The role of exchange interactions in the formation of the magnetic structure in rare-earth orthoferrites RFeO$_{3}$ (R=Ho, Tb, Yb)

Ovsianikov, Aleksandr; Roth, Georg (Thesis advisor); Brückel, Thomas (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023, Kumulative Dissertation

Abstract

The rare earth orthoferrite family RFeO3, where R is a rare earth element, demonstrates a remarkable variety of magnetic properties. Its compounds crystallize in an orthorhombic perovskite structure with the space group Pnma. Different combinations of Dzyaloshinsky-Moriya interactions (DMI) and rare-earth ions with different ionic radii and filling of outer shells lead to a variety of magnetic effects. Rare earth orthoferrites are nowadays well known as multiferroics - materials with typically large magnetoelectric (ME) coupling and show the magnetocaloric effect (MCE). This cumulative dissertation investigates various orthoferrites RFeO3 (R=Ho, Tb, Yb) using neutron scattering methods. The orthoferrite HoFeO3 was studied by single crystal inelastic neutron scattering. It was shown that the spin dynamics of the Fe subsystem does not change through the spin reorientation transitions. The observed spectrum of magnetic excitations was analyzed in the framework of linear spin-wave theory. Within this approach the antiferromagnetic exchange interactions of nearest neighbors and next nearest neighbors were obtained for the Fe subsystem. Parameters of DMI at the Fe subsystem were refined. The temperature dependence of the gap in the Fe spin-wave spectrum indicates the temperature evolution of the anisotropy parameters. Estimations for the values of the Fe-Ho and Ho-Ho exchange interactions were made as well. Using the new polarized neutron diffraction (PND) setup of the instrument POLI at MLZ the spin reorientation transition in the HoFeO3 was studied at different wavelengths. The various experiments provided reproducible results demonstrating high reliability of the used setup. It was shown that during the phase transition at TSR=53 K in an external magnetic field applied along the crystal c-axis, the ordered magnetic moment of the Fe sublattice rotates from the crystallographic direction b to a not just in the ab plane, but through the z axis. This means that the applied field breaks the orthorhombic symmetry allowing some magnetization parallel to z within a small temperature region. Interestingly, this is the same temperature region where the large magnetocaloric effect for HoFeO3 was previously reported. A general model of the magnetic structure of HoFeO3, unconstrained by the orthorhombic symmetry, would allow the magnitudes and directions of the moments on each of the 8 magnetic sublattices in the unit cell to be independent of one-another, leading to 24 independent magnetic parameters. PND measurements were used to determine the absolute sign of the DMI in the ab plane for the Fe magnetic sublattice at 65 K. DMI plays an important role in the energy balance of the system. Neutron diffraction studies of HoFeO3 single crystals were performed under external magnetic fields. The interplay between the external magnetic fields, Dzyaloshinsky-Moria antisymmetric exchange, isotropic exchange interactions between Fe and Ho sublattices and within the Fe sublattice provides a rich magnetic phase diagram. As result of the balance of exchange interactions inside the crystal and external magnetic fields, eight different magnetic phases were found, which are induced or suppressed dependent on the external field. Investigations of the orthoferrites TbFeO3 and YbFeO3 were performed by neutron inelastic scattering and neutron single crystal diffraction in magnetic fields. The low temperature evolution of energy gaps was explored for both compounds and considered from the point of view of changes of rare earth ion anisotropy. Exchange parameters between nearest neighbors for Fe3+ in TbFeO3 were obtained. The magnetic phase diagram for YbFeO3 was obtained and discussed as a result of the energy balance between Heisenberg exchange interactions, Dzyaloshinsky-Moriya interaction, anisotropy and external magnetic field.

Institutions

• Division of Earth Sciences and Geography [530000]
• Applied Crystallography and Mineralogy Department [542220]