Investigation of the Structural, Magnetic, Ferroelectric Properties and their Correlations in YFeO3 and its Modified Systems

Abstract

In the search of an efficient magnetoelectric material other than the rare earth orthomanganates, (in which the magnetic transitions and magnetoelectric coupling is at very low temperatures), rare earth orthoferrites has gained considerable attention for their high temperature paramagnetic to canted antiferromagnetic transitions 640-740K for RFeO3 (R=Y, La-Lu). Yttrium orthoferrite (YFeO3) is a simplest of its class for having a non-magnetic Y3+ ion. Since spin reorientation in RFeO3 mainly arises from the magnetic R3+ -Fe3+ interactions, the weak ferromagnetism in YFeO3 apparently arises from the Fe-Fe interactions alone. No evidence of spin reorientation either at low or at high temperatures are found to be present in it. Previous studies have shown that the TN of this system can be markedly reduce below by suitable doping of other transition metal ions such as Cr or Mn in place of Fe. In the present thesis, the various physical properties such as structural, surface morphology, electrical, magnetic, ferroelectricity and magnetoelectric effects have been thoroughly investigated in the modified YFeO3 systems. This includes the investigation on parent YFeO3 through Raman spectroscopy and magnetization measurements that revealed the existence of spin-lattice (phonon) coupling, which influences its ac electrical response. With doping of low percentage Cr (10 at. %), the system exhibited weak ferroelectricity above liquid N2 temperatures while the weak ferromagnetic ordering retained. The magnetization studies at high temperatures also revealed the ordering from Fe-Fe nearest neighbour interactions at T nearly at TN of YFeO3 in this composition. The doped specimen also revealed unusual cluster glass states for T<200K as probed from the ac and dc magnetometry at low temperatures. Astonishingly, a strong magnetoelastic coupling at TN and an isostructural transition is revealed from the high temperature XRD studies. Low temperature XRD also highlighted substantial lattice anomalies at the spin freezing transitions validating the associated magnetoelastic coupling at low temperatures. Temperature dependent Raman spectroscopic studies reveals significant spin –phonon interaction in the medium also which suggests the relaxor like dielectric anomaly can produce the weak ferroelectricity in YFe0.9Cr0.1O3. Similar studies on the codoped specimen Sm0.5Y0.5Fe0.58Mn0.42O3 reveals multiple magnetic transitions at around room temperature, ferroelctricity, strong spin-phonon and electron phonon interactions in the specimen. Further high temperature SXRD studies reveals the anisotropic negative thermal expansion in the Sm0.5Y0.5Fe0.58Mn0.42O3, but absence of magnetostriction. Our study suggests that the parent and modified systems of YFeO3 can be envisaged in multiple technologically enhanced applications.