There have been many recent advances in the synthesis, manipulation, and study of complex atomic-scale structures and interfaces. Among these, understanding the fundamental changes in physical properties in areas of reduced dimensionality has played an important role and has led to improvements in spintronics, digital memory, and transistor technologies. A study from the University of Geneva, Switzerland, published in a recent issue of Nature Communications (doi: 10.1038/ncomms11227), demonstrates the complex evolution of magnetic phenomena occurring at the interfaces in multilayered perovskite oxides.
The researchers focused their efforts on [111]-oriented superlattices of LaNiO3 (LNO)/LaMnO3 (LMO). M. Gibert, the lead author, states “We focused on interface engineering, studying new properties that emerge at the interfaces of these oxide materials.”
Model of the magnetic arrangement of 7-monolayer LaNiO3 in the LaNiO3/LaMnO3 superlattice. The exchange bias sign reversal is exclusively observed for the superlattice with 7-monolayer LNO (blue), that is, not for 5-monolayer LNO or 8-monolayer LNO. Credit: Nature Communications.
At low temperatures, most materials in the LNO family of perovskite nickelates are both electrically insulating and antiferromagnetic. However, LNO is known to be both metallic and nonmagnetic at all temperatures. Researchers have shown that growing thin epitaxial LNO films in the [111] direction in between layers of LMO can promote a dimensionality-induced insulating character as well as antiferromagnetic ordering of the magnetic moments. As Gibert describes it, “LNO in these specific conditions behaves more like all the other nickelates.” Furthermore, by restricting the LNO layer to only 7 monolayers (ML), they were able to promote a number of complex temperature-dependent exchange coupling phenomena.
In a layer this thin, the LNO monolayers were found to couple antiferromagnetically to each other creating a 4-unit-cell stack that equates to a net zero magnetic moment, just like the rest of the perovskite nickelates. However, if only 7 ML of LNO are sandwiched between the LMO layers, the 4-unit-cell sequence is unfinished and the magnetic moments at the ends of the LNO layer (at the interfaces) are pointing in opposite directions; see Figure. Since the LMO layer couples ferromagnetically (parallel) to the LNO layer by a charge-transfer mechanism, successive LMO layers are then antiferromagnetically coupled to one another through the LNO layer. This ferromagnetic coupling at the LNO/LMO interface in turn contributes to stabilizing the antiferromagnetism in the LNO layer.
The researchers also demonstrated the exchange bias effect in the LNO/LMO heterostructures, a phenomenon that finds tremendous use in applications such as magnetic recording media. This unique effect, which originates from a balance between anisotropy energy (keeping magnetic moments aligned in a preferred direction) and exchange energies (coupling magnetic moments to one another) at an interface, produces a shift along the field axis of the magnetic hysteresis measurement. At low temperature (T < 15 K), a negative shift occurs in the LNO/LMO superlattice. However, at slightly higher temperatures (15 K < T < 30 K), there is a sign reversal of the exchange bias field that shifts the magnetic hysteresis measurement toward the positive field axis. Finally, at T > 30 K, this effect makes way for the antiferromagnetically coupled state described earlier.
This complex magnetic behavior is explained through an asymmetry in the interfacial energy terms at the top and bottom LNO/LMO interfaces that arises from the intermixing of Ni and Mn atoms. The competition between the anisotropy energy of the LNO layer and the interfacial energies at each of the LNO/LMO interfaces, all of which vary with temperature, leads to the complex evolution of the exchange bias phenomena.
These results demonstrate that the restriction of dimensionality that occurs at oxide interfaces can promote unique magnetic phenomena in a material that is typically nonmagnetic.
