Magnetic Characterisation of Nanostructured Magnetic Systems Using SQUID Magnetometry

  • Suzanne Bennington-Gray

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

In this thesis SQUID magnetometry was employed as the main characterisation tool to investigate
the magnetic behaviour of a number of materials on the bulk, nano- and atomic scale. In particular
magnetic characterization was conducted on a selection of doped lanthanum manganate (LMO)
magnetic nanoparticles (MNPs), a family of lanthanide ionic liquids, and finally on three distinct
magnetic crystals synthesized using an acetate ionic liquid in place of a more conventional solvent.

Investigation into the effect of doping on the magnetic properties of LMO MNPs was conducted with
the aim of synthesizing a suitable mediator for magnetic hyperthermia treatment. The magnetic
heating exhibited by the MNPs was shown to be enhanced when the dopant ion radius was larger
than that of the La3+ ion, and reduced when the dopant ion radius was smaller. La0.60Sr0.40MnO3
proved to show the most promise as a potential mediator for mild hyperthermia, showing magnetic
heating of 36.5°C and a specific absorption rate (SAR) of 46Wg-1Mn. Further investigations into the
effect of strontium dopant concentrations on the magnetic properties of the MNPs showed that as
the dopant concentration was increased the magnetic heating achieved also increased until a maximum was reached for La0.65Sr0.35MnO3. Maximum heating of 46.7°C and an SAR of 56Wg-1Mn was
achieved for this material. Finally the effect of using microwave heating compared with conventional
heating techniques during the synthesis process of the MNPs was investigated. This synthesis process was shown to improve the crystallinity of the MNPs over samples prepared using conventional heating. The improved crystallinity of the MNPs resulted in enhanced magnetic properties, which made them more attractive as potential mediators for mild hyperthermia treatments.

Magnetic characterization of a family of monomeric lanthanide ionic liquids showed them to possess
magnetic moments comparable with those of their constituent ions, as quoted in the literature. Further studies into Dy and Nd dimeric ionic liquids showed these materials to exhibit antiferromagnetic interactions between neighbouring atoms below 100K.

In the final section of this thesis, three distinct magnetic complexes, a hexameric Ni(II) complex,
trimeric Co(II) complex and polymeric Mn(II) complex, were all synthesized using the same process,
employing the 1-methyl-3-ethylimidazolium ionic liquid. All three magnetic crystals exhibited
splitting of the isofield lines in the reduced magnetization measurements, suggesting the presence
of sizeable zero-field splitting. Fitting to the data returned values of D=-2.34 cm-1 and D=16.66 cm-1 for the Ni(II) and Co(II) complexes respectively. Fitting to the susceptibility data of these two complexes confirmed these results and returned a spin ground state of 2 for the Ni(II) complex and
1.5 for the Co(II) complex. Stepping of the hysteresis loop of the Ni(II) complex was observed in the
temperature range 1.8-10K, suggesting that this complex exhibits single molecule magnet behaviour.

The work in this thesis leverages the flexibility of SQUID magnetometry to characterise interesting magnetic materials across a variety of size regimes and fields of interest. The results presented provide a firm foundation for further work in the area of ionic liquids, single molecule magnetism (SMM) and also demonstrate a convincing case for the application of LMO MNPs for magnetic hyperthermia treatment.
Date of AwardDec 2019
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorRobert Pollard (Supervisor) & Solveig Felton (Supervisor)

Cite this

Magnetic Characterisation of Nanostructured Magnetic Systems Using SQUID Magnetometry
Bennington-Gray, S. (Author). Dec 2019

Student thesis: Doctoral ThesisDoctor of Philosophy