Suspensions of magnetic nanoplatelets in external fields

Gregorin, Žiga (author), Osterman, Natan (mentor)

Ferrofluids are colloidal suspensions of magnetic nanoparticles in liquid media. Ferromagnetic ordering – with spontaneous magnetization in the absence of external fields – has been described in a system of permanently magnetised nanoplatelets suspended in n-butanol, concentrated enough to undergo the transition to the nematic phase. Such material shows magnetic domain structures analogous to those of solid-state ferromagnets. By optimization of fundamental interactions, the threshold concentration for the nematic ordering was significantly reduced. The optimized material allows reproducible and controllable analysis of domain structures and their formation dynamics, with domains forming on the time scale of seconds. Additionally, the effect of an external applied magnetic field on the fundamental nematic director fluctuations inside single domains has been studied with differential dynamic microscopy. The obtained results provide an estimation of the ferrofluid's spontaneous magnetization in the absence of external magnetic field. In an oscillating magnetic field, the platelets follow the magnetic field at low frequencies but tend to form stripes at high enough amplitudes and frequencies of the applied field. The stripe regime is examined optically and by X-ray and neutron scattering methods to obtain insight into the collective behaviour of the constituents in oscillating magnetic fields. The shape anisotropy of the platelets and the coupling between magnetic and optical properties lead to the effect of flow-induced ordering. We analyse the effect of flow on the generation of a non-zero net magnetisation, resulting in a measurable jump in the magnetic flux density outside of the capillary with flowing suspension and vice versa, the effect of an applied magnetic field on the flow velocity.

• Language: English
• Keywords: ferrofluids, liquid magnets, magnetic nanoplatelets, domains, domain growth dynamics, differential dynamic microscopy, SAXS, SANS, stripe regime, microfluidics, flow, magnetic field generation with flow
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-146910
• COBISS.SI-ID: 153537027
• Publication date in RUL: 16.06.2023

Secondary language

• Language: Slovenian
• Title: Suspenzije magnetnih nanoploščic v zunanjih poljih

Abstract

Ferofluidi so koloidne suspenzije magnetnih nanodelcev v tekočini. Feromagnetna ureditev - s spontano magnetizacijo v odsotnosti zunanjih polj - je bila opisana kot sistem trajno magnetnih nanodelcev, suspendiranih v n-butanolu, dovolj koncentriranih, da preidejo v nematsko fazo. Tak material tvori magnetne domenske strukture, podobne tistim pri feromagnetih v trdni snovi. Z optimizacijo osnovnih interakcij smo zmanjšali mejno koncentracijo za nematsko urejanje. Optimiziran material omogoča ponovljivo in nadzorovano analizo domenskih struktur in dinamike njihovega nastajanja, pri čemer se domene oblikujejo na časovni skali reda sekund. Poleg tega smo z diferencialno dinamično mikroskopijo preučili vpliv zunanjega magnetnega polja na osnovne fluktuacije nematskega direktorja znotraj posamične domene. S pomočjo rezultatov lahko ocenimo spontano magnetizacijo ferofluida v odsotnosti zunanjih magnetnih polj. V oscilirajočem magnetnem polju ploščice pri nizkih frekvencah sledijo magnetnemu polju, pri dovolj visokem polju in frekvenci pa težijo k oblikovanju črt. Črtast režim smo preučili z optičnimi metodami ter z metodami rentgenskega in nevtronskega sipanja. S tem smo pridobili vpogled v kolektivno obnašanje gradnikov v oscilirajočih magnetnih poljih. Anizotropna oblika ploščic ter povezava med magnetnimi in optičnimi lastnostmi privede do učinka urejanja v tekočinskem toku. Analizirali smo vpliv toka na nastanek neničelne skupne magnetizacije, ki povzroči merljiv skok gostote magnetnega polja zunaj kapilare, po kateri je tekla suspenzija, in obratno, vpliv zunanjega magnetnega polja na hitrost toka.

• Keywords: ferofluidi, tekoči magneti, magnetne nanoploščice, domene, dinamika domenske rasti, diferencialna dinamična mikroskopija, SAXS, SANS, črtast režim, mikrofluidika, tok tekočine, generacija magnetnega polja s tokom