The interface in complex oxide heterostructure provides a rich playground to explore the emergent interfacial phenomena that arise due to electronic, spin, or orbital reconstruction.[
Chinese Physics B, Volume. 29, Issue 9, (2020)
Tuning magnetic anisotropy by interfacial engineering in La2/3Sr1/3Co1 – xMnxO2.5 + δ/La2/3Sr1/3MnO3/La2/3Sr1/3Co1 – xMnxO2.5 + δtrilayers
Grouping different oxide materials with coupled charge, spin, and orbital degrees of freedom together to form heterostructures provides a rich playground to explore the emergent interfacial phenomena. The perovskite/brownmillerite heterostructure is particularly interesting since symmetry mismatch may produce considerable interface reconstruction and unexpected physical effects. Here, we systemically study the magnetic anisotropy of tensely strained La2/3Sr1/3Co1 – xMnxO2.5 + δ/La2/3Sr1/3MnO3/La2/3Sr1/3Co1 – xMnxO2.5 + δ trilayers with interface structures changing from perovskite/brownmillerite type to perovskite/perovskite type. Without Mn doping, the initial La2/3Sr1/3CoO2.5 + δ/La2/3Sr1/3MnO3/La2/3Sr1/3CoO2.5 + δ trilayer with perovskite/brownmillerite interface type exhibits perpendicular magnetic anisotropy and the maximal anisotropy constant is 3.385 × 106 erg/cm3, which is more than one orders of magnitude larger than that of same strained LSMO film. By increasing the Mn doping concentration, the anisotropy constant displays monotonic reduction and even changes from perpendicular magnetic anisotropy to in-plane magnetic anisotropy, which is possible because of the reduced CoO4 tetrahedra concentration in the La2/3Sr1/3Co1 – xMnxO2.5 + δ layers near the interface. Based on the analysis of the x-ray linear dichroism, the orbital reconstruction of Mn ions occurs at the interface of the trilayers and thus results in the controllable magnetic anisotropy.
1. Introduction
The interface in complex oxide heterostructure provides a rich playground to explore the emergent interfacial phenomena that arise due to electronic, spin, or orbital reconstruction.[
In previous works, most of the emergent interfacial phenomena were realized in the perovskite/perovskite (P/P) heterostructures. Due to advances in epitaxial synthesis techniques, grouping oxide materials together with different atomic/electronic configurations is accessible.[
2. Experiment
LSCMO(6 nm)/LSMO(6 nm)/LSCMO(6 nm) (x = 0–0.7) trilayers, [LSCO(4 uc)/LSMO(4 uc)]5 and [LSCMO(4 uc) / LSMO(4 uc)]5 (x = 0.7) superlattices were grown on (001)-SrTiO3 (STO) single crystal substrates (3 × 5 × 0.5 mm3) by pulsed laser deposition, using a KrF excimer laser with a wavelength of 248 nm. The fluence of the laser pulse was 2 J/cm2 and the repetition rate was 2 Hz. The deposition was carried out at 680 °C in an oxygen atmosphere of 30 Pa for the LSMO layer and LSCMO layers. The distance between the polycrystalline target and substrate is ∼ 4.8 cm. The growth rate is ∼ 2 nm/min for the LSMO layer and ∼ 1.4 nm/min for LSCMO layers. After deposition, the samples were cooled to room temperature at a rate of 10 °C/min in an oxygen atmosphere of 100 Pa. The film thickness was determined by the number of laser pulses, which has been carefully calibrated by the technique of small angle x-ray reflectivity.
