Monte Carlo study of a geometrically frustrated rare earth magnetic compound: SrGd2O4
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Date
2017
Authors
Hasan, Emrul
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Abstract
We study the low temperature magnetic phase transitions and phase diagram
of one member of the lanthanide family of frustrated compounds, SrGd2O4, using
Monte Carlo simulation techniques. Frustrated magnetism is the study of competing
interactions between the neighbouring spins. Frustration occurs when the lattice
geometry of the system is such that, with antiferromagnetic interactions, the system is
unable to find an unique ground state to minimize the energy of the system. Recently,
frustration has been identified in a rare earth family of compounds with the formula
SrLn2O4 where Ln = Ho, Gd, Er, Dy, Tm, and Y b. The two dimensional honeycomb
structure of SrLn2O4 in the ab plane is connected by triangular chains running along
the c direction, which leads to frustration. In this thesis we focus on one of these
frustrated materials, SrGd2O4.
A detailed experimental study of two members of the SrLn2O4 family of compounds,
SrHo2O4 and SrGd2O4, has recently been carried out by Young. In her
extensive studies, magnetic bulk properties are measured with both single crystal
and powder samples. Both compounds have the same structure but their magnetic
behaviour is quite different. SrHo2O4 exhibits complex crystal fi eld effects and an
Ising anisotropy at low temperatures. In contrast, in the ground state of SrGd2O4,
the orbital angular momentum L = 0 which allows us to neglect the spin orbit coupling
interaction and crystal field effects and study this material with only Heisenberg
exchange and dipole interactions. For SrGd2O4, two magnetic phase transitions in
zero applied fi eld are identifi ed at two different temperatures from both specifi c heat
and magnetisation measurements. Measurements of the magnetisation in an applied
fi eld also indicate two transitions at low temperatures. A complex phase diagram of
SrGd2O4 was mapped out in the field-temperature (H - T) plane from magnetisation,
susceptibility and specifi c heat measurements and several ordered phases were
identifi ed. However, the detailed nature of these phases remains unknown. Dipolar
interactions are believed to play an important role.
We have used a model of classical Heisenberg spins to investigate the low temperature
behaviour. We have studied the cases of pure exchange and pure dipole
interactions as well as their combined effects. Our simulation results qualitatively
agree with the experimental findings. In zero applied fi eld, two phase transitions
are identifi ed at two different temperatures from the specifi c heat and magnetisation
measurements as a function of temperature T. Measurements of these quantities as
a function of an applied field H also indicate several transitions at low temperatures
Finally, by collecting data from all measured thermodynamic quantities, a H - T
phase diagram is constructed. It reveals four separate regions of phases with unique
magnetic ordering. We have identifi ed the nature of the order in each of these phases.
Description
Keywords
Geometrical frustration, Rare earth magnetic compound, Magnetic phase transition