Real-Space Infrared Spectroscopy of Ferroelectric Domain Walls in Multiferroic h-(Lu,Sc)FeO3

We employ synchrotron-based near-field infrared spectroscopy to image the phononic properties of ferroelectric domain walls in hexagonal (h) Lu_0.6Sc_0.4FeO₃, and we compare our findings with a detailed symmetry analysis, lattice dynamics calculations, and prior models of domain-wall structure. Rather than metallic and atomically thin as observed in the rare-earth manganites, ferroelectric walls in h-Lu_0.6Sc_0.4FeO₃ are broad and semiconducting, a finding that we attribute to the presence of an A-site substitution-induced intermediate phase that reduces strain and renders the interior of the domain wall nonpolar. Mixed Lu/Sc occupation on the A site also provides compositional heterogeneity over micron-sized length scales, and we leverage the fact that Lu and Sc cluster in different ratios to demonstrate that the spectral characteristics at the wall are robust even in different compositional regimes. This work opens the door to broadband imaging of physical and chemical heterogeneity in ferroics and represents an important step toward revealing the rich properties of these flexible defect states.

Magnetic, electric and toroidal polarization modes describing the physical properties of crystals. NdFeO3 case

The structure and the physical phenomena that occur in a crystal can be described by using a suitable set of symmetry-adapted modes. The classification of magnetic modes in crystals presented in Fabrykiewicz et al. [Acta Cryst. (2021), A77, 327-338] is extended to a classification of electric and toroidal (anapole) modes in crystals. These three classifications are based on magnetic point groups, which are used in two contexts: (i) the magnetic point group of the magnetic crystal class and (ii) the magnetic site-symmetry point group of the Wyckoff position of interest. The classifications for magnetic, electric and toroidal modes are based on the properties of the three generalized inversions: space inversion 1, time inversion 1' and the space-and-time inversion 1'. It is emphasized that none of these three inversions is more important than the other two. A new notation for symmetry operation symbols and magnetic point group symbols is proposed; each operation is presented as a product of one proper rotation and one generalized inversion. For magnetic, electric and toroidal orderings there are 64 modes: three pure ferro(magnetic/electric/toroidal) modes, 13 mixed ferro(magnetic/electric/toroidal) and antiferro(magnetic/electric/toroidal) modes, and 48 pure antiferro(magnetic/electric/toroidal) modes. The proposed classification of modes leads to useful observations: the electric and toroidal modes have many symmetry limitations similar to those already known for the magnetic modes, e.g. a continuous reorientation of the magnetic or electric or toroidal moments is possible only in triclinic or monoclinic symmetry. An antiferro(magnetic/electric/toroidal) ordering with a weak perpendicular ferro(magnetic/electric/toroidal) component is possible only in monoclinic or orthorhombic symmetry. The general classifications of magnetic, electric and toroidal modes are presented for the case of NdFeO₃.

The magnetic structure of DyFeO3revisited: Fe spin reorientation and Dy incommensurate magnetic order

High resolution and high intensity neutron powder diffraction is used to study the ground state magnetic order and the spin reorientation transition in the orthoferrite DyFeO₃. The transition from the high temperaturek= 0 Γ₄(G_xA_yF_z) to the low temperature Γ₁(A_xG_yC_z) type order of the Fe-sublattice is found atT_SR= 73 K and does not show any thermal hysteresis. BelowT_N2= 4 K the Dy-sublattice orders in an incommensurate magnetic structure withk= [0, 0, 0.028] while the Fe-sublattice keeps its commensurate Γ₁type order. DyFeO₃is the first orthoferriteRFeO₃to possess an incommensurate magnetic order of the rare earth sublattice under zero field conditions; an important piece of information neglected in the recent discussion of its multiferroic properties.

Determination of the magnetic structures in orthoferrite CeFeO3by neutron powder diffraction: first order spin reorientation and appearance of an ordered Ce-moment

High resolution and high intensity neutron powder diffraction are used to determine the temperature dependence of the crystallographic and magnetic structure of the orthoferrite CeFeO₃. The high temperatureG_x-type magnetic coupling of the Fe-sublattice described by the Γ₄(G_xA_yF_z) irreducible representation changes at the spin reorientation temperatureT_SR= 228 K to aG_y-type coupling of Γ₁(A_xG_yC_z). The spin reorientation is of first order and sees a hysteresis of about 2.5 K atT_SR. Below 35 K faint magnetic peaks reflectingC_ztype magnetic coupling appear and are argued to be related to the Ce-sublattice. Magnetic moments at 2 K amount toμ_Fe= 4.15 μ_Bandμ_Ce= 0.11 μ_B. CeFeO₃is only the secondRFeO₃compound after DyFeO₃showing this ground state magnetic structure of the Fe-sublattice. The orthorhombic structurePbnmis kept over the whole temperature range.

Study of short range correlations and two-fold spin reorientation in NdFe0.5Mn0.5O3

Detailed powder neutron diffraction studies as a function of temperature is performed on NdFe0.5Mn0.5O3in the temperature range 400-1.5 K. Diffused magnetic scattering is observed due to three dimensional short range ordering (SRO), between Fe3+/Mn3+spins, over the whole temperature range 400-1.5 K. The presence of SRO is independent of long range ordering (LRO) in this compound which has never been observed in any Fe3+/Mn3+based compounds. Further, in this compound two-fold spin reorientation is discussed in the temperature range 300-1.5 K. Development of long range ordering at 300 K is due to the mixture of Γ4and Γ1magnetic structure, not like other orthoferrites which have Γ4structure at 300 K. This occurs due to the presence of large single ion anisotropy of Mn3+ions. Volume fraction of Γ4decreases with temperature leading to pure Γ1magnetic structure in the temperature range 150-90 K. Another spin reorientation of Fe3+/Mn3+spins occurs from Γ1to Γ2in the temperature range 70-25 K.