Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt](n-1)/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

We study layered synthetic antiferromagnets (SAFs) with out-of-plane interface anisotropy, where the layer-wise antiferromagnetic (AF) alignment is induced by interlayer exchange coupling (IEC). By applying low energy He+ focused ion beam irradiation to the SAF, a depth-dependent reduction of the IEC and anisotropy can be achieved due to layer intermixing. As a consequence, after irradiation, a specific field reversal sequence of the SAF is energetically preferred. When tuning the pristine SAF to exhibit an inverted field reversal, we are thus able to create AF domains in the irradiated regions. When irradiated with a fluence gradient, these AF domains can be further deterministically manipulated by an external magnetic field. Among other applications, this could be utilized for engineering a controllable and local magnetic stray field landscape, for example, at AF domain walls, within the otherwise stray field free environment provided by the SAF.

He+-ion irradiation enables controlled postdeposition modification of layered magnetic thin-film systems. The degree of modification and its depth profile can be tuned by the irradiation dose and energy. Here, we use magnetometry and magnetic force microscopy to explore the impact of gentle He+-ion irradiation on synthetic antiferromagnets, consisting of ferromagnetic Co/Pt multilayers with perpendicular magnetic anisotropy, which are antiferromagnetically (AF) coupled via Ru interlayers. This system shows a rich variety of magnetic domain patterns due to the strong competition between different magnetic energies. We show that AF interlayer exchange and perpendicular interface anisotropy energy are gradually reduced by the ion irradiation while the demagnetization energy is mainly preserved, which thus results in multiple successive magnetic-phase transitions.

Interfaces separating ferromagnetic (FM) layers from non-ferromagnetic layers offer unique properties due to spin-orbit coupling and symmetry breaking, yielding effects such as exchange bias, perpendicular magnetic anisotropy, spin-pumping, spin-transfer torques, and conversion between charge and spin currents and vice versa. These interfacial phenomena play crucial roles in magnetic data storage and transfer applications, which require the formation of FM nanostructures embedded in non-ferromagnetic matrices. Here, we investigate the possibility of creating such nanostructures by ion irradiation. We study the effect of lateral confinement on the ion-irradiation-induced reduction of nonmagnetic metal oxides (e.g., antiferro- or paramagnetic) to form ferromagnetic metals. Our findings are later exploited to form three-dimensional magnetic interfaces between Co, CoO, and Pt by spatial-selective irradiation of CoO/Pt multilayers. We demonstrate that the mechanical displacement of O atoms plays a crucial role in the reduction from insulating, non-ferromagnetic cobalt oxides to metallic cobalt. Metallic cobalt yields both perpendicular magnetic anisotropy in the generated Co/Pt nanostructures and, at low temperatures, exchange bias at vertical interfaces between Co and CoO. If pushed to the limit of ion-irradiation technology, this approach could, in principle, enable the creation of densely packed, atomic-scale ferromagnetic point-contact spin-torque oscillator (STO) networks or conductive channels for current-confined-path-based current perpendicular-to-plane giant magnetoresistance read heads.