Ultrafast magnetization dynamics of Mn-doped L10 FePt with spatial inhomogeneity

Abstract One of the persisting goals in the field of femtomagnetism is to study and understand the light-induced ultrafast magnetization dynamics of magnetic model systems and multi-compound materials. The latter offers the intriguing opportunity to tune and control the static as well as the transient magnetic properties of the materials according to the desired field of application. Here, we focus on the ultrafast magnetization dynamics of the technologically relevant FePt alloy doped with different concentrations of manganese (Mn) atoms. We find that the demagnetization time constant τ m of FePt decreases significantly upon Mn doping coinciding with a more energy efficient demagnetization process, i.e., the required laser fluence for a certain quenching of the magnetization reduces with increasing Mn concentration. Even more interesting, the characteristic demagnetization time vs. quenching curve does not reveal a single-peak curve as expected from the microscopic three-temperature model, but exhibits a double-hump signature. This peculiar behavior can be explained by the existence of a depth dependent variation of the Mn concentration in the FePt host material. It is caused by the sample preparation procedure using rapid thermal annealing as demonstrated experimentally by layer-sensitive magneto-optical Kerr effect experiments and secondary ion mass spectrometry. Our findings underline the crucial role of the sample preparation procedure for the transient magnetization dynamics of complex magnetic alloys.