Localization of Curie Temperature in Compositionally Graded Ferromagnetic Films*

Although ferromagnetism (FM) is in general a long-range collective phenomenon, it is possible to induce local spatial variations of magnetic properties in FM materials. For example, systematic variation of the exchange coupling strength can be used to create systems that behave as if they are composed of virtually independent segments that exhibit “local” Curie temperatures (Tc). While it is obvious that such localization should be possible across some lengthscale, the magnitude of that lengthscale is not so intuitive, nor is the expected behavior of a material exhibiting a nearly continuous varitation in exchange strength. We have explored these questions in real materials by using neutron scattering and mean-field simulations to study novel compositionally graded transition metal multilayer films as model systems [1-3].

Remarkably, we have found that non-local magnetic effects are significant only over distances less than 3 nm (possibly much less). Beyond that the structures behave as a continuum of decoupled layers with distinct Tc. This leads to fascinating functionality, including FM phase boundaries that can be reversibly moved up and down the thickness of a film with temperature, and modified with an applied magnetic field. Since we have demonstrated this for an itinerant metal, we assert that for virtually any modulated magnetic material system, collective effects should be suppressed to down to nanometer length scales, so that magnetic behavior overall can be well described in terms of local material properties.

[1] PRL 116, 0047203 (2016).
[2] PRB 95, 134445 (2017).
[3] PRB 98, 064404 (2018)