arXiv subjects

R. Vilarinho

Publications and source records attributed to R. Vilarinho.

8 recordsLinked to original sources

arXiv2025

Strong impact of low-level substitution of Mn by Fe on the magnetoelectric coupling in $TbMnO_{3}$

The correlation between static magnetoelectric coupling and magnetic structure was investigated in $TbMn_{0.98}Fe_{0.02}O_{3}$ with magnetic field up to 8 T and down to 2 K. Single-crystal neutron diffraction experiments reveal a substantial increase in the temperature dependence of the incommensurate modulation wave vector of the antiferromagnetic phase as the magnetic field strength increases. Magnetic field-dependent pyroelectric current measurements revealed significantly higher magnetoelectric coupling at magnetic fields below 4 T than in pure TbMnO3. This is due to the higher sensitivity of the incommensurably modulated cycloid structure to weak magnetic fields. Detailed analysis of our data confirmed that the ferroelectric polarization is induced by inverse Dzyaloshinskii-Moriya interaction for magnetic field strength up to 4 T, but at higher fields a departure from theoretical predictions is ascertained, giving evidence for an additional, as yet misunderstood, contribution to magnetoelectric coupling. It shows that a small 2% substitution of Mn3+ by Fe3+ has a strong impact on the magnetic structure, promoting the destabilization of the incommensurably modulated magnetic cycloidal structure of $TbMnO_{3}$ in a magnetic field above 5 T. We demonstrate that the magnetoelectric coupling magnitude can be tuned through suitable substitutional elements, even at low level, inducing local lattice distortions with different electronic and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2024

Two Displacive Ferroelectric Phase Transitions in Multiferroic Quadruple Perovskite $BiMn_{7}O_{12}$

We report on the microwave, terahertz (THz), infrared and Raman spectroscopic studies of $BiMn_{7}O_{12}$ ceramics, shedding more light into the nature of two structural phase transitions and their possible relation with ferroelectricity in this compound. We observed a softening of one polar phonon in the THz range on cooling towards 460 and 300 K, i.e., temperatures at which $BiMn_{7}O_{12}$ undergoes subsequent structural phase transitions from monoclinic $\textit{I2/m}$ to polar monoclinic $\textit{Im}$ and triclinic $\textit{P1}$ phases. The soft phonon causes dielectric anomalies typical for displacive ferroelectric phase transitions. Microwave measurements performed at 5.8 GHz up to 400 K qualitatively confirmed not only the dielectric anomaly at 300 K, but also revealed two other weak dielectric anomalies near the magnetic phase transitions at 60 K and 28 K. This evidences the multiferroic nature of the low-temperature phases, although the relatively high conductivity in the kHz and Hz spectral range prevented us from directly measuring the permittivity and ferroelectric polarization. Some Raman modes sense the magnetic phase transitions occurring near 60 and 25 K, showing that spin-phonon coupling is relevant in this compound and in this temperature range. The deviation of the Mn-O stretching mode frequency from the anharmonic temperature behavior was successfully explained by the spin correlation function calculated from the magnetic contribution to the specific heat.

cond-mat.mtrl-sci↗

arXiv2023

Modifying the magnetoelectric coupling in TbMnO$_3$ by low-level Fe$^{3+}$ substitution

We report a comprehensive study of the low-level substitution of Mn$^{3+}$ by Fe$^{3+}$ effect on the static and dynamic magnetoelectric coupling in TbMn$_{1-x}$Fe$_x$O$_3$ ($x=0$, 0.02 and 0.04). The cationic substitution has a large impact on the balance between competitive magnetic interactions and, as a result, on the stabilization of the magnetic structures and ferroelectric phase at low temperatures. Low-lying electromagnon excitation is activated in the cycloidal modulated antiferromagnetic and ferroelectric phase in TbMnO$_3$, while it is observed up to TN in the Fe-substituted compounds, pointing for different mechanisms for static and dynamic magnetoelectric coupling. A second electrically active excitation near 40 cm$^{-1}$ is explained by means of Tb3+ crystal-field effects. This excitation is observed up to room temperature, and exhibits a remarkable 15 cm$^{-1}$ downshift on cooling in Fe-substituted compounds. Both electromagnon and crystal-field excitations are found to be coupled to the polar phonons with frequencies up to 250 cm$^{-1}$. Raman spectroscopy reveals a spin-phonon coupling below TN in pure TbMnO$_3$, but the temperature where the coupling start to be relevant increases with Fe concentration and reaches 100 K in TbMn$_{0.96}$Fe$_{0.04}$O$_3$. The anomalies in the T-dependence of magnetic susceptibility above TN are well accounted by spin-phonon coupling and crystal-field excitation, coupled to oxygen motions.

cond-mat.mtrl-sci↗

arXiv2019

Crossover in the pressure evolution of elementary distortions in RFeO3 perovskites and its impact on their phase transition

