First-Principles Study of Non-Collinear Magnetism and Spin-Phonon Coupling in Al-Doped Multiferroic YMnO₃

Hexagonal YMnO₃ is a multiferroic material in which improper ferroelectricity coexists with a noncollinear 120° antiferromagnetic (AFM) ground state arising from geometric frustration on the triangular Mn³⁺ sublattice. We employ first-principles density functional theory (DFT+U) with a noncollinear spin formalism to investigate the spin configuration and lattice dynamics of pristine YMnO₃ and Al-substituted supercells over a range of concentrations. Our simulations show that substituting Mn with nonmagnetic Al disrupts the long-range 120° order and induces local distortions with extended spin textures around impurity sites. This dilution of the magnetic lattice systematically weakens the overall AFM order. By computing the phonon density of states (pDOS) and phonon dispersion relations via the finite-displacement method, we analyze impurity-induced shifts in vibrational frequencies and changes in the overall phonon spectrum. This provides an atomic-level view of how perturbations to magnetic order correlate with spin–phonon coupling signatures in hexagonal manganites.

In this talk, I will discuss how nonmagnetic impurities perturb the coupled magnetic and vibrational properties in the class of geometrically frustrated multiferroics.