Interlayer coupling through a dimensionality-induced magnetic state

Dimensionality is known to play an important role in many compounds for which ultrathin layers can behave very differently from the bulk. This is especially true for the paramagnetic metal LaNiO3, which can become insulating and magnetic when only a few monolayers thick. We show here that an induced antiferromagnetic order can be stabilized in the [111] direction by interfacial coupling to the insulating ferromagnet LaMnO3, and used to generate interlayer magnetic coupling of a nature that depends on the exact number of LaNiO3 monolayers. For 7-monolayer-thick LaNiO3/LaMnO3 superlattices, negative and positive exchange bias, as well as antiferromagnetic interlayer coupling are observed in different temperature windows. All three behaviours are explained based on the emergence of a (¼,¼,¼)-wavevector antiferromagnetic structure in LaNiO3 and the presence of interface asymmetry with LaMnO3. This dimensionality-induced magnetic order can be used to tailor a broad range of magnetic properties in well-designed superlattice-based devices.

-oriented (LNO7/LMO7)15 superlattice with antiferromagnetic spirals in each LNO layer. With this real stack, the structural scattering associated with the different periods completely takes over. The (¼ ¼ ¼)-peak is just vaguely distinguished, both in the reflectivity curves and in the asymmetry ratio. This demonstrates that a clear Bragg peak cannot be expected in our superlattices, since the structural intensity in reflectivity geometry is large and it amplifies the magnetic components at the q values corresponding to structural periods.

Supplementary Note 1: SQUID-magnetometry measurements
Supplementary Figure 1 presents the temperature-evolution of the squareness of the magnetization-field loops for several (111)-oriented LaNiO3/LaMnO3 (LNO/LMO) heterostructures, as represented by the ratio between remnant (Mrem) and saturation (Msat) magnetizations. The superlattice with 7-monolayer-thick-LNO is singled out as the one clearly displaying the fastest decay of the Mrem/Msat ratio with temperature. Such behavior is a strong indication of the increasing strength of the interlayer antiferromagnetic coupling above 30 K, as already evidenced in the resonant reflectivity measurements. It is also consistent with a strengthening of the global magnetic behaviour at lower temperature due to the exchange bias field. The decrease of Mrem/Msat with temperature observed for the superlattices compared to a bare LMO film (also shown in Supplementary Figure 1) is explained by a reduction of the LMO magnetization and ordering parameter at the interface.
Training effects have been investigated by cycling the system through several consecutive hysteresis loops at a given temperature. Supplementary Figure 2 reports the magnetization-field loops for a (LNO7/LMO7)15 superlattice acquired at T=5 K and T=18.5 K, two temperatures representative of the negative and positive biasing regimes, respectively. The exchange bias sign does not change upon cycling, which demonstrates that the positive exchange bias is an intrinsic effect of our LNO/LMO heterostructures.

Supplementary Note 2: Reflectivity measurements at Mn L2-edge
Supplementary Figure 3 presents the field-dependence of the half-order peak q/2 indicative of an antiferromagnetically-coupled state in (LNO7/LMO7)15 superlattices. Its behaviour is consistent with the SQUID measurements that show a low remanence and a rather slow saturation in field. When sweeping the field from saturation, antiferromagnetic coupling takes over leading to a maximum intensity between 0.05 and 0.1 T (depending on the sweep direction). Then, upon reversal of the field, the entire magnetic structure flips and produces a minimum in the q/2 peak intensity. The asymmetric positions of the field of these maximum and minimum are attributed to the tilt of the small ferromagnetic component. The two independent measurements of the q/2 peak asymmetry ratio and the net magnetization at each field step allow the 'high' field induced folding of the magnetization vectors to be reconstructed. This is represented in Supplementary Figure 3 where saturation can be expected to occur near 0.3 T from a rough extrapolation of the linear and reversible part. The evolution of the 3q/2 peak intensity as function of temperature suggests that the antiferromagnetic-coupled state vanishes around 120 K.
The absence of half-order peaks in reflectivity measurements at Mn L3-edge for superlattices with LNO thickness N7 monolayers is evidenced in Supplementary Figure 4.