R. Vilarinho

On the ferroelectric and magnetoelectric mechanisms in low Fe3+ doped TbMnO3

Abstract This work addresses the effect of substituting Mn 3+ by Fe 3+ at the octahedral site of TbMnO 3 on the magnetic phase sequence, ferroelectric and magnetoelectric properties, keeping the Fe 3+ concentration below 5%. The temperature dependence of the specific heat, dielectric permittivity and electric polarization was studied as a function of Fe 3+ concentration and applied magnetic field. From the experimental results a strong decrease of the electric polarization with increasing Fe 3+ substitution is observed, vanishing above a concentration of 4%. However, within this range, a significant increase of the magnetic sensitivity of the electric polarization is obtained by increasing Fe 3+ concentration. For Fe 3+ concentration above 4%, a non-polar phase emerges, whose spin structure prevents ferroelectricity according to the Dzyaloshinskii-Moriya model. The experimental results here reported reveal the crucial effect of B-site substitution on the magnetic phase sequence, as well as, on the polar and magnetoelectric properties, evidencing the important role played by the e g electrons on the stabilization of the magnetic structures that are suitable for the emergence of electric polarization.

Dzyaloshinskii–Moriya nature of ferroelectric ordering in magnetoelectric Gd1−xYxMnO3 system

Abstract The phase diagram of Gd 1− x Y x MnO 3 , 0≤ x ≤0.4, is presented and the role of the driving mechanisms in stabilizing both magnetic and ferroelectric orderings is discussed in the framework of the Dzyaloshinskii–Moriya model. The ( x , T ) phase diagram reflects the effect of lattice distortions induced by the substitution of Gd 3+ ion by smaller Y 3+ ions, which gradually unbalances the antiferromagnetic against the ferromagnetic exchange interactions, enabling the emergence of ferroelectricity for higher concentrations of yttrium.

Heat capacity, magnetic and lattice dynamic properties of TbMn1-xFexO3

The effect of substitution of Fe3+ ions by Mn3+ ions on crystal structure, lattice dynamic, heat capacity and magnetic properties in TbMn1-xFexO3 ceramics has been studied. X-ray powder diffraction and Raman spectroscopy revealed that lattice distortion can be mainly attributed to Jahn-Teller distortion and tilting of octahedrons for samples with x < 0.4; for higher Fe concentration, the distortions are dominated by the octahedra tilting with less contribution of the Jahn-Teller effect. The anomalies in heat capacity of parent compounds (TbMnO3 and TbFeO3), which are associated with magnetic transitions, are smeared out by ion substitution. Magnetization measurements indicate that magnetic ordering persists in whole concentration range. The butterfly-type magnetic hysteresis loops suggest that the magnetic ground state of the whole system is complex and thence interesting for next experimental and theoretical studies.

Scaling spin–phonon and spin–spin interactions in magnetoelectric Gd{sub 1−x}Y{sub x}MnO{sub 3}

We present a systematic study of the structure and lattice dynamics at room temperature, and the phonon behavior at low temperatures in orthorhombic Gd{sub 1−x}Y{sub x}MnO{sub 3} manganites, with 0≤x≤0.4. Powder x-ray diffraction and Raman scattering at ambient conditions reveal that the Jahn–Teller distortion is composition-independent, while the Mn–O1–Mn bond angle increases by about 0.5° from x=0 to x=0.4. The spin–phonon coupling was ascertained from the relation between the renormalized wavenumber of the in-plane O2 stretching mode, associated with the spin–spin correlation function. The balance between competitive magnetic interactions is discussed based on the temperature dependence of the phonon frequency renormalization. - Graphical abstract: Wavenumber of the in-plane O2 stretching mode as a function of the spin–spin correlation function, for GdMnO{sub 3} (a), and Gd{sub 0.8}Y{sub 0.2}MnO{sub 3} (b). - Highlights: • Crucial role of structural distortions induced by ion substitution. • Tuning of magnetic interactions through ion substitution. • Facing spin–phonon coupling and competitive exchange magnetic interactions. • Scaling of spin–phonon and spin–spin mechanisms.