Moumita Kundu

Nanoscale miniaturization has revolutionized the field of spintronics by enabling exponential growth in areal bit density. A similar leap is also expected in device speeds through successfully harnessing femtosecond magnetization dynamics. However, combining this with the miniaturization of realistic devices is challenging. To address this, we studied the effect of dimensional confinement on the femtosecond demagnetization of Fe. By gradually increasing the level of confinement while keeping excitation conditions constant, we found that Fe layers thinner than 10 nm exhibit enlarged demagnetization amplitudes, reaching a ∼75% increase at 2 nm. By combining ultrafast experiments sensitive to the spins, the charge carriers, and the phonons, we establish that this finite‐size effect is magnetic in origin and is not phonon‐driven. With the support of ab initio calculations and atomistic spin dynamics simulations, we identify the enhancement effect as due to local weakening of spin order at the Fe's interface, which becomes significant upon increased confinement.

The control of electron spins in materials that are simultaneously ferromagnetic and insulating opens up a wealth of quantum phenomena in spin-based electronics. Thin films of europium oxide (EuO) are ideal for the generation and manipulation of spin-polarized states, but so far, there are no experimental literature reports on the magnetic domain patterns of EuO. However, at these microscopic length scales, magnetic relaxation between the remanent and demagnetized states takes place in any spintronic device. This relaxation process involves the displacement of magnetic domain walls and can therefore be strongly influenced by the film's structure and thickness. Here, we present an investigation of the temperature-dependent behavior of magnetic domains and hysteresis in bulk-like (25 nm) and ultrathin (3 nm) EuO films. Magneto-optical Kerr microscopy is used, a technique that is a valuable tool for exploring microscopic features such as spin dynamics and magnetic domain walls. Significant Kerr rotation in EuO led to high-contrast magnetic domain images in thick films, facilitating the observation of domain dynamics. The Curie temperature ( T C ) and coercivity show thickness-dependent variations. The analysis and comparison of hysteresis loops and domain imaging in EuO and EuO/Co reveal proximity effect-induced antiferromagnetic coupling of both layers. To elucidate the magnetization reversal dynamics in EuO, micromagnetic simulations using MuMax 3 were performed below and above T C . This comprehensive approach was designed to comprehend the impact of magnetism and magnetic proximity effects in EuO on the micromagnetic scale, potentially extending its magnetic ordering beyond T C .