Professor Paolo G. Radaelli OSI

Abstract:

Structural and magnetic chiralities are found to coexist in a small group of materials in which they produce intriguing phenomenologies such as the recently discovered skyrmion phases. Here, we describe a previously unknown manifestation of this interplay in MnSb2O6, a trigonal oxide with a chiral crystal structure. Unlike all other known cases, the MnSb2O6 magnetic structure is based on co-rotating cycloids rather than helices. The coupling to the structural chirality is provided by a magnetic axial vector, related to the so-called vector chirality. We show that this unique arrangement is the magnetic ground state of the symmetric-exchange Hamiltonian, based on ab-initio theoretical calculations of the Heisenberg exchange interactions, and is stabilised by out-of-plane anisotropy. MnSb2O6 is predicted to be multiferroic with a unique ferroelectric switching mechanism.

Abstract:

Reaction of the anion-deficient, cation-ordered perovskite phase Ba 2YFeO5 with 80 atm of oxygen pressure at 410 C results in the formation of the Fe4+ phase Ba2YFeO5.5. The topochemical insertion of oxide ions lifts the inversion symmetry of the centrosymmetric host phase, Ba2YFeO5 (space group P2 1/n), to yield a noncentrosymmetric (NCS) phase Ba 2YFeO5.5 (space group Pb21m (No. 26), a = 12.1320(2) Å, b = 6.0606(1) Å, c = 8.0956(1) Å, V = 595.257(2) Å3) confirmed by the observation of second-harmonic generation. Dielectric and PUND ferroelectric measurements, however, show no evidence for a switchable ferroelectric polarization, limiting the material to pyroelectric behavior. Magnetization and low-temperature neutron diffraction data indicate that Ba2YFeO5.5 undergoes a magnetic transition at 20 K to adopt a state which exhibits a combination of ferromagnetic and antiferromagnetic order. The symmetry breaking from centrosymmetric to polar noncentrosymmetric, which occurs during the topochemical oxidation process is discussed on the basis of induced lattice strain and an electronic instability and represents a new strategy for the preparation of NCS materials that readily incorporate paramagnetic transition metal centers. © 2013 American Chemical Society.

Abstract:

Giant tunability of ferroelectric polarization (ΔP=5000  μC/m2) in the multiferroic GdMn2O5 with external magnetic fields is discovered. The detailed magnetic model from x-ray magnetic scattering results indicates that the Gd-Mn symmetric exchange striction plays a major role in the tunable ferroelectricity of GdMn2O5, which is in distinction from other compounds of the same family. Thus, the highly isotropic nature of Gd spins plays a key role in the giant magnetoelectric coupling in GdMn2O5. This finding provides a new handle in achieving enhanced magnetoelectric functionality.