Date of Award

9-1990

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Supervisor

M.F. Collins

Abstract

The critical behaviour of CsMnBr₃ has been studied by magnetic neutron scattering, magnetic susceptibility measurements, and Monte Carlo simulations. The magnetic Mn²⁺ ions in this insulating material form a simple hexagonal lattice. In the absence of an applied magnetic field me Mn²⁺ magnetic moments order in a 120º structure with the spins confined to the ab-plane.

Neutron scattering measurements of the temperature dependence of the paramagnetic critical scattering and the antiferromagnetic order parameter have found critical exponents y = 1.01 ± 0.08, ν = 0.54 ± 0.03, and β = 0.21 ± 0.02. These exponents do not correspond to any of the standard universality classes. This is a consequence of the Z₂ x S₁ symmetry of the order parameter arising from its XY (S₁) and chiral (Z₂) degeneracy.

Elastic neutron scattering has been used to determine the magnetic phase diagram of CsMnBr₃. The application of a magnetic field along the <100> direction splits the zero field transition and results in a intermediate phase (ll) of spin-flop character. The zero field transition is a tetracritical point with cross-over exponents Ψ(p-II) = 1.21 ± 0.07 and Ψ(II-I) = 0.75 ± 0.05. The fact that both exponents are less than two reflects the narrowness of the temperature range over which the intermediate phase is stable. These exponents are not in agreement with the theoretical prediction Ψ(P-II) = Ψ(II-I) ≅ 1.04.

Magnetic susceptibility (x) measurements near the tetracritical point have shown that the phase transition is marked by a discontinuous change in the slope of X. This is in contrast the predictions of scaling theory that there should be a singularity at TN where X goes to zero.

Monte Carlo simulations of CsMnBr₃ have been performed to determine to what extent the magnetic Hamiltonian is consistent with the observed phase diagram. The results reproduce the qualitative features of the phase diagram including the tetracriticality of the zero field transition and the increase of the Neel temperature with increasing magnetic field. A substantial renormalization of the Néel temperature with the size of the lattice along the c direction due to the quasi-one-dimensional nature of the system is observed. This is consistent with a strong suppression of the Neel temperature when the system is diluted with non-magnetic impurities.

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