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Neutron scattering provides new insights into nanoparticles

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Publié le vendredi 11 septembre 2020

Researchers from the University of Luxembourg, the University of Salamanca and from General Numerics Research Lab in Germany have investigated the spin structure of nanoparticles. Their findings have been published in the renowned scientific journal Physical Review Letters.

Magnetic nanoparticles are promising candidates for many technological and biomedical applications, e.g., as constituents of temperature-sensitive ferrofluids, for usage in magnetic hyperthermia, or as contrast-enhancing agents in magnetic resonance imaging. Besides their size and shape, it is their internal chemical and magnetic structure which determines their potential for a particular application. Very small magnetic particles tend to be uniformly magnetised and are said to be in a single-domain state. “This is because strong quantum-mechanical exchange forces hold all atomic spins in parallel on a length scale of the interatomic spacing. Increasing the size of nanoparticles increases the influence of the magnetostatic stray field energy, which causes deviations from the single-domain state. As a result, complex spin configurations such as vortex-type structures may form in larger nanoparticles”, explains Andreas Michels, Professor within the Department of Physics at the University of Luxembourg.

Magnetic neutron scattering, more specifically small-angle neutron scattering (SANS), is one of the most important techniques for investigating the spin structure of nanoparticles. The dipolar interaction between the magnetic moment of the neutron and the magnetic field which is related to the spin and orbital motion of unpaired electrons allows one to access the structure and dynamics of matter on a wide range of length and time scales. The conventional SANS data analysis is largely based on the geometrical concept of the form factor, which embodies a uniform or a stepwise uniform magnetic structure of the nanoparticle. This discontinuous macrospin picture ignores the smooth and continuous spatial variation of the magnetisation vector field, which results from the competition between the relevant magnetic interactions such as the exchange energy, Dzyaloshinskii-Moriya interaction, magnetic anisotropy, and the magnetodipolar energy.

Researchers from the Universities of Luxembourg and Salamanca together with their colleagues from General Numerics Research Lab in Jena have studied the transition from single-domain states to complex spin configurations in nanoparticles and its implications for the ensuing magnetic SANS cross section. Above a certain particle size they find that the cross section and the related correlation function cannot be described anymore with the uniform particle model. This results e.g. in deviations from the well-known Guinier law, which is commonly employed for estimating the particle size. In their computations a clear signature for the occurrence of a vortex-like spin structure is found, which may help to identify this magnetisation distribution in future neutron experiments.

 

Link to the paper in Physical Review Letters: Toward Understanding Complex Spin Textures in Nanoparticles by Magnetic Neutron Scattering