Exchange bias and inverted hysteresis in monolithic oxide films by structural gradient

Abstract

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. The 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_{0.67}Sr_{0.33}MnO_{3} on SrTiO_{3} which exhibits inverted hysteresis, spontaneous magnetic reversal, and exchange bias due to a structural gradient in the oxygen-octahedral network. Varying the growth conditions, 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 a continuous structural gradient that creates three distinct regions within the monolithic 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 metal-oxygen bond angle. This study illustrates the importance of octahedral geometry in shaping the physical properties of the materials.