Strain-induced high-temperature perovskite ferromagnetic insulator

• ferromagnetic insulator
• strain
• defect
• pulsed-laser deposition
• X-ray absorption

The realization of a dopant-free ferromagnetic insulator (FMI) is critical to the fabrication of versatile spintronic devices due to the potential for dopants acting as undesirable inelastic scattering centers. In low-symmetry magnetic materials, an FMI may be realized through charge ordering in which magnetic ions occupy different atomic sites. Unfortunately, integrating such low-symmetry FMIs (1⇓⇓–4) into film devices presents significant challenges due to large lattice mismatch between the film and commonly used substrates, which almost universally possess high-symmetry structures. In high-symmetry structures, however, magnetic ions likely occupy identical crystallographic sites so that charge-ordering–stabilized FMI states are highly disfavored. Thus, undoped high-symmetry FMIs remain extremely rare (5⇓–7). In fact, chemical doping has been always a prerequisite for making high-symmetry FMIs such as the colossal magnetoresistance oxides (8⇓⇓⇓–12) and magnetic semiconductors (13, 14). The two well-known undoped high-symmetry FMIs, EuS and strained EuTiO₃, exhibit low Curie temperatures of about 16 and 4 K, respectively (5, 7). Therefore, engineering an undoped, FMI with high symmetry and high transition temperature (T_C) is highly desirable.

Normally, realizing an FMI requires a nonzero energy cost (ΔE) associated with electron hopping from an occupied metal orbital to a nearby unoccupied metal orbital through the bridging ligand orbital, so that the “hops” remain virtual events (15⇓–17). Recently, the diamagnetic perovskite LaCoO₃ has been suggested as a possible high-symmetry FMI candidate due to the near-degeneracy of various spin states of the Co³⁺ ion, so that spin-state ordering may be readily achieved (18, 19). Such an ordering of the spin states, which may be induced through strain effects, was first theoretically proposed by Hsu et al. (20) and later by Demkov and coworkers (21) to explain the observed FM in the LaCoO₃ thin films (22, 23). However, previously fabricated LaCoO₃ thin films are susceptible to oxygen vacancy (O_v) ordering as observed in scanning transmission electron microscopy (STEM) studies (24⇓–26). More recently, such O_v ordering was reported to be critical for the observed FM in nonstoichiometric LaCoO_3−δ films (25, 26). Currently, the nature of the FMI state in LaCoO₃ films remains unresolved, and the roles of strain and O_vs remains hotly debated. In addition to resolving these questions, it is critical to elucidate whether the LaCoO₃ film can in fact yield an undoped, high-temperature FMI with the high symmetry which enables high-quality growth.

In this work, we fabricated nearly stoichiometric LaCoO₃ films through pulsed-laser deposition under a maximal oxygen pressure of 25 Pa, which resulted in layer-by-layer growth. Details of the sample information can be found in Materials and Methods and SI Appendix, section I. We also fabricated various LaCoO₃ thin films in which the Co²⁺ concentration was systematically varied by tuning the oxygen pressure and film thickness. The latter increases the Co²⁺ defect density when the thickness is reduced below 10 unit cells (u.c.). Further experiments and first-principles calculations demonstrated that the film with minimal Co²⁺ under tensile strain exhibited the strongest FM state. The observed FM eventually disappears for Co²⁺ concentrations above ∼10%. These results indicate that LaCoO₃ films under tensile strain may represent the unusual case of a high-symmetry undoped FMI with a T_C above liquid nitrogen temperature.

In Fig. 1A, we show magnetization and resistivity vs. temperature measurements for a 30-u.c. LaCoO₃ film grown under 25-Pa oxygen pressure on (001) SrTiO₃ substrate. The FM transition for this film appears at about 85 K. The resistivity increases with decreasing temperature from 390 to 70 K. Below 70 K, the resistance is over the measurement limit. A high-angle annular dark-field (HAADF) image, shown in Fig. 1B, was taken along the (010) direction by an aberration-corrected STEM. Long-range–ordered dark stripes associated with Co²⁺ state (25) or La–La elastic interactions (24) were not observed in either the present HAADF images or a variety of images taken in different areas and directions of the film, although we occasionally observed short, randomly distributed stripes in some areas (SI Appendix, section II). Co and Ti L-edge electron energy-loss spectroscopy (EELS), shown in Fig. 1C, reveals that the film–substrate interface is very sharp. Furthermore, the thickness oscillation fringes in the X-ray diffraction shown in Fig. 1D and the atomic force microscopy image in Fig. 1E both demonstrate the high quality of the fabricated LaCoO₃ film.

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Fig. 1.