The surface morphology of the trilayers was measured by an atomic force microscope (SPI 3800 N, Seiko). X-ray diffraction (XRD) and reciprocal space mapping (RSM) was determined by Bruker x-ray diffractometer (D8 discover). High-angle annular dark-field (HAADF) images were recorded by a high-resolution STEM with double Cs correctors (JEOL-ARM200 F). Magnetic measurements were conducted by a quantum designed vibrating sample magnetometer (VSM-SQUID) in the temperature interval from 10 K to 380 K. The x-ray absorption spectra (XAS) were performed at Shanghai Synchrotron Radiation Facility, in the total electron yield mode. The spectra were measured at the Mn L-edge for the two polarizations in a geometry. The x-ray incident angle was rotated to 0° and 60° from the sample normal, which correspond to the in-plane (E||a, IIP) and out-of-plane (E||c, IOP) directions, respectively. The x-ray linear dichroism spectra, defined by IIP – IOP, was the intensity difference of normalized XAS along two polarizations, which provide information about the orbital occupancy of Mn-3d states. The measurement temperature was 300 K.
3. Results and discussion
High-quality LSCMO/LSMO/LSCMO (x = 0–0.7) trilayers are epitaxially grown on single-crystalline STO substrate, and the schematic diagram of the trilayers is shown in Fig. 1(a). Here, the thicknesses of the LSCMO layers and LSMO layer are fixed to 6 nm, which will highlight the interfacial effect of the trilayers.[
Figure 1.(a) The sketch diagram of LSCMO/LSMO/LSCMO (
To obtain detailed information about the lattice structure of LSCMO/LSMO/LSCMO trilayers, figures 2(a)–2(c) presents the typical HAADF images of the cross-section of the LSCMO/LSMO/LSCMO (x = 0, 0.3, 0.7) trilayers, recorded along the [100] zone axis. Here, the brighter spots are La/Sr ions and the fainter spots are Mn/Co ions. At first glance, the interface of LSCMO/LSMO in the trilayers is sharp and the LSMO layer displays the perovskite structure without any defects. However, dark stripes appear every two columns in the LSCO layers, indicating the formation of the brownmillerite LSCO phase.[
Figure 2.Typical high-angle annular dark-field image of the cross-section of the LSCMO/LSMO/LSCMO trilayers with (a)
It is interesting to see whether the different interface structures will influence the MA of the LSCMO/LSMO/LSCMO trilayers. So we pay our attention to the magnetic properties of the trilayers. Figure 3(a) shows the thermomagnetic (M–T) curves of LSCO/LSMO/LSCO trilayer with P/BM interface along in-plane (IP) and out-of-plane (OP) applied fields at various magnetic fields with field-cooling mode, respectively. Taking the data with the field of 0.05 T as an example, the magnetic moment first undergoes an obvious increase at the Curie temperature (Tc) of the LSMO layer (the increase-to-decrease crossover at ∼ 235 K is an indication of the spin reorientation), and then rapidly drops along the IP direction upon cooling. Meanwhile, the magnetic moment of OP direction exhibits monotonous increase upon cooling, and the magnetic moment of OP direction is about 4 times larger than that of the IP one at 10 K. As the magnetic field increases, the IP M–T curves gradually approach that of OP direction, and the spin reorientation phenomena can still be clearly seen in the M–T curve under the applied field of 1.5 T. The M–T curves clearly manifest an easy axis of the LSCO/LSMO/LSCO trilayer along OP direction. Compared to the PMA of the LSCO/LSMO/LSCO trilayer, the LSCMO/LSMO/LSCMO (x = 0.7) trilayer with the P/P interface displays a totally different MA. Figure 3(b) presents the M–T curves of LSCMO/LSMO/LSCMO (x = 0.7) trilayer along IP and OP applied fields at various magnetic fields. The magnetic moments of two directions monotonously increase upon cooling and no spin reorientation phenomena are observed. The magnetic moment of OP direction gradually approaches that of IP direction as the applied fields increase, and they become almost coincide at 1.5 T. These results manifest the easy axis of the LSCMO/LSMO/LSCMO (x = 0.7) trilayer along the IP direction.
