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Register a new userMetastable antiphase boundary ordering in CaFe$_{2}$O$_{4}$
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CaFe$_{2}$O$_{4}$ is an $S=5/2$ antiferromagnet exhibiting two magnetic orders which shows regions of coexistence at some temperatures. Using a Greens function formalism, we model neutron scattering data of the spin wave excitations in this material, ellucidating the microscopic spin Hamiltonian. In doing so, we suggest that the low temperature A phase order $(uparrowuparrowdownarrowdownarrow)$ finds its origins in the freezing of antiphase boundaries created by thermal fluctuations in a parent B phase order $(uparrowdownarrowuparrowdownarrow)$. The low temperature magnetic order observed in CaFe$_{2}$O$_{4}$ is thus the result of a competition between the exchange coupling along $c$, which favors the B phase, and the single-ion anisotropy which stabilizes thermally-generated antiphase boundaries, leading to static metastable A phase order at low temperatures.
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CaFe$_{2}$O$_{4}$ is an anisotropic $S={5over 2}$ antiferromagnet with two competing $A$ ($uparrow uparrow downarrow downarrow$) and $B$ ($uparrow downarrow uparrow downarrow$) magnetic order parameters separated by static antiphase boundaries at low temperatures. Neutron diffraction and bulk susceptibility measurements, show that the spins near these boundaries are weakly correlated and a carry an uncompensated ferromagnetic moment that can be tuned with a magnetic field. Spectroscopic measurements find these spins are bound with excitation energies less than the bulk magnetic spin-waves and resemble the spectra from isolated spin-clusters. Localized bound orphaned spins separate the two competing magnetic order parameters in CaFe$_{2}$O$_{4}$.
We use fermion mean field theory to study possible plaquette ordering in the antiferromagnetic SU(4) Heisenberg model. We find the ground state for both the square and triangular lattices to be the disconnected plaquette state. Our mean field theory gives a first order transition for plaquette ordering for the triangular lattice. Our results suggest a large number of low lying states.
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