# Ferromagnetism of LaCoO$_3$ films

### Submission summary

 As Contributors: Andrii Sotnikov Arxiv Link: https://arxiv.org/abs/2004.00488v2 (pdf) Date accepted: 2020-05-28 Date submitted: 2020-05-13 02:00 Submitted by: Sotnikov, Andrii Submitted to: SciPost Physics Academic field: Physics Specialties: Condensed Matter Physics - Theory Condensed Matter Physics - Computational Approaches: Theoretical, Computational

### Abstract

We study ferromagnetic ordering and microscopic inhomogeneity in tensile strained LaCoO$_3$ using numerical simulations. We argue that both phenomena originate from effective superexchange interactions between atoms in the high-spin (HS) state mediated by the intermediate-spin excitations. We derive a model of the HS excitation as a bare atomic state dressed by electron and electron-hole fluctuations on the neighbor atoms. We construct a series of approximations to account for electron correlation effects responsible for HS fluctuations and magnetic exchange. The obtained amplitudes and directional dependence of magnetic couplings between the "dressed" HS states show a qualitative agreement with experimental observations and provide a new physical picture of LaCoO$_3$ films.

### Ontology / Topics

See full Ontology or Topics database.

Published as SciPost Phys. 8, 082 (2020)

We thank the referee for the detailed feedback and helpful suggestions. These have been very useful for improving the manuscript. Please find below our replies in the same order as in the referee's report with a list of changes and the revised manuscript.

1. We have added the missing references to the studies on the structural relaxation in the strained compound. One paragraph with a corresponding discussion was added in the end of the first paragraph of Sec. 2 including Refs. [35-38]. The point ii) was addressed there as well, while in response to the point i), the second sentence in the first paragraph in Appendix A was extended to specify the structural details in the DFT approach.

2. We agree with the point raised by the referee and thus simplified Eq.(5) by writing it in terms of density operators and omitting redundant multipliers in the spin-exchange term.

3. We have added Ref. [41] and provided an additional explanation on the in-plane orientation of magnetic moments in the second sentence below Eq.(4).

4. To make the paper more accessible for a broader audience, we have extended the introduction part, in particular, the second and the fourth paragraphs in the corresponding section. In Figure 1, we have also provided a schematic comparison of the low-energy states in the bulk and the strained compound. To emphasize the difference to the ferromagnetism of metallic La_{1-x}Sr_xCoO_3, we also added one sentence in the end of Sec. 3.

5. Our statement on the absence of a superstructure in the bulk compound in Sec. 3.5 refers explicitly to the origin of the low population of HS states in the T=0 limit compared to the strained compound. Therefore, in our opinion, there must be still energy-related arguments, as it is given in the text.

### List of changes

1. Introduction was extended, in particular the text was added to the second and the fourth paragraphs of Sec. 1.
2. Fig. 1 was updated: the right panel now contains a comparison of local energies of the lowest states in the bulk and the film structures. The text in the figure caption was updated correspondingly.
3. One sentence with a discussion of the structural relaxation with Refs. [35-38] was added in the end of the first paragraph of Sec. 2.
4. One sentence with a discussion of the direction of magnetic moments in the film compound with Ref. [41] was added below Eq.(4).
5. Notations in Eq.(5) were simplified for a better accessibility.
6. One sentence emphasizing the difference to the ferromagnetism of metallic La$_{1-x}$Sr$_x$CoO$_3$ was added in the end of Sec. 3.
7. Details on the employed structure in the DFT approach were added in the second sentence of Appendix A.

### Submission & Refereeing History

Resubmission 2004.00488v2 on 13 May 2020
Submission 2004.00488v1 on 2 April 2020

## Reports on this Submission

### Anonymous Report 2 on 2020-5-23 (Invited Report)

• Cite as: Anonymous, Report on arXiv:2004.00488v2, delivered 2020-05-23, doi: 10.21468/SciPost.Report.1708

### Report

The authors present a theoretical study of emerging long-range ferromagnetic ordering in tensile strained LaCoO3 using numerical simulation. Building a model accompanied with a series of approximation to account electron correlation effects originating from Co3+ ions, authors tried to answer ferromagnetism and atomic-scale inhomogeneity in LaCoO3 films. A qualitative agreement with experimental evidence is claimed.

In my opinion the results are interesting and correct. The authors present manuscript in systematic way, but yet not well in pedagogical manner. The paper can be published as it is. My suggestion for the authors is just to double-check the paper for possible typos, and also give a clear reference about “Site 0 is the crossing point” statement mentioned in the caption of figure.3

• validity: good
• significance: high
• originality: high
• clarity: ok
• formatting: good
• grammar: good

### Anonymous Report 1 on 2020-5-14 (Invited Report)

• Cite as: Anonymous, Report on arXiv:2004.00488v2, delivered 2020-05-14, doi: 10.21468/SciPost.Report.1688

### Report

The authors made the revision in accordance with my previous report, so I am glad to recommend this manuscript for publication

• validity: -
• significance: -
• originality: -
• clarity: -
• formatting: -
• grammar: -