The relation between ferromagnetism and structure in half-metallic transition metal oxides

Interface-driven magnetic properties such as exchange bias and inverted hysteresis are highly sought after in modern functional materials, where at least two magnetically active layers are required. Ability to achieve these functionalities in a single layer thin film (monolithic) reduces the dimensionality while enriching the magnetism. Here we uncover a previously unseen part of the phase diagram of a monolithic epitaxial thin film of La_0.67Sr_0.33MnO₃ on SrTiO₃ which exhibits inverted hysteresis, spontaneous magnetic reversal and exchange bias due to a structural gradient in oxygen octahedral network. We have mapped the phase diagram of this material and discovered that at a specific oxygen pressure and above a critical thickness, a complex magnetic behavior appears. Atomic-scale characterization shows that this peculiar magnetism is closely linked to continuous structural gradient that creates three distinct regions within the film, each with a different magnetism onset. Extracting oxygen octahedral geometry by electron microscopy, we found that the Curie temperature is directly correlated with the bond angle. This study illustrates the importance of octahedral geometry in the magnetic properties of the materials.