that LSMO and La0.67Sr0.33MnO3/SrCoO2.5 (LSMO/SCO) films epitaxially grown on (110)-oriented La0.3Sr0.7Al0.65Ta0.35O3(LSAT) substrates exhibit different MA behaviors.By using an aberration corrected transmission electron microscope, we analyzed the microstructures of the two films at the atomic scale and investigated the reason for causing the different MA.

Experiments
The LSMO and LSMO/SCO films were grown epitaxially on (110)-oriented LSAT substrates by using the pulsed laser deposition (PLD) method with a KrF eximer laser (λ = 248 nm).The LSMO layer is deposited at a temperature of 730 °C in an oxygen pressure of 30 Pa accompanying a laser fluence of 1.6 J/cm 2 .And the SCO layer is deposited at 700 °C in an oxygen pressure of 12 Pa accompanying a laser fluence of 1.1 J/cm 2 .The film thickness is determined by the time of deposition.The magnetic measurements were carried out in a Quantum Design vibrating sample magnetometer superconducting quantum interference device (VSM SQUID).The details of the fabrication and properties of the films can be found in Ref. [28].
Thin specimens for transmission electron microscopy (TEM) were prepared by mechanical polishing accompanied by Ar ion milling at liquid nitrogen temperature or focused ion beam (Helios 600I, FEI) technique.The selected area electron diffraction (SAED) observations were performed on a transmission electron microscope (CM200, Philips) with a field-emission gun (FEG) at 200kV.The high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) spectra were acquired on a scanning transmission electron microscope (STEM) equipped with double Cs correctors (CEOS) for the condenser lens and objective lens (ARM200F, JEOL) and a cold FEG at 200kV.Double tile holders were adopted in TEM studies and controlled by a TEM operate system to ensure the zone axis parallel to the electron beam in all TEM experiments.

Results and discussion
Two types of films were grown epitaxially on the (110)-oriented LSAT substrates, as shown schematically in Figs.1c and d It is well known that LSAT is a diamagnetic material with a perovskite structure (space group of Pm-3m) [29,30] and SCO has a G-type antiferromagnetic orthorhombic structure with a space group of Ima2 [31].Therefore, the ferromagnetism only originates from the LSMO film.Meanwhile, there is an extraordinarily strong double exchange interaction between Mn 3+ and Mn 4+ ions in the LSMO below TC [32].Because of the effect of the crystal field in the LSMO, d-orbitals of Mn ion will split into eg and t2g orbitals [33].Moreover, the Jahn-Teller effect [34] degenerates eg orbitals into x 2 -y 2 and 3z 2 -r 2 orbitals.The electronic occupation state of Mn ions is closely related to the crystal structure of LSMO.In order to explore the microstructures and transformation of the easy magnetization axis in the LSMO film, we carried out TEM observations for the film samples along the cross-section direction at the atomic scale.Fig. 2 shows HAADF STEM images of the two samples, indicating the thickness of 6.5 nm for the LSMO and a sharp boundary between the LSMO and LSAT.A bulk LSMO has a pseudo-cubic structure with R-3c space group [35].
When it is grown epitaxially on a cubic structure substrate, it will retain the pseudo-cubic structure.Thus, grown on LSAT, the LSMO film exhibits a perovskite structure, as shown in Figs.2a and b.Since the contrast intensity is approximately proportional to Z 2 (Z is the atomic number) in HAADF STEM images [36], the brightest spots in the LSMO layer in HAADF images represent La(Sr) atomic columns, and the fainter spots correspond to Mn atomic columns.The O atoms could not be observed in the HAADF image since their scattering is too weak to be acquired at acceptance angles of 90-370 mrad (HAADF).
However, there is an indistinct borderline between the LSMO and SCO layers, which instructs a mixed trace of the two components closing to the interface.Besides, parallel dark stripes occur in the SCO layer and have an angle of 45° relative to the interface, as shown in Fig. 2d.These dark stripes indicate that the SCO film has a typical brownmillerite structure [37,38] rather than a perovskite structure.Thus, we deduce that a perovskite-brownmillerite interface plays a significant role in the MA in LSMO [2,40].In SLF: In BLF: As we mentioned above, crystal distortion of perovskite degenerates eg orbitals into x 2 -y 2 and 3z 2 -r 2 orbitals.The magnetic moment of LSMO mainly comes from the spin moment of Mn ions, but the direction of the magnetic moment is affected by the occupied orbital direction.In SLF, ξ[001] > ξ[11̅ 0] causes an elongated octahedral along the [001] direction , as shown in Fig. 5f, and the lower energy of 3z 2 -r 2 orbital compared to x 2 -y 2 orbital.In this case, the eg electron of Mn 3+ preferentially occupies 3z 2 -r 2 orbital.According to Bruno model [24], this induces an easy magnetization axis along the [001] direction.Relatively, in BLF, ξ[001] < ξ[11̅ 0] leads to a compressed octahedral along the [001] direction , as shown in Fig. 5f, and the lower energy of x 2 -y 2 orbital, thus, the eg electron prefers to occupy x 2 -y 2 orbital.In the LSMO film plane, the [11 ̅ 0] direction is along the projection of the x 2 -y 2 orbital.So, the occupation of eg electron in x 2 -y 2 orbital induces an easy magnetization axis along the [11 ̅ 0] direction.This is consistent with the result of x-ray linear dichroism [28].Charge transfer between Mn and Co ions was mentioned to explain the phenomenon of MA in an LSMO film [40].In Fig. 6, we present the EELS spectra of the BLF film in an energy range from 625 eV to 875 eV.Each spectrum is an integration of the line scan profiles along the [11 ̅ 0] direction when the sample was observed along the [001] zone axis.We obtained EELS spectra from the LSMO layer to the SCO layer across the interface as shown in Fig. 6.Obviously, near the interface between LSMO and SCO, both of the Mn and Co peaks appear simultaneously, indicating a mixture of several atomic layers.This is consistent with the blurry borderline in Fig. 2d and causes stronger crystal distortions.However, there is no obvious shift of Mn L edge from the LSMO layer to the interface, indicating no change of valence state for Mn ions.On the contrary, a shift of the Co L3 peak toward low energy can be noticed from the Co layer to the interface, suggesting a decrease of valence state for Co ions.This may be caused by the existence of SrCoO3-δ (0<δ<1) in local areas as an impurity in SrCoO2.5 film [28,41].Therefore, there may be no charge transfer between Mn and Co ions in the BLF.Through the quantitative statistical analyses, we found that the Mn-O octahedral distortions are different in these two film samples.In SLF, the octahedra are elongated, while in BLF, the octahedra are compressed.This gives rise to the preferential occupation of eg electron in the 3z 2 -r 2 orbital of Mn in SLF while in the x 2 -y 2 orbital in BLF.Thus, it is concluded that the anomalous MA transformation of LSMO from the [001] direction in SLF to the [11 ̅ 0] in BLF originates from a spin orientation of Mn ions.Our EELS spectra reveal that there is no detectable charge transfer between Mn and Co ions.
Please see the Manuscript PDF le for the complete gure caption Integrated EELS spectra from the LSMO layer to the SCO layer . Fig.1cindicates a single layer LSMO film with a thickness of 6.5 nm and Fig.1dis a bilayer film with 6.5 nm LSMO and 40 nm SCO.The magnetizations of the two samples in a field cooling with H=100Oe as a function of temperature are shown in Figs.1a and b, respectively.The direction of the applied magnetic field is along the [001] and [11 ̅ 0] axes, respectively.The corresponding magnetizations are represented by pink and blue curves, respectively.In Figs.1a and b, an obvious increase occurs around 300K, indicating a ferromagnetic transition.It should be noted that the magnetization along the [001] axis is higher than that along the [11 ̅ 0] axis below 280K in single layer film (SLF) [Fig.1a],however, that is inverse in bilayer film (BLF) [Fig.1b].This means that the easy magnetization axis is along the [001] direction in the SLF whereas along the [11 ̅ 0] direction in the BLF.

