Using high frequency A.C. Susceptibility to probe frustrated magnetic systems

Cafolla-Ward, Clara 2024. Using high frequency A.C. Susceptibility to probe frustrated magnetic systems. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Utilising a unique high frequency A.C. susceptometer, the heretofor unquantified magnetic dynamics of select frustrated magnetic systems are measured. This equipment can measure frequencies up to 3MHz, temperatures down to 2K and magnetic fields up to 9T. For Dy_2Ti_2O_7, a component b(T) that quantifies the dynamic constraints is measured. Values of b ≈ 1.76 at 2K, a b ≈ 1.58 minimum at 10K and a non-constrained b ≈ 2 above ≈20K were found. At 10K, many relaxation processes are present that potentially constrain the dynamics. Low temperature values conflict with published results: this is attributed to limited high frequency data in the literature. For Ca_3Co_2O_6, spin dynamics are present beyond the T_{c1} ≈ 24K critical temperature and at 1.85T, a region considered static. In addition to the global relaxation (GR), evidence of an additional local relaxation (LR) between 13K and 23K, affected by temperature history alone, is observed. The LR disappears with increasing magnetic field. Activation energies of the GR is found to be ≈ 134K below T_{c2} ≈ 13K and ≈ 230K above T_{c2} ≈ 13K: the same temperature the GR and LR merge/diverge. The LR indicates a meta-stable zero-field state. For LiY_{0.99}Ho_{0.01}F_4, the system is pushed to 2MHz to try and measure the isolated susceptibility, a fully quantum quantity. This was successfully done in the similar crystal environment Y_{1.9975}Ho_{0.0025}Ti_2O_7. However, at 2K and 2MHz, the system is not fully in the isolated limit. A simplified two-state approximation of the ground state Ising doublet can still successfully model the response. However, a portion of the response is overestimated, indicating a non-Boltzmann population distribution is more appropriate.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	A.C. susceptibility', 'Frustrated magnetism', 'frustrated spin systems', 'Spin ice', 'Isolated susceptibility', 'High frequency A.C. susceptibility', 'Ca3Co2O6', 'LYF4:Ho3+', 'Dy2Ti2O7'
Funders:	EPSRC
Date of First Compliant Deposit:	8 October 2024
Last Modified:	08 Oct 2024 15:19
URI:	https://orca.cardiff.ac.uk/id/eprint/172700

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