Strain Anisotropy and Magnetic Domains in Embedded Nanomagnets.

Nanoscale modifications of strain and magnetic anisotropy can open pathways to engineering magnetic domains for device applications. A periodic magnetic domain structure can be stabilized in sub-200 nm wide linear as well as curved magnets, embedded within a flat non-ferromagnetic thin film. The nanomagnets are produced within a non-ferromagnetic B2-ordered Fe60 Al40 thin film, where local irradiation by a focused ion beam causes the formation of disordered and strongly ferromagnetic regions of A2 Fe60 Al40 . An anisotropic lattice relaxation is observed, such that the in-plane lattice parameter is larger when measured parallel to the magnet short-axis as compared to its length. This in-plane structural anisotropy manifests a magnetic anisotropy contribution, generating an easy-axis parallel to the short axis. The competing effect of the strain and shape anisotropies stabilizes a periodic domain pattern in linear as well as spiral nanomagnets, providing a versatile and geometrically controllable path to engineering the strain and thereby the magnetic anisotropy at the nanoscale.

Laser-Rewriteable Ferromagnetism at Thin-Film Surfaces.

Manipulation of magnetism using laser light is considered as a key to the advancement of data storage technologies. Until now, most approaches seek to optically switch the direction of magnetization rather than to reversibly manipulate the ferromagnetism itself. Here, we use ∼100 fs laser pulses to reversibly switch ferromagnetic ordering on and off by exploiting a chemical order-disorder phase transition in Fe60Al40, from the B2 to the A2 structure and vice versa. A single laser pulse above a threshold fluence causes nonferromagnetic B2 Fe60Al40 to disorder and form the ferromagnetic A2 structure. Subsequent laser pulsing below the threshold reverses the surface to B2 Fe60Al40, erasing the laser-induced ferromagnetism. Simulations reveal that the order-disorder transition is regulated by the extent of surface supercooling; above the threshold for complete melting throughout the film thickness, the liquid phase can be deeply undercooled before solidification. As a result, the vacancy diffusion in the resolidified region is limited and the region is trapped in the metastable chemically disordered state. Laser pulsing below the threshold forms a limited supercooled surface region that solidifies at sufficiently high temperatures, enabling diffusion-assisted reordering. This demonstrates that ultrafast lasers can achieve subtle atomic rearrangements in bimetallic alloys in a reversible and nonvolatile fashion.