Ti-doped YMnO3: Magnetic and thermal studies at low temperature and dielectric properties at high temperature

Abstract

We study the magnetic, thermal, and dielectric properties in Ti-doped YMnO3. Crystal structure analysis showed that as Mn3+ is replaced by Ti4+ ions, a phase transition from hexagonal symmetry with P63cm space group to rhombohedral symmetry with R3c space group takes place at around x = 0.14 of Ti4+ doping. The local deformation, as well as the partial charge compatibility of the Ti+4 ion at the Mn+3,+4 site, allows solubility up to 20% in the YMnO3 matrix. The magnetic analysis at a low temperature showed that the effective moments, μeff, and the Curie-Weiss temperature, ƟCW, drop rapidly as Ti+4 replaces the Mn+3 ion. This fact indicates that the Mn3+ magnetic moments in the geometrically frustrated antiferromagnetic array are strongly decoupled by the Ti+4 substitution. Also, the magnetic contribution of the specific heat showed that the magnetic transition observed at about 75 K for the pristine sample decreases down to ∼40 K with increasing Ti+4 up to x = 0.20, which indicates that the frustrated (hexagonal) magnetic phase coexists with the rhombohedral phase. On the other hand, the dielectric properties from room temperature to 800 K showed an increase in the dielectric loss with increasing Ti4+ doping, which is detrimental to the ferroelectric properties. Furthermore, the AC conductivity measurements showed three thermally activated relaxation behaviors following the Arrhenius law in three different temperature ranges in the pristine sample. We found that the conductivity behavior is dominated by a single slope from room temperature to 800 K when Ti-doping reaches 20% in the YMnO3 matrix. The local lattice deformation plus hole addition (small polaron) by Ti+4 at the Mn+3 site is the dominant mechanism of conduction in the rhombohedral phase. The gradual increase of holes as charge carriers with increasing of Ti4+ ions in the YMnO3 matrix is also responsible for the increase in the dielectric loss. The results shown here infer that the decoupling of the frustrated AFM lattice and the increase of charge carriers in the magnetically disordered phase by Ti substitution imply a destruction of the ferroelectric state and the magnetoelectricity below the Néel temperature.

Influence of the orthorhombic phase content on the dielectric and magnetic properties of YMnO3

“…With regard to the electrical properties, there are several works where the p-type conductivity in YMnO 3 is discussed in terms of different sources showing inconsistent results between them: while some authors have related this p-type conductivity with a thermally activated small polaron hopping between localized Mn 3+ and Mn 4+ sites, others have pointed to interstitial oxygen or oxygen vacancies as a possible source of this conductivity in hexagonal YMnO 3 [29][30][31][32] . Previous works have reported on the electrical characterization by using impedance spectroscopy, but they are mainly focused on the characterization of the hexagonal phase and offer disagreeing interpretations 16,20,33,34 . As far as we know, there are no published results about the influence of the simultaneous presence of both h-YMO and o-YMO phases on the dielectric properties in polycrystalline YMnO 3 .…”