Nanosecond laser-induced synthesis of nanoparticles with tailorable magneticanisotropy

• Published in: Journal of magnetism and magnetic materials Vol. 323; no. 3; pp. 356 - 362
• Main Authors: Krishna, H., Gangopadhyay, A.K., Strader, J., Kalyanaraman, R.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-02-2011
• Subjects: Arrays; Magnetic anisotropy; Magnetization; Nanocomposites; Nanomaterials; Nanoparticle; Nanoparticles; Nanostructure; Nickel; Orientation; Strain; Nanoparticle; Magnetic anisotropy; Strain
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2010.09.041

Controlling the magnetic orientation of nanoparticles is important for many applications. Recently, it has been shown that single domain ferromagnetic hemispherical Co nanoparticles prepared by nanosecond laser-induced self-organization, show magnetic orientation that was related to the negative sign of the magnetostrictive coefficient
λ
S
[J. Appl. Phys. v103, p073902, 2008]. Here we have extended this work to the Fe
50Co
50 alloy, which has a positive
λ
S
and Ni, which has a negative
λ
S
. Patterned arrays of ferromagnetic nanoparticles of Fe
50Co
50, Ni, (and Co) were synthesized from their ultrathin metal films on SiO
2 substrate by nanosecond laser-induced self-organization. The morphology, nanostructure, and magnetic behavior of the nanoparticle arrays were investigated by a combination of electron microscopy, atomic force microscopy, and magnetic force microscopy techniques. Transmission electron microscopy investigations revealed a granular polycrystalline nanostructure, with the number of grains inside the nanoparticle increasing with their diameter. Magnetic force measurements showed that the magnetization direction of the hemispherical Co and Ni nanoparticles was predominantly out-of-plane while those for the Fe
50Co
50 alloy was in the plane of the substrate. Finite element analysis was used to estimate the average residual strain in the nanoparticles, following laser processing. The difference in behavior is due to the dominating influence of magnetostrictive energy on the magnetization as a result of residual thermal strain following fast laser processing. Since
λ
S
is negative for polycrystalline Co and Ni, and positive for Fe
50Co
50, the tensile residual strain forces the magnetization direction to out-of-plane and in-plane, respectively. This work demonstrates a cost-effective non-epitaxial technique for the synthesis of magnetic nanoparticles with tailored magnetization orientations.
► Pulsed laser self-organization of ferromagnetic films produces an array of single-domain magnets with tailorable magnetic orientation. ► Direction of magnetic orientation of as-prepared array is shown to be in-plane or out-of-plane ► Sign and magnitude of magnetostrictive coefficient determines the direction of magnetic orientation.