Tuning magnetic anisotropy by continuous composition-gradients in a transition metal oxide

Compositional tuning of correlations between electrons, spins, and lattices in solids is a key strategy for exploring quantum phenomena. In contrast to uniform chemical substitutions that are widely used for the compositional tunings, continuous variations in compositions along a given direction of materials (composition gradients) are expected to modulate translational crystal symmetries and electronic band structures, leading to unique properties not seen in homogeneously substituted materials. Preparing compositionally graded materials, however, is still challenging as it requires elaborate fabrication processes, and how composition gradients affect materials' properties remains elusive. Here, we show that composition gradients can be made by simply depositing films under continuously varying oxygen pressures by pulsed laser deposition. Applying this technique to the inverse-spinel-structured ferrimagnet NiCo2O4, we found that composition gradients in NiCo2O4 epitaxial films tune the magnetic anisotropy and stabilize the magnetizations modulated along the gradient, which are not seen in compositionally uniform films. The spatially modulated magnetization can also be reversed in controlled manners through distinct antiferromagnetically coupled magnetization configurations in the single material. Our results demonstrate the potential of composition gradients to tune materials' properties and explore a new class of materials with added functionalities.