Strain-driven spin-state transition and superexchange interaction in LaCoO3:Abinitiostudy

Abstract

Using spin density functional theory with the Hubbard correction we investigate the magnetic structure of strained LaCoO 3 . We show that beyond biaxial tensile strain of 2.5%, local magnetic moments originating from the high spin (HS) state of Co 3+ emerge in a low spin (LS) Co 3+ matrix. In contrast, we find that compressive strain is not able to stabilize a magnetic state due to geometric constraints. LaCoO 3 accommodates tensile strain via spin state disproportionation resulting in an unusual sublattice structure. In tensile-strained LaCoO 3 , the first nearest neighbor (n.n.) exchange coupling is ferromagnetic (FM) while the second n.n. interaction is stronger and antiferromagnetic (AFM). This unusual feature of the exchange parameters is qualitatively verified with a model superexchange calculation. Due to the competition between the FM and AFM couplings in the system, we find that the most probable magnetic structure of tensile-strained LCO is a canted-spin structure, which may explain the relatively small observed magnetic moment of 0.7μ B /Co 3+ .

The effects of Co3O4on the structure and unusual magnetism of LaCoO3

“…Seo et al found in their theoretical calculations that tensile stress can induce ferromagnetically interacting spins on some Co ions at the film/substrate interface. [15] The importance of tensile stress in inducing long-range ferromagnetic order is most convincingly demonstrated by the spintronic device discussed in Hu et al [20] By growing a thin film of LCO on a substrate of SrTiO 3 , the piezoelectric property of the substrate can be used to apply tensile stress to the LCO film to induce ferromagnetism.…”

“…In a study using dynamical mean-field theory in a local density approximation, Krapek et al found that a magnetic state is favored by lattice expansion and that the latter acts as positive feedback for the appearance of local moments [14]. Seo et al argued that strained LCO heterostructures can induce thermal spin state transitions [15]. By depositing LCO thin films onto different substrates, Fuchs et al successfully tuned the ferromagnetic ordering of LCO; [16] an increase in epitaxial strain between the substrate and film was observed to correlate with an increase in the effective magnetic moment µ B /Co.…”

Role of oxygen vacancies in room-temperature ferromagnetism in cobalt-substitutedSrTiO3

“…) electrons in SrTi 1-x Co x O 3-δ and the U eff value can significantly affect both the spin-state and the energy of the occupied 3d levels [36]. Therefore, the choice of U eff has to be consistent with the observed spin state and the valence band density of states.…”

Lattice mismatch accommodation via oxygen vacancy ordering in epitaxial La0.5Sr0.5CoO3-δ thin films

“…LaCoO 3 provides a good example as it adopts a low-spin "non-magnetic" ground state in bulk, but exhibits strain-stabilized ferromagnetism in films. [1][2][3] In the doped case, e.g., in bulk La 1-x Sr x CoO 3-δ (LSCO), substitution of Sr 2+ for La 3+ changes the Co valence, inducing metallic ferromagnetism, 4 but also O vacancies. Without the use of high pressures, these vacancies form in high concentrations for x > 0.5-0.6, 5 eventually forming ordered superstructures, as exemplified by Brownmillerite SrCoO 2.5 (SCO).…”

“…For STO(001)/LSCO and LAO(001)/LSCO, the (013) reflection was chosen. For STO(110)/LSCO, the fourfold in-plane symmetry is broken, and we chose the (310) and (222) reflections to probe two orthogonal high symmetry in-plane directions ( [1][2][3][4][5][6][7][8][9][10] and [001]). In all cases, white crosses mark the positions of the fully strained (pseudomorphic) and fully relaxed (bulk) LSCO reflections.…”