Monte Carlo study of a geometrically frustrated rare earth magnetic compound: SrGd2O4

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Hasan, Emrul
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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.
Geometrical frustration, Rare earth magnetic compound, Magnetic phase transition