Figure 3.Thermomagnetic curves of the (a) LSCO/LSMO/LSCO and (b) LSCMO/LSMO/LSCMO (
To obtain the quantitative description of MA, the anisotropy constant (KA) is calculated. Figure 3(c) presents the magnetic moment as a function of the magnetic field (M–H curves), extracted from the M–T curves of LSCO/LSMO/LSCO trilayer along IP and OP directions at 10 K. Along the OP direction, the magnetic moment increases rapidly with the applied field and saturates in a field about 0.2 T. In contrast, the magnetic moment exhibits a smooth growth with applied field along the IP direction. The energy required to force IP magnetic moment to align with the OP direction equals the shaded area encircled by the M–H curves. The calculation gives ∼ 3.385 × 106 erg/cm3 of the KA for LSCO/LSMO/LSCO trilayer at 10 K, which is more than one order of magnitude larger than that of bare LSMO film with magnetoelastic coupling interaction (∼ 104 erg/cm3).[
To explore the MA variation of the tensile-strained LSCMO/LSMO/LSCMO (x = 0–0.7) trilayers on the gradually changed interface structure from P/BM to P/P-type, a series of M–T curves of other LSCMO/LSMO/LSCMO (x = 0–0.7) trilayers are measured at different applied fields (
Figure 4.(a) Thermomagnetic curves of the LSCMO/LSMO/LSCMO (
It is already known that the electron filling of 3d-orbitals of transition metal oxides plays a crucial role to determine the electric and magnetic properties of oxide heterostructures.[
Figure 5.Normalized Mn-XAS spectra for the (a) [LSCO(4 uc)/LSMO(4 uc)]5 and (b) [LSCMO(4 uc)/LSMO(4 uc)]5 (
The above results strongly suggest that the MA and the orbital reconstruction are closely related to the interfacial coupling effect in the LSCMO/LSMO/LSCMO trilayers. As already demonstrated, the interface type changes from P/BM to P/P as the content of Mn increases. For the P/BM type interface, the interfacial MnO6 octahedra share the apical oxygen with neighboring CoO4 tetrahedra at the interface (Fig. 2(a)). This would cause the interfacial MnO6 octahedra to elongate along the [001] axis and tilt around the [110] axis to accommodate the symmetry mismatch between the CoO4 layer and the MnO6 layers.[
A further issue to be addressed is how the orbital reconstruction modifies the MA of the manganite oxide heterojunctions. The LSMO film owns strongly coupled spin, charge, and orbital degrees of freedom, and the MA stems from the strong spin–orbital interaction. According to the Bruno model, the anisotropy of the spin–orbit energy is directly related to the anisotropy of the orbital moment[
4. Conclusion
In summary, we systemically research the MA of tensile-strained LSCMO/LSMO/LSCMO (x = 0–0.7) trilayers with the interface structure changing from P/BM to P/P-type. The initial LSCO/LSMO/LSCO with typical P/BM structure exhibits huge PMA and the maximal value of KA is ∼ 3.385 × 106 erg/cm3 at 10 K, which is more than one order of magnitude larger than that of tensile-strained LSMO film. Using Mn ions to substitute Co ions of LSCO layer to reduce the CoO4 tetrahedra concentration of the LSCMO layers, the value of KA of trilayers changes from 3.385 × 106 erg/cm3 to –3.501 × 106 erg/cm3. When x ≤ 0.3, PMA dominates and the value of KA monotonously decreases as Mn concentration increases. However, in-plane MA dominates as x ≥ 0.5, i.e., the easy magnetic axis of trilayers changes from OP to IP directions. The orbital reconstruction occurs at the interface of the LSCMO/LSMO/LSCMO trilayers by interfacial engineering, resulting in the change of MA due to the strong spin–orbit coupling effect. Thus, this work demonstrates the great potential to tune the electromagnetism properties of oxide heterostructure by interfacial engineering.
[1] S Okamoto, A J Millis. Nature, 428, 630(2004).
