First-principles modeling of multiferroic RMn2O5

First-principles study of magnetic interactions and excitations in antiferromagnetic van der Waals material MPX3(M=Mn, Fe, Co, Ni; X=S, Se)

Transition metal phosphorus trichalcogenides MPX3(M = Mn, Fe, Co, Ni; X = S, Se), as layered van der Waals antiferromagnetic (AFM) materials, have emerged as a promising platform for exploring two-dimensional (2D) magnetism. Based on density functional theory, we present a comprehensive investigation of the electronic and magnetic properties of MPX3. We calculated the spin exchange interactions as well as magnetocrystalline anisotropy energy. The numerical results reveal thatJ3is AFM in all cases, andJ2is significantly smaller compared to bothJ3andJ1. This behavior can be understood with regard to exchange paths and electron filling. Compared to other materials within this family, FePS3and CoPS3demonstrate significant easy-axis anisotropy. Using the obtained parameters, we estimated the Néel temperatureTNand Curie-Weiss temperatureθCW, and the results are in good agreement with the experimental observations. We further calculated the magnon spectra and successfully reproduce several typical features observed experimentally. Finally, we give helpful suggestions for the strong constraints about the range of non-negligible magnetic interactions based on the relations between magnon eigenvalues at high-symmetrykpoints in honeycomb lattices.

Micro-Raman and infrared studies of multiferroic TbMn₂O₅

We have studied the Raman and infrared spectral response of TbMn2O5 under an applied magnetic field parallel to the easy magnetic a-axis at 4.2 K. Strong spin-lattice coupling in TbMn2O5 is evidenced by a frequency shift of Raman and infrared phonons as a function of magnetic field compared to the phonon response of BiMn2O5 that remains unaffected. The magnetic field behavior of the highest frequency phonons retraces the polarization switching in TbMn2O5 and shows an important frequency softening below 3 T that is modulated by the J 3 and J 4 exchange parameters. The role of the Tb(3+) spin alignment with H is interpreted in terms of a local lattice striction and the contribution of the charge transfer mechanism to the magnetoelectric process is evaluated.

Replica exchange molecular dynamics optimization of tensor network states for quantum many-body systems

Tensor network states (TNS) methods combined with the Monte Carlo (MC) technique have been proven a powerful algorithm for simulating quantum many-body systems. However, because the ground state energy is a highly non-linear function of the tensors, it is easy to get stuck in local minima when optimizing the TNS of the simulated physical systems. To overcome this difficulty, we introduce a replica-exchange molecular dynamics optimization algorithm to obtain the TNS ground state, based on the MC sampling technique, by mapping the energy function of the TNS to that of a classical mechanical system. The method is expected to effectively avoid local minima. We make benchmark tests on a 1D Hubbard model based on matrix product states (MPS) and a Heisenberg J1-J2 model on square lattice based on string bond states (SBS). The results show that the optimization method is robust and efficient compared to the existing results.

Molecular-spin dynamics study of electromagnons in multiferroic RMn2O5

We investigate the electromagnon in magnetoferroelectrics RMn(2)O(5) using combined molecular-spin dynamics simulations. We confirm that the origin of the electromagnon modes observed in the optical spectra is due to the exchange-striction interaction between the magnons and the phonons, and the dielectric step at the magnetic phase transition is due to the appearance of the electromagnon in the low-temperature phase in these materials. The magnetic anisotropy breaks the rotational symmetry of the magnetic structures and, as a result, the electromagnon splits into three modes in RMn(2)O(5). We find that the electromagnon frequencies are very sensitive to the magnetic wavevector along the a direction q(x). Therefore, the electromagnon frequencies of TmMn(2)O(5) (q(x) ~ 0.467) are expected to be much higher than those of other materials of the family, such as R= Tb, Y, Ho, etc (q(x) ~ 0.48). We further calculate the electromagnons in the magnetic field, and find a new mode appearing in the magnetic field. Although the modes' frequencies change significantly under magnetic field, the total static dielectric constant contributed from the electromagnons does not change much in the magnetic field, suggesting that the colossal magnetodielectric effects in these materials may not be caused by the electromagnons.

Origin of ferroelectricity in high-T(c) magnetic ferroelectric CuO

Cupric oxide is a unique magnetic ferroelectric material with a transition temperature significantly higher than the boiling point of liquid nitrogen. However, the mechanism of high-T(c) multiferroicity in CuO remains puzzling. In this Letter, we clarify the mechanism of high-T(c) multiferroicity in CuO by using combined first-principles calculations and an effective hamiltonian model. We find that CuO contains two magnetic sublattices, with strong intrasublattice interactions and weakly frustrated intersublattice interactions. The weak spin frustration leads to incommensurate spin excitations that dramatically enhance the entropy of the multiferroic phase and eventually stabilize that phase in CuO.