This work reports on the pressure dependence of the octahedra tilts and mean Fe-O bond lengths in RFeO3 (R=Nd, Sm, Eu, Gd, Tb and Dy), determined through synchrotron X-ray diffraction and Raman scattering, and their role on the pressure induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti- and in-phase octahedra tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). EuFeO3 stands at the borderline, as the tilts are pressure independent. For the compounds where the tilts increase with pressure, the FeO6 octahedra are compressed at lower rates than for those ones exhibiting opposite pressure tilt dependence. The crossover between the two opposite pressure behaviors is discussed and faced with the rules grounded on the current theoretical approaches. The similarity of the pressure-induced isostructural insulator-to-metal phase transition, observed in the whole series, point out that the tilts play a minor role in its driving mechanisms. A clear relationship between octahedra compressibility and critical pressure is ascertained.

cond-mat.str-el↗

arXiv2017

On the ferroelectric and magnetoelectric mechanisms in low Fe$^{3+}$ doped TbMnO$_3$

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. Above that value, a non-polar, weak ferromagnetic phase emerges, in good agreement with the predictions of the Dzyalowshinskii-Moriya model. This behavior reveals the crucial effect of Fe$^{3+}$ substitution in octahedral sites on the magnetic phase sequence, polar and magnetoelectric behavior of the TbMn$_{1-x}$Fe$_x$O$_3$ system. From the results obtained in this work, it is understood that this behavior is not associated with the eg electronic configuration, but instead to the competition between ferromagnetic and antiferromagnetic interactions, which is very sensitive to both local fields and distortions.

cond-mat.str-el↗

arXiv2016

Magnetic phase diagram of TbMn$_{1-x}$Fe$_x$O$_3$ $(0 \leq x \leq 1)$ substitutional solid solution

We present the magnetic phase diagram of TbMn1-xFexO3 substitutional solid solution in the whole concentration range 0 <= x <= 1 as determined from magnetization and specific heat measurements. We have found that the dominant magnetic ion in the concentration range 0 <= x < 0.3 is manganese, while iron ions do not create independent magnetic structure, but strongly affect magnetic properties of the parent compound by reducing transition to magnetically ordered state and transition into a cycloidal phase. The magnetism in the concentration range 0.3 < x <= 1 is driven by the Fe sublattice. The manganese ions again do not order in long range magnetic ordered state, but stabilize four different magnetic structures of Fe sublattice above 2 K. The magnetic ordering of Tb sublattice was observed only on parent compounds TbMnO3 and TbFeO3 and for doping level below 0.1, or over 0.9.

cond-mat.str-el↗

arXiv2014

Structural properties and spin-phonon coupling in orthorhombic Y-substituted GdMnO$_{3}$

We present a systematic study of the structure and lattice dynamics at room temperature, and the phonon behavior at low temperatures in orthorhombic Gd$_{1-x}$Y$_{x}$MnO$_{3}$ manganites, with 0 $\leq$ x $\leq$ 0.4, using powder x-ray diffraction and Raman scattering. A thorough analysis towards the correlation between both structural and Raman modes parameters was undertaken. The data obtained at room temperature reveal structural distortions arising from the Jahn-Teller distortion and octahedra tilting. The Jahn-Teller distortion is apparently x-independent, while an increase of Mn-O1-Mn bond angle of about 0.5° could be ascertain when x changes from 0 to 0.4. Spin-phonon coupling was evidenced from the Raman results. The temperature dependence of the B1g in-plane O2 stretching mode of the MnO6 octahedron has revealed either a positive or negative shift regarding the pure anharmonic temperature dependence of the phonon frequency, which strongly depends on the Y-concentration. The frequency renormalization is explained in terms of a competition between ferro and antiferromagnetic interactions. The ratio between the spin-phonon coupling constant and the effective magnetic exchange integral per spin was determined from the renormalized wave number of the in-plane O2 stretching mode, associated with the spin-spin correlation function.

cond-mat.str-el↗

arXiv2014

Dzyaloshinskii-Moriya nature of ferroelectric ordering in magnetoelectric Gd$_{1-x}$Y$_{x}$MnO$_{3}$ system

This work reports on magnetic, dielectric, thermodynamic and magnetoelectric properties of Gd$_{1-x}$Y$_{x}$MnO$_{3}$, with 0 \leq x \leq 0.4, with emphasis on the (x, T) phase diagram, towards unraveling the role of the driving mechanisms in stabilizing both magnetic and ferroelectric orderings. The (x, T) phase diagram reflects the effect of lattice distortions induced by the substitution of $Gd^{3+}$ ion by smaller $Y^{3+}$ ion, which gradually unbalances the antiferromagnetic against the ferromagnetic exchange interactions, enabling the emergence of ferroelectricity for higher concentrations of yttrium. For $x \leq 0.1$, the paramagnetic phase is followed by a presumably incommensurate collinear antiferromagnetic phase, then a weak ferromagnetic canted A-type antiferromagnetic ordering is established at lower temperatures.For $0.2 \leq x \leq 0.4$, a different phase sequence is obtained. The canted A-type antiferromagnetic arrangement is no more stable, and instead a pure antiferromagnetic ordering is stabilized below T$_{lock}$ $\approx$ 14 - 17 K, with an improper ferroelectric character. From these results, a cycloid modulated spin arrangement at low temperatures is proposed, accordingly to the inverse Dzyaloshinskii Moriya model. Anomalous temperature dependence of the dipolar relaxation energy and magnetization evidence for structural and magnetic changes occurring at $T* \approx 22 - 28 K$, for $0.1 \leq x \leq 0.4$.

cond-mat.str-el↗