[2] J Chakhalian, J W Freeland, H U Habermeier, G Cristiani, G Khaliullin, M van Veenendaal, B Keimer. Science, 318, 1114(2007).
[3] P Zubko, S Gariglio, M Gabay, P Ghosez, J M Triscone. Annu. Rev. Condens. Matter Phys, 2, 141(2011).
[4] H Y Hwang, Y Iwasa, M Kawasaki, B Keimer, N Nagaosa, Y Tokura. Nat. Mater, 11, 103(2012).
[5] B Cui, C Song, F Li, G Y Wang, H J Mao, J J Peng, F Zeng, F Pan. Sci. Rep, 4, 4206(2015).
[6] A Bhattacharya, S J May. Annu. Rev. Mater. Res, 44, 65(2014).
[7] F Hellman, A Hoffmann, Y Tserkovnyak et al. Rev. Mod. Phys, 89(2017).
[8] B Dieny, M Chshiev. Rev. Mod. Phys, 89(2017).
[9] C Chappert, A Fert, F N Van Dau. Nat. Mater, 6, 813(2007).
[10] J H Ngai, F J Walker, C H Ahn. Annu. Rev. Mater. Res, 44, 1(2014).
[11] A D Kent, D C Worledge. Nat. Nanotechnol, 10, 187(2015).
[12] J He, A Borisevich, S V Kalinin, S J Pennycook, S T Pantelides. Phys. Rev. Lett, 105(2010).
[13] J M Rondinelli, S J May, J W Freeland. MRS Bull, 37, 261(2012).
[14] R Aso, D Kan, Y Shimakawa, H Kurata. Sci. Rep, 3, 2214(2013).
[15] R Aso, D Kan, Y Shimakawa, H Kurata. Adv. Funct. Mater, 24, 5177(2014).
[16] Z Liao, M Huijben, Z Zhong, N Gauquelin, S Macke, R J Green, S Van Aert, J Verbeeck, G Van Tendeloo, K Held, G A Sawatzky, G Koster, G Rijnders. Nat. Mater, 15, 425(2016).
[17] D Kan, R Aso, R Sato, M Haruta, H Kurata, Y Shimakawa. Nat. Mater, 15, 432(2016).
[18] D Yi, C L Flint, P P Balakrishnan, K Mahalingam, B Urwin, A Vailionis, A T N’Diaye, P Shafer, E Arenholz, Y Choi, K H Stone, J H Chu, B M Howe, J Liu, I R Fisher, Y Suzuki. Phys. Rev. Lett, 119(2017).
[19] S Ismail-Beigi, F J Walker, A S Disa, K M Rabe, C H Ahn. Nat. Rev. Mater, 2(2017).
[20] L F Wang, Q Y Feng, Y Kim, R Kim, K H Lee, S D Pollard, Y J Shin, H B Zhou, W Peng, D Lee, W J Meng, H Yang, J H Han, M Kim, Q Y Lu, T W Noh. Nat. Mater, 17, 1087(2018).
[21] J F Ding, F Cossu, O I Lebedev, Y Q Zhang, Z D Zhang, U Schwingenschlogl, T Wu. Adv. Mater. Interfaces, 3(2016).
[22] J Zhang, Z Zhong, X Guan, X Shen, J Zhang, F Han, H Zhang, H Zhang, X Yan, Q Zhang, L Gu, F Hu, R Yu, B Shen, J Sun. Nat. Commun, 9(2018).
[23] J E Zhang, F R Han, W Wang, X Shen, J Zhang, H Zhang, H L Huang, H R Zhang, X B Chen, S J Qi, Y S Chen, F X Hu, S S Yan, B G Shen, R C Yu, J R Sun. Phys. Rev. B, 100(2019).
[24] B C Behera, S Jana, S G Bhat, N Gauquelin, G Tripathy, P S Anil Kumar, D Samal. Phys. Rev. B, 99(2019).
