Antiferromagnetic Interfacial Coupling and Giant Magnetic Hysteresis in La0.5Ca0.5MnO3-SrRuO3 Superlattices

Abstract

Superlattices are of great importance due to their potential as new materials genome to synthesize new functional materials. Thus, tuning of the ground state of superlattices is crucial to further control their physical properties. In this study, superlattices (SLs) consisting of alternating layers of SrRuO₃ (SRO) (5 nm) and La_0.5Ca_0.5MnO₃ (LCMO) (5 nm) are epitaxially grown on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates with 10-unit-cell periods. A variation in the substrate-induced-strain for this choice of SLs triggers observation of remarkable properties, such as magnetic anisotropy and large magnetic hysteresis. The strain states experienced by LCMO and SRO in these SLs result in strong ferromagnetic interlayer coupling and weak antiferromagnetic interlayer coupling at low temperatures in SLs of LCMO-SRO/STO and a strong antiferromagnetic interlayer coupling in SLs of LCMO-SRO/LAO. Besides, a large magnetic hysteresis resulting from the predominant magnetic anisotropy of SRO together with the strength of magnetic coupling is observed in SLs of LCMO-SRO/LAO along the out-of-plane direction of the LAO substrate. These four different magnetic behaviors along four different directions of substrate orientations are interpreted in terms of preferential orbital occupation and competing magnetic exchange coupling together with magnetic anisotropy. This study demonstrates the subtleties in controlling the strength of magnetic coupling at the interface and stands as a model system to realize fascinating magnetic phenomena in layer-by-layer hetero-epitaxial oxide films.

Figure 1

(a) Out-of-plane (OOP) XRD profiles of LCMO–SRO SLs epitaxially grown on STO(001) and LAO(001) were recorded and represented by red and blue curves, respectively. The Bragg reflections of substrates as well as the satellite peaks of the superlattice are indicated. (b) Low-magnification diffraction-contrast cross-sectional TEM image of the SLs grown on STO and LAO substrates; alternating bright and dark layers are SRO and LCMO layers, respectively. (c, d) Selected area EDPs of the SLs epitaxially grown on LAO and STO in the cross-section sample along the [100] zone axis, respectively.

Figure 2

(a) Z-STEM micrograph of LCMO–SRO SL showing the interfaces between a 5 nm LCMO layer and two adjacent 5 nm thin SRO layers. (b) Intensity scan along the yellow and purple lines as indicated in (a).

Figure 3

(a) and (b) are the in-plane and out-of-plane lattice parameters of the superlattice near the STO substrate, respectively, that are displayed as a function of the distance along the c-axis of the substrate. (c) and (d) are the in-plane and out-of-plane lattice parameters of the superlattice near the LAO substrate, respectively, that are displayed as a function of the distance along the c-axis of the substrate. (e) and (f) are the stress distribution of the SLs grown on STO and LAO, respectively, in the two-dimensional space.

Figure 4

Temperature dependence of magnetization along IP and OOP directions for LCMO–SRO/STO and LCMO–SRO/LAO under a magnetic field of 100 Oe (top panel). OOP magnetization loops, M(H), for the SLs grown on STO and LAO (bottom panel) at temperatures around the magnetic anomalies and at 10 K. Inset: temperature-dependent magnetization for SRO (5 nm) and LCMO (5 nm) on STO.

Figure 5

(Top panel) MnL_2,3-edge linear dichroism (E//ab–E//c) of LCMO–SRO superlattice samples grown on STO and LAO, in the temperature ranges 300–50 and 200–50 K, respectively. (bottom panel) MnL_2,3-edge XMCD of LCMO–SRO superlattice samples recorded at 80 K grown on STO and LAO.