RESUMEN
This study provides a comprehensive insight into the effects of controlled off-stoichiometry on the structural and multiferroic properties of the hexagonal manganite LuMn1-xO3+δ (x = 0.02; δ â¼ 0), supported by neutron powder diffraction measurements confirming single phase P63cm symmetry and evidencing a relevant ferromagnetic component, below TN â¼ 90 K, which breaks the archetypal geometrically frustrated antiferromagnetic state typically ascribed to LuMnO3. The perturbations in the triangular disposition of spins prompt an additional electric polarization contribution and a clear enhancement of the magnetoelectric coupling which are in good agreement with the results of first principles calculations. In addition, Raman spectroscopy, dielectric permittivity, pyroelectric current and magnetic measurements as a function of temperature point out the precursor effects of the magnetic phase transitions involving a strong coupling between spins, lattice and electric order, even above the Néel temperature.
RESUMEN
This work reports a detailed infrared reflectivity investigation of the phase transitions in single crystals of sodium ammonium sulfate dihydrate (SASD). The polarized reflectivity spectra allow us to follow the temperature dependence of the polar vibrational modes and detect the critical behaviour of the vibrational parameters through the two low temperature structural phase transitions observed in the compound. The results obtained show that the mechanism of the transitions in SASD is complex, involving a strong coupling between pseudo-spins and phonons. In the paraelectric phase, the driving mechanism of the first phase transition (T(c1) = 95 K) appears to be related to a relaxation with a characteristic frequency that is much lower than the phonon frequencies. In the temperature range corresponding to the first ferroelectric phase (T(c1)>T>T(c2) = 79 K), the dynamics of the lattice change considerably and the parameters characterizing several vibrational modes display anomalous temperature dependences. The second phase transition occurring at T(c2) is marked by an important and discontinuous change of the spectral shape, indicating that a considerable lattice distortion is involved.
RESUMEN
In this work, we have studied in detail the temperature dependence of the electric polarization of Eu(0.8)Y(0.2)MnO(3) aimed at clarifying the controversial issues concerning the ferroelectric nature of the lower temperature magnetic phases and hence its multiferroic character. The existence of a spontaneous polarization in 30 K < T < 22 K, provides clear evidence for the ferroelectric character of the re-entrant non-collinear spiral-antiferromagnetic phase, stable in that temperature range. Contrary to results published in previous works, our experimental data clearly show that the weak-ferromagnetic, canted antiferromagnetic phase stable below 20 K is not intrinsically ferroelectric. The misinterpretation, regarding the polar character of the lower temperature magnetic phases, stems from the existence of an induced polarization occurring below 30 K. The mechanisms associated with polar and magnetic properties, and their correlation with both spin and lattice structures are also discussed.
RESUMEN
Eu(1-x)Y(x)MnO(3), compared against other magnetoelectric systems, exhibits very distinctive features. Its magnetoelectric properties are driven by the magnetic spin of the Mn(3+) ion, but they can be drastically changed by varying the content of Y(3+), which does not carry any magnetic moment. Although the x = 0.40 composition has been studied extensively, some basic areas still remain to be thoroughly understood. Thus, this work is aimed at studying some of its polar properties and their magnetic field dependence as well. The experimental results reported here show that this material is very easily polarizable under external electric fields, and so, whenever the polarization is obtained from time integration of the displacement currents, an induced polarization is superposed on the spontaneous one, eventually masking the occurrence of ferroelectricity. We have found clear evidence for the influence of a magnetic field in the polar properties of Eu(0.6)Y(0.4)MnO(3). The study of electric polarization of Eu(0.6)Y(0.4)MnO(3) under an external magnetic field yields a value with the same order of magnitude of the remanent polarization as was determined from polarization reversal experiments. The comparison of the magnetically induced changes in the polarization obtained for polycrystalline samples and single crystals confirms the threshold magnetic field value for the polarization rotation from the a-direction to the c-direction, and provides evidence of the importance of the granular nature of the samples in the polar response to the magnetic field.