[25] B Liu, Y Q Wang, G J Liu, H L Feng, H W Yang, X Y Xue, J R Sun. Phys. Rev. B, 93(2016).
[26] J Li, J Wang, H Kuang, H R Zhang, Y Y Zhao, K M Qiao, F Wang, W Liu, W Wang, L C Peng, Y Zhang, R C Yu, F X Hu, J R Sun, B G Shen. Nanoscale, 9(2017).
[27] J E Zhang, X X Chen, Q H Zhang, F R Han, J Zhang, H Zhang, H R Zhang, H L Huang, S J Qi, X Yan, L Gu, Y S Chen, F X Hu, S S Yan, B G Liu, B G Shen, J R Sun. ACS Appl. Mater. Interfaces, 10(2018).
[28] K Steenbeck, R Hiergeist. Appl. Phys. Lett, 75, 1778(1999).
[29] K Steenbeck, T Habisreuther, C Dubourdieu, J P Sénateur. Appl. Phys. Lett, 80, 3361(2002).
[30] H W Yang, H R Zhang, Y Li, S F Wang, X Shen, Q Q Lan, S Meng, R C Yu, B G Shen, J R Sun. Sci. Rep, 4(2015).
[31] P Bruno. Phys. Rev. B, 39, 865(1989).
[32] H B Huang, T Shishidou, T Jo. J. Phys. Soc. Jpn, 69, 2399(2000).
[33] M Huijben, L W Martin, Y H Chu, M B Holcomb, P Yu, G Rijnders, D H A Blank, R Ramesh. Phys. Rev. B, 78(2008).
[34] A Tebano, C Aruta, S Sanna, P G Medaglia, G Balestrino, A A Sidorenko, R De Renzi, G Ghiringhelli, L Braicovich, V Bisogni, N B Brookes. Phys. Rev. Lett, 100(2008).
[35] D Yi, N P Lu, X G Chen, S C Shen, P Yu. J. Phys.: Condens. Matter, 29(2017).
[36] D J Huang, W B Wu, G Y Guo, H J Lin, T Y Hou, C F Chang, C T Chen, A Fujimori, T Kimura, H B Huang, A Tanaka, T Jo. Phys. Rev. Lett, 92(2004).
[37] C Aruta, G Ghiringhelli, A Tebano, N G Boggio, N B Brookes, P G Medaglia, G Balestrino. Phys. Rev. B, 73(2006).
[38] D Pesquera, G Herranz, A Barla, E Pellegrin, F Bondino, E Magnano, F Sanchez, J Fontcuberta. Nat. Commun, 3, 1189(2012).
[39] J Peng, C Song, F Li, B Cui, H Mao, Y Wang, G Wang, F Pan. ACS Appl. Mat. Interfaces, 7(2015).
[40] B Cui, F Li, C Song, J J Peng, M S Saleem, Y D Gu, S N Li, K L Wang, F Pan. Phys. Rev. B, 94(2016).
[41] P Bruno, Magnetismus Von Festkörpern, . KFA: Jülich Germany, 24, 1(1993).
Get Citation
Copy Citation Text
Hai-Lin Huang, Liang Zhu, Hui Zhang, Jin-E Zhang, Fu-Rong Han, Jing-Hua Song, Xiaobing Chen, Yuan-Sha Chen, Jian-Wang Cai, Xue-Dong Bai, Feng-Xia Hu, Bao-Gen Shen, J-Rong Sun. Tuning magnetic anisotropy by interfacial engineering in La2/3Sr1/3Co1 – xMnxO2.5 + δ/La2/3Sr1/3MnO3/La2/3Sr1/3Co1 – xMnxO2.5 + δtrilayers[J]. Chinese Physics B, 2020, 29(9):
Received: Jun. 4, 2020
Accepted: --
Published Online: Apr. 29, 2021
The Author Email: J-Rong Sun (jrsun@iphy.ac.cn)