Sprecher
Beschreibung
Research into magnetic nanoparticles is being propelled by their promising applications across diverse fields such as medicine, biology, and nanotechnology. Numerous studies have highlighted their potential in targeted drug delivery, imaging, and hyperthermia treatment. However, assumptions about the uniformity of the magnetization distribution within nanoparticles, often made in application-oriented studies, can fall short in capturing the true complexity of these systems. This complexity is exemplified in magnetic hyperthermia, where microstructural defects may lead to enhanced specific absorption rates compared to defect-free particles. Understanding the spin structure of nanoparticles thus becomes pivotal not only for fundamental scientific inquiry but also for optimizing technological applications.
To delve into the magnetic microstructure of nanoparticles and its relation to macroscopic properties both observational and computational methods are crucial. Among experimental techniques, magnetic small-angle neutron scattering (SANS) stands out as it can probe spin structures on the mesoscopic length scale and within the volume of magnetic materials. Consequently, numerous experimental SANS investigations, including those on ferrofluids, have been conducted to date, revealing a plethora of nonuniform spin configurations within nanoparticles. These configurations range from canted to vortex-type or core-shell-type arrangements, underscoring the rich complexity of magnetic behavior at the nanoscale.
Complementing experimental efforts, numerical micromagnetic simulations are increasingly valuable in predicting nanoparticle spin structures and their related scattering signatures. These simulations consider various factors influencing the magnetic ground state of nanoparticles, such as particle size, shape, defects, and magnetic interactions. Here, by combining SANS with numerical micromagnetic computations, we discuss the transition from single-domain to multi-domain behavior in nanoparticles and its implications for the ensuing magnetic SANS cross section. Above the critical single-domain size we find that the cross section and the related correlation function cannot be described anymore with the uniform particle model, resulting, e.g., in deviations from the well-known Guinier law. In the simulations we identify a clear signature for the occurrence of a vortexlike dipolar-energy-driven spin structure at remanence---the magnetic correlation function exhibits a damped oscillatory behavior. Recent experimental SANS data on an isotropic Nd-Fe-B magnet seem to support the occurrence of mesoscaled flux-closure patterns in the magnetic microstructure that give rise to the corresponding feature in the correlation function. The resulting field dependence of the correlation length (see figure) exhibits a power-law behavior that varies with a different exponent than the numerically-computed data, which poses a challenge to theory.
