Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films

Abstract The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain‐induced ferromagnetism using element‐specific X‐ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it is unambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low‐spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high‐spin and low‐spin states with a ratio close to 1:3. From the identification of this spin state ratio, it is inferred that the dark strips observed by high‐resolution scanning transmission electron microscopy indicate the position of Co3+ high‐spin state, i.e., an observation of a spin state disproportionation in tensile‐strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. The study highlights the importance of spin state degrees of freedom, along with thin‐film strain engineering, in creating new physical properties that do not exist in bulk materials.

impurities, will reduce the peak intensity around 530 eV because the highly ionic nature of the Co 2+ impurities forms the weaker hybridization with O 2p states.In Figure S11(a) we show the O K edge EELS spectra of LCO/LAO.It can be seen that the spectra got only a small drop in intensity around the 530 eV peak by taking with higher dose of electron beam irradiation, indicating that the compressive LCO film is less prone to the electron beam irradiation damage, i.e. the formation of oxygen vacancies.In contrast, as shown in Figure S11(b), the intensity around the 530 eV peak of LCO/STO decreases more when the higher dose electron beam is used to take the EELS data.It is noted that the EELS spectrum taken in the dark stripe has a lower intensity of the 530 eV pre-peak than that recorded in the bright stripe, indicating more oxygen vacancies in the dark stripe.The spectra around 530 eV are usually assigned to the O 2p band hybridized with Co 3d state. [32]The presence of the oxygen vacancies caused by the electron beam irradiation damage, which form the Co 2+ impurities, will reduce the EELS peak intensity around 530 eV because the highly ionic nature of the Co 2+ impurities forms the weaker hybridization with O 2p states.These findings indicate that the oxidation state of Co along the dark stripes has been reduced more by a relatively high dose of electron beam.Previously studied has suggested that the HS state is more prone to the oxygen vacancies than the LS state, [38][39] indicating that the location of the dark stripes is where HS Co 3+ is present.

Figure S1 .
Figure S1.The schematic diagram of electron configurations for (a) LS Co 3+ , (b) IS

Figure S6 .
Figure S6.Real space sketch to the LCO film structure grown in average pseudomorphically on top of a rigid, STO or LAO substrates.

Figure S7 .
Figure S7.(a-d) The experimental Co L2 XAS spectra of LCO/LAO and LCO/STO at

Figure S8 .
Figure S8.The total and atomic projected densities of states (DOS) for LaCoO3 thin films under different substrates.(a) The low-spin state of LaCoO3 thin films under LaAlO3 substrate.The black, red and blue curves are total, Co-d and O-p projected DOS, respectively.(b) The mixed-spin state of LaCoO3 thin films under SrTiO3 substrate.The black, red, green and blue curves are total, high-spin (HS) Co-d, lowspin (LS) Co-d and the O-p projected DOS, respectively.In both panels, EF is the Fermi level.

Figure S9 .
Figure S9.High-resolution HAADF STEM image of a LCO film grown on a LAO substrate with different electron dose condition; (a) low-and (b) high-dose conditions.

Figure S10 .
Figure S10.The energy level diagram as a function of 10Dq calculated under the three configurations model (a) and the single configurations model (b).

Figure S11 .
Figure S11.(a, c) O-K edge and (b, d) Co-L edge EELS spectra from line scans taken along the bright and dark contrast planes for LCO/LAO and LCO/STO under low and high dose electron beam conditions.It can be seen that there is a considerable difference in the peak around 530 eV between LCO/LAO and LCO/STO, which is due to the fact that LCO/STO is more prone to electron beam damage.The spectra around 530 eV are usually attributed to the O 2p hybridized with Co 3d.The presence of the oxygen vacancies caused by the electron beam irradiation damage, which form the Co 2+