Itinerancy-dependent noncollinear spin textures in SrFeO3, CaFeO3 , and CaFeO3/SrFeO3

Abstract

Non-collinear, multi-q spin textures can give rise to exotic, topologically protected spin structures such as skyrmions, but the reason for their formation over simple single-q structures is not well understood. While lattice frustration and the Dzyaloshinskii-Moriya interaction are known to produce non-collinear spin textures, the role of electron itinerancy in multi-q formation is much less studied. Here we investigated the non-collinear, helical spin structures in epitaxial films of the perovskite oxides SrFeO3 and CaFeO3 using magnetotransport and resonant soft x-ray magnetic diffraction. Metallic SrFeO3 exhibits features in its magnetoresistance that are consistent with its recently proposed multi-q structure. Additionally, the magnetic Bragg peak of SrFeO3 measured at the Fe L edge resonance energy asymmetrically broadens with decreasing temperature in its multi-q state. In contrast, insulating CaFeO3 has a symmetric scattering peak with an intensity 10x weaker than SrFeO3. Enhanced magnetic scattering at O K edge prepeak energies demonstrates the role of a negative charge transfer energy and the resulting oxygen ligand holes in the magnetic ordering of these ferrates. By measuring magnetic diffraction of CaFeO3/SrFeO3 superlattices with thick CaFeO3 layers, we find that the CaFeO3 helical ordering is coherent across 1 unit cell-thick SrFeO3 layers but not 6 unit cell-thick layers. We conclude that insulating CaFeO3 supports only a simple single-q helical structure in contrast to metallic SrFeO3 that hosts multi-q structures. Our results provide important insight into the role of electron itinerancy in the formation of multi-q spin structures.

Charge disproportionation and Hund's insulating behavior in a five-orbital Hubbard model applicable to $d^4$ perovskites

“…In order to stabilize the high-spin state of the Fe cation, a Hubbard-corrected DFT+U treatment and the presence of antiferromagnetic (AFM) order is required. We find that moderate values of U = 4 eV and J = 1 eV and A-type antiferromagnetic order (as proxy for the more complicated helical spin structure in CaFeO 3 [35]) result in structural parameters that are in good agreement with experimental data (e.g., the calculated unit cell volume is 210.34 Å…”

Spin excitation spectra in helimagnetic states: proper-screw, cycloid, vortex crystal, and hedgehog lattice

“…In addition to the above substances, recently, multiple-Q spin states in centrosymmetric systems, for which the antisymmetric interactions of the Dzyaloshinskii-Moriya type are inactive, have been attracting considerable attentions, for example, GdRu 2 Si 2 (D 4h ) [19,75], Gd 2 PdSi 3 (D 6h ) [76][77][78][79][80][81], Gd 3 Ru 4 Al 12 (D 6h ) [82,83], and SrFeO 3 (O h ) [84][85][86][87]. In our model, however, when the antisymmetric interactions are absent, the ground states are always given by superpositions of sinusoidal spin density waves, inconsistent with the experimental observations.…”

Topological spin crystals by itinerant frustration

“…Another important issue is to construct the effective spin models for the materials hosting the multiple-Q topological spin crystals. Although it was shown that the effective bilinear and biquadratic model and its extensions well explain the SkXs observed in GdRu 2 Si 2 [104,105], and Gd 3 Ru 4 Al 12 [102,103], and the HXs in MnSi 1−x Ge x [27,28,29,106], there remain various topological spin crystals whose mechanisms are still missing, e.g., the SkXs in EuPtSi [94,95,96], and Gd 2 PdSi 3 [97,98,99,100,101], and the HX in SrFeO 3 [107,108,109,110]. The 4 f -electron compound EuPtSi with the chiral lattice structure, which belongs to the same space group as MnSi, exhibits the SkX with extremely short magnetic period in the wide range of the temperature and the magnetic field [94,95].…”

“…As the underlying mechanism is generic in itinerant magnets and is irrespective of lattice structures, it has garnered attention for understanding the microscopic origins of various topological spin crystals, especially the recently discovered ones that are hard to understand by the conventional scenarios because of the extremely short magnetic periods and the centrosymmetric lattice structures. Indeed, the itinerant frustration has been intensively discussed, e.g., for the VXs in MnSc 2 S 4 [89,90], CeAuSb 2 [91,92], and Y 3 Co 8 Sn 4 [93], the SkXs in Co-Zn-Mn alloys [71], EuPtSi [94,95,96], Gd 2 PdSi 3 [97,98,99,100,101], Gd 3 Ru 4 Al 12 [102,103], and GdRu 2 Si 2 [104,105], and the HXs in MnSi 1−x Ge x [27,28,29,106] and SrFeO 3 [107,108,109,110].…”