Functionality of materials can be enriched by the anisotropy. Crystalline and/or shape anisotropy of materials results in orientationally dependent physical properties. For example, a class of materials, named hexaferrites, shows uniaxial magnetic anisotropy due to their anisotropic crystal structure of magnetoplumbite type. Hexaferrites crystallize in the shape of thin hexagonal platelets with a magnetic easy in the direction of the c-crystal axis, i.e., perpendicular to the basal crystal plane. Consequently, a single hexaferrite platelet forms a nanomagnet, with a direction dependent response to an applied magnetic field. In order to exploit the platelets' specific magnetic properties they should be synthesized in wet and used in the form of stable suspensions. Subsequently, plate-like nanomagnets can be: (i) embedded in different matrices to form composites, (ii) assembled from the suspensions into higher structures, films or bulk materials, or (iii) hybridized with functional organic moieties; all resulting in new advanced materials. In this contribution we present our original approach for the synthesis of hexaferrite plate-like nanomagnets, including the possibilities for tuning different interparticle forces that allowed for the development of self-biased thick films, new magneto-optic composites and ferromagnetic suspensions. A specific role of the magnetocrystalline and shape anisotropy of these nanomagnets for the realization of the new advanced material was be elucidated and their potential applications presented.
B.04 Guest lecture
COBISS.SI-ID: 30765863Presented at Gordon Conference, GRC Liquid Crystals: Soft Order and Topology Motives in Biomedicine, Nanoscience, Cosmology, Living Matter and Emergent Industries, 7-12 July 2019, New London, NH, USA Suspensions of magnetic nanoplatelets in isotropic solvents are very interesting examples of ferrofluids. It has been shown that above a certain concentration such suspensions form ferromagnetic nematic phase, which makes this system a unique example of a dipolar fluid. The formation of nematic phase is driven by screened anisotropic electrostatic and long-range dipolar magnetic interactions. When a layer of such suspension is exposed to an alternating magnetic field, above a certain frequency stripe-like magnetic domains form in the direction perpendicular to the magnetic field. The stripes are observed even in suspensions which are initially isotropic, and, consequently, paramagnetic. The width of the stripes depends on the concentration of the platelets in the suspension, but not on the thickness of the layer. Additionally, macroscopic flow is observed along the stripes. Using polarization microscopy and correlation video microscopy, we studied formation of the stripes, collective orientation of the platelets in the stripes, and measured macroscopic flow. We found out that the collective orientation in the neighbouring stripes oscillates in opposite directions around the direction of the stripes, indicating that the magnetic orientation in the neighbouring stripes is opposite. The flow direction depends on the position, so that the velocity is zero at the centre of a stripe and reaches maximal, but opposite value at each of stripe’s edges. This dynamic formation of magnetic domains shows, that orientational correlations between the neighbouring platelets is present even in the isotropic phase, and point to a strong coupling between the collective orientation and the flow, which is most likely a consequence of the shape of the platelets.
B.04 Guest lecture
COBISS.SI-ID: 32517159Presented at : Optimal design of soft matter - including a celebration of Women in Materials Science (WMS) : workshop Optimal design of complex materials, 14th January 2019 to 18th January 2019, Cambridge. Cambridge: Isaac Newton Institute for Mathematical Sciences. 2019 (https://www.newton.ac.uk/seminar/20190118110011451) Polar order, i.e., ferromagnetic or ferroelectric, in 3D liquids is experimentally rarely observed. In this talk I will discuss the reason for this and show two examples of how shape of constituents can promote polar order. The first example is a ferromagnetic liquid phase, which emerges in a suspension of magnetic nanoplatelets in isotropic solvent as a result of platelets’ shape. The second example is antiferroelectric splay nematic phase, which appears in materials made of wedge-shaped molecules with large electric dipole moments.
B.04 Guest lecture
COBISS.SI-ID: 32516903Supervision of a master thesis: A suspension of ferromagnetic particles which forms a ferromagnetic phase is called a ferromagnetic ferrofluid. In this work the results, observations and measurements on a ferromagnetic suspension of nanoplatelets dispersed in terc-butanol are presented. This includes measurements of magnetic hysteresis with the help of a vibrating sample magnetometer, measurements of diffusion constants by dynamic light scattering and measurements of dynamics of a striped structure which appears in the presence of an alternating external magnetic field.
D.10 Educational activities
COBISS.SI-ID: 3259236A high surface-to-volume ratio of nanoparticles with the related high chemical reactivity allows us to tune their surface chemistry for a variety of applications. However, the high surface reactivity also lowers the chemical stability of nanoparticles in comparison to bulk materials of the same composition and structure. We presented beneficial and harmful effects of surface ligands on the physico-chemical properties of nanoparticles. A more in depth presentation included two examples from our studies. Firstly, we presented lanthanide-doped NaYF4 nanoparticles, i.e., upconverting nanoparticles (UCNPs). They have been studied as alternative multimodal biomarkers but their usage is being questioned due to surface quenching of the luminescence and chemical instability in aqueous media. We propose the optimum surface chemistry for keeping the UCNPs bright and chemically stable. Secondly, we presented a surface chemistry optimization of barium hexaferriet (BHF) magnetic nanoplatelets (NPLs) that led to the development of the first ferromagnetic liquid and a room-temperature liquid magnet. We showed the first evidence of the dissolution of BHF NPLs since BHF is considered as practically insoluble in water. A possible dissolution mechanism is proposed.
B.04 Guest lecture
COBISS.SI-ID: 32423719