Strain- and thickness-dependent magnetic properties of epitaxial La0.67Sr0.33CoO3/La0.67Sr0.33MnO3 bilayers

Abstract

Magnetic properties and interfacial phenomena of epitaxial perovskite oxides depend sensitively on parameters such as film thickness and strain state. In this work, epitaxial La0.67Sr0.33CoO3 (LSCO)/La0.67Sr0.33MnO3 (LSMO) bilayers were grown on NdGaO3 (NGO) and LaAlO3 (LAO) substrates with a fixed LSMO thickness of 6 nm, and LSCO thickness (tLSCO) varying from 2 to 10 nm. Soft x-ray magnetic spectroscopy revealed that magnetically active Co2+ ions that strongly coupled to the LSMO layer were observed below a critical tLSCO for bilayers grown on both substrates. On LAO substrates, this critical thickness was 2 nm, above which the formation of Co2+ ions was quickly suppressed leaving only a soft LSCO layer with mixed valence Co3+/Co4+ ions. The magnetic properties of both LSCO and LSMO layers displayed strong tLSCO dependence. This critical tLSCO increased to 4 nm on NGO substrates, and the magnetic properties of only the LSCO layer displayed tLSCO dependence. A non-magnetic layer characterized by Co3+ ions and with a thickness below 2 nm exists at the LSCO/substrate interface for both substrates. The results contribute to the understanding of interfacial exchange spring behavior needed for applications in next generation spintronic and magnetic memory devices.

Tuning In-Plane Magnetic Anisotropy and Interfacial Exchange Coupling in Epitaxial La2/3Sr1/3CoO3/La2/3Sr1/3MnO3 Heterostructures

“…In addition, several general trends can be observed: (1) as t LSCO increases to 8 nm, the prepeak intensity (shaded in pink color) decreases and fully disappears when t LSCO = 10 nm; (2) the intensity ratio of the doublet peak (marked with “▼”) reduces as t LSCO increases from 1 to 4 nm; and (3) the Co- L 3 peak positions shift to higher energies as t LSCO increases. These gradual changes in XA spectral shape indicate that the valence state of Co ions is gradually changing from Co 2+ -dominated to mixed Co 3+ /Co 4+ -dominated with increasing t LSCO , which is consistent with the results in previous work. ,, This t LSCO dependence again supports the result of decoupling of H EB and H c , and the enhancement of H c values in the LSMO layer as shown in Figure …”

“…In Figure c, RSMs of bilayer C4M6N are shown as an example, where vertical alignment of film peaks with substrate peak indicates that both layers are coherently strained to the NGO substrate. Therefore, no strain relaxation was found for bilayers with t LSCO up to 10 nm . In addition, a small tilt is observed between the film and substrate peak locations in the (103) pc RSM (marked as the white dashed line), indicating that the film has a unit cell angle deviation away from 90° along this direction.…”

“…For bilayers with t LSCO > 4 nm, a significant lateral shift of the hysteresis loops in the direction opposite to the biasing field is observed along both in-plane directions due to the different in-plane MA and larger difference in H c values between the LSCO and LSMO layers, resulting in a large EB effect. Such behavior indicates that the magnetic moments of the soft LSMO layer are pinned by a hard FM layer, and the interfacial FM/FM exchange coupling is similar to the behavior at FM/AFM interfaces. , Unfortunately, experimental limitations due to the extremely low luminescence yield of NGO substrates and the finite probing depth of TEY detection mode prevent us from acquiring reliable biased Co-XMCD loops from the buried LSCO layer.…”

“…29 In a recent study, LSCO/LSMO bilayers on NGO substrates showed a similar Co ion distribution throughout the LSCO thickness: a nonmagnetic layer characterized by Co 3+ ions at the LSCO/ NGO interface, a bulk-like h-LSCO layer with mixed Co 3+ / Co 4+ ions in the middle of the layer, and an FM s-LSCO layer at the LSCO/LSMO interface. 30 Moreover, the formation of the Co 3+ /Co 4+ ions was suppressed as t LSCO decreased, leaving only an s-LSCO layer with strong ferromagnetism. However, the interfacial exchange coupling behavior and MA for the bilayers on NGO substrates were not explored.…”

“…Therefore, no strain relaxation was found for bilayers with t LSCO up to 10 nm. 30 In addition, a small tilt is observed between the film and substrate peak locations in the (103) pc RSM (marked as the white dashed line), indicating that the film has a unit cell angle deviation away from 90°along this direction. This deviation is only observed in the (103) pc RSM meaning that this angle variation is only occurring for one of the unit cell angles, resulting in monoclinic unit cells for the LSCO and LSMO films on NGO substrates.…”