Toward a Systematic Discovery of Artificial Functional Magnetic Materials

Abstract

Magnetic materials play a major role in many spintronic and other technological applications such as magnetic storage, logic devices, magnetic field sensors and magnetic random access memory. Materials with strong intrinsic ferromagnetic (FM) order above room temperature, such as the transition metals Fe, Ni or Co and their alloys, are rather unusual among the magnetic materials known today and there is still the need for new functional materials with magnetic order above room temperature. In the past two decades, a method of creating artificial ferromagnetic materials has emerged and a multitude of so-called defect-induced ferromagnets were reported. Since the first prediction of an artificial ferromagnetic material with transition temperature above 300~K, based on Mn doped ZnO appeared twenty years ago, the field has substantially evolved. First, it was realized that doping with magnetic impurities was not at all necessary in order to induce a robust FM order in the non-magnetic host matrix, rather all kinds of lattice defects were at the origin of the measured magnetic signals. This realization promised great possibilities to construct new functional magnetic materials, as any non-magnetic material could potentially host a certain kind of defect, turning it into an artificial ferromagnet. The hunt was on and the result was a plethora of reports ranging from oxide, nitride, carbon-based, 2D van der Waals and many more materials showing signals of ferromagnetism upon introducing all kinds of nominally non-magnetic defects. One of the most promising and versatile methods for introducing these defects is the irradiation with non-magnetic ions, owing to the availability of ion sources ranging over the whole periodic table and energies from few eV to hundreds of MeV.Although many experiments were accompanied by theoretical studies, such as electronic structure calculations based on density functional theory (DFT), the search was mostly guided by blind trial and error and a brute force approach. It is therefore not very surprising that most of the reported materials only showed very tiny magnetic signals, which soon led to debates about the nature of the effect and raised the question of whether this route could eventually lead to a robust magnetic order above room temperature, comparable with intrinsic ferromagnets. Furthermore, the measurement of the magnetization of such artificial ferromagnetic samples turns out to be quite difficult due to the inherent uncertainty of the magnetic volume, leading to largely underestimated values in the literature. Considering the enormous amount of host material candidates and lattice defects, a more systematic search method and better selection criteria are highly needed.In this presentation, a computational scheme for the systematic discovery of candidate artificial magnetic materials that can be created by ion irradiation is introduced. The scheme is based on first principle calculations, guided by experimental constraints, automatically restricting the potential defects to those accessible experimentally and can readily be implemented for high throughput material discovery. We further provide a method to determine the defect distribution created within the host lattices, allowing to obtain accurate magnetization values. We demonstrate the predictive power of the scheme by comparing systematic experimental investigations to the calculations and provide corrections to the magnetization, that reach values as high as those of typical intrinsic ferromagnets.We show that the method is applicable to a wide range of host materials and ion irradiation parameters (100 eV - 200 MeV) and yields artificial functional magnetic materials, e.g. in TiO2, SiC and CeO2 hosts, in a controllable way, ranging from thin magnetic films down to 2D magnetic monolayers. The artificial magnetic materials can be tailored to application requirements, exhibiting properties like, e.g., strong perpendicular magnetic anisotropy. **