FMI state in a nearly stoichiometric LaCoO₃ film under tensile strain. (A) Temperature dependences of the magnetization and resistivity of a 30-u.c. LaCoO₃ film grown on (001) SrTiO₃ at p(O₂) = 25 Pa. The magnetization was measured by a superconducting quantum interference device (SQUID) along the in-plane direction with a 500-Oe field after field cooling. (Inset) Schematic of the LaCoO₃/SrTiO₃ heterostructure. (B) Cross-section STEM-HAADF image of the film taken along the (010) direction. (C) Ti and Co layer-by-layer distributions across the interface measured by STEM-EELS. (D) Synchrotron XRD measurement of the (00L) peaks. (E) Atomically flat LaCoO₃ surface with terraces measured by atomic force microscopy.

To further examine the nature of the observed FMI, we intentionally introduced controlled levels of oxygen vacancies into the film by reducing the oxygen growth pressure and by reducing the film thickness, yielding LaCoO_3−δ films. The corresponding reductions in T_C and magnetization are shown in Fig. 2A. We found that reducing the oxygen pressure causes T_C to decrease and a dramatic reduction occurs between 10 and 15 Pa. X-ray diffraction analyses in Fig. 2B show that by reducing the oxygen pressure the out-of-plane lattice constant increases dramatically, in agreement with a previous report (27). All films discussed in this paper are coherently strained to their substrates, either SrTiO₃ or (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 (LSAT). We define an effective strain by ε_eff =1 − c/a. In Fig. 2C the effective strain was found to match the T_C variation extremely well. Thus, reducing the oxygen pressure is associated with a reduction in T_C and magnetization which we attribute to a reduction in the effective tensile strain and increased nonstoichiometry.

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Fig. 2.

Suppressed FM in LaCoO_3−δ films on SrTiO₃ substrates with released effective strain or with nonstoichiometry. (A) Temperature dependence of magnetization in LaCoO_3−δ films measured along the in-plane direction with a 500-Oe applied field after field cooling. (B) XRD scans of films grown with varied pressures. The fringes are total thickness oscillations indicating high-quality growth. (C) Dependences of T_C and effective strain on the growth pressure. (D) XAS spectra of films with varied thicknesses. (E) XANES spectra of films with varied thicknesses alongside reference spectra of bulk samples. Reprinted with permission from ref. 32, copyright (2006) by the American Physical Society. (F) Thickness dependences of T_C and Co valence measured by both XAS and XANES. (G) STEM-EELS oxygen K-edge spectra of three representative LaCoO₃ films. (H) EELS oxygen K edge of the three different blocks, each about 10 u.c., in the 30-u.c. nearly stoichiometric film.

Reducing the film thickness (at a constant growth pressure of 15 Pa) allows the T_C to be tuned more systematically while the effective strain remains relatively constant at ∼3.5% in thicker films and ∼3.3% in the 5-u.c. film. To probe the Co valence, we measured the Co L-edge and K-edge using soft X-ray absorption (XAS) and X-ray absorption near-edge structure (XANES) as shown in Fig. 2 D and E, respectively. In a LaCoO₃ film under tensile strain, Co³⁺ ions may exist in either the low-spin t_2g⁶e_g⁰ state (LS, S = 0) or the high-spin t_2g⁴e_g² state (HS, S = 2) (19). The introduction of O_vs, however, could stabilize Co²⁺ HS states (S = 1.5) with an orbital occupancy of t_2g⁵e_g² (26, 28, 29). Quantitative XAS spectral analysis using atomic multiplet plus the crystalline-field calculation (30, 31) was performed to extract the individual concentrations of the three spin states (more details in SI Appendix, section III). Additionally, the Co valence was quantitatively analyzed by comparing the measured XANES spectra to the reference spectra with a well-determined Co valence in a pseudocubic structure (32). The dependences of T_C and Co valence (both from the XAS and XANES) on the layer thickness are shown in Fig. 2F. Both decrease drastically as the thickness is reduced below 10 u.c. The coincidence of T_C and Co valence transitions suggests that the introduced nonstoichiometry suppresses the FM.

Since the oxygen K edge is very sensitive to changes in the valence of the transition metal, we performed its STEM-EELS measurements to double check the Co valence obtained from XAS and XANES. We studied three representative samples on SrTiO₃ substrates (more details in SI Appendix, section II), including a thick film (30 u.c., 25 Pa), a thin film (7 u.c., 15 Pa), and a poorly oxygenated thick film (30 u.c., 1 Pa). The average oxygen K-edge spectra in Fig. 2G show that the 7-u.c. film exhibits a very weak preedge peak likely due to rich concentrations of O_v. From the line-by-line scans in the well-oxygenated 30-u.c. film, we do not observe clear trends of this prepeak weakening as the layer approaches the interface. To improve the signal quality and enable better comparison, we average the spectra of the top 10 layers, middle 10 layers, and interfacial 10 layers of the 30-u.c. films. We observe that the middle block has a slightly stronger prepeak than the surface and interfacial blocks, as shown in Fig. 2H.