Fig. 1 a
Fig. 1 a and b Magnetizations as a function of temperature in a field cooling with H=100Oe corresponding to the LSMO and LSMO/SCO films, respectively.c and d Schematic diagrams of LSMO and LSMO/SCO films (purple arrows represent the

Fig. 2 a
Fig. 2 a and b Cross-sectional HAADF images of LSMO SLF along the [001] and [11 ̅ 0] axes, respectively.d and e Cross-sectional HAADF images of LSMO BLF along the [001] and [11 ̅ 0] axes, respectively.c and f Schematic diagrams of SLF and BLF, respectively (subscript "C" in the HAADF images indicates cubic structure)

Fig. 3 a
Fig. 3 a and b Cross-sectional SAED patterns of SLF corresponding to Figs. 2a and b. c and d Cross-sectional patterns of BLF corresponding to Figs. 2d and e (spots marked by red arrows come from the SCO film)

Fig. 4 a
Fig. 4 a The HAADF image of cross-sectional LSMO BLF along the [001] zone axis.Yellow arrows indicate dark stripes in

Fig. 5 f
Fig. 5 The statistical interatomic distances of Mn-Mn.a ξ[11̅ 0] and ξ[110] in SLF obtained from the HAADF image along the [001]C zone axis.b ξ[001] and ξ[110] in SLF obtained from the HAADF image along the [11 ̅ 0]C zone axis.c ξ[11̅ 0] and ξ[110] in BLF obtained from the HAADF image along the [001]C zone axis.d ξ[001] and ξ[110] in BLF obtained from the HAADF image along the [11 ̅ 0]C zone axis.Each data point is averaged by the Mn-Mn distances along a row or a column.The dotted lines are the average values of all corresponding data.e Schematic Mn-Mn distances of ξ[001], ξ[11̅ 0] and ξ[110] in the structure.f The elongated and compressed Mn-O octahedral of LSMO along the [001]C direction

Fig. 6
Fig. 6 Integrated EELS spectra from the LSMO layer to the SCO layer

Figures
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