Talk given by

Prof. Marco Belaggia

DTU Nanolab

Technical University of Denmark

Lyngby, Denmark

Abstract:

Magnetic nanoparticle superstructures, where exchange coupling between macrospins is either absent or weaker than dipolar coupling, feature a variety of magnetic ground states and responses to applied fields (hysteresis) not found in continuous magnets. Examples are core-less vortices, zero-width longitudinal domain walls [1], toroidal moment analogues, and in the special case of eight dipoles at the corner of a cube, a degenerate continuum of ground states [2]. Driving the emerging phenomenology is the interplay between (super)crystal structure, packing and structural order, and the magnitude of dipolar coupling. The latter, in contrast to exchange coupling in regular magnets, can be conveniently tuned by the choice of the macrospins' size, composition and lattice parameter, which brings new and exciting opportunities for applications, especially in the field of biomedics and engineering.

Unveiling the physics of these systems requires imaging techniques that combine single-particle resolution and sensitivity to tiny magnetic moments to yield a quantitative map of the magnetic topography: how each moment is oriented, and what its magnitude is. Electron holography is one among the few techniques capable of providing such information. Examples will be shown of low-dimensional superstructures of 10-15 nm Cobalt nanoparticles [3] where electron holography captured the distribution of magnetic moments, which was then used to assess magnetic correlations [4] and to establish a connection between magnetic and structural order.

The model-independent method introduced in Ref. [5] enables the quantification of the magnetic moment of a particle with just two ingredients: the experimental holographic phase image with a sufficient field of view around the object, and Gauss theorem. No prior knowledge is required on shape or composition, nor on the distribution of field sources, the determination of which is ultimately the target of our measurements. Examples will be shown on the magnetic moment measurement in clusters of iron-oxide nanoparticles, referred to as nanoflowers, which are promising candidates for high-efficiency magnetic hyperthermia.

Higher order multipoles, carrying information on, e.g., magnetization non-uniformities, as well as toroidal moments --indicators of vorticity associated with a magnetic configuration-- are also accessible via generalizations of the data integration scheme. Determination of the magnetic quadrupole moment of Skyrmions in a thin-film tilted with respect to the electron beam allows the direct assessment of their chirality, while information on their toroidal moment yields their polarity.

We are currently focusing our efforts on dynamical aspects, in particular the self-assembly process of nanoparticle superstructures, which requires combining time-resolved imaging methods and liquid-cells for transmission electron microscopy and electron holography [6]

[1] M. Varon, et al., Scientific Reports 5, 14536 (2015).

[2] M. Kure, M. Beleggia, C Frandsen, J. Appl. Phys. 122, 133902 (2017).

[3] M. Varon et al., Scientific Reports 3, 1234 (2013).

[4] M. Beleggia and C. Frandsen, J. Phys. Conf. Ser. 521, 012009 (2014).

[5] M. Beleggia, T. Kasama, R.E. Dunin-Borkowski Ultramicroscopy 110 425 (2010).

[6] M.N. Yesibolati et al., Science Advances, submitted (2019).