To better understand the interplay between strain, off-stoichiometry, and FM in LaCoO₃ we performed first-principles density-functional theory (DFT) calculations, the details of which are shown in Materials and Methods. Consistent with previous investigations, the ground state of the strained LaCoO₃ is FM for a = 3.905 Å due to the high concentrations of HS Co³⁺ and FM coupling between the nearest HS and LS Co³⁺ (20, 21). Our calculations show that the FM in LaCoO₃ will indeed be suppressed by increasing the O_v density. Such O_vs introduce extra electrons which generate HS Co²⁺ atoms containing half-occupied e_g orbitals which support antiferromagnetic (AFM) superexchange between half-occupied orbitals, which decreases the magnetization. LaCoO₃ films with three different O_v concentrations were investigated using 22×22×4 (1O_v/32Co), 22×2×4(1O_v/16Co), and 22×2×2 (1O_v/8Co) u.c. (Fig. 3). Stoichiometric LaCoO₃ without O_v was also explored using three structural models discussed above, all yielding similar results. One can see that when the concentration is only 1O_v/32Co (9.4% Co²⁺), the magnetization only decreases slightly from 2 μ_B/Co to 1.6 μ_B/Co. This is because the AFM coupled HS Co²⁺ atoms only affect the local magnetic state. When the concentration increases to 1O_v/16Co (12.5% Co²⁺), the magnetization decreases dramatically to 0.2 μ_B/Co as the two Co²⁺ atoms introduce a long-range AFM coupling into the u.c. (more details in SI Appendix, section IV). For an even higher concentration of 1O_v/8Co (25% Co²⁺), the magnetization is close to zero. This behavior may be well described by a step function with an inflection point near a Co²⁺ concentration of 10%, in excellent agreement with our experimental results. The O_v-ordered LaCoO_3−δ thin films have been studied by Biškup et al. (25). Here we studied films with O_v concentrations much less than that of the previous work. Although details did not agree, both studies showed fewer magnetizations in O_v-occupied films than O_v-free films.

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Fig. 3.

Theoretically calculated magnetizations of the tensile-strained LaCoO_3-δ films with increased O_v concentrations. The three atomic pictures from left to right show the DFT calculated spin-state distribution in tensile-strained LaCoO_3−δ films of 1O_v/32Co, 1O_v/16Co, and 1O_v/8Co, respectively. The atoms without density isosurface (blue colored) are LS Co³⁺. The rest are HS Co³⁺ and HS Co²⁺. The cyan- and yellow-colored isosurfaces have opposite spin directions.

In addition to the studies of films on SrTiO₃ described above, we have performed similar experiments using LaCoO₃ films grown on (001) LSAT substrates with varied thicknesses, and found similar results. In Fig. 4, we summarize the T_C of all three thin-film groups, in addition to examples of films in the literature (23, 24, 26), as a function of effective strain and Co valence to create a phase diagram. Higher T_Cs are associated with the phase region with tensile strain and the lowest O_v concentrations. Away from this region, T_C is sharply reduced or the films are nonmagnetic. The universality and substrate-independent reproducibility of these trends supports the validity and generality of this study. Therefore, the experimental findings strongly suggest that tensile strain of nearly stoichiometric LaCoO₃ thin film results in a high-temperature FMI.

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Fig. 4.

Phase diagram of the FM T_C. The T_C, effective strain ε_eff, and Co valence are all from experimental results. In the white-colored region, T_C is below 30 K and is difficult to be quantified due to the paramagnetic background in SQUID measurements. Open circles, triangles, and squares with dots inside represent the three series of samples fabricated in this study (SI Appendix, Table S1). The p series refer to the varying pressure films on SrTiO₃. m and n series refer to the varying thickness films on SrTiO₃ and LSAT, respectively. Solid-color symbols represent films from the literature (23, 24, 26).

Relative to other examples of undoped high-symmetry FMIs, the LaCoO₃ Curie temperature is surprisingly high. A detailed understanding of this behavior is not yet clear and will require further exploration. One possible explanation is the larger overlap between the Co 3d and oxygen 2p orbitals. In other FMIs such as EuS, the FM originates in localized 4f electrons at the Eu sites and their orbital overlapping with the S 2p orbital or Eu 5d orbital (33, 34). On the other hand, our theoretical calculations show that the Co 3d orbital is much broader than the Eu 4f orbital and the superexchange interaction in LaCoO₃ films (21) is more than one order magnitude larger than that in the Eu chalcogenides (34). The availability of an undoped high-symmetry and high-T_C FMI is an important step toward bringing quantum-spintronic devices into the practical operation regime. Our results here indicate that the tensile-strained LaCoO₃ thin film is a much better technological candidate than previous low-temperature FMIs, thanks to the material’s high T_C of nearly 90 K, its high cubic symmetry, and the ability to fabricate high-quality films over large areas on conventional substrates. The realization of such an FMI provides a solid foundation for the growth and design for next-generation device fabrication.

• Copyright © 2018 the Author(s). Published by PNAS.