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Magnetic frustration is a route for novel ground states, including spin liquids and spin ices. Such frustration can be introduced through either lattice geometry or incompatible exchange interactions. Here, we find that epitaxial strain is an effective tool for tuning antiferromagnetic exchange interactions in a square-lattice system. By studying the magnon excitations in La2NiO4 films using resonant inelastic x-ray scattering, we show that the magnon displays substantial dispersion along the antiferromagnetic zone boundary, at energies that depend on the lattice of the film's substrate. Using first principles simulations and an effective spin model, we demonstrate that the antiferromagnetic next-nearest neighbour coupling is a consequence of the two-orbital nature of La2NiO4. Altogether, we illustrate that compressive epitaxial strain enhances this coupling and, as a result, increases the level of incompatibility between exchange interactions within a model square-lattice system. Frustration in magnetic systems may lead to exotic quantum phases such as spin liquid and spin ice state. Here the authors demonstrate that compressive epitaxial strain in La2NiO4 films deposited on different substrates can tune antiferromagnetic exchange interactions and increase the degree of frustration through the increased level of incompatibility between exchange interactions. [ABSTRACT FROM AUTHOR]

Magnetic frustration is a route for novel ground states, including spin liquids and spin ices. Such frustration can be introduced through either lattice geometry or incompatible exchange interactions. Here, we find that epitaxial strain is an effective tool for tuning antiferromagnetic exchange interactions in a square-lattice system. By studying the magnon excitations in La2NiO4 films using resonant inelastic x-ray scattering, we show that the magnon displays substantial dispersion along the antiferromagnetic zone boundary, at energies that depend on the lattice of the film's substrate. Using first principles simulations and an effective spin model, we demonstrate that the antiferromagnetic next-nearest neighbour coupling is a consequence of the two-orbital nature of La2NiO4. Altogether, we illustrate that compressive epitaxial strain enhances this coupling and, as a result, increases the level of incompatibility between exchange interactions within a model square-lattice system. Frustration in magnetic systems may lead to exotic quantum phases such as spin liquid and spin ice state. Here the authors demonstrate that compressive epitaxial strain in La2NiO4 films deposited on different substrates can tune antiferromagnetic exchange interactions and increase the degree of frustration through the increased level of incompatibility between exchange interactions. [ABSTRACT FROM AUTHOR]