The discovery of ferromagnetism in two-dimensional (2D)
monolayers
has stimulated growing research interest in both spintronics and material
science. However, these 2D ferromagnetic layers are mainly prepared
through an incompatible approach for large-scale fabrication and integration,
and moreover, a fundamental question of whether the observed ferromagnetism
actually correlates with the 2D crystalline order has not been explored.
Here, we choose a typical 2D ferromagnetic material, Fe3GeTe2, to address these two issues by investigating its
ferromagnetism in an amorphous state. We have fabricated nanometer
thick amorphous Fe3GeTe2 films approaching the
monolayer thickness limit of crystallized Fe3GeTe2 (0.8 nm) through magnetron sputtering. Compared to crystallized
Fe3GeTe2, we found that the basic ferromagnetic
attributes, such as the Curie temperature which directly reflects
magnetic exchange interactions and local anisotropic energy, do not
change significantly in the amorphous states. This is attributed to
the short-range atomic order, as confirmed by valence state analysis,
being almost the same for both phases. The persistence of ferromagnetism
in the ultrathin amorphous counterpart has also been confirmed through
magnetoresistance measurements, where two unconventional switching
dips arising from electrical transport within domain walls are clearly
observed in the amorphous Fe3GeTe2 single layer.
These results indicate that the long-range ferromagnetic order of
crystallized Fe3GeTe2 may not correlate to the
2D crystalline order, and the corresponding ferromagnetic attributes
can be utilized in an amorphous state which suits large-scale fabrication
in a semiconductor technology-compatible manner for spintronics applications.
Fabrication of perpendicularly magnetized ferromagnetic films on various buffer layers, especially on numerous newly discovered spin–orbit torque (SOT) materials to construct energy-efficient spin-orbitronic devices, is a long-standing challenge. Even for the widely used CoFeB/MgO structures, perpendicular magnetic anisotropy (PMA) can only be established on limited buffer layers through post-annealing above 300 °C. Here, we report that the PMA of CoFeB/MgO films can be established reliably on various buffer layers in the absence of post-annealing. Further results show that precise control of MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer, is the key to obtain the as-deposited PMA. Interestingly, contrary to the previous understanding, post-annealing does not significantly influence the well-established as-deposited PMA but indeed enhances unsaturated PMA with a thick MgO layer by modulating oxygen distributions, rather than crystallinity or Co– and Fe–O bonding. Moreover, our results indicate that oxygen diffusion also plays a critical role in PMA degradation at high temperatures. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials.
Fusing oxygen heterocycles into an indacenodithiophenebithiophene core contributes to realizing low bandgap non-fullerene molecular acceptors with a power conversion efficiency exceeding 8%.
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