Probing ultra-slow magnetisation relaxation in transition metal fluorides through DC and AC magnetometry

Abstract

Certain antiferromagnets (AFMs) show the phenomenon of spin canting when it is symmetry allowed. Aside from being responsible for exotic spin structures like skyrmions, the phenomenon of weak ferromagnetism also shows a potential for technological applications in next generation devices. This project aims to investigate a very specific trait that pertains to an ultra-slow magnetization relaxation phenomenon observed in a number of Dzyaloshinskii-Moriya Interaction (DMI) driven AFMs through Superconducting Quantum Interference Device (SQUID) magnetometry. In this project, our aim is to investigate whether such unique magnetization dynamics is also shown by Single Ion Anisotropy (SIA) driven spin canted systems which are also symmetry allowed piezomagnets. For this purpose we have explored two of these SIA driven Weak Ferromagnets (WFMs), NiF\textsubscript2 and CoF\textsubscript2, in powders consisting of regular shaped micro-crystallites \& in Single Crystal (SC) form. The microcrystals are prepared using solvothermal route by Sri Pragna in our lab. The single crystals of CoF\textsubscript2 and NiF\textsubscript2 have been provided by Dr. D. Prabhakaran, Oxford Physics. The Thermo-remanent Magnetization (TRM) in NiF\textsubscript2 and CoF\textsubscript2 microcrystallites, obtained through SQUID magnetometry, is presented in this thesis. These data show ultra-slow magnetization relaxation dynamics which is intimately connected to the spin canted phase. This enigmatic result is strikingly similar to previous results on DMI driven WFMs, however, unlike the DMI driven WFMs, the TRM does not show a peak when its magnetic field variation is investigated, at least for field till 10 kOe. Analysis of the DC magnetometry data reveals that spin canting appears to play an important role in the magnetization dynamics of not only NiF\textsubscript2, but also CoF\textsubscript2. The inverse susceptibilities for both the samples show huge deviations from linearity in the paramagnetic region. This deviation exists much away from the magnetic transition region, wherein deviations from linearity due to correlations are typically not observed. To further investigate the magnetization relaxation, AC-Susceptibility data on CoF\textsubscript2 SC is taken. AC-Susceptibility is one of the best-known techniques for probing relaxation phenomenon. Towards this, a fully functional Liquid Helium (LHe) compatible AC-Susceptometer was constructed, and AC-Susceptibilty data on these SCs above 77 K (Liquid Nitrogen (LN\textsubscript2) temperature) was obtained. The construction of the AC-Susceptometer involved designing a LHe compatible insert with coils wound on the Quartz tubes themselves for improved sensitivity (by about 1.6 times). This is noteworthy, as winding of secondary in the double walled glass-insert itself -- which makes sample closer to the secondary coil -- is not reported before to the best of our knowledge. This is extremely significant, especially if AFMs are to be probed. This work endeavors to summarize this, and other peculiar results.